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I've noticed that among audio codecs, Variable Bit Rate (VBR) is typically the default. In fact, Vorbis and AAC do not support a truly constant CBR mode, instead offering a 'constrained' or 'managed' mode.

On the other hand, The MP3 encoder LAME defaults to CBR 128 kbit/sec, and Opusenc has a "hard-cbr" option, which seems to be true CBR. I have also heard murmurings that "VBR is superior" but without an objective explaination.

Can anyone care to explain why CBR seems to be unavailable or not recommended in many codecs?

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Constant bit rate is just that - constant, whether it's needed or not.
It wastes data when it's not needed & it can't keep up when it is.

In areas of low complexity, therefore, CBR is just spinning its wheels not really doing anything. In areas of high complexity the rate isn't high enough to capture all the detail.

VBR avoids this.

CBR was useful in the early days (90s), so streaming & decoding always had time to keep pace. These days it really not needed any more, especially as almost everything can buffer to cope with higher rates in complex areas. No computer or phone player these days would have any issue at all keeping pace.

See https://en.wikipedia.org/wiki/Variable_bitrate

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