1-26. SIGNALS PRODUCED IN TRANSMIT SIGNAL PATH
The preceding figure shows the various frequencies produced by the transmitter module for use in the transmit signal
path. The blocks, representing circuits, correspond to similar blocks in the transmit signal path detailed block diagram
(fig. FO-4). Within the blocks are tables that show the output of that circuitry as a function of the setting of the MODE
SELECTOR switch. Signals are produced in mark-space generation circuitry and in the mixer oscillator (para 1-22, 1-23)
for combination in the mixer and application to filters and transmit output circuitry. The frequencies subtractively mixed
are shown within the mixer, amplifier, and switch block. In 85-Hz modes, the output of the mark-space generation is
applied direct to the 2,805-Hz section of FL1; in 85-Hz diversity, it is also mixed in the mixer section to obtain a 425-Hz
nsk signal. Also, the mixer oscillator supplies receiver module A4 mixer injection in 85-Hz diversity.
Section V. DETAILED BLOCK DIAGRAM ANALYSIS OF RECEIVE SIGNAL PATHS
A simplified description of the receive signal path is given in paragraph 1-16. This section contains a detailed
description of the function of each stage in the signal path. (Description of the functions of components within each stage
is covered in section VIII.) The receive signal path consists of two main branches: a tty signal path, which converts mark
and space tones into mark and space pulses for operation of tty machines, and a voice and tty monitor signal path, which
separates voice signals from tty tones and amplifies the voice (or tty tones in all modes but 85 Hz + voice) for
loudspeaker or headset.
1-28. RECEIVE SIGNAL PATH INPUT
The receive signal path input consists of audio tty tones and/or voice received by radio, or over landlines. In one-way
operation, only one radio transmitter or landline is required; but in duplex operation, the simultaneous transception of
signals requires an auxiliary receiver or second landline for reception, so that the other may be used for transmission.
ONE/WAY DUPLEX switch S1 is used to select the source of the audio input to the modem. The input signals are
applied simultaneously to identical isolation amplifiers, one for the tty channel and the second for a voice channel. The
isolation amplifier provides a constant input impedance and, prevents interaction between signals of the tty signal path,
and the voice signal path. The output of tty channel isolation amplifier A7Q1 is applied to MODE SELECTOR switch
S4D, which divides the signal path into four paths; one is for all settings of S4D involving 85-Hz modes. The 2,805-Hz
nsk tty tones are passed through the 2,805Hz nsk pass section of FL4, and applied to tty channel 2. In 85-Hz diversity,
the input signal also contains 425-Hz nsk tones. The 425-Hz nsk tones are passed through the 425-Hz nsk pass section
of FL4 and applied to tty channel 1. The third signal path provided by MODE SELECTOR switch S4D is for 2,000-Hz fsk
signals in the 850-Hz mode. This path bypasses FL4, and is applied direct to the input of tty channel 1. The fourth signal
path grounds the output of tty channel isolation amplifier A7Q1 in voice mode. Discussion of receive tty channel 2 (para
1-29) precedes that of the tty channel 1 (para 1-30) because the operation of channel 2 is more easily understood.