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Measuring Audio Gear Without Spending Big Dollars

You can do it with your PC...

By Jim Rowe

In the old days, checking the performance of audio equipment like amplifiers and preamps usually involved a fair bit of test gear: an audio generator, an audio millivoltmeter (or better still a distortion and noise meter), a scope to keep an eye on clipping and hum and some resistive ‘dummy loads’ to provide the audio equipment with its correct loading (standing in for things like speakers, with their complex impedances).

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If your PC sound card is not up to it, the SILICON CHIP USB Recording/Replay Interface (published June 2011) would be an ideal partner for any PC-based audio equipment measurement setup.

And even when you had all of this test gear on hand, the actual testing was a rather tedious and time-consuming operation.

Now, thanks to galloping digital technology, you do can do your own tests at much lower cost, using a recent-model PC with a decent full-duplex sound card (or a USB Recording/Playback interface) plus a low cost digital audio analyser software package.

Before we start telling you how it’s done, we don’t want to mislead you about the kind of measurement results you can expect.

Although the performance of the digital analyser software tends to be excellent in the digital domain (ie, inside the PC itself), the performance ‘outside’ in the analog domain will inevitably depend very much on the circuitry inside the PC’s sound card or USB sound interface – in particular, on the ADCs in the audio input circuitry and on the DACs in the audio output circuitry.

The analyser software can automatically correct for things like sound card frequency response variations and gain or loss in both the input and output analog circuitry but it can’t really compensate for high noise and/or distortion levels in this circuitry.

The same qualifications tend to apply in the case of crosstalk inside the sound card/interface – both inter-channel crosstalk between the right and left channels (within both the input ADCs and the output DACs) and also crosstalk directly between the ADCs and DACs.

With an ‘el cheapo’ sound card (such as that included in budget PCs or integrated with their motherboards) the results are likely to be fairly modest, while with a ‘top of the range’ sound card or interface they may well approach what could be achieved using a dedicated digital audio analyser system.

So to ensure the best possible results use the highest performance sound card or interface that you can afford.

What you’ll need

I’d recommend as a minimum using a Pentium 4 system (or better) running at 1.5GHz or better, with at least 512MB of RAM and a 80GB or larger hard drive. It should also be running either Windows XP, Vista or Windows 7 – which will again increase the demands for RAM and hard drive capacity.

Of course if the PC is already fitted with a top-quality sound card, so much the better. If it isn’t, your best bet would be to buy or build a good USB sound interface, such as that described in the June 2011 issue of SILICON CHIP.

Software

You will also need a digital audio analyser software package, as mentioned earlier. There are a few of these around but the one that seems to have the best reputation is a package called ‘TrueRTA’, written by John L. Murphy, a former space systems software analyst for the US Air Force and an audio design engineer with over 20 years’ experience.

Details of this software are summarised in the panel at right. After trying out the free Level 1 version myself for a few days, I decided to upgrade to Level 4 and the next day I was able to use this after entering the registration code. I’ll be using the Level 4 version of TrueRTA to illustrate each aspect of doing audio testing with a PC throughout this article.

During the preparation of this article I’ve used TrueRTA Level 4 with a number of PC’s as well as sound cards and USB interfaces. The PCs include a Compaq V2000 laptop running Windows XP, a Compaq D380mx desktop machine also running Windows XP and an Acer Aspire AX1800 machine running Windows 7 (64-bit version).

The SILICON CHIP USB Recording/Playback interface was used with the Compaq V2000 and the Acer AX1800, while I used a Creative Extigy USB interface with the Compaq D380mx machine for comparison.

The basic set-up

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Fig.1: The basic setup for PC-based audio testing. The audio line outputs provide the test signals, while the line inputs connect to the meter, scope and analyser.

Fig.1 shows the basic set-up for using a PC for audio equipment testing. The laptop or desktop PC is running an audio analyser package like TrueRTA, while the analog outputs and inputs of either the internal sound card or the USB sound interface are used as the system’s interfaces to the gear being tested.

The line outputs are used to provide the test signals (from the software audio generator), while the line inputs are used to feed the output signals from the equipment you’re testing back into the PC for analysis.

You can see from Fig.1 that in order to use the PC and its USB sound interface for audio testing, the operating system (ie, Windows) must be set up not only to use the USB interface as its ‘default’ sound card but also to configure it so that the ‘recording’ signals being fed in via the line inputs are NOT ‘looped back’ internally by the software to the line outputs (this is often done to allow ‘record monitoring’).

So as well as going into the Windows Control Panel and making sure that your USB Record/Playback interface is set as the default audio device for both recording and playback, it’s also quite important to go into the Windows Playback Mixer dialog and make sure that the Line Inputs are not selected for playback – only the WAVE signals. This is usually the best way to ensure that the input (‘recording’) and output (‘playback’) functions are kept isolated from each other.

Another important step in your initial system set-up is to ensure that the recording and playback volume controls are each set to a known and easily repeatable level.

Usually with Windows XP and earlier operating systems this is ‘maximum’ – ie, with the software sliders set at their upper limit. However with Windows 7, you need to go into Control Panel/Sound/Recording and then select the Microphone function of your USB Audio Codec and open its Properties dialog.

Then you need to click on the Levels tab and move the Microphone volume slider almost fully to the left, until numeral ‘1’ is being displayed in the box just to the right of the slider itself. This sets the ‘recording’ gain to unity, corresponding to a ‘line level’ input.

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