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A Simple Test For Good A/D D/A Converters.

INTRODUCTION

Evaluating converters is a somewhat subjective art like evaluating amplifiers and speakers. But hitherto, no one to my knowledge has ever published a quick and easy way that just happens to be the best way… to evaluate converters.

I know you're probably expecting some hi tech talk with an abundance of specs and details heaped on your head. Well, the true test for any audio device is NOT its specs which may or may not reflect the quality of the unit. It's what it sounds like to the average bear who's going to hear your work and hopefully buy it.

From the very beginning of the digital audio era, it was obvious that the CD spec of 16 bits and 44.1 khz sample rate had many problems with sound quality. Over the years most of the enhancements to make the CD sound better have been centered on introducing distortion modeled from old analogue recorders and tube electronics. For tube lovers, there's just something about the sound of billions of electrons smashing onto a highly charged plate that makes a sound that we like better than the dull, sterile sounding, miniaturized solid state circuitry of today that has little appeal unless doctored with some good old fashioned distortion.

The conversion in and out of the digital realm is at the heart of the sound. Converters are everything. It IS your sound. Running at 44.1 kHz, the critical component is the design of the anti aliasing filters that keep out unwanted spectral ghosts that trash up the sound. The better the design of the filters the more pleasing the high end is. Cheap filters may also introduce a slightly synthetic sounding high end noticeable on some tracks.

THE TEST-

So how do we test converters without an array of sophisticated testing tools? Fortunately, the best test doesn't require expensive sound systems, no anechoic chambers... and a totally inexperienced listener can determine which converters are better with this easy test.

The key to checking converters is to ALWAYS TEST CONVERTERS IN STEREO... NEVER MONO. Test's should be conducted at 44.1 kHz sampling rate.

Testing any sound system with your voice... won't tell you a thing especially about converters. Even cheezy converters will sound about the same as good ones because the freq response is the same as an expensive converter. Listen for hiss if you can hear any at all. Typically DAT machines, even pro line DATs will have the noisiest preamps around. For almost a decade NPR has used top of the line DAT recorders only to find they have to buy expensive pre amps because the built in pre's are so noisey.

For our test, all you need is a raw stereo sound source. The best converters will be the ones that maintain (or minimize the shrinkage of)... the stereo field or stereo width of the sound. Cheezy converters will nearly always shrink or collapse the field width enough for even a casual listener to notice if they are asked to compare. The shrinkage can be the result of poorly designed circuitry and/or cheap components which create phase and cross talk problems between channels bringing about some critical mixing problems for the unsuspecting user.

The stereo test can be a spread of two mics on a piano or other instrument. If that's not available, a nice wide stereo string patch from a synthesizer will do nicely. Pre-recorded material is not as effective for testing. Make certain that you have a perfectly aligned A to B test. You should monitor one source as the sound passing through the converters and associated electronics and the other that totally bypasses the converters and all associated electronics. By associated electronics we mean for example, all the mixing channels on a digi 002, or all the mixing channels on a Roland digital recorder… those are associated electronics. Go directly to the speakers or the mixer not associated with the converters... to the main amp and speakers. The reason for this is to eliminate the tendency that some units have to channel sound through their converters even when you aren't using the recording features.

Volumes should be very carefully adjusted between the channels unaffected by the converters and the channels through which the converters sound.

If you hear a collapsed field or an odd sounding stereo sound, then assuming no problems elsewhere in your system, you might want to look around for a better set of converters or chunk of electronics that house them.

Don't be fooled by a good name, or a review that sounds too good to be true. We have found lousy converters in very popular “Pro” gear. So forget the hype, forget the specs, forget who's supposedly using it… AND USE YOUR EARS. They are the best test instrument you have.

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