Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 331

synthesized speech was compared against that from 5.3 kb/s ITU G.723.1,
6.3 kb/s ITU G.723.1, and 8 kb/s ITU G.729 coders. In all the tests, stationary
voiced segmentswere quantized at 4 kb/s, and silence andunvoiced segments
are quantized at 1.5 kb/s. The speech material used for each test consists of
eight sentences, four from male and four from female talkers, filtered by
modified IRS filter; a pair of headphones was used to conduct the test.
Twelve listeners were asked to indicate their preferences for randomized
pairs of synthesized speech. Both experienced and inexperienced listeners
participated in the test. The subjective test results are shown in Tables 9.9,
9.10, and 9.11.
For the speech material used in the subjective tests, after discarding the
silence frames, about 64% of the frames used harmonic excitation, 22% used
ACELP, and 14% used white-noise excitation. The 4 kb/s, 6 kb/s, and 8 kb/s
ACELP mode hybrid coders give average bit-rates of 3.65 kb/s, 4.1 kb/s, and
4.53 kb/s, respectively. The 4 kb/s ACELP version performs slightly better
than G.723.1 at 5.3 kb/s. The 6 kb/s ACELP version achieves similar quality
to G.723.1 at 6.3 kb/s. The quality of the 8 kb/s ACELP version is also similar
to G.729 at 8 kb/s, with an overall average bit rate of 4.53 kb/s.

Table 9.9 4 kb/s ACELP hybrid vs 5.3 kb/s G.723.1
Better

Slightly better Same Slightly worse Worse

Male (%) 6.2 34.4 28.2 31.2 0.0

Female (%) 9.4 31.2 37.5 18.8 3.1

Average (%) 7.8 32.8 32.8 25.0 1.6

Table 9.10 6 kb/s ACELP hybrid vs 6.3 kb/s G.723.1
Better Slightly better Same Slightly worse Worse

Male (%) 0.0 31.3 43.7 18.8 6.2

Female (%) 6.3 28.1 37.5 21.9 6.2

Average (%) 3.2 29.7 40.6 20.3 6.2

Table 9.11 8 kb/s ACELP hybrid vs 8 kb/s G.729
Better Slightly better Same Slightly worse Worse

Male (%) 0.0 9.6 65.4 23.1 1.9

Female (%) 1.9 11.5 55.8 30.8 0.0

Average (%) 1.0 10.6 60.5 26.9 1.0