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    • Anton Khirnov's avatar
      lavf: replace AVStream.codec with AVStream.codecpar · 9200514a
      Anton Khirnov authored
      Currently, AVStream contains an embedded AVCodecContext instance, which
      is used by demuxers to export stream parameters to the caller and by
      muxers to receive stream parameters from the caller. It is also used
      internally as the codec context that is passed to parsers.
      
      In addition, it is also widely used by the callers as the decoding (when
      demuxer) or encoding (when muxing) context, though this has been
      officially discouraged since Libav 11.
      
      There are multiple important problems with this approach:
          - the fields in AVCodecContext are in general one of
              * stream parameters
              * codec options
              * codec state
            However, it's not clear which ones are which. It is consequently
            unclear which fields are a demuxer allowed to set or a muxer allowed to
            read. This leads to erratic behaviour depending on whether decoding or
            encoding is being performed or not (and whether it uses the AVStream
            embedded codec context).
          - various synchronization issues arising from the fact that the same
            context is used by several different APIs (muxers/demuxers,
            parsers, bitstream filters and encoders/decoders) simultaneously, with
            there being no clear rules for who can modify what and the different
            processes being typically delayed with respect to each other.
          - avformat_find_stream_info() making it necessary to support opening
            and closing a single codec context multiple times, thus
            complicating the semantics of freeing various allocated objects in the
            codec context.
      
      Those problems are resolved by replacing the AVStream embedded codec
      context with a newly added AVCodecParameters instance, which stores only
      the stream parameters exported by the demuxers or read by the muxers.
      9200514a
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    • Nicolas George's avatar
      ALSA demuxer: use av_gettime and a timefilter. · 5d35b279
      Nicolas George authored
      The PTS for captured audio was measured using snd_pcm_htimestamp.
      
      snd_pcm_htimestamp hangs when the input is a dsnoop plugin.
      
      Furthermore, at some point, snd_pcm_htimestamp started returning monotonic
      timestamps rather than wall clock timestamps, in most but not all
      situations.
      Monotonic timestamps are fine, but ffmpeg uses wall clock timestamps
      everywhere else, and we have no API to inform the user which kind of
      timestamps it is.
      
      A separate snd_pcm_htimestamp is only slightly less accurate than
      snd_pcm_htimestamp: the standard deviation for the difference between two
      consecutive timestamps is (on my hardware):
      - ~13 µs with snd_pcm_htimestamp;
      - ~35 µs with av_gettime;
      -  ~5 µs with av_gettime and a timefilter.
      5d35b279
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