Databases Reference
In-Depth Information
3
2
1
0
−
1
−
2
−3
2400 2500 2600 2700 2800 2900 3000 3100 3200
F I GU R E 18 . 3
The sound /s/ in
test
.
From analysis filter 1
.
.
.
Synthesis
filter 1
From analysis filter 2
Synthesis
filter 2
Noise
source
.
.
.
Pitch period
Pulse
source
Voiced/unvoiced decision
Synthesis
filter
n
From analysis filter
n
F I GU R E 18 . 4
The channel vocoder receiver.
occur. For example, the first formant occurs in the range 200-800Hz for a male speaker, and in
the range 250-1000Hz for a female speaker. The importance of these formants has led to the
development of
formant vocoders
, which transmit an estimate of the formant values (usually
four formants are considered sufficient) and an estimate of the bandwidth of each formant. At
the receiver the excitation signal is passed through tunable filters that are tuned to the formant
frequency and bandwidth.
An important development in the history of the vocoders was an understanding of the
importance of the excitation signal. Schemes that require the synthesis of the excitation signal
