Smpte 259-c

[Tetra1]

Hi all,

Looking for some advice/help to aid my general understanding.

I've come across products on the market which convert from 1080p30 to NTSC composite output and I was wondering how the downconversion would work?

If it took a letterbox approach, would I be right in thinking that the first operation would be to add horizontal black bars along the top and bottom to make the overall frame size 1920x1440 to make the aspect ratio 4:3? Next step would be to rescale to 640x480?

The last step is the part that is confusing me. SMPTE-259C has 720x480 (I gather this is to do with rectangular pixels from 'back in the day'). Is the horizontal pixels scaled to 704 with 16 pixels of blanks added to make up the 720x480 to make it overall compliant with the SMPTE standard?

Hopefully that makes sense.

Many thanks in advance

[Cornucopia]

The best quality would be to do the resizing in one step and the padding in another step.

Composite NTSC is analog, so you need a digital to analog converter as the last step. What digital resolution is required as input depends on the device and what it supports, what its settings are.
Most commonly, it would support 601, so you should probably end up with 704x480 as you last dig rez. Since analog likely wouldn't know what to do with widescreen signalling, it makes more sense to letterbox it.

Padding, then resizing seems easier to laypeople, but it actually would be better quality to resize then pad.

Use the universal formula of

DAR = PAR * H rez / V rez

Plug in what you know for each end, and use the common figures for both image1 (start) and image2 (end), so you have an equivalency that you can use for substitution.

1.7777 = 1 * 1920 / 1080 image1 wanting to find 1.333 = x * 704 / y image2

1.3333 = 1 * 1440 / 1080

1.3333 = x * 704 / 480
so
1440 / 1080 = x * 704 / 480
or
X = 1440 * 480 / 1080 / 704 = 0.9090909

1.7777 = 0.9090909 * 704 / y
so
Y = 0.9090909 * 704 / 1.777 = 361.53, or 362 (even # divisor) or 360 (divisible by 4)

Then pad height (symmetrically each at top, bottom) to fill out to 704 x 480.


Scott

[jagabo]

The equation:

Y = 0.9090909 * 704 / 1.777

leaving the original fractions (to avoid rounding errors) is:

Y = (10 / 11) * 704 / (16 / 9)

results in exactly 360.

So the 1920x1080 frame should be reduced to 704x360 and padded to 704x480 or 720x480 depending on what the device requires.

But all of this is theoretical. There's no telling what a particular device is going to do.

[Cornucopia]

Yeah, I was using a calculator on the fly in 2 stages. No surprise about the rounding. The rest is still a good way to approach it generally.

Agree the device may not do things optimally.


Scott