The real V99x8 Pixel Aspect Ratio (PAR)

It's apparently the only thing I know how to talk about, but my emulator includes a full emulation of a CRT, which should mean that the aspect ratio is correct even though I've never bothered to work out what it is. For comparison, 1:1 copying from another emulator versus mine (showing composite video, so a bit fuzzier, but check out the aspect ratio):

Doing the arithmetic, sticking to the TMS modes because my knowledge is limited, I get:

256 cycles of pixels, at 2/3rds of a colour cycle per pixel = very close to 75% of the complete line as pixels. Or, each pixel is 1 / (227.5 * 1.5) of a line. Around 52/64ths of the line are visible, so:
64 / (227.5 * 1.5 * 52)
= 64 / 17745

Each line is 1/240th of the visible area tall.

Given the screen is 4:3, the aspect ratio of a pixel is:
(4 * (64 / 17745)) / (3 * (1 / 240))
= (256 / 17745) / (3 / 240)
= (256 / 17745) / (1 / 80)
= 4096 / 3549
~= 1.154127923358693

So we agree!

@Grauw

Thanks!

But then there's the "well calibrated NTSC monitor" issue. They're calibrated using a standard calibration card (electronic card, these days). This sets the monitor to the exact standardized image dimensions. (it's also used to calibrate the gain, hue and many other parameters, but those are out of the scope right now).

The monitor will then impose the following constraints to the image:

1) Only 480i lines (240p) will be drawn. So the extra three lines that the V99x8 renders will fall off of the screen, as overscan
2) The image period of a line is exactly 52.6uS long. Any pixels sent after this will fall off of the screen, as overscan

On the V99x8, the leftBorder+ActiveArea+rightBorder = 1139 cycles = 53.03281uS

This means that the last 4.647799 pixels of the right border will fall off of the screen, as overscan.

So, after the monitor crops the image, we have an effective resolution of 564.8522 x 480i. Yes, the numbers have decimal points because this is all analog. NTSC had no real concept of pixels.

(4/564.8522)/(3/480) = 1.133039756

So I think this seems to be (finally!) the definite pixel aspect ratio for the MSX on an NTSC monitor. What do you think? :)

I'm not sure you can be absolutely definite:

• practically, because CRTs were usually calibrated to a test card manually by factory staff; and
• strictly, because both the TMS and V99x8 are off-spec by 0.2% in horizontal timing.

A real NTSC line is 227.5 colour clocks long — that's the period between horizontal syncs. Both the TMS and V99x8 run for 228 colour clocks between horizontal syncs. Within tolerance, but some screens might compress to make up the difference and others might not. You're still getting the same aspect ratio to two decimal places, but everything after that is questionable even if you discard the calibration concern as being a manufacturing problem.

Two decimal places is good enough I reckon, and we’re not far apart . Somewhere between 9:8 and 8:7.

Btw, do you have sources for that 52.6uS and 240 lines where I can read up on it?

I have my suspicion that the visible horizontal line time may have been redefined down to more like 50.2µs in the same way that the vertical visible area was redefined down to 240 on the grounds that the standard for digitisation of composite video is to sample at four times the colour subcarrier for Nyquist reasons, there should be 227.5 cycles of that in each 63.5µs period, and DVDs went with 720 pixels across.

720/4 is 180; (180/227.5)*63.5 = approximately 50.2µs.

Of course there's no inherent reason why DVDs would necessarily be aligned exactly with the preservation standard (e.g. they're definitely not in PAL), it just seems very unlikely to me that they'd make it 95% of the number for the same period and not just slightly redefine the visible region; it also feels improbable that what DVDs consider the visible region wasn't based on real empirical evidence.

All circumstantial though.