[linux-audio-dev] Direct Stream Digital / Pulse Density Modulation musing/questions

[Martijn Sipkema]

[...]
> Conventional PCM techniques are unable to reproduce high frequencies
> correctly. And the explanation is very simple.

Actually a correct explanation isn't that simple. Yours is much _too_
simple.
Theoretically a 20 kHz bandlimited signal can be represented _exactly_ as a
40 kHz PCM stream. In order to not have to use a very steep lowpass filter
in
the DAC it is better to use a somewhat higher sampling frequency. 48 kHz
should
be enough most of the time.

> If you record a sound at 44.1
> kss, you get a theorical frequency response of 0 - 22050 Hz. BUT to
describe
> frequencies from 11050 to 22050 Hz, you can only play with a 4-sample long
> period.

These are interpolated using a lowpass filter.

> A 22050 Hz sine could be really accurate (one sample up, one sample down
> every 1/22050th second), and so is 11025. But intermediary frequencies
> introduces temporal aliasing, some metallic feeling due to temporal
> quantization. This is inherent to the very low sampling rate (96 kHz is
just
> a bit better, but no miracle), which is unable to describe waveforms at
high
> frequencies.
>
> Bad high frequencies temporal definition means bad transients. Anyone can
> notice it when he _actually_ hear and compare PCM and DSD.
>
> Stop speculative talking and try to get some real demo...

This has been done and it is very hard if not impossible to hear the
difference.
24bit 96 kHz is most likely better than DSD, _especially_ at high
frequencies.
The demos for SACD often compare normal stereo PCM to surround on a
SACD...

Actually, DSD suffers deteriorating sample resolution
at higher frequencies whereas the resolution doesn't depend on the frequency
for
PCM. Also signal processing will have to be done in PCM. A SACD player with
an EQ has DSD -> PCM and PCM -> DSD convertors and these convertions
are not lossles.

The DACs and monitors you use probably make a lot more difference than
44 kHz PCM or 96 Khz or DSD.

There are also people who buy very expensive oxygen free cables and claim
to hear the difference. There are thousands of religious people who _know_
God exists...

DSD is a bad thing IMHO.

--ms