What the listener hears is not the original voice message, but an artificial voice signal that is reconstructed from
the signal sources in the local vocoder. The result is an improvement in the S/N over conventional voice
transmission.
d. Disadvantages. The primary disadvantage of the vocoder system of voice transmission is the relatively
large bandwidth required to achieve naturalness of voice. Although satisfactory vocoder voice transmission is
feasible in the 3,200-Hz bandwidth of a telephone carrier channel, the reproduced sound does not have enough
natural timbre to allow voice recognition by the listener. The problem may be solved by using the band space
occupied by a number of transmission channels to gain additional bandwidth for the vocoder signal pulse train,
or by using prearranged identifying codes so the listener can establish positive speaker identification.
Secondary disadvantages include more circuit complexity and the need for absolute system synchronization.
2. Security. Security of the system can be improved and simplified by coding the sequence of vocoder pulses
in a security device designed to perform this function. In the integrated communications system described, it is
necessary to secure each channel of communications at the source of signals. For example, each of the four
teletypewriters, the two telephones, and the facsimile set must have its own security device. One method of
overcoming this problem is to secure the digital intelligence by one security device. One arrangement is shown
in Figure 1-3.
a. Securing channels. Each of the four telephone channels has its own vocoder to digitize the voice
signals. The security device then combines the digital signals from all four channels into a sequence determined
by the characteristics of the security device. In this way all four channels are secured simultaneously.
b. Teletypewriter application. It is not presently possible to combine the several channel frequencies of an
FDM telegraph carrier set with a vocoder. The speech-plus-duplex (S/D) system must therefore be employed to
achieve teletypewriter transmission when using a vocoder. One teletypewriter is applied to each telephone
circuit. Since the security device secures all channels simultaneously, the teletypewriters do not need individual
security devices.
SS0031
1-18
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