[flac-dev] FLAC specification clarification

[Stephen F. Booth]

Is this a case where something allowed by the specification isn't
implemented by the reference encoder/decoder (such as 25-32 bits per
sample) but could be in a different implementation? If so, I am not sure
whether it makes sense to change the specification based on the reference
implementation.

Stephen

On Wed, Jun 17, 2020 at 3:22 PM Martijn van Beurden <mvanb1 at gmail.com>
wrote:

> Hi all,
>
> When trying to better understand the way LPC exactly works, I stumbled
> upon something which, after some digging, was already reported and (partly)
> fixed: https://sourceforge.net/p/flac/bugs/424/
>
> Apparently, the FLAC specification has a LPC shift that can be both
> positive and negative, but the encoder specifically makes sure that only
> positive shifts are encoded and the decoder only accepts positive shifts.
> The ffmpeg FLAC encoder and decoder show the same behaviour.
>
> Now, the documentation in the source code is fixed, the documentation on
> the website (which I was looking at) isn't yet. The website format.html
> states: "Quantized linear predictor coefficient shift needed in bits (NOTE:
> this number is signed two's-complement)." The source code format.html says
> "Quantized linear predictor coefficient shift needed in bits (NOTE: this
> number is signed two's-complement; but, due to implementation details, must
> be non-negative)."
>
> I was thinking of submitting a patch to the FLAC website git to get the
> website format.html up-to-date (there have been more changes than just this
> one), but I feel the line above isn't clear enough. Maybe change it to
> something like this, to make the wording more similar to the rest of the
> specification
>
> Quantized linear predictor coefficient shift needed in bits (NOTE: these
> bits must be 00000-01111. Originally this was a signed integer, but
> negative shifts were never implemented).
>
> Or perhaps:
>
> Quantized linear predictor coefficient shift needed in bits (NOTE: First
> bit must be zero. Originally this was a signed integer, but negative shifts
> were never implemented).
>
> Any thoughts?
>
> Kind regards, Martijn van Beurden
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