Radio Shack TRS-80 Reference Handbook page 49

Cassette Audio Input
If the Recorder could faithfully give back what
was sent to it, we could eliminate a quad opera-
tional amplifier and a handful of passive compo-
nents. But, it doesn't; so Z4 stays in. Matter of
fact. the Recorder adds stuff to the signal.
Motor noise and
60
cycle hum complicate signal
processing considerably.
Upon a CLOAD instruction from the CPU. the
Recorder motor turns on and cassette audio is
applied to pin 4 of J3. This signal, called CAS-
SI N, is tied to capacitor C24 and resistor R67 at
the input of the audio processor section.Z4, pins
1, 6 and the output pin 5, form an active filter.
This part of the circuit is used to filter out un-
desired noise and hum from CASSI N. It is a high
pass filter, with about a 2 kHz roll off.
If you looked at CASSIN using an oscilloscope,
you would see the data pulses riding atop a
60
Hz hum signal. After passing through the high
pass filter, the resulting waveform would have
the 60 Hz hum removed and only the data
pulses would be left. The signals are swinging
above and below a base line of about 2.0V. Fig-
ure 11 shows some idealized cassette signals. The
signal drawn at Line A is the type that could be
expected at the output of the active filter.
48
Once filtered, the next section of Z4 is used as
an active rectifier. CR4 and CR5, together with
the biasing resistors around pins 2, 3 and 4 will
full-wave rectify the data pulses. A typical
output on the cathode side of CR4 is shown on
Line B of Figure 11.
After rectification, the signal is inverted and
amplified. Z4, pins 8,13 and 9, is wired to form
an inverting amplifier circuit. The ratio of
R41
and R42 gives the amplifier a gain of about 2.
Line C in Figure 11 shows a typical output at
Z4, pin 9.
The last stage of Z4 is used as a level detector.
CR6 and CR7, together with C39. form a power
supply of sorts. The amplified audio signal from
Z4, pin 9, is applied to the anode of CR6. CR6
and CR7 decrease the voltage level of the incom-
ing signal by about (i'l.8 to 1.0 volt. C39 filters
the resulting voltage and creates a DC signal like
the one shown on Line
D
of Figure 11. If the
signal output from Z4, pin 9, drops below the
reference voltage level at
C39, Z4,
pin
10
will go
low. It will stay low as long as the voltage on pin
12 of Z4 stays below the reference. Line E
shows the resulting output from Z4, pin 10.
Notice that we lost a couple of pulses of audio
because the signal did not swing toward ground
enough to trigger Z4, pin 10. The negative
transaction at pin 10 will be used to set flip-
flop Z24. Cassette data will be converted into
program data by the software in ROM and the
CPU.