54
CIRCUIT DESCRIPTION
circuit monitors the 1/2 way point of luminance, (50 IRE or 357mV) and pulls the signal towards pure black
or the 7.5 IRE level. This increases the distance from Black Peak to White Peak, which is contrast.
Dynamic Noise Reduction Circuit:
This circuit again monitors the area from 50 IRE down and subtracts noise. This circuit is dynamic meaning
that it characteristics change. In other words, the subtraction process is greater near black level that it is
near 50 IRE. The subtraction is 6dB at maximum, meaning that there would be some frequency loss near
black, but the noise, which is seen as white speckles, would be reduced.
Time Compression Circuit:
Any time an analog signal is passed through a capacitor, its high frequencies are reduced. To replace these
high frequencies, Zenith uses Time Compression. This circuit is on the order of Aperture Compensation,
however it differs in the fact that it uses 5 delay lines. The actual signal should look like Figure 2. Figure 3,
however after passing through a capacitive circuit, it looks like Figure 4. After Time Compression takes
place, the beginning rise is advanced. Just before white peak, the signal is delayed. Just before the signal
falls the signal is advanced and just before the signal reaches black peak the signal is delayed. This causes
the signal to appear more like the actual signal and thus restores the high frequencies lost through capaci-
tance.
Sharpness:
During the Time Compression process, switching pulses that are detected at the transition point, (A transi-
tion is the point at which the luminance signal goes for black to white or white to black) are used in the
sharpness circuit. This signal is the routed through a sort of variable resistor and according to how much
sharpness the customer has selected, determines how much of the transition signal is added to the original
signal. The greater the sharpness setting, the greater the transition signal added.
Controlling the Audio Circuits when directed by the Customer.
The customer has control over how the set accesses audio information for all of its inputs. The tuner for
example is an integrated type. This not only means that held within the Main Tuner is all the necessary
components for Reception and Video detection. It also has a built in audio and MTS decoder. The Main
Tuner outputs Left Total and Right Total signals. (Left Total and Right Total means that the encoding for
Pro-Logic is held within the individual signal.) The customer can select first, how the Tuner decodes its
audio. Stereo, Mono, or SAP can be selected. The Main Tuner must tell the Microprocessor what signal it
is receiving. The Main Tuner has a ST LED output at pin (19), which tells the Microprocessor it is receiving
MTS Stereo and a SAP LED output at pin (20), which tells the Microprocessor it, is receiving Second
Audio Program. How these are selected by the consumer via the Main Menu determines the output from
the Microprocessor.
ST LED
is routed from the Main Tuner at pin (19), through Q204, to the DAC1 I003 pin (10). The DAC1
outputs Clock and Data via pins 15 SCL1 and 14 SDA1 signals to the Microprocessor input on pin 3
SCL1 and pin 2 SDA2. The Microprocessor knows how to switch the tuner decoder circuit by making
judgment upon these inputs. Then the Microprocessor can us Clock, Data and Enable lines to control the
Tuner.
SAP LED
is routed from the Main Tuner at pin (20), through Q203, to the DAC1 I003 pin (9). The DAC1
outputs Clock and Data via pins 15 SCL1 and 14 SDA1 signals to the Microprocessor input on pin 3
SCL1 and pin 2 SDA2. The Microprocessor knows how to switch the tuner decoder circuit by making
