I'm experimenting with using HLS to serve audio on demand to web
clients. The server has a weeks-long continuous audio recording. The
user chooses an arbitrary point in time in the recording to start
listening. The audio plays until the user stops it or unloads the web
page from the browser.
When the source of the audio recording is mp3 encoded (from a Shoutcast
live stream) I'd be interested to avoid transcoding to AAC in producing
the HLS files.
Iiuc, FFmpeg's hls muxer supports only mpegts and fmp4 segment types. Do
these formats accept mp3 audio streams?
Can I create Packed Audio[1] with FFMpeg using the segment muxer? The
tricky part seems to be where the HLS spec says
Each Packed Audio Segment MUST signal the timestamp of its first
sample with an ID3 Private frame (PRIV) tag [ID3] at the beginning of
the segment. The ID3 PRIV owner identifier MUST be
"com.apple.streaming.transportStreamTimestamp". The ID3 payload MUST
be a 33-bit MPEG-2 Program Elementary Stream timestamp expressed as a
big-endian eight-octet number, with the upper 31 bits set to zero.
I don't think segment muxer will do that. But could a script add the
timestamp to a finished segment via ffmpeg cli using metadata key
"id3v2_priv.com.apple.streaming.transportStreamTimestamp"[2]?
I'm not sure using either mp3 or Packed Audio segments in HLS are good
for breadth of client compatibility but I'm interested to try it out.
Tom
[1]
https://tools.ietf.org/html/draft-pantos-hls-rfc8216bis-07#section-3.1.3
[2]
https://github.com/FFmpeg/FFmpeg/commit/8a4cc0a2567fa8418709f75af5539cdf76fefb99
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