ML1 Signal Balance, Cable Testing
Contents
1. To calculate the balance error the ML1 measures amplitudes and signs of Live and Return signals The detailed calculation is listed in the Appendix Example a Live and Return signal being of same amplitude but opposite in sign gt a signal balance error value of 0 displayed b Live signal amplitude being two times the amplitude of Return i e a 6 dB difference will result in a balance error of 33 c One signal broken will result in display value near 99 9 page 2 of 8 Application Note Signal Balance Error 3 Applications By the measurement of the signal balance error discrepancies concerning the way of signal transmission or signal generation show up immediately a Example unsymmetrical amplifier Unbalance can also originate at the source As an example Live and Return output stages of an amplifier may produce unsymmetrical voltages This is no matter of concern as long as the sum of both voltages is same to the perfect symmetric case and as long as phases are unchanged But attention at high signal levels the output carrying the greater level may start to clip So better unsymmetrical should be avoided E g the level of pin 2 gt pin 3 is causing reducing signal levels and loss of sound quality of the outgoing analog audio signal To determine and localize this problem take the Minilyzer select the BALANCE mode and the non symmetric o2. being almost identical as well in amplitude as in phase Subtraction here will cancel out the same amplitude and same phase slice of the interferences only leaving the small rest originated by the difference between almost identical and identical So using a perfect balanced connection the signal to noise ratio of the transmitted signal will be recovered even if severe same amplitude same phase interferences act on the transmission line for example hum originated by a ground loop XLR Input Fig 2 2 Minilyzer XLR input connector 3 The Signal Balance Measurement As per our considerations we conclude that balance conditions are of major importance in noisy environments The measurement results level signal to noise ratio S N total harmonic distortion plus noise THD N which is closely related to S N even polarity of a signal may be altered considerably with improper balanced connections To get a quick idea on the actual balance conditions the ML1 includes signal balance measurement This is not only applicable to check XLR connections Unexpected balance states may also point to improper measurement set ups as provoked by unsuitable or even broken cables Being so important to the signal transmission the signal balance is measured in detail in the BALANCE mode and it is also monitored during Level THD N and Polarity measurements How does the balance error measurement is accomplished
3. scale e Press enter to confirm your setting Input Level RVUS page 7 of 8 Signal Transmitter XLR Application Note Signal Balance Error b Calculation of signal balance error Signal balance error is calculated including the Value and sign U2 is the voltage at pin2 U3 the voltage at pin3 both voltages are measured relative to pin 1 Fig A 4 schematic diagram U1 U2 Signal Balance Error U1 U2 sign U1 U2 Operating the ML1 within BALANCE mode the Value is displayed in and the sign is indicated as an arrow In case the level at pin 2 is greater than the level at pin 3 the display will indicate 2 gt 3 Balance indication within other measurement modes Level THD N Polarity will show a filled triangle shifted from middle position toward the 2 end of a horizontal line You may imagine a pair of balances with more weight pushed to the pin 2 scale Strictly looked into the signal we should not only respect sign but also deviations from 180 phase differences ML1 assumes that these deviations are small enough to be neglected an assumption justified for purpose of signal balance measurement XLR Minilyzer ML1 Accuracy Measuring Accuracy Typically impedances of the ML1 XLR inputs Live and Return relative to ground are 20 kOhm But this impedances feature tolerances within 1 and 1 margins The measuring accuracy
4. 0068088088088 8O886 e ee e o e o o e eee eee 6 coe ooo e ee e 9068S OHOSOSHSOO Product Subject This application note describes the signal balance error as measured by the Minilyzer ML1 It introduces the reader to the backgrounds and advantages of balanced audio signal transmissions list abilities and benefits of the ML1 signal balance error analysis and explains how the signal balance error is calculated Contents 1 Balanced unbalanced connections 2 XLR connection balanced signal 3 The signal balance measurement 4 Applications Appendix a User manual signal balance error b Calculation of signal balance error 1 Balanced unbalanced connections An unbalanced connection is a two wire system Of course both wires transmit the audio signal but one called shield reduces interference of external fields on signal transmission to a great extend This shield normally is connected to ground A balanced connection is a three wire system Two wires carry the signal one in original polarity one in inverted polarity the third is the shield which as with unbalanced connections normally is connected to ground The balanced system is more immune to noise and is by far the preferred method in professional audio The most common balanced connector is the XLR type Minilyzer ML1 Signal Balance Error Application Note The unbalanced system is more susceptible to noise pick up and it is generally n
5. larity measurement function XLR input only the Minilyzer ML1 permanently displays the input signal balance in qualitative terms The graphical indicator for this measurement is located in the upper right area of the LCD LEWEL 9 989bH2 LIHEAR SETUP _ 2 8 88 dBu 4A HRH PE U U U U U 15 0 S O d u DIU 10 0 Fig A 1 ML1 screenshot level This field shows the deviation from level match of the incoming balanced signal between pin 2 and pin 3 in percent The position of the arrow indicates the following Arrow in center the input signal is balanced e Arrow out of center linear indication of a balancing problem e g arrow moves left nearer to the number 2 shows the signal level on pin 2 is higher than on pin 3 e Left or right end the signal balance error is 33 or higher 33 equals a difference in the signal level of 6 dB UNBAL the signal balance symbol changes to UNBAL at the signal balance error exceeding 90 For detailed indication the user may select the Balance mode ird OCT SCOPE 5 0dEurtl 10 0 Fig A 2 ML1 screenshot menu In addition to the permanent indicator ML1 offers an exact Signal Balance Error measurement To activate this mode proceed as follows e Switch ON the ML1 e Press the enter key to open the measurement functions submenu and select the entry Balance by using the cursor keys e Confirm by pressing the enter key The signal balance e
6. of the Minilyzer ML1 depends on the impedance of the signal transmitter s output resistors Rou Figure A 5 outlines the error margins of balance error measurement with 1 tolerances of ML1 input impedances at equal generator output impedances Rur The margins are calculated assuming a balanced source with equal open circuit voltages This assumption yields the broadest error bounds As source voltages get more unbalanced the error margins will be smaller Tr E e a errr 100 1k 10k 100k Impedance of Rout ohm Fig A 5 measurement accuracy The preceding calculation assumed purely resistive impedances In reality capacitive loads may play in inductive components can be neglected for short cable length This evokes phase shifts As pointed out the signal balance error measurement does not include phase measurement only sign is respected Anyhow different capacitive loads at pin 2 and pin 3 will show up if significant Low passes formed by resistors involved and capacitive loads will alter absolute voltage values at pin 2 and pin 3 differently page 8 of 8
7. ot preferred in professional work The most common unbalanced connector is the 4 phone connector also known as RCA connector 2 XLR connection balanced signal 3 pin XLR cables are used as standard electrical interconnections between professional audio devices The significance of this interconnection type rests on the so balled balanced mode Figure 1 shows the essentials of the balanced signal interconnection The three XLR pins are assigned as follows 1 Shield ground X 2 Live L 3 Return R EEn N ean dv Ly Fig 2 1 balanced mode signal transmission For a perfect interconnection in balanced mode the two signals Live and Return both measured with reference to ground X have to show two characteristics signal amplitude has to be identical phase difference between the two has to be 180 degrees NTI AG www nti audio com page 1 of 8 V1 1 May09 Application Note Signal Balance Error By subtracting Return from Live as performed by the receiver results at the receiver output will in the transmitted signal with doubled amplitude Why do we need two connection paths for one signal Let s see what happens in the case of a perfect balanced signal if external interferences act on the connection path In most cases interferences affecting the interconnection will couple disturbing signals to Live and Return
8. rror reflects the deviation from the perfect balance status No signal balance error indicates that the absolute levels at XLR pin 2 and pin 3 relative to pin 1 ground are identical and opposite in polarity In the BALANCE mode the signal balance error is shown in a numerical value which is the deviation from the optimum in percent page 6 of 8 Application Note Signal Balance Error Frequency of Sgnal Balance Input Ggnal Error Balance Indicator ete SETUP S Bargraph Zoom Mode Bargraph Zoom Control 0 00 Dee Dil 1 00 Bargraph with Scaling Information Fig A 3 ML1 screenshot balance menu Balance Indicator The direction of the deviation is indicated by the arrows like 2 lt 3 or 2 gt 3 whereby the numbers 2 and 3 represent the signal at XLR pin 2 and pin 3 In the Level THD N and Polarity electrical input only measurement function the Minilyzer permanently monitors the balance of the input signal by a graphical indicator This enables e g to check whether cable connections are made correctly Bargraph The bargraph shows an analog display of the signal balance error The scaling may be controlled automatically or manually e Select manual M or automatic A scaling by the bargraph zoom mode field e Within the manual scaling press the left right keys to scroll through the actual range or the up down keys to increase or decrease the range sensitivity of the bargraph
9. utput will be immediatelly indicated on the display by signal balance error reading greater than 1 Is the signal level on pin 3 for example smaller than on pin 2 by half so a signal balance error of 33 will be indicated by the Minilyzer v e v Fig 3 1 amplifier output defective b Example impedance match The balance error may also point you to other imperfections e g different output loads may play in or different output impedances If unexpected happens where s the origin The Minilyzer will help you to localize it c Example cable check You expect a balanced signal but being in the LEVEL measurement mode the signal balance indicator shows UNBAL indicating a unbalanced signal Somewhat wondering you switch to BALANCE measurement mode There you find that signal at pin 2 is much greater than at pin 3 and you read 99 9 balance error po op p Pal Fig 3 2 cable broken Explanation You might be erroneous in your assumption that the original signal is balanced Or you just detected that something goes wrong In many cases the cause of the problem is the cable or the connection to the connector Broken audio cables may be found often in the praxis caused due to ageing short circuits or external force The following figures 3 3 to 3 12 lists level and balance values for some bad cable formations The test signal is produced by the NTI Minirator MR1
10. with a sine signal selected at 0 dB and 1 kHz As you can recognize the extent and direction of balance error give a hint which type of cable failure you deal with page 3 of 8 Application Note Signal Balance Error Test setup MR1 out ML1 in 2 f 2 z gt lt 90 See Fig 3 3 wire 1 and 2 crossed MR1 out ML1 in Cae Fig 3 4 wire 1 and 3 crossed MR1 out ML1 in 2 2 hB o gt lt o Fig 3 5 wire 2 and 3 crossed MR1 out ML1 in C Fig 3 6 wire 1 and 2 shorted MR1 out ML1 in O THE Fig 3 7 wire 1 and 3 shorted MR 1 out ML1 in CTT Fig 3 8 wire 2 and 3 shorted Balance 33 2 lt 3 33 2 gt 3 0 2 gt 3 Level 6 dBu 6 dBu 0 dBu select the MR1 polarity test signal and the ML1 will indicate negative polarity 99 100 2 lt 3 99 100 2 gt 3 0 2 gt 30r2 lt 3 6 dBu 6 dBu 90 100 dBu page 4 of 8 SOREA Application Note Signal Balance Error Test setup Balance Level MR1 out ML1 in 2 2 1 OC EA 99 100 2 lt 3 6 dBu O Fig 3 9 wire 2 broken at ML1 side MR1 out ML1 in 2 2 OCT EE 99 100 2 gt 3 6 dBu Fig 3 10 wire 3 broken at ML1 side MRi1 out ML1 in 2 2 1 Fig 3 11 wire 2 broken at MR1 side MR1 out ML1 in 98 99 2 gt 3 6 dBu a ES Fig 3 12 wire 3 broken at MR1 side page 5of8 aot Application Note Signal Balance Error Appendix a User manual signal balance error In the Level THD N and Po
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