Source Selection - Hitachi C1422R Service Manual

Source Selection

Source selection is controlled by the microcontroller via I²C bus commands. The video processor I501 can
select between internal demodulated CVBS video on its pin 13, external CVBS video (AV1) from pin 20 of
P501 on its pin 17 or, if available, from external CVBS video (AV2) from the yellow phono socket of PA501,
which is applied to pin 11. The internally demodulated CVBS video is always available on pin 19 of P501.
External RGB is selected within I501. Fast blanking pulses from pin 16 of P501 are passed via an OR-ing
circuit of Q301-304 and associated components to pin 26 of I501. This pin controls the state of the RGB
outputs to the tube base pins 19, 20 and 21. It has three possible states:
1) Less than 0.4V
2) Between 0.4V and 4.0V
3) Greater than 4V
When teletext or OSD is displayed, pin 35 of I703 goes from ground to 5.0 Volts. This signal is passed via
Q301 and causes pin 26 of I501 to enter the third state above, independent of the state of the other inputs to
the OR circuit. During mixed TV/Text mode or whilst the OSD is showing on part of the screen this line will
be switching at a high rate. RGB mode can be selected manually by the user and in this case the open-drain
output on pin 8 of the micro-controller will be switched off and will be pulled up to 1.7 volts by R307 and
R306. Q302 then applies a voltage of about 1.0 volts to pin 26 of I501. In the absence of a higher voltage via
Q301 and Q303 from the fast blanking input, pin 16 of P501, this is still sufficient to enter the external RGB
mode.
Audio source switching is controlled via the microprocessor I701. The audio is switched from internal to
external source via I²C in I501. Internal source is fed to I501 at pin 1 and the external source at pin 2. On
models equipped with front AV, the external source is provided either from pins 2 and 6 of P501 or the white
phono socket PA501.The external source selection is controlled by pin 3 of I701 and Q703, then switched by
I602. On models without front AV, I602 is omitted and bypassed via R601. On models equipped with a
headphone socket, the speaker feed from the audio amplifier (I601) is diverted to the headphone when a
headphone jack is plugged into PA601.
Internal RGB from colour decoder.
External RGB from pins 7, 11 and 15.
Output blanked for OSD/Text insertion.
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