Analog tape recorder Elex
Contents
1. 350 nw m 250 nw m 02 Figure 11 Over Bias something that is rarely mentioned but often causes grief Distortion in the bias waveform is not a bad thing in itself because the bias waveform is not reproduced However any even order harmonic distortion results in the recording of random noise When there is any non symmetry in the signal the even order distortion products yield the equivalent of a DC signal that will offset the reproduced zero axis This means that all drop outs and surface irregularities become modulation elements resulting in pop corn and bias rock We often see the same result with a magnetized erase head The cogniscenti in my audience are familiar with the sound and it is not pleasant Symmetry problems are most often traced to problems with electronic clipping or magnetic saturation In al most all newer tape recorders there exists some form of adjustment to care for or cause this problem The vari ous stages that require inspection cor rection and alignment are master os cillator master bias bus master erase bus individual channel bias outputs and individual channel erase outputs An oscilloscope is necessary for all AAR aroy EE eae a of this work as is a low ca2. CS A Road Map For Maintenance Engineers The process of reproducing sound from a recorded tape is very similar in function to the operation of a hydro electric generator They both operate on the principle that when a magnetic field cuts across a wire current flows in the wire The recording process works using the inverse of the princi ple when a current flows in a piece of wire magnetic lines of flux are created around that wire Magnetic tape is com posed of very fine particles of mag netizable material glued onto a long chunk ofplastic In the recording proc ess where analog audio is con cerned these particles are mag netized along the plane of travel of the tape They are stacked perpendicular to the plane of travel across the face of the gap that contacts these particles When these stacks of horizontally polarized bar magnets travel across the face of the reproduce head the lines of magnetic flux that surround them are carried by the head across a winding inside the head and a current flows in that winding That current is our audio signal and getting it on and off the tape with the least amount of destruction is our goal HOW HEADS WORK Reproduction The playback head is a transducer that changes the magnetic flux reversals of particles on tape moving past it into electrical signals This can be viewed simplistically as a transformer that uses magnetic tape in motion across the core gap pole pieces
3. rents within the core of a magnetic head They flow in the same direction as the core windings and if permitted to circulate unhampered would be have as if there were a number of short ed turns within the core Such a short circuit would dissipate a considerable amount of energy and decrease the head s efficiency It can be said that these losses are best visualized as a reduction in the cross sectional area of the pole pieces which reduces the amount of flux carrying area resulting in a loss of signal Eddy currents limit the penetration of flux lines into the core center and the flux lines are con fined to the core surface As the fre quency increases this sheath of flux carrying material is no thicker than the skin depth of the core material A lami nated structure is used in most magnet ic heads to increase the number of flux carrying sheaths 3 Eddy currents increase with frequency In the record ing process eddy currents limit the amount of current available to the windings because of the increasing impedance of the head This results in a loss that increases with frequency Core permeability loss at high frequen cies is not as well known as eddy cur rent loss but contributes to its effect In practice the permeability of metal heads changes with frequency partial ly due to eddy current losses and parti ally due to core permeability loss This results in the head changing induct ance as the frequency incre
4. length of the pickup transducer then we only are able to retrieve the maxi mum differential available at the trans ducer contact points See Fig 6 The signal we retrieve from a playback head is directly proportional to the rate of change of the magnetic field as well as the strength of the field Since a 30 Hz signal at 30 ips is over one inch in length the wavelength is longer than the head dimension and therefore the response falls Thirty Hertz at 15 ips has a wavelength of one half inch This usually is within the width of the secondary gap of the reproduce head and the response is still robust Coupling the loss of wavelength resolution to the attenuation we pro vide to correct for head bumps and fringing we see a rapid fall off in the low frequency response of the repro ducer Methods for improving head bumps include increasing the width of the head making the incoming and outgoing wrap angle of the tape non symmetrical about the gap decreas ing the wrap angle of the tape to head ABA KAY SITY IDED 1007 jer gt Tracks Channel in Record 1a Al I ji b Erase Coupling without shield contact area and ensuring that all of the angles of the head around the face of the head are radius d curves and not sharp angles If the head penetration is not field adjustable there is not much the screwdriver wielding maintenance person can do about these low end problems other than taking th
5. SMPTE Vol 74 2Lowman Magnetic Recording 3Finn Jorgensen Magnetic Recording 4 w Scott RE P 5 Sakamoto and Kogure AES 61st Con vention 6 Gundry AES 64th Convention 7 Jorgen Selmer and Jensen AES 70th Convention 8 Staros AES 66th Convention Garrett Studioworks I always suggest that all new machines be used for about a month to let the electronics settle down and stabi lize Tweak it whenever you want then after that it s starting to stable out it holds its setting then you align it After that every day the engineer should clean the machine demagnetize it tweak it up but learn about it before diving in Keep the service manual handy along with service bulletins and any spare parts necessary to keep it running silly little things that cost three four dollars Voltage regula tors pinch rollers lightbulbs springs small items that can put the whole thing out of commission And keep track of how much time you have on a pinch roller Once a month or every two months clean the relays and belts Keep every connection and switch contact as clean as possible Many machines have edge connectors that are alwaysgoing bad so keep a pencil eraser handy to shine em up Cramolin is a product that s good to put on all edge connectors contacts anything else you can get CONTINUED ON PAGE 204 PROFESSIONAL lj lea
6. Subic e a mx width x tan angle wavelength See Fig 2 Azimuth error makes itself obvious with wider track widths such as half inch 2 track format With a 24 track 34 MIX OCTOBER 1987 machine the azimuth can be out as much as 360 degrees between tracks one and 24 and high frequency atten uation due to that error is negligible Thickness of Coating Loss Thickness loss is the primary deficien cy that reproduce equalization is meant to overcome and to understand it we first must explain spacing loss Spacing Loss Spacing loss occurs when the magnetic oxide of the tape is not in contact with the gap of the head This loss iswavelengthdependent For a given separation distance the shorter the wavelength the greater the loss Consider the case of the cassette the tape speed is slow and the high fre quencies have short wavelengths so a Figure 3 spacing of 25 micro inches 000025 will produce 11 dB of loss at 15 kHz The formula for calculating this loss is Spacing Loss in dB distance 3 ao Spacing loss probably is the most significant element that a techie will deal with It rears its ugly head in many forms Most of the time spacing loss occurs because of incorrect head rota tion coupled with wear of the head at the gap This can be corrected in the field whereas the majority of other types of losses mentioned here are a function of head manufacture Spacing loss appears in other ways suc
7. by the electric company or heavy loads from nearby industrial facilities but you should have a line monitor conditioner on the power supply line because in one fell swoop much damage can be incurred Forman Tekcom Starting out with a properly aligned tape deck ensures that you won t have to relap or replace the heads prematurely If the studio doesn t have a mainte nance tech who does daily align ment and calibration they should purchase a series ofalignment tapes so they can check that and do any minor level adjustments needed to keep it running properly If it s a 4 or 8 track format check the align ment every week or two If it s a larger format being used heavily check it on a daily basis standard operation of the machine As can readily be observed from the preceding it is a wonder that we get audio on and off tape at all let alone as well as we do It is a tribute to the art and skill of the design engineers of today s recording equipment that we obtain in the analog realm results that can be so artistically satisfying I hope that this article has armed you with a few tools with which you can optimize your recording system and give continued life to the analog re cording domain Happy tweaking W Formerly chief engineer at Wally Heider Studios L A and SF and technical di rector of Audiotechniques Greg Hanks now heads New York Technical Support References 1K Johnson and D P Gregg
8. lengthy and can be found in the IEFE transcripts authored by Shun Ichi and Neal Bet tram Bias Erasure This is the phenom enon whereby the short wavelength signals top end are reduced in level more rapidly than the long wavelength signals with increasing bias current There are a number of different ex planations for this occurrence Some early investigations explain the proc ess by mapping the geometry of the bias field at the gap and state that the shape of the bias field at the trailing edge of the head or transition zone is responsible for the reduc tion in high frequencies 1 Let me take a step back and note that the recording process takes place as the tape exits the head where the bias field is decay ing past the remnance point of the tape There is a zone where the bias level is at a flux level which no longer will re orient the magnetic domains of the particles of the tape This is called the recording zone If the bias field is of a geometry that allows this zone to be large in relation to the wavelength of the recorded signal then the result ant flux is reduced to zero It has also been said that it can be viewed as though the short wavelengths are being pushed into the tape coating and that thickness loss prevents the signal from being available to the reproduce heads Eddy Current Loss Revisited and Core Permeability Loss at High Frequencies Eddy currents can be defined as the circulating cur
9. other numbers above The preceding begs the question 6 relative to what and that is what makes this reference sensi ble The use of 6 requires contem plation of the reference level in use and choosing either to go to twice that fluxivity or to match an arbitrary level that is recorded on the tape What this reference generally means in the U S is to align the reproducing circuitry of the tape machine to read 3 VU at the reference frequency of 1 kHz when reproducing a 250 nW m test tape conforming to the equalization stand ard in effect for the chosen operating speed This number corresponds to an actual flux level of 353 13 nW mz2 or 5 85 dB above the original Ampex operating level of 185 nW m 510 nW m This 510 nW m is a somewhat newer reference used to align DIN or CCIR reproducers to 0 on a peak reading meter This level corresponds to 9 dB above 180 nW m2 which you will recall is the effective flux of a 1 kHz tone refer enced to a 700 Hz 185 level This effec tively represents a 3 dB increase over the original Ampex level or roughly corresponds to our 3 level of 250 nW m Dolby Level Not to confuse things any more than they are already Dolby labs attempted to influence operating levels on machines that em ployed their noise reduction system The reasoningwent something like If we tell em to use 185 nW m then they won t be limiting the signal with the tape itself
10. record equaliza tion are meant to overcome The slow Figure 5a 36 MIX OCTOBER 1987 er the speed the shorter the wave lengths and the more thickness loss we have The above loss illustrates why newer chrome tapes have better top end The tape surface is now much smoother than older tapes and this means the oxide can have much more intimate contact with the head Eddy Current Loss In the repro duce chain this loss probably contri butes the least to the difficulties of getting quality reproduction of music Eddy currents are created by the lines of flux generated by the windings of the head When the flux from the tape is converted into electrical energy this generates a current flow in the wind Critical Recording Z Debris Backing Oxide Layer ing The current flow in the windings of the head causes lines of flux to be generated around the winding and these are coupled into the core and around and around it goes The net result is that the flux at high frequen cies is confined to the surface of the core Modern heads consist of many laminations of core material This re duces eddy current losses by increas ing the number of surfaces thus in creasing the amount of available flux The net result the slope of the rising response of 6 dB oct is reduced to 5 e dB oct Typically this results in 1 4 to 2 dB loss at 20kHz This is a frequen cy dependent loss so is unnoticeable at lower sp
11. able permeability with a consistent inductance Most newer designs incorporate a divider in the master bias oscillator section to achieve an erase signal that is 1 3 or 1 4 of the bias frequency We need to develop some signifi cant erase current to fully erase a track on a multi track machine and we are constrained by the amount of area that we have available to us to accomplish that task Modem erase heads are built in a checkerboard type of pattern to allow for extra area in which to wind the wire and house the pole pieces This has given us levels of erase pre viously unobtainable with conven tional inline erase heads Incorpo rated in almost all erase heads are multiple gaps which in effect erase the signal twice When we align the erase drivers we want as much signal as possible to get to the head without saturating the pole pieces or clipping the driver am plifier Care must be taken with some of the checkerboard type heads be cause early versions did not include a guard band between vertically adjacent tracks which can result in transformer coupling of the erase signal to the winding s of the tracks above and below the track that is in record This results in partial erasure of the high frequency signal on the tracks that are two tracks above and two tracks below a track that isin record See Fig 10 for a graphic illustration of what I m trying to relate Another concern to be aware of when doing the e
12. and our compandor sys tem will work within its proper dynam ic law and with the improvements offered by our noise reduction system the reduction of operating level will not be negatively intrusive Well as it works out what in fact happens is that Dolby Level refers not to the fluxi vity but rather to the operating level of the facility When the tape recorder reads 0 then the Dolby is aligned to read in the center of the Dolby dot They had the right idea but studio personnel are forever going to do it their own way anyway What falls out of the above discus sion is the determination that operat ing level must be tied to the volume units in use and the alignment method must be specified if true tape inter changeability is to be fully realized Thus a master reel with tones should include a note indicating what meter type should be used such as VU or peak reading and what the reference level should be set to OK so now you understand where some of the confusion about operating level comes from and why losses play such a prominent role in the recording reproduction chain So at this point you can seewhywe have equalization applied both to the playback and re cord systems Let s discuss some of the details Equalization An important item of note the current equalization standards are applied to reproduction only The equalization applied to the re cording and reproduction system is designed t
13. as the primary winding The head is an inductive source This means that with a constant flux level presented to the pole pieces the frequency response rises at 6 dB BY GREG HANKS per octave The output of the head is similar to that ofa miniature generator and it is important to note that the voltage generated not only is propor tional to the amount of flux but also to its rate of change This results in the same response curve as you get with a capacitor in series with a load but with a major difference With a capaci tor in series you have 90 degrees of phase shift but with the head there is no accompanying phase shift Because of this response all tape reproduce pre amplifiers start with a falling 6 dB per octave response which mandates an included 90 degree phase shift The equalization curve necessary to ac complish conformance to the appro priate standard is then applied to this falling 6 dB oct curve Seems simple enough right See the sidebar on EQ curves for an explanation of why most published curves look the way they do If the output of the head is rising at 6 dB oct then why is there high fre quency boost applied to the record signal and why are the EQ curves so different for different speeds Well the reasons are based in trying to over come the losses that are an integral part of the record reproduction chain Among these are gap loss azimuth loss spacing loss thickness loss head bu
14. ases The slope of the high frequency losses is increased by the coupling of these two losses combined Ferrite heads do not show decreasing permeability at high frequencies and the eddy cur rents are negligible which illustrates why there are often difficulties in the simple exchange of ferrite heads for metal heads in the record chain The end result is 15 to 20 kHz being down in record all other factors being equal The only corrective action possible is the replacement of the head with a different type showing lower losses Modern studio recorder heads do not usually demonstrate this property and if they do the integrity of their manu facture is suspect Spacing Loss Revisited The geometry of the bias field is one of the determinants of the high frequency response of the recording chain Spac ing loss in the record side of the sys tem whether caused by debris or a film of air results in a dynamic change of the shape of the recording zone The effect of this change is the same as under bias which results in an in crease in distortion and an elevation of the high frequencies See Fig 5 So now you see how difficult it is to get audio on tape and how much more difficult it is to get the stuff back Enough about losses Let s get into some of the meat and potatoes issues that we face every day Operating level One of the more confusing aspects of studio life for the aspiring Techie is the mathema
15. ble flux The rising response of the record equalization is intended mainly to overcome the rising imped ance of the record head with frequen cy to provide us with the ability to drive the record head with a constant current Because some of the losses of the system are frequency dependent while others are wavelength operat ing speed dependent the EQ require ments change with the operating speed The trade offs between head room and noise were made long be fore we had the need to adjust or repair our equipment so I won t belabor the philosophies involved in the decision as to where boosts and cuts belong but rather will try to illustrate where they are and how to look at the system when we have response problems to address Equalization standards are ex pressed most often as two different time constants such as 3 180 and 50 psec These numbers refer to the time constants of the combination of resis tors and capacitors comprising the fil ters that accomplish the equalization for example T RxC Therefore for the NAB 15 ips standard T 50 usec and C 0luF R 5 KQ Since the 3dB point of 5 kQ in series with OluF is calculated with the formula l Remember the frequency response of the reproduce pre amplifier starts at a falling 6 amp per octave and the above time constants refer to break points on this curve This means the low fre quency EQ results in a flat response from the pre amp but this trans
16. e When we align the erase drivers we want as much signal as possible to get to the head without saturating the pole pieces or clipping the driver am plifier Care must be taken with some of the checkerboard type heads be cause early versions did not include a guard band between vertically adjacent tracks which can result in transformer coupling of the erase signal to the winding s of the tracks above and below the track that is in record This results in partial erasure of the high frequency signal on the tracks that are two tracks above and two tracks below a track that is in record See Fig 10 for a graphic illustration of what I m trying to relate Another concern to be aware of when doing the erase alignments is heat Most modem erase heads re quire the presence of tape on the front of the head while it is energized be cause the head utilizes the tape as a heat sink to help keep the internal temperature of the pole pieces within operational limits If the head gets too warm the inductance of the pole pieces shifts As almost all recorders are designed to resonate the erase head for maximum signal when the inductance changes so does its reso nant frequency This can result in an alignment that is incorrect for the FROM PAGE 37 MAINTENANCE EXPERTS ment should be cool and dry around 40 relative humidity The deck should be on all the time The danger in this would be AC power line disturbances caused
17. e heads to a service facility that can change the geometry of them There are a few things that can be done electronically to improve the very low frequency response of a reproducer and one of them was put forward by J M amp night in a paper presented at the 1976 AES convention MCI took him up on this one and incorporated this change into their JH 114 and JH 24 series machines This change is responsible for the problems associated with the LF con trol affecting the wide band gain when it is at the end of its attenuation range See Fig 9 Bias and Related Arguments Let s re examine some knowns The mechanism aiding the transfer of the current that s an analog of our audio signal to the strip of iron oxide we call tape familiarly is termed bias This is a high frequency high amplitude sig nal added to the audio signal The benefits of employing bias are well known and many Some of them in clude increased recording efficiency reduced noise residual reduced signal distortion and improved high frequen cy response The bias current causes a high fre quency magnetic field to exist about the vicinity of the recording head gap To effectively function as an aid to re cording the bias field must fully satu rate the oxide as it passes over the record head gap As the tape enters the flux zone it alternately is saturated in both polarities by the bias field The tape then proceeds to the critical recording z
18. e to the listener was limited by a scandalous set of rules better suited for raising poultry these standardswere responsi ble for practically annihilating an al ready endangered species the crea tive musician For further information and a cata log write to New Music Distribution Service 500 Broadway New York NY 10012 or call 212 925 2121 a FROM PAGE 44 ANALOG ELECTRONICS we can conclude that the trailing edge s of the record head gap s are not in perfect vertical alignment This may be caused by gap scatter or gap width differentials that are symmetrical about the gap center line Gap scatter is easily identified in playback whe reas varying gap width is not The net result is that the optimization of the record process may be at the expense of sync reproduction and vice versa Distortion vs Levels The amount Why EQ Curves look the Way They Do Most of the time when we see the response of the reproduce section of a tape recorder illustrated graph ically as in the case of test tape curves or in the back pages of the service manual these curves look like those in Fig 7 What you are seeing in these curves is what response would be if there were a constant flux of chang ing frequency present in an ideal reproduce head However if you disconnect the head and attach a signal generator that has a constant output and sweep it you will see a curve similar to the one in Fig 8 This is due to
19. eeds while observable at 15 and 30 ips Head Bump Loss When the re corded wavelength on tape approach es the overall dimensions of the two head pole pieces the pole pieces be gin to act as a second gap This leads to a rise in the output of the head When the wavelength is twice the length of the contacted pole pieces there is a maximum addition of available flux Conversely when the wavelength is equal to the combined second gap we have a cancellation of output be cause the additional flux output is zero The resulting curves can be seen in Fig 3 Recording Getting the signal back off the tape is much more difficult than getting it on there in the first place The recording process operates on the principle that when a current flows through a wind ing lines of flux are generated around that winding These lines of flux are carried by the pole pieces of the head to the surface of the tape There is a gap perpendicular to the travel of the tape and this gap has a magnetic field across it that is polarized along the plane of tape travel Being that we are glossing over the basic physics princi ples of the recording process will simply state that obviously we are all well aware of the non linearities of tape and thusly are cognizant of the role of bias in the recording process Recording takes place at the trailing edge of the gap and not across the gap itself The above mentioned phenom enon is what accounts
20. f number recon ciliation These tapes employ fringing compensation and use 1 kHz as the reference frequency 250 nW This was the first 3 or elevated level reference fluxivity The reference frequency was 1 kHz and was first introduced by Standard Tape Laboratories This number was 2 85 above 180 nW m which is the fluxivity of a 1 kHz tone from a 185 nW m tape referenced to 700 Hz at 7 or 15 ips This is rounded to 3 for convenience MRL also introduced a 250 nW m test tape because this number finally conforms to the ANSI standard number sequence The level differential between the STL and MRL tapes is due to the fringing compensa tion incorporated in the MRL tapes This has become the DIN standard for cassette operation using 333 Hz as a center frequency The Japanese have adopted this standard for cassette production with the reference fre quency conforming to the ANSI pre ferred number of 315 Hz 320 nW This is the DIN standard level for 19 to 76 cm s 7 to 30 ips First established as peak recording level way back when this level was to correspond to 0 on a peak program meter This is curiously enough 6dB over the approximate levelof the orig inal NAB standard These days this level often is referenced to 0 VU or 6dB on a peak reading meter This level is referenced to 1 kHz 6 Ambiguous as it may seem this number actually makes more sense than trying to refer to the
21. for the drastic phase shifts that occur in multi track machines with their bias aligned at 1kHz or below Let me move into a discussion of the losses that affect the recording Critical Recording Zone eS Figure 5b process to better illustrate the reasons for the equalization that we apply There are a number of losses that occur in recording as well as reproduc tion including demagnetization bias erasure eddy current revisited and spacing revisited Demagnetization When record ing takes place we are orienting and magnetizing the magnetic particles rust that are glued onto the tape Refer to Fig 4 for a simplistic picture of a representative magnetic flux pat tern on tape Upon inspection we see the patterns from one group of parti Tape Backing Oxide Layer cles interact with adjacent groups of particles This occurs in both longi tudinal and perpendicular magnetiza tion The result is a reduction of the flux available to the head The de magnetization loss for longitudinal orientation is greater at short wave lengths and perpendicular losses are greater for long wavelengths A bell shaped curve is the result Modem tapes use longitudinally oriented par ticles so the predominating loss is at shorter wavelengths Low frequency boost is the result of demagnetization losses The formulae that describe this Wavelength sia a NAAA Available Flux 38 MIX OCTOBER 1987 O loss are
22. h as registration error The manner in which this usually distinguishes itself is in meter bounce although this also occurs with racking error men tioned above Registration error is the same thing as head height errors be tween the recorded track and the re produce head gap location Registra tion error common to all tracks in a multi track head can be corrected by changing the height of the head whereas the situation where only a couple of tracks are off can be cor rected only by head replacement When the reproduce head track width is not centered on the recorded track there is less total flux available to the reproduce head and the overall signal level is reduced Thickness Loss Revisited When the tape is recorded the oxide coating depth is completely penetrat 10 000 20 000 Oo ed by the bias field Thus the entire depth of the tape is recorded When the tape is played back the flux field s of all the layers of the magnetic parti cles that comprise the tape contribute unequally to the reproduced signal Because of the spacing loss phenome non noted above only the longest of wavelengths fully contribute to the re produced signal with only the surface layers of magnetized particles contri buting to the high frequency reproduc tion This gives us a falling frequency response that is tape speed depen dent It is this high frequency loss that both reproduce and
23. ing to strain your resources you ll never have enough money to keep it in print be cause the money won t come back quick enough If you have an unsuc cessful record then you have a prob lem too because you have all these records lying around and no one s pay ing for them So you ve got to hit this middle ground a weird way of looking at things One unique side effect of NMDS s continued albeit modest success is that its very existence serves as encour agement to outcast musicians whose chances in a conservative record indus try are next to nil When it started there weren t many musicians making their own records Evelev reports We showed that if you can make your own record and there s a place to take it it doesn t have to sit in your living room propping up your coffee table You can take it here we ll send it out to stores to press people to radio peo ple try and promote it in some limited way and try and help you sell the records In essence NMDS has helped gen erate a Warholian utopia everyone once authorized has a shot at 15 minute celebrity In addition NMDS has admirably bucked the monolithic record industry and shown that inde pendence and artistic self sufficiency are possible Carla Bley writes Until the recent unruly springing up of musicians la bels began in defense against corpo rate impenetrability the selection of interesting new music availabl
24. lates to low frequency attenuation The high frequency response also is flattened out with EQ but this translates to high frequency boost We also have a boost requirement in record so we have high frequency boost in both record and reproduction This overcomes all those losses we described earlier Much of what we re discussing deals with fixed EQ We have adjust ments in our machines both for play back and record The playback adjust ment control has the affect of changing the short time constant or high fre quency turn over point of the equal izer Since most of the high end prob lems we encounter in a studio environ ment deal with spacing loss if we crank up the high end EQ to make up for dirt on the heads gap loss or reso nance losses we end up bringing the mid band up in level as well Remember that the turnover point of the standard for 15 ips is 3 180 Hz The result of the turnover frequency being so low coupled with the varia ble element changing the turnover point means that turning up the equal izer for 3 dB at 16 KHz can end up bringing 4 kHz up about 1 as well The net result of all this is that we must find out which one of the many losses is responsible for the problems we face For example I have encountered machines that when new heads were put on them you could not turn down the top end Upon inspection I found there was an EQ limiting resistor in series with the high frequency cont
25. le 021 on Reader Service Card It Should Sound Better Bryston has always thought so Their latest product the 6B is a professional monaural power amplifier that can be switched between bridged and parallel operation With enormous current handling capability even in bridged mode the 6B sets a new standard for certain demanding monitor applications Bridged it delivers 500 watts into 8 ohms 800 watts into 4 ohms In parallel it delivers 500 watts into 2 ohms 800 watts into 1 ohm As with Bryston s renowned 4B 3B and 2B LP the reliability and sound quality are superb There s more to an amplifier than a spec sheet call Studio Consultants to find out why Bryston is the only amplifier we sell and to discuss a demonstration in your facility AMS AudioFile API Audio Developments Audio Digital Audio Kinetics Bryston Fostex Professional Rane Valley People Westlake White and other fine products stu pont tant 321 West 44th Street New York NY 10036 212 586 7376 Equipment support and design services for professional audio facilities and broadcasters Circle 022 on Reader Service Card OCTOBER 1987 MIX 39 20 Figure 7 the confusion There are a number of standards adhered to including NAB The NAB operating level was specified as 8dB below 3 distortion using an appropriate tape such as Scotch 111 There was no fluxivity specified but it generally fell in the 160 nW
26. lements resulting in pop corn and bias rock We often see the same result with a magnetized erase head The cogniscenti in my audience are familiar with the sound and it is not pleasant Symmetry problems are most often traced to problems with electronic clipping or magnetic saturation In al most all newer tape recorders there exists some form of adjustment to care for or cause this problem The vari ous stages that require inspection cor rection and alignment are master os cillator master bias bus master erase bus individual channel bias outputs and individual channel erase outputs An oscilloscope is necessary for all AND MAIV ACTADECD 1007 of this work as is a low capacitance probe The specifics vary from machine to machine but the concept remains the same We must insure that the bias and erase signal s that are getting to the head are as free of even order distortion as possible While we re on the subject of satur ation it is a good time to discuss bias and erase frequency We need the bias signal to be at least five times greater than the highest audio frequency of interest This is obvious for any fifth order harmonic of the highestfrequen cy will produce a beat frequency with the bias signal and this beat product will become an integral part of our recorded signal Whistles do not agree with flutes or drums In practice this has resulted in most machines using a frequency of 120 kH
27. m range This has evolved into the current 160 nW m specifica tion used by the NAB for the tape car tridges commonly used in broadcast ing Most broadcasters are now using 200 to 250 nW m as their in house operating level 185 nW This is the original Am pex operating level established as the amount of flux per unit area that was 6dB below the 3 distortion level of the tapes in common use at that time This fluxivity refers to a 700 Hz tone recorded at 7 or 15 ips where the playback head pole pieces cover the full recorded track Mono machines were the norm and this operating level and the test tapes utilizing it were fully sufficient for the purposes of align ment This level has also been adopted by the NAB for the purposes of 7 and 15 ips open reel formats There are no compensations employed in the test tapes that use this level 200 n w Magnetic Reference Iab oratories introduced this as a means of conforming the reference fluxivity of their test tapes to the ANSI preferred number series There is much conster nation and argument about the rela tionship between 185 and 200 nW m It s said that this differential is due to the differences of open and closed magnetic measurements But accord 40 MIX OCTOBER 1987 ing to Magnetic Reference Laborator ies J McKnight who is the person responsible for the introduction of this level the move to 200 nW m was only for the purpose o
28. mp loss and eddy current loss Gap Loss This is probably the most widely comprehended loss in the reproduce chain The head re sponds to the changing magnetic pat tern on the tape as it is moved across the head When thewavelength is equal Theoretical Ra Wavelength Head OCTOBER 1987 MIX 33 Theoretical 6db Oct is Wavelength Head Gap Figure 2 to the gap size the output of the head is zero This is the familiar function sin X The response that accompanies gap loss is shown in Fig 1 Azimuth Loss Itis well known that the high end will get a bit mud died when the playback head is on a tilt This is known as azimuth loss The reason the top end disappears is that this is a wavelength sensitive loss The mechanism of the loss is best ex plained by remembering that the amount of current which flows in the head winding is proportional to the amount of flux differential which ap pears across the gap When the head is not perpendicular to the plane of travel of the line of magnetism remem ber there is a line of magnetic particles perpendicular to the travelof the tape then there are fewer particles contri buting to the differential of flux at the gap The amount of loss is greater when the track width is larger It also is greater when the wavelengths are shorter The formula that describes this loss is Alignment Loss in dB sin 7x width x tan angle wavelength AEO ann
29. o overcome the above men tioned losses as well as to provide an improvement in the signal to noise ratio of the system This task is broken into pre equalization applied during recording and post equalization ap plied during reproduction The divi sion of the quantities of equalization used between recording and playback should not be an arbitrary one The 6 db oct curve 74 20 Figure 8 trade off that occurs is between high frequency headroom and high fre quency noise Unfortunately these two goals are mutually exclusive The reproduce equalization is mandated by the various standards that are in place and the equalization applied to the recording chain is that correction field V 10 000 20 000 required to create recordings that reproduce in conformance to the above mentioned standard This means that the record EQ is tailored to the machine tape and head characteris tics of the recorder and tape used See sidebar Remember when we were talking 703 455 8188 tix 510 0281 898 Circle 023 on Reader Service Card studio c nsultants ine 321 West 44th Sheet New York NY 10036 242 586 7376 OCTOBER 1987 MIX 44 about the reproduce head having a response that rises at 6 dB oct with a constant flux Well to get a constant flux level we need a constant current in the record head Remember eddy currents and the other losses of the record head that deprive the head of availa
30. one which contains a field intensity below the tape satura tion level corresponding to the linear magnetization characteristic of the oxide Our audio signal is amplitude modulating the bias field in this re gion When the dimensions of the wavelength of the modulating signal are significantly longer than the size of the recording zone the magnetic un dulations representative of the ampli tude modulation of the bias field our audio signal in other words remain on the tape as it leaves this zone Sig nals that modulate the bias field whose wavelength is shorter than the length of the critical recording zone will not remain on the tape as it passes out of the recording zone and will be of sig nificantly lower level This in a nut shell is how and why recording takes place on the trailing edge of the gap When we set the bias on a machine it is common practice to set it while observing the playback monitor and recording a 1OkHz signal at 15 ips We then set the bias so that we overbias the tape by 3 dB Or when operating at 30 ips we over bias the same amount with a 20 kHz signal or 10 kHz over bias 1 dB This method best estab lishes the size and shape of the critical recording zone for if the bias level is established say at the maximum 1kHz sensitivity level minor reproduction level variations represent major varia tions in the shape and size of the recording zone Any variations in the size of the rec
31. ording zone represent a shift in time for the signal recorded This results in an apparent phase shift of the recorded signal and will result in high frequency losses of the summed reproduction of the record ed signals The above mentioned phenome non may be put to use in determining the accuracy of manufacture of the re cord head in a multi track machine If we set the record head azimuth using the summing method in sync playback we have aligned the center line of the gaps to be perpendicular to the record ed field on the test tape When we then inspect the azimuth of the record signal on the same machine and find that the azimuth has shifted slightly CONTINUED ON PAGE 200 MUSIC NOTES go to record good places for pressing Everybody should start a label Evelev exclaims in fact Evelev s own Icon label was picked up for national distri bution by the prestigious Nonesuch label Among his projects have been records by Daniel Lentz the Javanese pop group Group Gapura and an elaborate setting of music by Ennio The Good the Badand the Ugly Mor ricone by John Zorn But it was no easy feat for Evelev to launch Icon I should know enough not to do it You need the funding to start and then you have to figure how to issue records and get money back quick enough or spend little enough so that you can issue more records and that s always been a problem Any time you have a successful record you re go
32. pacitance probe The specifics vary from machine to machine but the concept remains the same We must insure that the bias and erase signal s that are getting to the head are as free of even order distortion as possible While we re on the subject of satur ation it is a good time to discuss bias and erase frequency We need the bias signal to be at least five times greater than the highest audio frequency of interest This is obvious for any fifth order harmonic of the highest frequen cy will produce a beat frequency with the bias signal and this beat product will become an integral part of our recorded signal Whistles do not agree with flutes or drums In practice this has resulted in most machines using a frequency of 120 kHz or greater The higher the bias frequency the higher the audio frequency response can extend There are practical limits on how high we can get the bias frequency and one of them is eddy current Eddy current losses result in heat which lowers the permeability of the head resulting in lowered magnetic effi ciency Newer head designs using ex tremely thin laminations and the en tire ferrite family exhibit very low eddy current loss This has enabled modem recorders to utilize bias frequencies of up to 500 kHz with 240 kHz being quite common The frequency of the erase signal on the other hand is kept much lower We desire a frequency which will have negligible eddy cur rent loss and a st
33. rase alignments is heat Most modem erase heads re quire the presence of tape on the front of the head while it is energized be cause the head utilizes the tape as a heat sink to help keep the internal temperature of the pole pieces within operational limits Ifthe head gets too warm the inductance of the pole pieces shifts As almost all recorders are designed to resonate the erase head for maximum signal when the inductance changes so does its reso nant frequency This can result in an alignment that is incorrect for the 10 4 1020 nwm 700 nw m 510 nw m 350 nw m Distortion 250 nw m 05 02 Figure 11 2 5 3 Over Bias something that is rarely mentioned but often causes grief Distortion in the bias waveform is not a bad thing in itself because the bias waveform is not reproduced However any even order harmonic distortion results in the recording of random noise When there is any non symmetry in the signal the even order distortion products yield the equivalent of a DC signal that will offset the reproduced zero axis This means that all drop outs and surface irregularities become modulation e
34. rn the techniques and t echnology of the professional recording studio m Recording and Mixing m Editing Tec hniquesi Signal Processing m Disc Recording Console amp Outboard Gear m Digital Logic m Audio Systems Design Professional Studio Intemships m Classes filling now Call Today 212 777 8550 Insttute of Audio Research 64 University Place Greenwich Village New York NY 10003 Licensed by NY State Dept Education Approved for Veterans Training Established 1969 Financial aid for those who qualify Circle X233 on Reader Service Cord OCTOBER 1987 MIX 203
35. rol By lowering its value by half we could bring the machine into alignment Most machines also incorporate some form of circuitry to overcome the effects of gap loss The simplest form of compensation is head damp ing You often will find that the head is terminated in a transformer and the secondary of that transformer will have an R and a C in parallel across it These elements provide critical damp ing of the head and may be varied for optimal flat response in the 12 to 20 kHz area There are many benefits in opti mizing this area of the circuitry not the least of which is the phase re sponse of the machine When the re producer is made flat the transient response of the machine sounds bet ter Take care when doing this change for the damping elements on the front end of the repro pre amp correct for gap loss which is a wavelength de pendent phenomenon in a manner that is frequency dependent This re sults in the optimization being valid only for one operating speed If this optimization is to be performed do it at the highest operating speed where the losses are at a minimum Other wise the extreme top end will take off and brittleness and hiss will result If you think about it you can see why the high end at 30 ips sounds so much better than 15 ips because the wavelength dependent losses are half as great there is much less high fre quency boost in the playback equaliz er But
36. ted Examination of each parameter af fected by bias current as specified on the manufacturers data sheets pro vides a much clearer picture hopeful ly Something missing on most spec sheets is the variability of bias current for minimum distortion vs operating level and the effects of self bias This phenomenon has been studied recent ly 67 and has resulted in products that appear mostly in cassette and home equipment such as the Dolby HX system Applying this to profes sional recorders requires looking at the study that was done by T Staros of MCTIB and relating his curves See Fig 11 to your operating level Bear in mind that music has a dy namic envelope that the VU meter does not see If we are operating at 46 then we have to assume that most of our signal content will actually have a flux density of anywhere from 370 to 1 100 nW m2 We therefore would like our minimum distortion point to be about 3 dB above our chosen test tape and sine wave operating point Gener ally this falls at around 3 dB over at 10 kHz at 15 ips By the way the over bias point is wavelength sensitive there fore it is very important to note that 3 dB over at 10 kHz at 15 ips is not the same bias point as 3 over at 30 It is much closer to 142 over Bias symmetty or lack thereof is 10 i020 nw m 700 nw m 510 nw m
37. the rising 6 dB oct curve mentioned in the accompany ing article Since all magnetic repro AMM riny PVT YADIE D 1007 of bias current that is applied to the record head will affect distortion high frequency response biased tape noise and recording sensitivity Many assume that these characteristics are tied together and that setting the bias current for minimum biased tape noise puts you at minimum distortion and near maximum record sensitivity This is not so it only will put you at the operating point of minimum bias tape noise The formulations of various tapes often cause the minimum noise duction systems exhibit this basic curve it is assumed in all literature and equalization discussions It seems obvious now but how many of us believed that the low fre quency EQ differences between CCIR and NAB were in fact a low frequency roll off in NAB that re quires a boost in recording Recording curves do not exhibit the inverse of the 6 dB oct re sponse In fact there is only a slight rise in the high end and low end for the NAB curve on the order of 2 at 10 kHz and 5 at 40 Hz When ever we look at reproduce equaliza tion curves unless it is otherwise stipulated assume that the curve is the result of the difference between a falling 6 dB octave line and the result of the equalization G H point and the minimum distortion points to overlap at the same bias cur rent but you can t take it for gran
38. this gain is not without payment and that occurs in the bottom end Low end problems at higher tape speeds are caused by a number of things including head bumps second ary gap effect and recorded wavelength versus pick up size Head bumps and secondary gap effects affect us just as much at 15 ips as they do at 30 but at 15 the frequencies F Zn RG Time Constant Tape Speed Standard Tape Speed i Low Frea High Frea hee 3180 90 psec NAB and IEC 3 75 ips The low frequency EQ point is calcu ma 70 psec IEC and CCIR 7 5 ips lated with the resistor and the capaci 3180 50 usec NAB 7 5 and 15 r tor in parallel The following time con oo 35 psec IEC and CCIR 15 o stants provide the appropriate operat ae 1 y ing EQ standards TA pes a 30 ips 42 MIX OCTOBER1987 ERASE HEADI S with shield Figure 10 of disturbance are an octave lower than at 30 and usually fall below the lowest frequencies of interest But at 30 these gremlins tend to jump out at the 50 to 125 Hz region and the extremely long wavelengths at the frequencies below this make reproduction extremely dif ficult to achieve Consider this we are able to extract the maximum signal from tape as long as the pickup transducer dimensions are greater than the wavelengths to be reproduced and the gap is smaller than the shortest wavelengths of inter est When the wavelength of the repro duced signal is greater than the overall
39. tical relationships of standard operating level I am fre quently asked why the 250 nW m test tape is called a 3 operating level when the standard operating level is 200 nW mz Also is 6 6 dB above 200 or 3 above 250 or is it based upon a European reference level Well I hope that the following list and expla nations of the most commonly used reference levels will explain some of J L Hartley Pro Audio Support 44831 Fremont Blvd Fremont CA 415 490 0818 QUALITY MOTOR REFURBISHING J L Hartley Pro Audio Support Group offers ultra precision machining for specialty on hand custom modification demands Our motor refurbishing is an involved process assuring a new long motor life for your recorder Your motor rebuilding will entail complete motor disassembly magnet fluxivity check charging if necessary shaft TIR checked capstan shafts blasted for fresh tape surface New high grade bearings are used in accordance with original manufacturing specifications The rotor pancake is chemically cleaned and treated Your motor is always precision handled and reassembled All prices include parts and labor for a standard motor refurb CAPSTAN MOTORS REEL MOTORS CINEMA MAC VIDEO MOTORS Offering Full Service Support For AMPEX MCI 3M and all pro audio recorders QUALITY MOTOR REFURBISHING Before J L Hartley Pro Audio Support 44831 Fremont Blvd Fremont CA 415 490 0818 Circ
40. z or greater The higher the bias frequency the higher the audio frequency response can extend There are practical limits on how high we can get the bias frequency and one of them is eddy current Eddy current losses result in heat which lowers the permeability of the head resulting in lowered magnetic effi ciency Newer head designs using ex tremely thin laminations and the en tire ferrite family exhibit very low eddy current loss This has enabled modem recorders to utilize bias frequencies of up to 500 kHz with 240 kHz being quite common The frequency of the erase signal on the other hand is kept much lower We desire a frequency which will have negligible eddy cur rent loss and a stable permeability with a consistent inductance Most newer designs incorporate a divider in the master bias oscillator section to achieve an erase signal that is vs or 4 of the bias frequency We need to develop some signifi cant erase current to fully erase a track on a multi track machine and we are constrained by the amount of area that we have available to us to accomplish that task Modem erase heads are built in a checkerboard type of pattern to allow for extra area in which to wind the wire and house the pole pieces This has given us levels of erase pre viously unobtainable with conven tional inline erase heads Incorpo rated in almost all erase heads are multiple gaps which in effect erase the signal twic
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