Thanks Pierre for the comments,
+Adding Patric Lai into loop,
On 02/07/2020 16:00, Pierre-Louis Bossart wrote:
On 7/2/20 6:11 AM, Srinivas Kandagatla wrote:
For gapless playback its possible that each track can have different
codec profile with same decoder, for example we have WMA album,
we may have different tracks as WMA v9, WMA v10 and so on
Existing code does not allow to change this profile while doing gapless
playback.
This patch adds new SNDRV_COMPRESS_SET_CODEC_PARAMS IOCTL to allow
userspace to set this new parameters required for new codec profile.
That does not seem fully correct to me. WMA profiles are actually
different encoding schemes - specifically the WMA 10 LBR.
The premise for gapless playback was that the same codec and profile be
used between tracks, so that the same internal delay was used. If you
look at the output data, it's made of zeroes for N samples, and then you
see decoded data. When you change tracks, the first N samples actually
come from the previous track.
If you change coding schemes between tracks, you cannot call this
gapless playback. You will both remove the last N samples of the
previous track and insert M zeroes (for the new decoder).
If you wanted to support such a mode, you would need to provide an
indication of the delay difference, e.g. by looking at the ID3 tags and
let firmware realign. Unfortunately, you don't send this information
with the new IOCTL? You would also need firmware tricks for the first
decoder to flush out its output and the new decoder to realign.
I also don't see how one might end-up with different profiles for an
album in the first place. The gapless use came mostly from ripping live
music recorded on audio CDs in different tracks, and it would have taken
a twisted mind to select different encodings between tracks.
If the 'album' is really a playlist, then the gapless playback as
supported by the ASoC compressed layer is nearly useless. What you would
really want is cross-fade but that's a different use case and
implementation that would be needed.
Patrick seems to have discussed this topic in detail at one of the audio
conf!
He might want to add more to this discussion.
Thanks,
srini
