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Original Message

That's a good idea...

Posted by Charles Hansen on January 1, 2006 at 18:12:17:

...but it almost certainly won't work.

As far as I know, there are no detailed specs on how WMP decodes HDCD discs. We've already seen one significant difference between WMP and the hardware implementation used for stand-alone CD players. But if WMP behaves the same way as the HDCD decoders used for stand-alone CD players, then dither is added to the signal sent to the DAC chip. Here is the relevant text from the datasheet for the PMD-100:

Dither Modes
The PMD-100 provides 8 different output dithers in Program mode. All dither levels are available for output data
widths from 16 to 24 bits at 8 Fs and 4 Fs oversampling rates only. Dither must be disabled if the 2 Fs oversampling rate
is selected (as well as 24 bit mode must be selected when the 2 Fs oversampling rate is selected). There are seven levels
of high-frequency weighted dither (modes 0-6) plus minimum amplitude white triangular PDF dither (mode 7). Modes
0 and 7 are minimum amplitude dithers which correct quantizing errors only, whereas modes 1 through 6 are increasing
levels of high-frequency weighted dither designed to smooth out non-linearity errors in multi-bit DAC’s (Modes 0 or 7
are appropriate for single-bit DAC’s). The HF weighted dithers put the dither energy above the audio spectrum, where
most of it is later filtered out by the analog low-pass filter following the DAC. In general, multi-bit DAC’s behave better
with high levels of dither, but some analog circuits following the DAC may have problems with transient intermodulation
distortion (TIM) when confronted with high levels of high frequency energy. The best dither level for a particular circuit
implementation must be determined empirically.
Dither Notes:
1) Dither level 6 is the highest level of high-frequency weighted dither available with the PMD-100.
2) In Stand-Alone Mode, dither mode 0 (minimum high frequency weighted) is available. Setting pin 4 DITH low
will disable all dither including programmed dither and should be used only for test purposes.

So assuming that WMP behaves like the stand-alone HDCD decoders, dither will be added to at least the LSB (and possibly more, although the datasheet is evasive on the maximum amount used).

Before you cry "foul" and say that this proves that some sort of HDCD decoding is taking place, please keep in mind two facts:

a) This dithering is applied to *all* discs, HDCD or not, that are played through the PMD-100.

b) Other digital filters also apply dither. For example although it is not mentioned anywhere in the datasheet, I was personally told by one of the Burr-Brown design engineers that the DF1704 uses 1/2 bit of triangular PDF dither. (One of the AES papers by Lipshitz and Vanderkooy proved that this was the mathematically optimal dither.)

It would still be interesting to try the experiment, as one might still be able to learn something. For example if the difference file were amplified and played back by itself, then you could probably tell what is happening just by listening to it. If it basically sounded like noise, then the difference must solely be due to dither. But if there were some sort of expansion happening in the decoder, the difference file should have a clearly audible music signal.