HDCD allows the use of three features: peak extend, gain adjustment and reconstruction filter switching. Windows media player supports peak extend and gain adjustment, and it has hence been possible to fully implement these features.
Filter switching, however, only makes sense when you are upsampling, something which wmp doesn't do, and there's hence no reference to implement it from. Investigation of various test CDs and various (usually contradictory) literature has led to the conlusion that there are only two filters available at playback, the normal filter and the transient filter.
The reason for this conlusion is that of the three bits of each hdcd code remaining after peak extend and gain adjustment, wmp abandons two, but explicitly stores the remaining one (suggesting that this is the only important remaing bit, giving only two options).
An ivestigation of when this bit was set was what lead to the conclusions that it is intended to cover transients. If you take a look at,
http://www.srcf.ucam.org/~cjk32/hdcd...3%20Panama.PNG
the right channel is untouched, and the left channel shows that value of this remaining bit. It is set to for a period covering each of the drum beats, and similar behaviour is observed with many other tracks.
The intention is to try to add upsampling to the code once the filter's responses are known. I'm hoping that someone with a hardware HDCD decoder (which should be upsample and switch filters) will be able to play the following test tracks through it to ascertain the impulse / frequency response,
http://www.srcf.ucam.org/~cjk32/hdcd/transient_filter/
These are a frequency sweep and an impulse train with hdcd codes embedded to either enable the normal or trainsient filter (_nf.wav, _tf.wav). If anyone does have a go with these, please don;t put the impulse train through speakers (or amp?), I can't imagine it'll do them any good.
Chris