McIntosh MR80 Owner'S Manual Download Page 15

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tions in load impedance. This prevents changes in
audio cable lengths outside the tuner from affecting
the detector performance.

FM STEREO MULTIPLEX

The heart of the multiplex section is a new third

generation phase locked loop (PLL) stereo decoder

integrated circuit (IC). This PLL IC incorporates two

special systems, an automatic variable separation
control circuit to reduce background noise when

receiving weak stereo stations, and tri-level digital

waveform generation which eliminates interference

from SCA signals and from the sidebands of adja-
cent channel FM signals.

The variable separation control is operated from

the IF amplifier's signal strength detector. A smooth
transition is provided from mono to stereo or from

stereo to mono at weak signal levels to provide the

optimum signal to noise ratio and best stereo
separation for the prevailing signal conditions. The
circuit operates only during stereo reception. It swit-
ches automatically to monophonic if the 19kHz pilot
tone is absent.

In the PLL the internal oscillator operates at

228kHz, locked onto the 19kHz pilot tone. The
228kHz feeds a 3 stage Johnson countervia a binary
divider to generate a series of square waves.

Suitably connected NAND gates and exclusive OR

gates produce the tri-level drive waveform for the

various demodulators in the circuit. The usual

square waveforms have been replaced in the PLL
and decoder sections by tri-level waveforms. These
tri-level forms contain no harmonics which are

multiples of 2 or 3. This eliminates frequency

translation and detection of interference from the

side-bands of adjacent stations since the third har-
monic of the sub-carrier (114kHz) is excluded. It also

eliminates interference from SCA broadcasts since
the third harmonic of the pilot tone (57kHz) is exclud-
ed. Unwanted spurious audible components and

phase jitter in the PLL are inherently eliminated by

this technique.

Additional advantages of the phase locked loop

stereo demodulation are the elimination of induc-

tors to minimize drift, integral lamp driving capabili-
ty to indicate the presence of the 19kHz pilot carrier,
excellent channel separation over the entire audio
frequency range, extremely low distortion, low out-

put impedance, and transient-free mono/stereo

switching.

AUDIO CIRCUITS

Following the MPX decoder is the three position

deemphasis switch. The three different positions

allow the MR 80 to be used in North America with

standard 75

s deemphasis and in Western Europe

and the Far East with 50

s deemphasis. The 25

position is for use with an external noise reduction

adapter.

An electronically switched f i l t e r circuit, im-

plemented with two J-FETs of a quad J-FET
package, is used to reduce out of phase noise when
in the stereo mode and tuned to a weak station. This

filter is actually a twin-T bandpass that blends the

high and low frequencies, leaving separation unaf-

fected at mid-frequencies. This results in a greatly

improved stereo image when the filter is required.

A combination of three inputs are used to control

the switch point of the filter. The first is the front

panel switch. The filter is only active when this
switch is in the AUTO or ON position. In the ON posi-

tion the filter is in the circuit for all stations mono or
stereo. In the AUTO position the filter will switch in
when the station is transmitting a stereo pilot signal
and the signal strength is below 100

In the same quad J-FET package are the two FETs

that are used for the muting circuit. These FETs are
controlled by the logical ANDing of three signals.

The first signal is the deviation or center tune, this

detects when the tuner is correctly tuned to the
center of the FM channel. The second signal is the

signal to noise ratio. Only when there is sufficient
signal will the tuner be allowed to unmute. Signal to

noise ratio is measured by a special bandpass

amplifier centered around 138kHz. This frequency is
used to prevent false triggering due to adjacent

channel and overmodulation products. The third
signal is the RF signal level that is derived from the

five signal detector circuits. A front panel control

(MUTING LEVEL) adjusts the reference level to a
voltage comparator. The other input to the com-

parator is the signal strength voltage. When the RF
level exceeds the preset reference level, a signal is

sent to the muting logic to be ANDed with the other
two signals. This adjustment allows the unmute