[opus] opus lossless?
Timothy B. Terriberry
tterribe at xiph.org
Wed Aug 22 17:12:40 PDT 2012
> 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.
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