McIntosh MR80 Owner's Manual page 16

harmonic distortion. Because of its extremely low
THD and power capability, it is also the main output
amplifier.
TOUCH SENSOR
The touch sensor consists of three functional
elements: 1) Input buffers and static protection cir-
cuits. 2) Latches, and 3) Encoder. The seven sensors:
KNOB, PRESET 1 to 4, UP and DOWN SCAN are in-
puts to the seven input buffer amplifiers. These per-
form a dual function. First the inputs are biased to a
high level by a very high resistance. Second the buf-
fers are designed to detect a negative transition
from a reference level. When a sensor is touched,
the stray capacity of the human body shunts some
of this reference signal to ground. This reference
signal is derived from the 228kHz oscillator on the
multiplex circuit board. This means a l t noise com-
ponents of the reference signal are a multiple of
19kHz, and will be filtered out by the 19 and 38kHz
filters. The second phase of this reference signal is
used to transfer the output of the buffer amplifiers
into the latches. The latches are used as temporary
storage to prevent false triggering due to static and
voltage transients.
The output from the latched inputs are supplied to
the encoder, which performs two functions. First, it
puts in order the inputs, assigning highest priority to
the tuning knob and the lowest to the down scan
sensor. Second, it encodes the input with the
highest priority into a binary coded decimal word
and outputs this to the decoder circuit. This decoder
reconverts the BCD word back into a 1 of seven line
code, indicating the sensor that is touched. An addi-
tional input to the decoder is the reference phase in-
put that inhibits the decoder during clock edge tran-
sitions. This is done to prevent false triggering due
to noise glitches and partial contact with the touch
sensor.
PRESET SCAN
The seven outputs from the touch control and
decoder go to the preset scan circuits. In the preset
scan is an electronic switch and a counter. A switch
on the rear panel selects whether the tuner scans all
stations or only the preset stations. The circuit
operates in the preset scan mode as follows: When a
pulse is received at the rear panel connector the
counter is indexed by one, then the seven pole dou-
ble throw switch connects the input to the tuning
control logic to the output of the counter. The con-
trol logic then selects the preset that is represented
by the counter output. At the end of the pulse the
counter is disconnected and the lock circuit auto-
matically fine tunes the tuner to the center of the
channel.
When in the scan all stations mode, the input
pulse is routed to the scan circuitry on the tuning
control logic board. The logic circuitry will then
select the auto scan circuit and start searching for
the next acceptable station. The direction of scan
will be the last selected scan direction and will
change when the band limit is reached.
The remote input to the tuner is a ground asserted
logic level that is normally at + 15 volts. When this
line is grounded the tuner will start scanning. The
use of this logic permits any number of remote
cables to be connected to the MR 80.
CONTROL LOGIC
The control logic portion of the tuner is made up
of very low power complementary MOS integrated
circuits. By using this logic family a very significant
power reduction was realized in the design. With the
lower power consumption, a very real reduction in
the operating temperature and an improvement in
the operating life is achieved.
A very important consideration in the design of a
digital tuner is the amount of radiated noise from the
digital integrated circuits. To achieve low noise the
control logic was made to operate in a static mode
to entirely eliminate any interference. This allows
the control logic board to be built without the
shielding necessary to reduce interference.
The seven touch sensor inputs from the preset
scan board are stored in eight C-MOS flip-flop's. The
state of the flip-flop's indicate which input was ac-
tive last. One bit is used for the main tuning knob, 4
bits are used to represent the 4 presets. The scan cir-
cuitry uses 3 bits to tell if the scan is on or off, set to
scan up or down, and the third is set to a zero when
the tuner reaches an acceptable station. This com-
plex group of circuits provides a level of sophistica-
tion and ease of operation not possible before in a
tuner.
Several timing circuits are used to provide the
various time sequences and delays for the proper
noise free operation of the preset and scan circuits.
The first of these monostable circuits is used as a
power on clear that resets the circuits to the main
tuning knob mode. During the four second delay the
lock circuit is inhibited and the high speed muting
input to the multiplex is held high to mute the tuner
and prevent any noise or transient from affecting the
rest of the users system. At the completion of this
four second delay the lock circuit is enabled, locking
the tuner to the incoming frequency and unmuting
the tuner.
When one of the four presets is activated, the
display and audio are inhibited for two seconds from
when the sensor is touched. The lock circuit is
disabled during the time the sensor is touched, to
allow the presets to be set to the proper frequency.
When the sensor is released the lock circuit then
tunes to the center of the FM station. During normal
operation (not setting of the presets) the touch sen-
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