|Advanced Audio Recording|
The advantage of 192kHz for audio processing
In recent years 192kHz sampling frequency was announced to be the standard for modern audio recording. It appears to have significant advantages over 96kHz, but is it worth a try?
Do we really need 192kHz in modern audio recording?
General advantage of higher sampling frequencies
From the point of view of an engineer doing signal processing, there is no doubt in any way: Measurement systems in the field of data processing and acquisition do perform oversampling at about 100-1000 times of the desired highest frequency to get all of the available information correctly, since Nyquist's theorem required an ideal anti aliasing filter which does not exist in reality. So the idea, "the higher the sampling frequency the better for quality" absolutely is right. Read an article about a comparison of different sample frequencies a more detailed view on this issue: The advantage of 96kHz.
Higher sampling frequencies in audio processing
From the point of sound recording it is a bit different, since there is not a big part of energy in the upper bands so a slight linear distortion from a "normal" AA-Filter (having it's edge frequency at around 15kHz to provide a good stop band rejection) does not so much "harm" to the signal ,as most people think. In fact, the "errors" are mostly smaller than those introduced by our monitors.
96kHz had one big advantage: You can record in 96kHz and more or less down sample to 48kHz and resample to 44,1kHz without the fear of significant quality loss. Changing 48kHz recordings to 44,1 is not nice to do! So the improvement from 48 to 96 years ago was THE big point (at least regarding this particular aspect, because on the other hand, the total improvement when comparing recordings of 44,1 and 96 directly was only little - even when having a full 96kHz system. And it is important to use a full 96kHz path with optimized anti aliasing filters at both ends of the recording chain: This refers to ADC and DAC and the whole digital path in between. The analog path has to be optimized that way, that filter edge frequency has to be far outside of the audible band.
Is 192 kHz "too high"?
So, 192kHz is even less important! As soon
as you record through an AA Filter with an edge frequency of e.g.
25kHz and above, as with most 96er systems, there is not much
relevant distortion anymore (you could hear) caused by the filter
curve. The plus of aliasing which occurs, when
going with only 96 instead of possible 192 is less than 20% of the
signal for a white noise primary source, meaning you have still 80%
of the remaining aliasing problem when increasing from 96 to 192
(and possibly still 50% when increasing from 192 to 384 and so on).
absolutely some theoretical advantage performing the recording in
192kHz with a ADC having an optimized AA-filter at e.g. 30kHz edge
frequency and very low distortion in the listening band and an
acceptable ripple in the stop band and then post processing this
with a digital filter later. This means:
If you want to make use of 192kHz: Get a newer, 192kHz only ADC and perform down conversion at the end of the process offline with a software tool using look ahead mechanisms - in terms of FIR filters with a large number of TAPs and strong overlay and optimized windowing. Processing e.g. a 192kHz stereo stream with effective FIR-Filters with e.g. 4096 TAPs (which was not too much for this quality demand) requires a calculation speed of nearly 1,6Mio MUL operations / secs which hardly can be done in a nowadays VST plugin. This has to be done offline with appropriate software or special real-time hardware like FPGAs.
Read about an example FPGA platform for 96kHz audio processing.
|© 2006 J.S.|