Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 341

Table 9.13 Hybrid vs 6.3 kb/s G.723.1 for noisy speech
Better Slightly better Same Slightly worse Worse

Male(%) 0.0 12.5 40.0 40.0 7.5

Female(%) 5.0 22.5 35.0 30.0 7.5

Average(%) 2.5 17.5 37.5 35.0 7.5

corrupted with babble noise (10 dB SNR) and the spectrum of the synthesized
speech, with 80% of the spectrum declared voiced and the remaining high
frequency components synthesized using filtered and scaled Gaussian noise.
The same informal listening test was conducted to compare the speech
quality. The informal test results are shown in Table 9.13. Comparing with
the results shown in Table 9.12, the introduction of the harmonic voicing
significantly improves the performance under background noise which indi-
cates that there is still some room to retune the harmonic coder for the hybrid
coding operation. The same is perhaps true for ACELP, and it should be
designed specifically for hybrid operation.

9.11.2 PerformanceUnderChannel Errors

The inherent robustness of the hybrid coder to mode bit errors was tested by
simulating all the possible mode errors. The hybrid coder has three modes,
hence there are six possible mode errors, i.e. each mode may be erroneously
decoded with the other two modes. The bit stream of the hybrid coder is
shown in Tables 9.14 and 9.15. For each parameter, the most significant bit
(MSB) is transmitted first. When erroneously decoding a lower-rate mode as
a higher-rate mode, e.g. decoding a white-noise excitation frame as harmonic,
the remaining bits are set to 1. Simulations show that setting the remaining
bits to 1 has the worst effect, since the higher indices are mapped to the
higher-energy levels in the gain quantizers. Using the LTP gain quantizer
shown in Table 9.3 results in blasts when the white noise or harmonic frames
are erroneously decoded as ACELP. Therefore the maximum LTP gain is
limited to 1.2.
All the modes quantize the LSFs using 23 bits, consequently they are
transmitted using the same bits. Therefore the LSFs are independent of the
mode and the mode bit errors can only affect the excitation parameters.
This is particularly attractive for the LSF interpolation and quantization with
first-order moving average prediction. The most significant bits of the gain
parameters are also transmitted using the same bits. However the gain of
each mode is estimated using different criteria. Hence the gain quantizers
of each mode have different dynamic ranges, and mode errors affect the
dequantization of the gain.