wjm@lcuxc.UUCP (B. Mitchell) (03/25/85)
The advantage to oversampling in CD players is that it simplifies the design of the low pass analog filters that follow the D/A converter. These filters are required to prevent aliasing - that is frequencies > 22.05 kHz created through the digital sampling process causing spurious signals in the analog output. Without oversampling, a VERY sharp cut-off at 22.05 kHz is required and such "brick wall" filters, have rather nasty phase responses. With oversampling a filter with a much more gentle roll off can be used since the oversampled D/A output will not contain spurious signals below 88.2 kHz. (The spurious signals only occur at frequencies greater than half the effective digital sampling rate - 44.1 kHz without oversampling, 176.4 kHz with it). Regards, Bill Mitchell (ihnp4!lcuxc!wjm)
prk@charm.UUCP (Paul Kolodner) (07/27/85)
Douglas Hamilton made two boo-boos in his response to media interest in cd players: 1. I think the original posting was in jest. I think the original poster knew that that is NOT how they do it! 2. About oversampling: The"staircase" waveform that results from straight reconstruction fron discrete samples exhibits white noise all over the frequency spectrum, not just at the high end as he suggests. This noise is inaudible with 14-bit sampling, so it's not the issue when it comes to oversampling. Oversampling does not shift this noise to higher frequencies, as he suggests. It merely allows for less phase distortion. A second thought: why they call the output filters "anti-alias filters" I'll never know. The aliases are either prevented by filtering before digitization, or else they can't be removed at all. Oversampling introduces new aliases, but they are above the original Nyquist frequency and are removed by output filters.
keithe@tekgvs.UUCP (Keith Ericson) (07/30/85)
In article an earlier article someone wrote:
1>A second thought: why they call
2>the output filters "anti-alias filters" I'll never know.
3>Oversampling introduces new aliases, but they are
4>above the original Nyquist frequency and are removed by output
5>filters.
<3-5 restated>: the (new) aliases are removed by the output filters.
And that's why they're called anti-alias filters...
--
Keith Ericson at TekLabs (resident factious factotum)
Tektronix, PO 500, MS 58-383
Beaverton OR 97077
(503)627-6042
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ARPAnet: keithe.tek@rand-relaykarn@petrus.UUCP (Phil R. Karn) (07/30/85)
> In article an earlier article someone wrote: > 1>A second thought: why they call > 2>the output filters "anti-alias filters" I'll never know. > > 3>Oversampling introduces new aliases, but they are > 4>above the original Nyquist frequency and are removed by output > 5>filters. > > <3-5 restated>: the (new) aliases are removed by the output filters. > > And that's why they're called anti-alias filters... Negative. The output of the D/A and sample-and-hold on a CD player contains multiple IMAGE SPECTRA. This is entirely distinct from the phenomenon of "aliasing", which is unique to A/D conversion, i.e., digital recording. Aliasing refers to the possibility of a signal above half the sampling rate "masquerading" or "aliasing" (hence the name) as a lower frequency. Such high frequencies MUST be filtered out before A/D conversion, because once they reach the A/D converter there is absolutely nothing that can be done afterwards to separate aliases from the desired audio signals. This is in contrast to the problem of removing the high frequency "image spectra" which are found in the "stairsteppy" output of a D/A converter. This is done by a "reconstruction" filter which can be anywhere between the D/A converter and the speaker, or even omitted entirely if you don't care about feeding all that supersonic crap to your tweeters and fetching all of the dogs in the neighborhood. Unlike aliases in the A/D process, the image spectra produced in D/A conversion do NOT directly affect the audible sound, unless your hearing is so good that you can hear well past the CD half-sampling rate. However, they can cause intermodulation distortion, burn out tweeters and waste amplifier power. Phil