Bose AV28 Manual page 25

Theory of Operation
7.5 TV Power Detector
The infrared power command for televisions toggles the power state of the television; in order to
automatically turn a customer's television on when a video source is selected, the power state of
the TV must be known to determine if sending a power toggle command is necessary. The circuit
detects the presence of the television's horizontal sweep frequency; standard NTSC and PAL
monitors use 15750 Hz, and line-doubled, HDTV, or 480p monitors use 31500 Hz.
An external dongle plugs into the tuner board at J105; the dongle contains 25 turns of wire inside a
plastic enclosure. When placed properly (at the back of the unit near the deflection coils) on the
television, the dongle will output 20~200 mV of signal.
The TV power detector circuit is on the tuner board. See sheet 3 of SD256131.
The power detection circuit is built around U303, an NJM3403 quad op-amp IC. C328 and L302
form a highly peaked low-pass filter about 15750 Hz; C324, C325, and L303 form a similar filter
about 31500 Hz. The filters boost the voltage of their passbands about 10 times. The outputs of
these filters are fed to active band-pass filters constructed of sections of U303; the active filters
have a Q of 5 (allowing for component variation) and a gain of 20. The outputs of these active
band-pass filters (if present) are rectified and summed by D302. D303 performs a voltage shift
equal to that of the rectifier so that a reference voltage for the comparator can be generated with
diode drop compensation.
The remainder of U303 serves as a comparator and a Schmidt trigger. The comparator compares
the rectified signal with 0.5V; if either sweep frequency present, the comparator output will go high.
The Schmidt trigger section, designed to minimize chatter at the circuit output, drives the TV_ON
signal to U1 to indicate the detected power state of a television.
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