Multi-Instrument 3.2
Contents
1. PE vintualinstrumentisoitware am INSerUInelites 2 Multi o Intrigued by the idea of using your PC as the engine for a suite of virtual audio test instruments Here s a run down on a powerful software package that will let you use it as a 2 channel audio scope combined with a powerful spectrum analyser a 2 channel audio signal function generator and an audio DMM which even includes a frequency counter ACK IN THE October 2011 issue of SILICON CHIP we explained how to test common domestic audio gear using a good quality sound card with your PC and running a virtual audio test instrument package like TrueAu dio s TrueRTA While we found that TrueRTA has many worthwhile features including the ability to make accurate frequency response and noise level plots it did have a few limitations with regard to things like distortion and crosstalk measurements and plots Recently though we became aware of another software package called 16 SILICON CHIP Multi Instrument 3 2 developed by a Singapore based firm called Virtins Technology Virtins has specialised in PC based virtual instrument tech nology since it was founded in the early 1990s and in addition to the Multi Instrument software package it currently markets a range of virtual DSOs together with its own real time audio analyser VMI 3 2 is the latest version of a software package first released in late 2004 for use with PC sound cards It supports all Windows2. siliconchip com au wea ef jiii E Me Poi f IE Fail EE Fig 1 the main screen has four horizontal function bars along the top with icons to activate the scope spectrum analyser DMM and generator Here the scope window is at upper left with the analyser window below it The smaller narrower window at lower right is for the signal generator and arbitrary waveforms which may be stored on hard disk In addition the generator can be set to provide any desired phase differ ence between the two output channels and it can mask their outputs in order to provide burst test signals It can also provide sinewave signals sweep ing either linearly or logarithmically between any two selected frequencies and at any desired speed Its also possible to set the exact output frequency to a value which minimises spectral leakage when you are using the spectrum analyser An AC multimeter able to display RMS volts dBV dBu dBrelative and dBA B C plus cycle RMS and cycle mean It can also function as a fre quency counter a tachometer RPM a straight counter a duty cycle indicator and a frequency voltage converter Calibration amp compensation VMI 3 2 also supports calibration of the input and output channels of your interface sound card so that absolute values in engineering units can be used for display analysis or export It is also able to account for external attenuator settings such as the inp
3. Pi EEN A Timi EA SL E a AATE i i Paani D sev g i mea K 2i WH Ane E A cir a Sov p i reel POH D sr g Pree k TNH pre Pail Emy _ era ky prk Pi IE eae ole ite Ame f o an PESO i Tiimi pc Nowy Ame i Mon EE tite cts E E POMS t 86m Fig 7 12 these scope grabs show sinewaves at 10kHz amp 20Hz square waves at 100Hz amp 10Hz a triangle wave at 20Hz and a sawtooth at 10Hz The overshoot visible on the square wave signals is probably due to the characteristics of the anti aliasing LP filter in the USB Virtual Test Instrument Interface output channels while the droop visible at the top and bottom of the 10Hz square wave is due to the low frequency roll off in the same output channels it shows the real results of the test the overall frequency response of the Interface s channel A in blue near the top and the crosstalk into the channel B input in red near the bottom It s not easy to read off the exact values of either curve from the display as shown although if you use VMI 3 2 s cursor reader facility you can get it to show the exact value of both curves numerically just at the top of the graph itself In addition the ana 20 SILICON CHIP lyser parameter toolbar at the bottom allows you to zoom into the graph in terms of frequency and also in terms of amplitude so you can expand either or both curves as much as yo
4. ch would give the sharpest FFT resolution in the analyser Because there are so many display options for each of the instruments in the VMI 3 2 package providing for siliconchip com au Fig 3 6 these four screen grabs show a 1kHz sinewave a 500Hz square wave a 500Hz sawtooth and a 500Hz triangular wave as generated by the VMI 3 2 software and displayed in the scope window on a PC their setting could easily take up a lot of screen space and encroach into the important data display area But the software designers have been clever because each of the main instrument display windows scope spectrum analyser and DMM has its own View Parameter Toolbar which only ap pears along the bottom of the screen when that instrument s display win dow has been selected So if you select the scope window its parameter toolbar appears at the bottom select the analyser window instead and its parameter toolbar appears that s the one you can see along the bottom of Fig 1 Similarly if you select the DMM window its own parameter toolbar is shown This makes it easy to adjust the function and display parameters for each instrument without sacrificing siliconchip com au a lot of screen area As you can see the spectrum analyser s parameter toolbar allows you to adjust many of the important parameters most of them via pop up menus which again minimises screen space Before we leave Fig 1 I should per
5. compatible sound cards and interfaces and Virtins own virtual instruments plus many industrial ADC DAC cards like the DAQmx series from National Instru ments Like TrueRTA an evaluation version of VMI 3 2 can be downloaded free from their website In this case it s a fully featured version which expires after 21 days unless you buy a licence from them online There are three performance levels which may be purchased Lite costing US 49 95 Standard costing US 99 95 and Pro costing US 199 95 There are also various add on functions like a Waterfall Plot Spectrogram Data Log ger LCR Meter and a Vibrometer plus siliconchip com au an option which allows you to create save and execute a series of Device Test Plans After downloading and trying out the evaluation version of Virtins Multi Instrument 3 2 VMI 3 2 for a week or two we were motivated to develop the USB Virtual Instrument Interface featured elsewhere in this issue A virtual instrument suite In Standard form VMI 3 2 is a suite of the following virtual instruments A 2 channel digital oscilloscope with a bandwidth from below 10Hz to 96kHz depending on the capabilities for your sound card or ADC hardware The sampling depth can be 8 16 or 24 bits again depending on your sound card There are a range of triggering modes and display modes such as A and B A B A B Ax Band Lissajous A against B Each frame of data ca
6. d then moved along to show just the range between 800Hz and 3 5kHz As aresult you can clearly see the fundamental peak of the 1kHz genera tor signal together with the second and third harmonic peaks generated by distortion in the USB Test Instrument Interface s channel A output and input siliconchip com au circuitry blue graph You can also see the crosstalk into the channel B input circuitry red graph Above the graphs the analyser dis plays the calculated THD 0 0279 and THD N 0 0720 figures for channel A blue text along with the SINAD SNR and NL It also does these calculations for channel B but these are not important because they represent crosstalk distortion Since I had placed the reading cur sor at the 1kHz peaks vertical red line on the graphs the analyser has also calculated the amplitude of the fundamental peaks in both channels and displayed them in dBu on the third line from the top The channel A peak is 8 00dBu while the chan nel B peak is 88 99dBu You don t have to subtract one from the other to work out the crosstalk at 1kHz either because the analyser does that as well and displays it at the end of the line 80 99dB Generator waveforms The remaining screen grabs from VMI 3 2 and the accompanying scope grabs Figs 7 12 indicate the variety of waveforms that can be delivered by VMI 3 2 s signal generator For example Figs 3 6 show a 1kHz sinewave a 500Hz square wa
7. haps explain the displays that you can see in the three instrument windows In this case VMI 3 2 had been set up to do an overall frequency response and inter channel crosstalk test of my new USB Virtual Instrument Interface with a short cable looping the channel A generator output back to the chan nel A input and the channel B input terminated in a shielded 50Q resistor As you can see the generator was set to produce sinewaves of 0 5V RMS sweeping linearly from 1Hz to 23 5kHz over a period of 20 seconds Up at the top you can see that the scope and analyser were set to sample both input channels A amp B at 48kHz and 16 bits for a total record length of 655 360 points which if you work it out takes 13 6 seconds 655 360 48 000 Down in the bottom analyser toolbar you can see that an FFT size of 32 768 had been selected resulting in a total of 20 FFT segments 655 350 32 768 and an analyser frequency resolution of 1 46484Hz 48 000 32 768 A rectan gular window was also selected with no overlap between windows Aliasing effects I should comment on the strange display in the upper scope window for this kind of test it is badly affected by aliasing because of the way the signal is sweeping between 1Hz and 22 3kHz over the sweep period But the spectrum analyser display below SEPTEMBER 2012 19 Freel VO 00hH2 TETY E A es ee a Ee Freel IH PRR 241 fo p Freel k 200M M
8. king on their icons with each instrument opening in its own win dow in the main part of the screen In the case of the scope and analyser windows you can adjust their size and position in the usual way In Fig 1 you can see the scope win dow at upper left with the analyser window below it The smaller and narrower window at lower right is for the generator which is deliberately designed to be as compact as possible so that most of the screen is free for you to make the other two windows as large as possible Unlike the scope and analyser win dows the generator window can t be continuously adjusted in terms of size but it can be truncated or shrunk into just the upper 20 or so by un ticking the Show Editor button once you have set up the generator In any case it automatically varies in terms of screen height according to the operat ing mode selected For example if you don t activate sweeping the window contracts up from the bottom to remove the bottom 20 or so The DMM If you activate the DMM it is dis played in its own window which can be adjusted in size and position You can see it at upper right in Fig 2 dis playing the generator frequency in use Note that in this case the generator has been set to a frequency very close to 1kHz but not exactly so This was done by selecting the No Spectral Leakage option in the generator window to change the frequency to that nearest 1kHz whi
9. n be provided with a date time stamp and the data can also be recorded continu ously on the PC s hard disk A 2 channel spectrum analyser with a selection of seven different display modes amplitude power spectrum phase spectrum auto correlation and cross correlation functions coherence function transfer function Bode plot and impulse response The FFT win dow size can be selected from 16 dif ferent options from 128 to 4 194 304 points while there is a choice of no less than 55 different windowing func tions including rectangular triangular Hanning Hamming Blackman Gauss ian cosine Poisson and so on The overlap between windows can also be set to any desired percentage while there s a choice of many differ ent display and scaling options for both the Y axis and the frequency axis Parameters which can be measured using the spectrum analyser include bandwidth crosstalk THD THD N SINAD SNR and noise level NL in a specified frequency band It s also possible to measure IMD SMPTE DIN CCIF etc A 2 channel digital signal genera tor with a wide choice of waveforms and associated functions Waveforms include sine rectangle with adjust able duty cycle triangle sawtooth and multi tones like DTMF There s also a choice of white or pink noise maximal length sequences with length adjustable between 127 and 16 777 215 samples unit impulse and unit step notes from the tempered musical scale
10. r T T ds Boor pr om i alc LTE L Fig 2 the DMM function is displayed in its own window shown here at upper a iHi FYE ibe oI Eahiri l u iii right and this window can be adjusted for size and position on the screen In this screen grab it is shown displaying the generator frequency in use sired screen layout and combination of instrument settings as the default skin for VMI 3 2 when you start it up each time In short it offers a high degree of customisation All data collected by the VMI 3 2 oscilloscope or spectrum analyser can be saved either as a wave file or a text file All graphs can also be exported as bitmap BMP files or printed out directly And waveform files saved in either WAV or TXT form can be im ported into the Generator to generate that waveform again System requirements VMI 3 2 is compatible with all versions of Windows from Windows 95 to Windows 7 either 32 bit or 64 bit Virtins suggest that for optimum results your monitor should have a horizontal resolution of at least 1024 pixels 18 ISSUESIOGHING WOU EAren As you ve no doubt gathered from the above VMI 3 2 has an almost bewildering array of functions and facilities Fortunately there is a 283 page user manual which can be down loaded in PDF format And there are tutorial articles on FFT spectrum analysis including one on FFT basics and another comparing
11. the umpteen different FFT windowing functions Trying it out I installed a copy of VMI 3 2 Stand ard on an Acer AX1800 desktop PC running Windows 7 Home Premium 64 bit It installed without a hitch and very soon I was looking at a main screen much like that shown in Fig 1 It has four horizontal function bars along the top starting with the usual Menu bar File Setting Instrument etc There s a scope triggering and sampling parameter toolbar for the REAL Kee OA OUmeO ESTO RSM EO NKEH Ite WitiMheSeuraln yes Available Aust only Price A14 95 plus 10 p amp p per order includes GST Just fill in and mail the handy order form in this issue or fax 02 9939 2648 or call 02 9939 3295 and quote your credit card number Buy tive and get them postage free SILICON CHIP pee es ee tad ki e Ax scope and spectrum analyser followed by an Instrument and Miscellane ous toolbar with icons to activate the scope spectrum analyser DMM and generator plus settings to ad just scope input sensitivity take into account whether there s an external input attenuator and so on There s also a coloured bargraph on the righthand end of this bar showing you the amplitude of any input signals at the scope analyser inputs Finally there s the row of hot panel toolbar buttons shown here with their pre programmed default functions You activate the various instruments by clic
12. u need shifting along to any frequency range and shifting each curve up or down so you can inspect them visually in minute detail Not bad eh There s no problem about exporting or printing any of the analyser displays at any time either All you have to do is pause the analyser and click the relevant buttons Other capabilities Now let s look at Fig 2 again as this illustrates a few more aspects of VMI 3 2 s capabilities in addition to the DMM window When this screen grab ie Fig 2 was taken I had been using VMI 3 2 to measure the overall distortion and noise performance of the new USB siliconchip com au Test Instrument Interface s channel A output and input at 1kHz As before these were linked viaa short cable and the channel B input terminated with a 50 resistor The generator had been set to pro duce a 999 024Hz sinewave of 0 5V RMS and the analyser to take 500 000 samples per record The FFT size had been set to 16 384 with a Kaiser6 window function This gave 30 FFT segments per record and a frequency resolution of 2 92969Hz Again you shouldn t take too much notice of the upper scope window be cause it s showing the severely aliased display of 500 000 samples of a 1kHz waveform taken over 10 4 seconds But there s quite a deal of information to be gleaned from the lower analyser window because just before the grab was taken I had zoomed the horizontal frequency axis by five times an
13. ut switch settings in the Virtual Instrument Interface as well as be ing able to compensate for hardware characteristics such as the frequency response deviations of the interface sound card Once measured these can be saved as reference curves and loaded in whenever they are needed Zooming and scrolling in both the X and Y axes are supported in all graphs displayed by the scope and spectrum analyser to allow inspection of fine details In any case VMI 3 2 provides a cursor reader for each graph al lowing you to determine the exact X and Y readings for any specific point just by clicking on it and holding the mouse button down There are also two markers which can be set by dou ble clicking for comparison purposes Another nice feature of VMI 3 2 isa row of 16 hot panel setting buttons along the top of the screen These are pre configured to set up all the instru ment panel controls and settings for specific tests However you are free to reconfigure any or all of these but tons as you wish for your own most frequently performed tests As part of the reconfiguring you are even able to change the legend on the button being reprogrammed It s also quite easy to save any de SEPTEMBER 2012 17 if to A Ek erg ee eet e oe E TT I lii Li fen a Tu oe one e m e Be ee ee Fs ne Hz i as 999 02 i z s kE ET i udi
14. ve a 500Hz sawtooth and a 500Hz trian gular wave as displayed on VMI 3 2 s own scope Figs 7 12 are a series of scope grabs showing sinewaves at 10kHz and 20Hz square waves at 100Hz and 10Hz a triangle wave at 20Hz and a sawtooth at 10Hz The overshoot visible on the square wave signals is probably due to the characteristics of the anti aliasing LP filter in the USB Virtual Test Instru ment Interface output channel while the droop visible on the top and bot tom of the 10Hz square wave is due to low frequency roll off in the same output channels There s no doubt that Virtins Multi Instrument 3 2 is capable of delivering professional grade results especially if you were to use it with the highest quality ADC and DAC interface hard ware It s extremely flexible yet at the same time quite user friendly Overall Virtins VMI 3 2 represents excellent value for money especially the Standard version at its current price of only US 99 95 For further information see virtins com SC SEPTEMBER 2012 21
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