Commit 47b5b871 authored by Jean-Marc Valin's avatar Jean-Marc Valin
Browse files

Documenting the pitch decoding

parent 905c67ee
......@@ -8359,7 +8359,35 @@ Adaptive codebook
\end_layout
\begin_layout Standard
Decode period (constant, offset, per sub-frame)
For rates of 8 kbit/s and above, the pitch period is encoded for each subframe.
The real period is
\begin_inset Formula $T=p_{i}+17$
\end_inset
where
\begin_inset Formula $p_{i}$
\end_inset
is a value encoded with 7 bits and 17 corresponds to the minimum pitch.
The maximum period is 144.
At 5.95 kbit/s (mode 2), the pitch period is similarly encoded, but only
once for the frame.
Each sub-frame then has a 2-bit offset that is added to the pitch value
of the frame.
In that case, the pitch for each sub-frame is equal to
\begin_inset Formula $T-1+offset$
\end_inset
.
For rates below 5.95 kbit/s, only the per-frame pitch is used and the pitch
is constant for all sub-frames.
\end_layout
\begin_layout Standard
Speex uses a 3-tap predictor for rates of 5.95 kbit/s and above.
The three gain values are obtained from a 5-bit or a 7-bit codebook, depending
on the mode.
\end_layout
\begin_layout Subsection
......
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