Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 318

Table 9.1 Unquantized hybrid model vs 128 kb/s linear PCM
Better Slightly better Same Slightly worse Worse

Male (%) 0.0 19.3 51.9 26.9 1.9

Female (%) 0.0 5.8 69.2 17.3 7.7

Average (%) 0.0 12.5 60.6 22.1 4.8

Table 9.2 Unquantized hybrid model vs 8 kb/s G.729
Better Slightly better Same Slightly worse Worse

Male (%) 1.9 30.8 51.9 15.4 0.0

Female (%) 0.0 34.7 44.2 17.3 3.8

Average (%) 1.0 32.7 48.1 16.3 1.9

also indicates the upper bound of the quality achievable by the designed
hybrid model. An informal listening test was conducted using 128 kb/s linear
pulse code modulation (PCM), which is the best narrow-band speech quality,
and 8 kb/s ITU G.729, a toll-quality speech coder, as the reference coders
[26]. The speech material used for the test consists of eight sentences, four
from male and four from female talkers, filtered by the modified IRS filter
and a pair of headphones was used to conduct the test. Twelve listeners were
asked to indicate their preferences for the randomized pairs of synthesized
speech. Both experienced and inexperienced listeners participated in the test.
The subjective test results are shown in Tables 9.1 and 9.2. As indicated by
these results, the unquantized hybrid model performs better than G.729 and
worse than 128 kb/s linear PCM. Therefore the quality of the unquantized
hybrid model can be classified as being higher than toll quality and lower
than transparent quality. In general, the speech encoded and decoded with
unquantized hybrid coder model parameters does not sound too different
from the original speech material. The perceived speech quality shows only
a slight degradation, even after quantizing the harmonic mode parameters at
4 kb/s and white-noise excitation at 1.5 kb/s, with unquantized transitions (at
128 kb/s linear PCM). The hybrid coder achieves toll quality when transitions
are quantized with 6 kb/s ACELP.

9.9 Quantization Issues of Hybrid Coder Parameters

9.9.1 Introduction
The above hybrid speech-coding model can be adopted for various applica-
tions with different quality requirements by quantizing the model parameters
at different bit-rates. For applications which support variable bit rates, the