1-22. MARK-SPACE TONE GENERATION
The mark-space generation circuitry consists of a 221.0-kHz oscillator, 227.8-kHz oscillator, switching circuitry for
grounding the output of one or the other oscillator, a divide-by-8 circuit, a divide-by-10 circuit, an isolation amplifier, and
a transmit REV/NORM switch for selecting reverse or normal transmit tty tones. Frequencies produced by mark-space
generation and following circuitry are discussed in detail and summarized below.
Transmit REV/NORM Switch at NORM. With transmit REV/NORM switch set at NORM, the positive send pulse
output of envelope detector-doubler A5A2CR15, CR16 is applied to phase reverse switch A3A1Q1. This causes phase
reverse switch A3A1Q1 to conduct, driving Its output to ground. When A3A1Q1 output is grounded (low state), 221.0-
kHz switch A3A1Q2 is biased off (high state), resulting in a strong positive output. This biases 227.8-kHz switch A3A1Q3
Into conduction, driving its output to ground (low state). Since the output of 227.8-kHz (space) oscillator A3A1Q2, 022 is
tied to the output of A3A1Q3, the space oscillator is grounded, biasing isolation diode A3A1CR4 into nonconduction.
Consequently, the output of the ungrounded 221.0-kHz (mark) oscillator Is applied through forward-biased Isolation diode
A3A1CR3 to shaper amplifier A3A1Q4. If no loop current flows in dc loop No. 1 corresponding to a normal space, phase
reverse switch A3A1Q1 Is in a high state, because no positive pulse Is applied from the send sensing circuit. This biases
221.0-kHz switch A3A1Q2 to a low state, effectively grounding the output of 221.0-kHz (mark) oscillator A3A1Q19, Q20.
The Input to 227.8-kHz switch A3A1Q3 is also grounded, switching Its output to a high state. With the 227.8-kHz switch
offering a high impedance to ground, the output of A3A1Q21, 022 Is ungrounded, and Is applied to shaper amplifier
A3A1Q4 through isolation diode A3A1CR4. The 221.0-kHz switch, A3A1Q2, back biases A3A1CR3, isolating grounded
221.0-kHz oscillator A3A1Q19, 020 from the shaper amplifier A3A1Q4 Input.
Transmit REV/NORM Switch at REV. With transmit REV/NORM switch at REV, phase inverter switch A3A1Q1 is
bypassed, reversing the operation described above. That Is, current in the dc send loop causes the send sensing
circuitry to produce a positive pulse, which Is applied directly to 221.0-kHz switch A3A1Q2. The positive pulse switches
A3A1Q2 to a low state, grounding the output of the 221.0-kHz (mark) oscillator, back biasing isolation diode A3A1CR3,
and switching A3A1Q3 to a high state. With A3A1Q3 in a high state, the 227.8-kHz (space) oscillator feeds shaper
amplifier A3A1Q4. Thus, with transmit REV/NORM switch at REV, a current in the dc send loop generates a normal
space tone (rather than the normal mark tone produced for a setting of NORM). Similarly, with the switch at REV, no dc
loop current will produce a normal mark tone (reverse space).
Frequency Division of Mark and Space Tones . Mark-space frequency division circuitry consists of shaper amplifier
04, divide-by-8 multivibrators A3A1Q5 through Q10; divide-by-10 multivibrators A3A1Q11 through Q18; sine wave pass
filter A3A2C1, C2, L1; and isolation amplifier A3A2Q1. The mark-space frequency division circuitry divides the mark or
space tone by 8 for fsk modes and by 80 for nsk modes.
· Fsk operation. The mark or space tone provided by the 221.0or 227.8-kHz oscillator is applied through the proper
isolation diode to amplifier A3A1Q4, which shapes the oscillator output to approximate a square wave for the input to
divide-by-8 circuit. The divide-by-8 circuit consists of three series-connected, divide-by-2 bistable multivibrators.
The square wave output of the divide-by-8 circuit is applied direct to sine wave pass filter A3A2C1, C2, L1, which
blocks the high-frequency components of the square wave, so that the output of the sine wave pass filter is
essentially a sine wave. (Capacitor C1 is switched out of the filter to widen the bandwidth for fsk operation.) The
MODE SELECTOR switch removes power from divide-by-10 circuitry A3A1Q11 through Q18 so this portion of the
frequency divider chain does not operate in the 850-Hz mode (fsk). The output of the sine wave pass filter is fed to
isolation amplifier Q1, which recovers filter losses and stabilizes load and source impedances. At this point, a mark
is 27,625 Hz and a space is 28,475 Hz. Additional processing of these tones is necessary to produce the required
2,000-Hz fsk mark-space tones.