Sound card quality

Sound card quality

Although sound card bargains certainly exist, our experience shows that the sound cards that consistently give good or excellent results are not the rock-bottom-priced, unlabeled types. Which multimedia sound card you buy and how much you spend is up to you. If all you want to do is conversions, you do not even need a sound card.

You need to be warned: there are substantial differences in quality between the various sound cards available on the market today. We have yet to see a very cheap card that sounds great and has low noise but we certainly have seen some very expensive junk. The most important aspects are:

Some multimedia sound cards work very well but only when recording or playing back at the standard multimedia sample rates of 11, 22 and 44 KHz. If you also want to play back telephony files at 6 or 8 KHz on your sound card the quality at low sampling rates is extremely important. If you hear superimposed distortion that follows the rhythm of the recorded speech you may be hearing aliasing problems. Good products incorporate anti-aliasing filters that work at low frequencies too; cheap clones and some famous products don't.

·	A sound card is not a must. You can play files back over a voice processing telephony card using your standard telephony software with your own voice processing application.

Select a card that can do 16-bit recording and playback, and do it well from 6 to 64 KHz. The 8-bit cards, although usable, naturally introduce more quantization noise and are often of lesser quality. They will never give you good results and should be avoided. Of course, we are talking about the resolution of the recordings here, not about the complexity of the wave tables used to generate synthetic music.

The better cards have lower idle channel noise levels and pick up less stray noise from the PC itself. Some products are so bad that you record hissing noises when the PC's mouse is moved around the desk! This is very simple to test with Vox Studio. Mute your microphone input and start recording from that input. Then watch the recorded "silence" in Vox Studio in dB mode (right-click on the waveform and select Vertical Scale/dB). If you see anything at all above -90dB, your card does not produce pure silence (it probably does not).

·	Some sound cards give better results (less noise) with microphones when the microphone is connected to the line input rather than the microphone input. Strange, but true. It is worth the experiment.

Make sure that your sound card records and plays back at the precise sampling rate you ask it to. We have encountered a sound card that was 9% off its 6,000 Hz sampling rate, in other words it was unusable. Some manufacturers cut corners and use simplified clocks. As a result the sample rate is sometimes not very accurate outside of the traditional 11, 22 and 44 KHz ranges. This is not audible if you record and play on the same card. This becomes very audible if you export the resulting sound file to another system. This is catastrophic if your files contain calibrated frequencies like the DTMF tones. As a test, record any input at 8 KHz for exactly one minute (use a stopwatch) then look at the recorded signal in Vox Studio. The bottom part of the window should indicate exactly 480,000 samples in the file.

·	Buy the best sound card you can afford. A few hours of messing-around because of a poor card will cost you more than the difference for a decent sound card. There are lots of very good cards at very affordable prices. It would be nice to be able to test it on Vox Studio and return it if it is not good enough for your telephony needs.

·	If you are using a cheap sound card and find it to be of impeccable quality, let us know.

·	Good multimedia cards are sold with decent accompanying software. Look for a quality mixer (level adjustment) applet; you need one to adjust the input levels under Windows. Vox Studio does not provide such a tool; it comes standard with sound cards.