“As I understand, the quality of sound we get using computer music technology greatly depends on the D/A converters of the sound cards, digital mixers and the like. So far, I thought that the only parameters affecting the converter’s performance was the word length and the sampling rate. A converter using 24/96 technology is supposed to be a better quality converter than an 18/44.1 for instance. Question: Is my perception correct? Are there other parameters that affect D/A converter performance as to the quality of sound? Are these parameters measurable? Why aren’t they included in the specifications of such machines?”
Okay, here we go with multiple questions disguised as one again. D/A and A/D converters certainly do have an impact on the audio quality we experience. But, as your question implies, there is much more to it than bits and samples. The quality of the converter circuits themselves is a pretty significant issue. Even more important is the quality of the analog electronics in and around those converter circuits. This even includes things like the power supply and in some cases even the outer case (for shielding reasons). Then there are also other digital issues. One huge one is the clock source. Jitter or other types of inconsistencies in the clocking can cause a variety of inaccuracies converting between digital and analog signals. These are just a few of the more obvious things people look for. Depending upon how deeply you like to dig there are other, more obscure aspects of audio reproduction that can be considered (I know a company that sells $700 power cords!).
Are these things measurable and are they included in the specs? Yes and no. Clock jitter is a spec that can be measured and in high-end converters there is usually a listed spec for this. The performance of the analog signal path can be measured (frequency response, dynamic range, signal to noise, linearity, phase coherency, etc.), but ultimately the “sound quality” is an elusive and subjective matter, and not measurable by any standard that will satisfy everyone. Some folks like brand A and some like brand B, and there may be no quantifiable reason why. I’ve seen a number of instances where listeners will pick a high quality 16-bit 44.1k converter in a blind test over a lower quality 24/96 converter. I’ve also seen quite a few people not be able to hear any difference between five or six different converter systems. Some simply seem to have a much better ear for what to listen for than others. It’s also important to keep in mind the relatively high quality of even inexpensive systems today. Back in the early 1980’s when digital recording first became widespread it got a deservedly bad rap for audio quality mostly because the available converter systems at the time were not very good.