OPUS format does not record on iOS, documentation conflicting

[jt274]

Package version
5.1.0

Environment

• OS: iOS 17.4.1

Describe the bug

Front page of documentation states OPUS encoding is not available on iOS: https://pub.dev/packages/record

AudioEncoder enum states under opus that it is supported with SDK 11+: https://pub.dev/documentation/record/latest/record/AudioEncoder.html

However, when recording on iOS it throws an error: PlatformException(record, Failed to start recording, opus not supported., null)

Either this is a bug, or the documentation is incorrect. However, iOS does indeed support an opus encoder since SDK 11, so it should be working.

Add your record configuration RecordConfig(...)

RecordConfig(
  encoder: AudioEncoder.opus,
  sampleRate: 16000,
  bitRate: 8000,
  numChannels: 1,
)

Expected behavior

Audio should record using opus encoder on iOS 11+.

Additionally, documentation states OPUS is recorded in MPEG_4 container, but CHANGELOG states with version 4.0.1 that OPUS is now in OGG containers, so documentation is out of date.

[llfbandit]

Opus audio format is not natively supported by iOS. Fixing documentation.

[jt274]

@llfbandit According to Apple Developer docs, opus codec is supported on iOS 11+ using kAudioFormatOpus for AVFormatIDKey.
https://developer.apple.com/documentation/coreaudiotypes/kaudioformatopus

record_darwin specifies this here:

record/record_darwin/darwin/Classes/RecorderFormat.swift

Line 80 in de508bb

AVFormatIDKey : kAudioFormatOpus,

[llfbandit]

Opus is available for decoding only.

[jt274]

Opus is available for decoding only.

@llfbandit I edited this file:

record/record_darwin/darwin/Classes/Recorder.swift

Line 127 in de508bb

func isEncoderSupported(_ encoder: String) -> Bool {

I added the line: AudioEncoder.opus.rawValue

I then tried recording using AudioEncoder.opus on iOS, and the recording was successful. Reading the first bytes of the output file I can see that it was recorded in a CAFF container, which is what is expected for iOS.

So I believe iOS does support opus as an encoder (and the documentation of the package was correct in saying SDK 11+), but the record_darwin package left out the opus format in the isEncoderSupported function.

[llfbandit]

Can you send me a simple audio file with this setup?
If I remember well, the underlying encoder was not opus. Maybe I'm wrong?

[jt274]

I used the following config:

RecordConfig(
  encoder: AudioEncoder.opus,
  sampleRate: 16000,
  bitRate: 8000,
  numChannels: 1,
)

After recording an audio file, I analyzed it with ffprobe:

Input #0, caf, from '1.caf':
  Duration: 00:00:08.66, start: 0.000000, bitrate: 37 kb/s
  Stream #0:0: Audio: opus (opus / 0x7375706F), 48000 Hz, mono, fltp

For some reason the bitrate and sample rate are not correct, but it is working in opus format.

[llfbandit]

Ok, that's good news!
I'm not very surprised if iOS does not support those values.

The only caveat here (before testing & re-introducing this encoding) is CAF. This container is only supported by Apple products/platforms.

[jt274]

Yes, ideally there would be a solution to convert to OGG container, but I can figure something out for that.

[llfbandit]

Another funny thing, Safari only supports opus in CAF container and not OGG.
Welcome to my world :)