Re: Upsampling?


"Barend" <barendh@xxxxxxxxx> wrote in message
news:MPG.1ea457793da19926989685@xxxxxxxxxxxxxxxxxxxxxx
Now For Something Not Religious!

Whaddayou think?
Upsampling in CD Players- good or nonsense?
Barend

It has been used for years; it is standard practice.
"Oversampling" and "upsampling" are mathematically the same thing, with
differences in the actual hardware.

For quite some years, a "digital filter" has been interposed between the PCM
stream and the actual D/A converter. The converter implements what is called
an "interpolation algorithm", converting the 16 bit PCM output to a stream
of higher bitrate and finer resolution, ie., 16/44 to 20/364. This adds no
information whatsoever. It does simply the design and reduce the effect of
the final stage of DAC output, the so-called "analog reconstruction filter."
These filters are analog, and therefore causal. Causal filters have phase
shift. Quite afew designers believe it is the phase shift of such a filter
that is responsible for much of the variation in sound between DACs. But
oversampling makes it possible to push the cutoff frequency of the analog
reconstruction filter far from the audio passband.

One "authority" decided to define "upsampling", as opposed to
"oversampling", as the case where the digital filter was more elaborate
and/or in a separate package from the DAC chip. This discrimination is
broken by a number of early examples with separate digital filters.

For whatever reason, there are a number of so-called upsampling DACs that,
at the least, provide very good sound. In some high end cases, the digital
filter may be elaborate, incorporating proprietary algorithms and even
lookup tables, in an attempt to fake a higher resolution source.
Subjectively, these efforts can be successful. Less expensive upsampling
DACs simply incorporate very good 96 or 192kbs D/A converters in conjunction
with a separately packaged sample rate converter. Converting from 44 to a
sampling rate that is not a multiple has the potential to introduce
artifacts. Nevertheless, many people like the sound of these units as well.

I have a Musical Fidelity A324 which is an example of an elaborate high end
DAC, a Perpetual Technologies P3a exemplifying the simple approach, and a
Sony EP9ES, which implements the DAC of the Sony XA7ES, a Stereophile Class
A CD player. The Sony is most neutral; the A324 "clarifies" older
recordings, while the P3a is revealing and lively. A switchable collection
of DACs can be a highly effective tweak for any audio system.

SHP


.

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