Voice-Message Size: Encoding, Bitrate, Samplerate

[MathJud]

The chat voice message is currently very big and it's size should be considerably reduced.

Current Message Size & Encoding

The message size was tested with voice messages of 1 minute (60 seconds) length.

qaul: 1min = 1M

• file: .m4a (MPEG-4 Audio only default file)
• Container: Quicktime
• Audio Codec: MPEG-4 AAC
• Audio Bit Rate: 128 kbps
• Sample Rate: 44,10 kHz
• Channels: Stereo

Other Messengers Message Size & Encoding

Signal: 1min = 250 kB

• file: .aac
• Container: MPEG-4 AAC
• Audio Codec: MPEG-4 AAC
• Sample Rate: 44,10 kHz
• Channels: Mono

Whatsapp: 1min = 140 kB

• file: .ogg
• Container: Ogg
• Audio Codec: Opus
• Sample Rate: 16 kHz
• Channels: Mono

Optimization

We currently use the flutter record plugin, for which the AAC-LC codec seems to be the best encoding which is supported on all platforms.

We currently use it's default configuration which has the following values:

  const RecordConfig({
    this.encoder = AudioEncoder.aacLc,
    this.bitRate = 128000,
    this.sampleRate = 44100,
    this.numChannels = 2,
    this.device,
    this.autoGain = false,
    this.echoCancel = false,
    this.noiseSuppress = false,
  });

I suggest to change the following values:

• numChannels = 1 for mono recording
• bitRate: 64000

And to test and document the following values for 60 seconds:

• bitRate: 64000, 56000, 48000, 32000
  • 56kB should be broadcast quality, 48kB should be fine for voice
  • according to this research: https://www.radioeng.cz/fulltexts/2018/18_01_0342_0352.pdf
• samplingRate: 44100, 32000

In order to compare the test results, the tests should contain the following results:

• Voice message of 1 minute (60 seconds) length
• Message size in KB
• Message quality

Codec Information

• supported codecs of record Plugin
• Recommended Sampling Rate and Bitrate Combinations for AAC codec
• Paper on Bitrate & Quality for AAC-LC Codec