Originally Posted by BIslander
Thanks, Roger. Your insight is invaluable, as usual. A few follow up questions, if I may.
Are there any circumstances where the Dolby stereo downmix is used by a processor and are there any circumstances where it is not used by a source device such as a disc player or set top box when the output is stereo? What rules govern whether or not the stereo downmix is done by the decoder?
I suspect you can find certain AVRs that use the DD downmixing -- I've learned to never say never. But I have not found one.
In a set top box (cable/sat) I have not seen anything but the barest of bones decoding. Cheap is the key factor, and downmixing to 2.0 in the DD chip is the cheapest solution by far.
As for disc players, it happens quite often that the DD decoder runs in full 5.1 mode and a separate downmixer is present. The original Panasonic DVD players had both 5.1 and 2.0 outputs running in parallel. Since players must deal with not only DD, but also PCM, they must have a separate downmixer anyway, so why not use it for all the codecs?
I am curious about TrueHD. You say LFE is not discarded. But, isn't TrueHD built on a 2 channel core that gets used for stereo playback, which would seem to suggest there's no LFE present to either discard or redirect?
When TrueHD carries a 5.1 program, it is based on a 2.0 downmix and a 3.1 extension. That downmix has whatever and however much of the source 5.1 channels as the content maker decides, including LFE.
One other curiosity. Your post says bass management happens downstream of the decoder while the Dolby 5.1 Production Guide quoted elsewhere in this thread says "The subwoofer signal is created in the decoder as needed for the particular speaker complement in use, using crossover filters. This signal is created using bass management, and all Dolby Digital decoders perform this function." Why the apparent discrepancy?
Because they are using the term Dolby Digital rather casually. The decoder being the metal box with the DD logo on it. They should have said "Dolby Digital decoder product." There never was any bass management in the decoder algorithm, per se, but it is in the decoder product.
Once again, thanks for helping out here.
Always happy to help on these arcane matters of Dolby history.