Back 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 TrueAudio’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 Multi-Instrument 3.2, developed by a Singapore-based firm called Virtins Technology. Virtins has specialised in PC-based virtual instrument technology 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 Windows-compatible sound cards and interfaces and Virtins’ own virtual instruments – plus many industrial ADC/DAC cards like the DAQmx series from National Instruments.
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 Logger, LCR Meter and a Vibrometer, plus 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, A x B and Lissajous (A against B). Each frame of data can be provided with a date/time stamp and the data can also be recorded continuously 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 window size can be selected from 16 different options, from 128 to 4,194,304 points, while there is a choice of no less than 55 different windowing functions including rectangular, triangular, Hanning, Hamming, Blackman, Gaussian, cosine, Poisson and so on.
The overlap between windows can also be set to any desired percentage, while there’s a choice of many different 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 generator, with a wide choice of waveforms and associated functions. Waveforms include sine, rectangle (with adjustable 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 and arbitrary waveforms (which may be stored on hard disk).
In addition, the generator can be set to provide any desired phase difference between the two output channels and it can mask their outputs in order to provide “burst” test signals. It can also provide sinewave signals sweeping either linearly or logarithmically between any two selected frequencies and at any desired speed.
It’s 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 frequency counter, a tachometer (RPM), a straight counter, a duty cycle indicator and a frequency/voltage converter.