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TRANSMIT SIGNAL PATH
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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RECEIVE SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM
TM 11-5805-387-34-2
1-15. TRANSMIT SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM
The transmit signal path consists of two channels; a tty channel, which converts mark and space dc pulses into mark-
space (tty) tones, and a voice channel, which processes voice signals for transmission alone or in combination with tty
tones.
Transmit Tty Channel. The transmit tty channel consists of a send dc loop, a send sensing circuit, mark-space tone
generation circuitry, a mixer circuit, mixer tty filter, and transmit output circuitry. The send dc loop provides a current
supply, or routes an externally supplied current to send tty equipment that is connected in series with the send dc loop.
When the send tty equipment is keyed, loop current flows for a mark pulse and does not flow for a space pulse, resulting
in a time synchronized dc pulse input to the send sensing circuit. The send sensing circuit isolates the send dc loop from
the rest of the modem and produces a pulse of the proper shape and magnitude to trigger the mark-space tone
generation circuitry. When the send sensing circuit applies a positive pulse to the mark-space tone generation circuitry, a
mark tone is produced; when no pulse is applied to the tone generation circuitry, a space tone is produced. The tones
produced by the mark-space tone generation circuitry are not of the frequency at which they will be transmitted; the tones
are frequency divided, and in 850 Hz, and 85-Hz diversity are subtractively mixed in a mixer circuit to establish the
center frequency.  After leaving the mixer circuit, the tty mark and space tones are at the proper frequency for
transmission. The tones are filtered to remove mixing harmonics and are applied to the transmit output circuitry where
they are amplified, and applied through the modem RCVR TRANS AUDIO jack to the associated radio transmitter or
landline.
Transmit Voice Channel. The transmit voice channel has a 50-ohm microphone (mike) input, and a 600-ohm input
for mike or line. The voice channel is operative only in the 85 Hz + voice mode and the voice mode.
In the 85 Hz + voice mode, the 600-ohm voice input is amplified by the 600-ohm voice amplifier and applied to
a transmit voice filter. The 50-ohm mike input is applied direct to the transmit voice filter. The transmit voice
filter limits the bandwidth of the voice signal to enable the upper portion of the radio transmitter sidebands to
be used for tty tones. The output of the transmit voice filter is applied to the transmit output circuitry, where it
is combined with tty tones generated in the transmit tty channel to produce the composite tty and voice
transmission.
In the voice mode, no tty information is transmitted, so the transmit voice filter is bypassed. The 600-ohm
voice input is amplified by the 600-ohm voice amplifier and is applied direct to the transmit output circuitry.
The transmit output circuitry further amplifies the 600-ohm voice signal, which is then applied through the
modem RCVR TRANS AUDIO jack to the associated radio transmitter or landline. In the voice mode, the 50-
ohm mike input is routed through the modem for switching purposes. The switched 50-ohm mike input is
applied through the modem RCVR TRANS AUDIO jack to the associated radio transmitter or landline.
1-16. RECEIVE SIGNAL PATH SIMPLIFIED BLOCK DIAGRAM
(fig. FO-3)
The receive signal path consists of two main channels: a tty channel which converts mark and space tones into mark
and space pulses for operation of tty machines, and a voice channel which separates voice signals from tty tones and
amplifies the voice for loudspeaker or headset.
1-7

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