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CHAPTER 3
OPERATION
3.1 Sample
~requency
The relation between synthesized speech message length
and bit rate when the ADPCM method is used is shown in
Table 3-1.
Table 3-1
Sampling frequency and Maximum Message Length
Sampling
Bit rate
Message
frequency
(kbps)
length
(sec. )
(kHz)
\lPD7755
\lPD7756
4
16
6
16
5
20
5
12
6
24
4
10
8
32
3
8
~or
normal speech with no background music,about 10% to 20% of
the message is made
~p
of silent frames. These frames are
compressed in the case of the \lPD7755 /7756
so the
actual bit rate is about 80% to 90% of that shown in
Table 3-1.
Even further reductions in the bit rate have been achieved
by combining the ADPCM and phoneme methods. The bit rates
for speech synthesis when this combined method is used
are shown in Table 3-2.
Table 3-2
Sampling Frequency and Maximum
~lessage
Length
(with data compression)
Sampl ing
Bit rate
Message
frequency
(kbps)
length (sec. )
(kHz)
IlPD7755
\lPD7756
4
8
12
32
5
10
9
25
6
12
8
21
8
16
6
16
3-1
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