2. Telegraph multiplexing.
a. Sending. If four teletypewriter channels are to be multiplexed by the telegraph carrier set, a VF is
assigned to each of the four channels at station A. As each channel of the send side is keyed by the telegraph
intelligence, the frequency shifts a small amount to either side of the assigned frequency. This is the frequency
shift keying (FSK) method. All of the channel output circuits are attached to a common line called the send
bus. All the channel output signals therefore appear on the send bus at the same time. Each channel is assigned
a separate carrier frequency to establish its position in the spectrum of the composite output signal. A send
filter in each channel output circuit passes only the carrier frequency and side frequencies needed to transfer
character information. Moreover, the send filter electrically isolates one channel output circuit from the others.
Both channel frequency assignments and filter characteristics are used to prevent interchannel interference in
the transmitted composite line signal.
b. Receiving. In the receiving process at the distant terminal (station B), each respective telegraph carrier
channel frequency is selected from the composite signal on the receive bus by means of filters that are very
similar in design to the sending filters. Each filter therefore passes the analog VF carrier as it shifts between
mark and space frequencies, together with the intelligence components needed to reconstruct the original
teletypewriter signal. Since each of the four channels operates at a discrete carrier frequency and the channel
intelligence sideband components accompany the carrier through the filter bandpass, there is effectively no
crosstalk or interchannel interference under normal circumstances. However, distortion of the signal, or
interference with it anywhere during transmission over any part of the complete system, can result in distortion
of the received signal pulse or interchannel interference.
3. Signal quality. The signal quality (circuit quality) of a communications system, which includes telegraph
(teletypewriter) transmission, is indicated as telegraph distortion that appears in the receiving telegraph loop.
When traffic is sent from station A to station B, signal quality is measured in the receive loop of station B.
Conversely, when traffic is sent from station B to station A, signal quality is measured in the receive loop of
station A.
a. Distortionless operation. A distortionless telegraph circuit produces no change in the length of any
received direct current (DC) signal pulse. A receiving teletypewriter attached to such a circuit receives DC
signal pulses of precisely the same length a those sent into the sending loop. For example, the sending
teletypewriter is operating at a speed of 60 words per minute (WPM), it produces DC unit signal pulses of 22
milliseconds (ms) each. If the received mark or space unit signal pulses are also 22 ms, the interconnecting
circuit or system is free from telegraph distortion. Notice that neither the pulse shape nor pulse amplitude is the
determining factor of telegraph distortion; the pulse length is the important value.
b. Amount of distortion. The amount of telegraph distortion is measured as the amount of change in unit
pulse length. If the received unit mark pulse length changes for some reason from 22 to 20 ms, the percentage
of telegraph distortion is (22-20)/22, or approximately 9 percent. Since the mark pulse has been shortened, the
space pulse has
SS0031
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