2010318163752818

ISO/IEC 14496-3:2005(E)
frames (VUV = 0). The subclauses below show how to generate the decoder parameters for back ground noise
interval.
2.5.9.4.1 LSP decoding
In the decoder, two sets of previously transmitted LSP parameters, prevLSP1 and prevLSP2, are held.
prevLSP1: transmitted LSP parameters
prevLSP2: transmitted LSP parameters before prevLSP1
“Background noise” mode occurs only after “unvoiced” or “background noise” mode. When the “background noise”
mode is selected, LSP parameters are transmitted only when the frame is “noise update frame” (UpdateFlag = 1). If
new LSP parameters are transmitted, prevLSP1 is copied to prevLSP2 and newly transmitted LSPs are copied to
prevLSP1 regardless of VUV decision.
LSP parameters for each frame during the “background noise” mode are generated by the interpolation between
prevLSP1 and prevLSP2 using the equation:
where
qLsp( i)
= ratio ⋅ prevLsp1(
i)
+ ( 1−
ratio)
⋅ prevLsp2(
i)
Li
= 1..
10
(2.5.9.1)
2 ⋅ ( bgnIntval + rnd)
+ 1
ratio =
2 ⋅ BGN _ INTVL
(2.5.9.2)
qLsp(i) is the i-th LSP to be used for decoding operation of the current frame, prevLsp1(i) is the i-th LSP of
prevLSP1, prevLsp2(i) is the i-th LSP of prevLSP2 ( 1 ≤ i ≤ 10). In this equation, bgnIntval is a counter which
counts the number of consecutive background noise frames, and is reset to 0 at the receipt of background noise
update frame. BGN_INTVL(=12) is a constant, and rnd is a randomly generated integer value between –3 and 3.
If counter bgnIntval reaches BGN_INTVL, bgnIntvl is set to BGN_INTVL-1, and if the ratio obtained by the equation
(2.5.9.2) is smaller than 0 or greater than 1, the value of rnd is set to 0 and ratio is recomputed.
2.5.9.5 Excitation generation
During the period of “background noise” mode, the Gain index (VX_gain[0]) transmitted in the noise update frame is
used for all the subframes, the values of Shape index (VX_Shape1[0,1]) are randomly generated between 0 and
NUM_SHAPE_L0-1. These excitation parameters are used with the interpolated LSP parameters as described
above to generate the signals of background noise mode.
LSP index
UpdateFlag
V/UV/BGN
Celp Gain Index
Inverse VQ of LSP Interpolation for BGN
Random Number
Generator
Figure 2.15 — Additional diagram for variable rate decoder
LSP Interpolation for UV
Celp Shape Index
60 © ISO/IEC 2005 — All rights reserved