I installed this circuit, and it seems to be producing signal at around 3Mhz, and at 1.5Mhz (as heard on an AM radio). I think that matches: 3.072 Mbit/s (48 kHz sampling rate, which I think is what they use with HDTV. I don't have a SPDIF receiver here, so I can't tell yet if it really works or not!
Someone over at this forum
http://www.avsforum.com/avs-vb/showt...004274&page=18
Built a similar circuit, and got SPDIF, but it was always muted.
If there's no solution to that, I'll just jumper out CD29 and change resistors RD61 to 75 ohms and RD62 to 2K, and remove RA7. The Y output of the STV6433 filter chip will now feed the base of QD10, thus making this circuit a baseband video line driver, to feed luma only to a modulator to drive my antique B&W TV sets. Thus "Not Used" becomes an RCA jack with Y out on it.