I’m out of town (been like that lot these days), but I dragged some equipment along with me (scope, sgx/duo, rom board). Did a few more tests on the SGX, Duo, and SuperCDROM (addon) video output. This time all under 75ohm receiver load. SGX and SuperCDROM are near identical under load. Duo is still over spec, but I suspect a bad resistor either after the amp or the VCE source before the amp.
Did a lot more reading about transmission line theory. I think I have enough understanding to build a ‘correct’ driver circuit ( from the looks of it, a lot of the s-video mods and drivers out there on the net aren’t exactly correctly setup). The VCE is directly capable of driving both composite and svideo signals, it’s just that you use an amp to buffer the chip. In case anything ever happens, the VCE will be protected (which is important, since the VCE isn’t just some off the shelf standard part).
The circuit will bypass the existing amp and add two separate amps for Luma and Chroma lines. But this brings me to a decision as how to handle IRE 0/7.5 issue. Csync is on the Luma line already, coming out of the VCE. The separate syncs coming out of the VCE aren’t exactly in sync (no pun intended) with Csync, IIRC. The point is, if you wanted to correct the IRE ‘black’ level issue for US NTSC TVs, you’ll have to strip sync from the Luma line, offset the line by adding a small amount of voltage. It gets a lot more complicated than that, because you need to simulate blanking vs black, which means another timed circuit with a fixed value between the rising edge of hsync and activating/switching to luma output. The more complicated part is vsync. The circuit would have to know when vsync is happening and not out the adjusting ‘black’ level. Pure black and blanking are the same level. See the problem? Most sets probably woundn’t care that the non sync part of vsync line had higher than blanking level voltage. But you never know for sure.
For now, I’m just going to skip over the issue. TG/SG/Duo/PCE fans having been turning up the brightness a bit to reclaim that lost black level colors/video, it would be no different now by leaving this issue alone. (Brightness adjusts were in the Luma line the offset from blanking level is for black level, Constast adjusts the compression/decompression of the Luma range.)
Another thing that I’m not 100% sure on, is driver/receiver divider. Transmission should be 2x voltage. The receiver divides it. Vout=r2/(r1+r2)*Vin. Unless I got my math wrong (the receiver has a fixed load of 75ohms), the only way to get 1/2 voltage after load – is to have both the driver and receiver use matching 75ohm resistor in circuit. And from what I understand, this matches the 75ohm (Z0) impedance of the transmission cable. A match is important to removing reflections back into the signal (which causes a number of artifacts like ghosting or ringing ,etc). I’ve seen DIY circuits out there with 33ohm or 27ohm (etc) drivers (with expecting 75ohm load on the receiver). Are they ignoring the cable impedance match up? Or am I missing something?
Anyway, I have little time each day – so this is progressing slowly. I have one amp built for Luma. May have some time to tap the VCE pins tomorrow or next day, to test out the circuit.