[opus] opus lossless?

[Timothy B. Terriberry]

LiZ wrote:
> Hi All,
>
> It is possible to make Opus/CELT a lossless coder if I allow
> a sufficiently high bit rate? We considered using FLAC, but FLAC's
> latency is well beyond the acceptable range.

No. The PVQ codebook used in the MDCT layer never includes code points 
at exact integer values at any rate, the allowable allocations for the 
bands aren't sufficient achieve 16 bits of depth should they contain 
enough energy to require this, the MDCT itself is not perfectly 
reversible (due to truncation and rounding in the arithmetic), and 
frequencies above 20 kHz are never encoded, to name just a few issues. 
There are similar amounts of truncation and rounding in the LPC layer 
that prevent it from being exactly reversible, as well.