On Saturday, July 21, 2018 at 11:19:55 AM UTC-7, Timothy B. Terriberry wrote: > raycarino--- via dev-media wrote: > > I have a follow on question. What's the best way to determine an Ogg Opus > > file's encoded sample rate? > > Opus is a lossy codec. The sample rate is something that is lost as part > of the lossy encoding. The original sample rate in the header gives you > a hint as to the best sample rate to run the decoder at to get all of > the information that was encoded, but it is perfectly possible to run it > any supported rate (8 kHz, 12 kHz, 16 kHz, 24 kHz, and 48 kHz), > regardless of how it was encoded. > > You _could_ get some idea of the content of the audio by looking at the > TOC byte in each of the packets (c.f. RFC 6716 Section 3.1). NB implies > 8 kHz, MB implies 12 kHz, WB implies 16 kHz, SWB implies 24 kHz, and FB > implies 48 kHz. But this is allowed to change on a packet-by-packet > basis, and the encoder may in fact change it based on the content of the > audio that was fed to it and the requested bitrate (regardless of what > sampling rate the encoder was running at). I'm not aware of a tool that > actually inspects and reports on this information, either.
Thank you again Timothy! That all makes sense. Hopefully these are my last follow-up questions. In general, I'm wondering whether there will be audio artifacts present when decoding at a different sample rate than what was encoded. Questions: 1. How does a high sample rate decoded at a lower sample rate behave? Example: will decode(48kHz.ogg 16kHz) === decode(16kHz.ogg, 16kHz), or can audio artifacts be expected? 2. Similarly, how does a low sample rate decoded at a higher sample rate behave? Example: will decode(16kHz.ogg, 48kHz) === decode(16kHz.ogg, 16kHz), or can audio artifacts be expected? I'm hopeful that decoding with a different sample rate either has no distortion, or minimal distortion because: (a) Firefox always seems to decode at 48kHz https://github.com/mozilla/gecko-dev/blob/f822a0b61631cbb38901569e69b4967176314aa8/dom/media/ogg/OpusParser.cpp#L46 (b) Chrome also seems to always decode at 48kHz https://github.com/chromium/chromium/blob/d196d28f53d37bad65feac0c0fb87a3b2c9480e9/remoting/codec/audio_decoder_opus.cc#L25 _______________________________________________ dev-media mailing list [email protected] https://lists.mozilla.org/listinfo/dev-media
