53
CIRCUIT DESCRIPTION
Automatically Controlling the Tuners when Channels are changed.
MAIN TUNER:
When channels are changed, the Microprocessor runs another routine. This routine detects the command if
the Remote Control or the Front keys input it. Whether its Scan Up/Down or direct access, and begins to
control the Tuner. First, the Microprocessor output a Mute command to blank the video, then data is sent to
the tuner to move it to the desired channel. After that, the Microprocessor again checks the coincidence
detector for active sync. If active sync is detected, the Microprocessor opens what is called the AFC Loop.
The AFC Loops comprises two cycles trying to lock the tuner to the specific IF frequency of 45.5 Mhz. A
DC voltage is sent from either the Main Tuner U201 pin (10) or the PinP Tuner U202 pin (21) back to the
Microprocessor pin (6). This DC voltage indicates the error between the IF detected and the IF frequency
reference. This error voltage tells the Microprocessor to do one of two things. First, if the error is large, the
Microprocessor changes the Programmable Dividers division rate to a larger or smaller degree to get
closer to the actual IF frequency desired. Second, move the Pulse Swallow division rate to either 1/32 or 1/
33. The Pulse Swallow tuning circuit is a second divider that is on the output from the Prescaler. The main
Prescaler takes the very high frequency output from the tuners mixer circuit which is produced when the
tuners main oscillator is beat against the incoming RF frequency. The Programmable Divider is instructed by
the Microprocessor exactly what division rate to apply to the Beat Frequency generating the IF frequency.
The IF frequency is then sent through the Pulse Swallow circuit which again divides the IF frequency at a
much smaller rate. This allows the IF output frequency to become much more finite and can correct for
much smaller errors between the Phase comparators reference frequency. The error voltage is routed back
to the main internal Oscillator in the front end to correct for Tuning errors.
(See the Microprocessor Data Communications Circuit Diagram Explanation for Details related to Data
Communication for controlling the Main Tuner).
Automatically Controlling the Tuners when Channels are changed.
PinP TUNER:
As far as the internal function of the PinP Tuner, it is the same as the Main Tuner. When the customer
presses the PinP button on the Remote Control, the Microprocessor outputs Clock, Data and Enable
controls to the Flex Converter. The Flex Converter also has the PinP circuit inside. The Clock, Data and
Enable pins on the Microprocessor are pins (20 Clock, 21 Data and 46 FCENABLE) These are routed to
the Level Shift IC, I014 pins (2, 3 and 4). They are output on pins (18, 17 and 16) to the Flex Converter
U205 connector PFC1 and input on pins (10, 11 and 12). The Flex Converters PinP unit is then switched
on and insertion is made into the regular Main Video line. This process controls the position of the PinP
window, the PinP window itself and other different display conditions. When SWAP is pressed on the
remote control, the channel or input that the PinP tuner was on, now becomes the Main Videos source and
the channel or input that the Main signal was on, now becomes the PinP source.
Automatically Controlling the Video Processor (Rainforest IC) when directed by the Customer.
The Rainforest IC has many enhancement circuits built in. These would include the Black Peak Expansion
circuit, the Dynamic Noise Reduction circuit, Time Compression and of course Sharpness, Black Level and
Contrast adjustments as well.
Black Peak Expansion Circuit:
This circuit is utilized to increase the contrast ratio. The standard video signal is 1-Volt Peak to Peak; the
actual video (Y) content is 730mVp/p. The Standard video signal is divided into units called IRE. The units
are equal to 140 total for the 1Vpp signal. Sync occupies 40IRE, which are negative and the Luminance
represents 100 IRE units. Each unit represents 7.1428mVp/p of information. The Black Peak Expansion
