Now that we no longer have Dolby Digital Passthrough, and instead have either PCM decode or DD re-transcode, there is a reasonably common (I believe) use-case which could be improved.
There are quite frequently broadcasts of 2.0 AAC and 2.0 DD/AC3 audio which contain Dolby Surround matrixed surround audio AND metadata in the stream that flags this. When bitstreamed this metadata automatically triggers AVRs downstream into ProLogic, THX etc. matrix surround decode automatically.
When Live TV supported passthrough of AC3 audio, this metadata just passed straight through (for AC3 audio).
However now 2.0 Dolby (and AAC) audio is permanently decoded to 2.0 PCM (even if transcode to Dolby Digital is enabled). This means the automatic detection of Matrix Surround content no longer operates, and AIUI there is also no OSD indication of the metadata-flagged presence of matrix surround (so at least you would know to manually enable matrix decode on your amp)
Given that it appears passthrough isn’t coming back anytime soon, and there appears to be no ProLogic or similar licence-compatible decode implementation (nothing in ffmpeg) (*) – is it possible for there to be an option to re-encode this audio to 2.0 Dolby (and encode AAC 2.0) to Dolby Digital 2.0 (not 5.1) WITH the ffmpeg “d_sur” metadata flagging the presence of matrix surround (and ideally passing through all other metadata)?
This would help mitigate a probably unforeseen side-effect of the lack of passthrough?
AIUI ITV HD on Freeview is an AAC 2.0 broadcaster that correctly flags matrix surround (and when transcoded to AC3/DD output correctly outputs 2.0 DD/AC3 with the matrix surround flag present to trigger off-board decoding in ProLogic/THX etc. modes) Apparently many French and German 2.0 AC3 and E-AC3 broadcasters do?