Receive Tty Channel. The receive tty channel consists of an isolation amplifier, an input filter, a mixer, diversity
switching circuitry, two frequency discriminators, mark-space pulse generation circuitry, a receive sensing circuit, a loop
current switch dc supply. Tty tones from the associated radio receiver or landline are applied to the tty channel isolation
amplifier, which prevents interaction between tty and voice channels and provides a constant input impedance. At the
output of the input filter, the receive tty channel is split into two channels, designated tty channel and tty channel 2. (In
the 850-Hz mode, tty channel 1 begins before the input filter; wideband noise and mixing harmonics are filtered after the
Tty chann el 1. Tty channel 1 functions in 850-Hz and 85-Hz diversity modes to process 2,000Hz fsk and 425-
Hz nsk tty tones, respectively. The 2,000-Hz fsk or 425-Hz nsk tones are fed to a mixer, which recenters them
to 2,805-Hz nsk. The centered tty tones are fed to the nsk fsk frequency discriminator, which puts out a dc
voltage of one polarity for a mark tone input, and a dc voltage of the opposite polarity for a space tone input.
Investigation of tty channel 2 will be made before continuing through the tty receiver path.
Tty channel 2. Tty channel 2 functions in all 85-Hz modes (including diversity) to process received nsk tty
tones centered about 2,805 Hz. The 2,805-Hz nsk is fed directly to the nsk only frequency discriminator, which
puts out a dc voltage of one polarity for a mark tone input, and the opposite polarity for a space tone input.
Tty receive path, reunited. The outputs of tty channels 1 and 2 are joined at the input of the mark-space
pulse generation circuitry. Combination of the output voltages of tty channels 1 and 2 does not occur. In all
modes but 85-Hz diversity, only one tty channel is operative; in 85-Hz diversity, diversity switching grounds the
weaker channel. The mark-space pulse generation circuitry shapes the output of the frequency discriminator
into a neat square wave for driving the receive sensing circuitry. The receive sensing circuitry isolates the
receive dc loop from the rest of the modem and produces a pulse of proper shape and magnitude to operate
the loop current switch. The loop current switch controls current supplied by the receive dc supply. The loop
current switch is biased on for a mark tone input to the modem, and off for a space tone input. Thus, receive
dc loop current flows for a mark tone input, and does not flow for a space tone input, keying the receive tty
equipment that is connected in series with the rest of the receive dc loop.
Receive Voice Channel. The receive voice channel consists of an isolation amplifier, receive voice filter FL5, and a
receive audio amplifier. Voice, or 2,805-Hz nsk combined with voice is applied to the modem voice input isolation
amplifier from the associated radio receiver or landlines. The voice input isolation amplifier, in conjunction with the tty
channel isolation amplifier, prevents interaction between tty and voice channels. In 85-Hz plus voice mode, the output of
the voice channel isolation amplifier is applied to receive voice filter FL5, which filters out 2,805-Hz nsk tty tones. The
filtered signal, now consisting of voice only, is amplified by the receive audio amplifier and applied to a monitor speaker
or headset. In all other modes, the unfiltered output of the voice channel isolation amplifier supplied directly to the
receive audio amplifier permits monitoring of voice circuits, or tty tones.