One of our faithful inSync readers, Dennis C asks: There seems to be three main variables in creating quality digital recordings: Converters, frequency, and bit depth. Comparing products is difficult because they often vary on all of these (Example: Is 44khz and 24 bit better than 48khz and 18 bit?). What are the combinations to look for and how does one make a meaningful comparison?
The easy answer on bit depth and sampling frequency is that "more is better always." This is more or less obvious to most people who have been working with digital audio for any length of time. Your question implies that you are asking which one is more important.
Keep in mind that sampling frequency (Fs) is going to determine the frequency response of your audio, and more importantly (if your Fs is above 44.1k) how much the digital filters impact the audio. Many engineers argue that we should be sampling at 96k or more (even though they know that sampling at 44.1k will yield 20 - 20k frequency response) because that would mean that all of the filtering that is necessary to prevent aliasing (see WFTD archive Aliasing) and other anomalies in the analog to digital process would be at a high enough frequency to have little or no effect on the sound we hear. It is known that filters, even if used above our range of hearing, can cause a variety of subtle problems with audio that we hear.
Bit depth is going to affect the ability to capture the full dynamic range of a signal, as well as reducing quantization error (see WFTD archive quantization error). More bits equal finer resolution and more potential dynamic range. For each extra bit of data you can capture 3dB more dynamic range. So the theoretical limit for 16 bit recordings is 96dB whereas a 24 bit recording can have a dynamic range of 120dB. Big difference.
In your specific example (44.1k/24bit versus 48k/18bit) I would say that the 44.1/24bit system would (arguably) sound better. The difference that the extra bit of sampling frequency makes (especially with today's modern converters) is pretty subtle compared to the difference between a well recorded 16 bit signal and 24 bit one. That is why there is so much buzz in the industry about 20 and 24 bit audio these days. It is a very audible difference on the right kind of program material.
There are some other important issues that can and will effect the performance of digital audio recording. The overall quality of the converters is a major issue. Don't forget the analog portion. There can be a big difference between the analog part of many popular converters, which many users tend to overlook. Clock stability is another factor. In most of the systems I know of the A/D converter supplies the clock for the system while recordings are being made through it. It is very important that this clock be very stable for maximum quality (see WFTD archive Jitter).
