Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 291

selected pulse

r(n)

centre of the
analysis frame

− 4 − 2 0 2 4

0 100 200 300

samples pitch pulse shape

α

(a) LPC residual (b) PDF of q

Figure 9.11 An analysis frame and the probability density function of α q

pitch pulse shape is fairly stationary and restricting the search range around
the previous value does not reduce the performance. Restricting the search
range has advantages such as reduced computational complexity and efficient
differential quantization of the pitch pulse shape. Furthermore, restricting the
search range avoids large variations in the pitch pulse shape. Large variations
in the pitch pulse shape introduce a reverberant character into the synthesized
speech.

2 π q L δ / 8 r α L = + − 1 ≤ δ ≤ 1 (9.18)

where,

α lst
2 π 8

1
2

q L = + for 0 ≤ α lst < 2 π (9.19)

Then R j and E j are estimated as before, substituting α q with α L , and all the
combinations of n p , α L ,and j for which E j

0 . 5 are excluded
from any further refinements. If no combination of n p , α L ,and j are left, the
search is extended to all the α q ,and q is estimated as before, otherwise the
remaining α L are chosen for further refinement, i.e. the remaining α L form the
set α q w . The pitch pulse closest to the centre of the analysis frame, i.e. closest
to the synthesis frame boundary for which R j >ξ is selected as the final pitch
pulse. The threshold value, ξ , is given by,

≤ 1 . 0or R j ≤

0 . 7max R j ξ = for α q ∈ α q w (9.20)

If more than one set of j and α q satisfy the condition R j >ξ for the same
pitch pulse closest to the synthesis frame boundary, the set of values which
maximizes R j is chosen. The pitch pulse shape and the integer pitch pulse
location are given by the chosen, α q and n p + j respectively. Figure 9.11a shows