On Tue, 2 May 2017 18:01:37 -0400 Shawn Singh <shawnsingh-at-google....@ffmpeg.org> wrote:
> Hello, > > We are trying to understand two fields in AVCodecParameters: > bits_per_coded_sample and bits_per_raw_sample. > > In the comments in libavcodec/avcodec.h, bits_per_coded_sample is described > as "the bitrate per sample". This sounds like (encoded bitrate / sample > rate), for example 128 kbps 44.1 kHz audio stream would be 3 bits per coded > audio sample. But, the code usage suggests that this field is actually > "the bit depth of each sample, if the sample was uncompressed", which is > also similar to the comments and usage for bits_per_raw_sample. For > example, the mov.c demuxing initializes bits_per_coded_sample when parsing > the "sample size" field of the mp4 AudioSampleEntry (in the stsd atom, for > both audio and video). > > Various codecs/formats initialize one value, or the other, or both in > different times. For example, pcm.c audio codec sets bits_per_coded_sample > on encoding, and bits_per_raw_sample on decoding. But the mov.c demuxer > and movenc.c muxer both use bits_per_coded_sample for muxing. > > So, what really is the difference between these two values? Is it > possible that these fields should just be merged into one field? Or if > there is a pattern we don't see, perhaps only the comments need to be > updated? That is indeed not very clear. Here's my guess (from my hazy memory and a superficial look at the code): bits_per_coded_sample is the "proper" value, and used as parameter for the decoder, while bits_per_raw_sample signals that not all bits in the _decoded_ data are used. For example, a codec could use bits_per_codec_sample as a mandatory parameter for the decoder (and that tells the decoder something about how a bitstream needs to be decoded). On the other hand, it might set bits_per_raw_sample to signal that the decoded PCM stream has a certain bit resolution. For example, some codecs support 24 bit audio, which is output as 32 bits, but with bits_per_raw_sample set to 24 (the 8 LSBs will be set to 0). Hopefully others have better explanations. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
