opus - Oct 2018 - Is OPUS_AUTO the default for an encoder's bitrate?

I had expected that the default bitrate for the encoder would be the same as
setting it to OPUS_AUTO, but I'm getting difference results:
>opusenc --comp 4 sample.wav sample.opus
Encoding using libopus 1.3-rc2 (audio)
-----------------------------------------------------
   Input: 8 kHz, 1 channel
  Output: 1 channel (1 uncoupled)
          20ms packets, 25 kbit/s VBR
 Preskip: 312

Encoding complete
-----------------------------------------------------
       Encoded: 2 minutes and 34.94 seconds
       Runtime: 1 second
                (154.9x realtime)
         Wrote: 588189 bytes, 7747 packets, 157 pages
       Bitrate: 29.7104 kbit/s (without overhead)  Instant rates: 1.2 to 50.4
kbit/s
                (3 to 126 bytes per packet)
      Overhead: 2.17% (container+metadata)

If I explicitly set the bitrate to OPUS_AUTO, it produces a stream with twice
the bitrate of the encoder's default from above.
>opusenc --comp 4 --set-ctl-int 4002=-1000 sample.wav sample.opus
Encoding using libopus 1.3-rc2 (audio)
-----------------------------------------------------
   Input: 8 kHz, 1 channel
  Output: 1 channel (1 uncoupled)
          20ms packets, 25 kbit/s VBR
 Preskip: 312

Encoding complete
-----------------------------------------------------
       Encoded: 2 minutes and 34.94 seconds
       Runtime: 1 second
                (154.9x realtime)
         Wrote: 1245829 bytes, 7747 packets, 157 pages
       Bitrate: 63.6644 kbit/s (without overhead)  Instant rates: 1.2 to 120.4
kbit/s
                (3 to 301 bytes per packet)
      Overhead: 1.03% (container+metadata)

As this is example 8kHz single channel audio, the encoder's default bitrate
seems more appropriate than the OPUS_AUTO bitrate. This presents a bit of a
problem because there are three possibilities for the bit rate, an explicitly
set value, a value set by OPUS_AUTO, and some mystery value from the default
encoder initialization. If the bitrate setting is changed, I don't see any
way to get back to the mystery value without re-initializing the encoder.  Can
anyone comment on why it works this way?

Kevin O'Connor

On 10/18/2018 10:42 AM, Kevin O'Connor wrote:
> I had expected that the default bitrate for the encoder would be the
> same as setting it to OPUS_AUTO, but I'm getting difference results:
libopus defaults to OPUS_AUTO for the bitrate, but opusenc is free to
have its own default bitrate. In this case, it appears that opusenc is
making a better decision than libopus (which IIRC doesn't take into
account the fact that your file is narrowband).

Cheers,

	Jean-Marc

>> opusenc --comp 4 sample.wav sample.opus
> Encoding using libopus 1.3-rc2 (audio) 
> ----------------------------------------------------- Input: 8 kHz, 1
> channel Output: 1 channel (1 uncoupled) 20ms packets, 25 kbit/s VBR 
> Preskip: 312
> 
> Encoding complete 
> ----------------------------------------------------- Encoded: 2
> minutes and 34.94 seconds Runtime: 1 second (154.9x realtime) Wrote:
> 588189 bytes, 7747 packets, 157 pages Bitrate: 29.7104 kbit/s
> (without overhead)  Instant rates: 1.2 to 50.4 kbit/s (3 to 126 bytes
> per packet) Overhead: 2.17% (container+metadata)
> 
> If I explicitly set the bitrate to OPUS_AUTO, it produces a stream
> with twice the bitrate of the encoder's default from above.
> 
>> opusenc --comp 4 --set-ctl-int 4002=-1000 sample.wav sample.opus
> Encoding using libopus 1.3-rc2 (audio) 
> ----------------------------------------------------- Input: 8 kHz, 1
> channel Output: 1 channel (1 uncoupled) 20ms packets, 25 kbit/s VBR 
> Preskip: 312
> 
> Encoding complete 
> ----------------------------------------------------- Encoded: 2
> minutes and 34.94 seconds Runtime: 1 second (154.9x realtime) Wrote:
> 1245829 bytes, 7747 packets, 157 pages Bitrate: 63.6644 kbit/s
> (without overhead)  Instant rates: 1.2 to 120.4 kbit/s (3 to 301
> bytes per packet) Overhead: 1.03% (container+metadata)
> 
> As this is example 8kHz single channel audio, the encoder's default
> bitrate seems more appropriate than the OPUS_AUTO bitrate. This
> presents a bit of a problem because there are three possibilities for
> the bit rate, an explicitly set value, a value set by OPUS_AUTO, and
> some mystery value from the default encoder initialization. If the
> bitrate setting is changed, I don't see any way to get back to the
> mystery value without re-initializing the encoder.  Can anyone
> comment on why it works this way?
> 
> Kevin O'Connor
> 
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>