--- In "Raimund" <> wrote:
> I beg to differ here. As someone who has already programmed
> (interpolation-based) sample rate conversion algorithms for both DSP chips
> and PC software, I see no reason why there should be any advantage to
> up-sample the signal to a higher intermediate sample rate in one just wants
> to convert the sample rate from 88.2 to 44.1 kHz.
Just to clarify things, Raimond.
1) The first part of my post was a statement of fact that we both agree with
(fortunately!): you need to apply a low pass filter when converting from
88.2ks/s to 44.1ks/s.
2) The second part of my post was in response to someone's previous post about
upsampling, followed by my own perceived benefits of upsampling (heard through
some very high quality equipment) along with what I believe are the reasons for
those benefits. I did not mean to imply that upsampling would be a necessity
for a better-sounding SRC... although if it makes an audible difference in EQ
algorithms in channel strip plug-ins, it seems reasonable to assume it would be
of benefit to the filter in an SRC. Perhaps not?
> Yes, this is true, but the required low-pass filter would be very
> straightforward and can just operate on the original sample stream of 88.1
> kHz.
Right... But how straightforward is it, *really*? If you look at the SRC comp's
site that I linked to earlier, it seems some/most coders have a very hard time
getting that filter right. And when it is not right, there are some unpleasant
side-effects getting into the audio. Personally, I take a lot of care choosing
and using my equipment; I don't want those efforts to be stymied by a poor
filtering algorithm.
Of course the SRC can operate at 88.2ks/s. But *if* upsampling from there
produced an even cleaner result, that would be worth chasing (in my opinion).
The question is, of course, whether that improvement is worth the additional
processing time and whether the user is willing to spend that extra time.
> Any intermediate up-sampling (to 7.056 MHz) would unnecessarily increase the
> amount of data that needs to be processed, which would take much longer to
> process.
Why 7.056MHz? I was talking 394ks/s. Or are we using different terminologies to
say the same thing?
Regardless... it might take more processing time, but is that additional
processing time *really* an issue these days when running a stereo file through
an SRC? If it takes me twice as long or more, but yields a better/cleaner/more
natural end result, I'll spend the time (unless it's a paid job and I'm
charging by the hour, in which case I'll inform the client of the options and
let him/her decide).
> <The mathematical process of filtering negates any theoretical benefits that
> an integer-related multiple sampling rate might have had.>
>
> No, this should not negate the benefits as there would be no additional
> interpolation or up-sampling required.
Right. Sorry, but that comment you are responding to was from the first part of
my message, before upsampling had even entered the conversation...
I was not referring to upsampling, just to the false argument that people use
(when supporting integer sampling rate relationships), which says that the
benefit of going from 88.2ks/s to 44.1ks/s (for example) is that you simply
delete every second sample. The supposed 'benefit' from this argument is that
there will be no loss in quality because the SRC'd signal still contains the
original sample values. That supposed 'benefit' is negated because the values
still must be filtered (before SRC), so the sampled values in the 44.1ks/s
stream are not the same as those in the original 88.2ks/s.
Just to clarify that further...
If you took an 88.2ks/s file and *supposedly* converted it to 44.1ks/s by
simply deleting every second sample (no LPF), the resulting file would not have
the same sampled values as if you took that 88.2ks/s file and correctly SRCd it
to 44.1ks/s.
- Greg Simmons
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause
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