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In Reply to: Re: If the base rate is 44.1K, you probably still have the steep filter posted by graemme on March 31, 2006 at 15:38:38:
The primary reason to upsample (as fmak pointed out) is to avoid brickwall filtering again in the DACThe digital filter in my DAC upsamples to 352K, so does that mean I don't need the rest of the digital filter performing the anti-alias for the 44.1K base sample rate? Seems like upsampling would be too good to be true if you could get something valuable like that for nothing :-)
For those that do the upsampling on their computer, here's what the Secret Rabbit Code FAQ says about it (bold is mine):
Q3 : If I upsample and downsample to the original rate, for example 44.1->96->44.1, do I get an identical signal as the one before the up/down resampling?
The short answer is that for the general case, no, you don't. The long answer is that for some signals, with some converters, you will get very, very close.In order to resample correctly (ie using the SRC_SINC_* converters), filtering needs to be applied, regardless of whether its upsampling or downsampling. This filter needs to attenuate all frequencis above 0.5 times the minimum of the source and desination sample rate (call this fshmin). Since the filter needed to achieve full attemuation at this point, it has to start rolling off a some frequency below this point. It is this rolloff of the very highest frequencies which causes some of the loss.
The other factor is that the filter itself can introduce transient artifacts which causes the output to be different to the input.
And on the hardware side, here's a blurb from the old CS8420 upsampler datasheet showing the digital filter passband is spec'd as .4535 Fsi (input sample rate).
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- Sounds too good to be true - Slider 16:26:31 03/31/06 (0)
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