Lets try this again since I am still thinking about output since I was mostly considering a audio component grade sound card.
Paul Mullen wrote:
The main reason that recordings are now in the 96kHz to 192kHz range is the effects of the low pass filter before the ADC
On output: The analog filter goes after the DAC. AFAIK, if a digital over-sampled filter is used before the DAC, it is always a linear phase filter.
is mainly confined to the frequencies beyond human hearing.
Actually, the phase response of a filter goes all the way down to 0 Hz which is why it needs to be linear.
It is possible to hear the artifacts of a 44.1kHz or 48kHz low pass against a 192kHz low pass if you have really good equipment.
Regarding input: It depends on the filter. If the filter has linear phase response up to 20 KHz you can't tell the difference. This is why the best equipment uses digital Bessel filters for output. But, yes if you use a high order Butterworth (analog) filter -- or worse a Chebechev filter -- a good ear can hear the difference between 22.05 KHz and 96 KHz. The problem is that the better the phase linearity the worse the amplitude cutoff and vice versa. I will have to dig out my DSP text but IIRC when using a sample frequency of 44.1 KSPS an input filter is needed only to filter out high frequency noise which could show up as lower frequencies in the reconstructed output. If you have a brick wall filter with a f0 of 20 KHz and liner phase all problems are solved. :-) If the objective is to make a commercial recording at 44.1 KSPS then using a higher sampling frequency for the input only begs the question since the sampled bit stream is going to contain high frequency noise that should not be contained in the recording which will be distributed at 44.1 KSPS. If you need an input filter that has a better cutoff than the SaH, then you can use a N^2 x input sampling rate with both an analog filter -- 3rd order active Bessel (with a small shelf added and feed forward isolation) -- and a digital filter. -- JRT _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
