why is oversampling in CD players considered bad?

[wa2ise]

Seems that many audiophiles don't like CD players that use oversampling ahead of the DAC chip. I think that I have a pretty good handle on why op-amps are not good for analog audio, and why some people like vacuum tubes for analog audio, but I don't see what the issue with oversampling is. Generally oversampling is just a FIR (finite impulse response) digital filter used to filter out the sharp clock edges after the sampling frequency is boosted, in hte digital domain. To make the reconstruction analog filter after the DAC chip easier to build with less impact on the audio. Otherwise you need a brickwall low pass filter there to get rid of the ultrasonic junk that could mess up the sound in your audio amp. Such a brickwall filter would impact the sound worse than any digital oversampling processing would do I would have thought...

[Peter Daniel]

You may check this link for some info: http://sakurasystems.com/articles/Kusunoki.html

[wa2ise]

Okay, I suppose that the "pre-echo" a symmetrical FIR filter would produce MIGHT be audiable. It's hard to imagine how you could get a pre-echo in the real world (say a real drum hit by a real drumstick heard by real ears). But we could create a non-symmetrical FIR or maybe an IIR (infinite impulse response) filter that would have no pre-echo but some trailing "echo". And still get the frequency filtering we want. Truncation of the digital filtered words can be a problem, partly avoided with DAC chips with more bits. Proper rounding techniques help too.

I'm going to ignore the PLL jitter issue as I would just house the DAC chip in the same box the transport mechanism (the actual CD drive) resides.

[carlosfm]

Originally posted by wa2ise
Otherwise you need a brickwall low pass filter there to get rid of the ultrasonic junk that could mess up the sound in your audio amp.

So much for digital filters, and then there's bitstream and delta-sigma dacs.

[SY]

Phase correction in the digital domain is trivial. If anti-imaging filters had been standardized, the CDs themselves could have had the correction "built in."

[Werner]

Originally posted by wa2ise
Okay, I suppose that the "pre-echo" a symmetrical FIR filter would produce MIGHT be audiable.

Guys, read Shannon closely . He specifies the reconstruction filter as the Sinc function, which has pre-echo. The pre-echo is required for proper reconstruction and does not make it to the actual output signal. In other words, the ringing filter results in a non-ringing analogue signal. If you don't believe this then please go through the math.


Since oversampling FIR filters (mostly) all try to approximate Sinc (and rightly so), they have to have pre-echo. The remaining question is: how good a Sinc approximation are they?


Now using ringing FIRs as anti-alias filters in an ADC is a different kettle of fish.

[Guido Tent]

Originally posted by wa2ise

I'm going to ignore the PLL jitter issue as I would just house the DAC chip in the same box the transport mechanism (the actual CD drive) resides.

Yes. Do that. If you ignore that, you ignore one of the key differences between oversampled and non oversampled systems.

best

[dddac]

The pre-echo is required for proper reconstruction and does not make it to the actual output signal. In other words, the ringing filter results in a non-ringing analogue signal. If you don't believe this then please go through the math.

mmmmhhhh, just 2 pics measured at the analog output of my CD player (oversampled) and my DAC (non oversampling)

It seems to me the pre ringing clearly made it to the output ....

Could you clarify what you actaully meant

regards
doede

[Werner]

Originally posted by dddac

It seems to me the pre ringing clearly made it to the output ....

That is because your stimulus signal on the disc was a 'perfect' digital square, not? Like -100 +100 -100 +100 ...

Ask yourself: is such a signal allowed to exist in the 44.1kHz sampled space?

The answer to that is: no. Is it not a band-limited signal, and as such
it can not exist in a legal way in the sampled signal space. The only thing such a test does for you is to provide you with a readout of the oversampling filter's coefficients.

Same for all those impulse response tests: invalid stimulus.


Remember that a Shannon/Nyquest recording channel consists of anti-aliasing, ADC, DAC, reconstruction, and when testing one should test the whole chain, and not inject illegal signals before the DAC.

Unless one knows and understands the implications and the limitations of such a test.

[Guido Tent]

Originally posted by dddac


mmmmhhhh, just 2 pics measured at the analog output of my CD player (oversampled) and my DAC (non oversampling)

It seems to me the pre ringing clearly made it to the output ....

Could you clarify what you actaully meant

regards
doede

Doede,

What remains is the result of imperfection in the filter. I won't claim you hear it or not, but the pre ringing shouldn't make it to the output when perfect implementation would be present.

Again, one of the many examples of concept versus implementation.

best