More composite signal analyzing

This time, I used a 8 step gray scale to test the signal. Looking for max white and pure black.

This interesting. Max white for both the SGX and the white PCE is 0.84v. That’s 0.14v above NTSC US, or IRE 100. There’s also some slight stepping distortion. I.e. all the steps aren’t the same space apart. This happens in pairs. If you divide all 8 levels into groups of 2; 0&1, 2&3, 4&5, 6&7. That’s how they are paired. And by paired, I mean the two steps are closer to each other, than from their neighbors. It’s slight, but it’s enough for me to see it with my eyes on the scope. And I see it on all three systems I tested: SGX, US Duo, white PCE.

Now, for something else that’s interesting. The Super CDROM2 addon. It normalizes the composite signal (from 0v to max, sync isn’t effected). Luma range is now 0.v to 0.76v (down from 0.84v. And it’s scaled, not clipped). Closer to spec to NTSC US, and probably what the Japanese spec is (I don’t have a direct reference, just assumption based on ‘whiter than white’ reports of Asian NTSC DVD players not correcting them the circuit/output on the US equiv models). The Chroma Burst PLL signal on the back porch, is also scaled up. On the white PCE, the amplitude of the CB is kind of small. There’s more amplitude on the SGX straight CVBS out. But on the SuperCDROM2 out, the amplitude is pretty large. I guess some TVs had problems phase locking to the burst signal. That’s the only reason I can think of it. It doesn’t effect saturation at all. It’s just a reference signal to lock to on every scanline.

So, over all brightness will be a little bit darker than the system itself – on the Super CDROM2. Though I don’t have a booster, so I can’t compare it against that (the white PCE). And the white PCE technically doesn’t have composite output ports, so who’s to say that the Booster wouldn’t perform the same normalization as the SCD addon. But the SGX is a different scenario. The SGX has composite output normally. So this is effectively a direct change. I can’t speak for the Core Grafx I or II, since I don’t have those. But it wouldn’t surprise me if they are set to the same levels as the SGX built in CVBS out.

I should bring out the ‘brief case’ CD addon and see if it does anything like the SCD addon is doing.

Back to the stepping of the DAC. From what I’ve tested, ALL the VCE DACs have this stepping distortion. Here’s how it translates:
I scaled the voltage for the pairs that I listed. They are scaled to 1.0v
The difference to its neighbor is a step of ~1.2v. See? Not an even scale.
Translate to into a linear range of 0-255: You get 0,+34,+40,+34,+40,+34,+40,+34. Instead of the linear steps of 36. So to use grayscale to simplify this: 0, 34, 74, 108, 148, 182, 222, 256(255). And, this pretty much matches what I saw when I set my capture card to full luma (you have to force this on for NTSC-J, else you get an incorrect clipped and scaled Luma range).

So, to recap. This isn’t just for Composite output. RGB DAC output has the same stepping pattern too. But here’s the kicker. The Y/Cr/Cb DACs aren’t 3bit like RGB (even though they output the same stepping). They’re higher res. 5bit DACs. So the next test would be to cut Chroma lines and test all 32 Luma levels. See if the stepping pattern shows something different for values finer than 8 levels.


4 Responses to “More composite signal analyzing”

  1. Peak white-level = 100 IRE (0.714 V) (ITU-R BT.601 level: 235)
    Peak level including chrominance signal = 120 IRE (0,857 V)
    blanking level = 0 IRE (0 V)
    ntsc-m(us) black level = 7.5 IRE (0.054 V) (ITU-R BT.601 level: 16)
    ntsc-j black level = 0 IRE (0 V) (ITU-R BT.601 level: 16)

    Note: the 120 IRE limit is the reason why the PC Engine scales the chroma at 75%.

    Maybe this is a stupid question… But with the oscilloscope are you using a 75 ohm termination or load?
    Otherwise you obtain wrong measures.
    It is incorrect to use even a termination AND a load, in this case you need a splitter.

    • The ITU-R BT.601 Level range (in 8bit quantization) shouldn’t apply to the equation given in the PDF. That’s specifically for digital transmission, where the receiver decodes the digital signal and outputs an analog signal (what encoded Y ranges correspond to). Noteably high speed digital serial YCrCb (it’s the only digital transmission in the pro field that I’ve ever seen. Our equipment has that option). So the additional range above Peak white is room for the Chroma peak amplitude level. Got it. Thanks 😀

      About the 75 ohm termination. I tried that already. I have a splitter for the composite cable and terminate the one end with a 75 ohm resistor, while the other end goes to the a BNC converter and directly into the scope. And the BNC connector as no resistance that I can measure (I’ve tested all my RCA to BNC connector converters). Appling a 75 ohm termination brings the whole signal down quite a bit(as in, it puts voltage range below spec). Max white level dropped from 0.81v to 0.66 (or something close to that). That’s pretty fair below the 0.714 ‘white level’. Not only that, but the Genesis composite output freaks out with the terminator on there, as the Y level moves up and down (it’s find/stable for max white – like Sonic 2 intro screen).

      Any suggestions?

    • Also, I checked the composite output with the system off. It’s already 120 ohms.

    • Heh, just spent an hour reading about load impedance. I see why it needs a ‘load’ (transmission output unit expects a load on the receiver side. Transmission line theory – meh). Guess I need to check my scope voltage levels.

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