Analog (Hi-8) capture and resize confusion

[Gav]

I just read the latest analog Capture Guide version 4.0 and to tell you the truth I am even more confused now regarding the appropriate resize values to maintain proper aspect ratios. Could someone please put me out of my misery and just tell me the appropriate way to accomplish what I want?

First off, I am in an NTSC part of the world. I am capturing from a Hi-8 camcorder using a BT8x8 based capture card with the latest BTWinCap drivers (5.3.8). I capture at 720x480. Now to maintain proper aspect ratios, what is the appropriate resize, adding/cropping amounts (if needed) if I want to:

1) make an XVID around the 640x480 size,

or

2) make a DVD with 720x480 size.

Please, please just help me out.

Gav

[Wilbert]

I just read the latest analog Capture Guide version 4.0 and to tell you the truth I am even more confused now regarding the appropriate resize values to maintain proper aspect ratios.
First you have to know your active capture window. You can measure it yourself as described in section 10, or you can look it up in the table in 5.4:

BT878, BTWincap v5.3.6.1: 52.80 µs (it's not actually the same driver version, but ok)

Note this means that you capped more than the 4:3 picture (which is 52.6555 µs). This will result in at least 2 pixels black borders (depends a bit on the capture size).

Just look at the examples in 5.6:

1) make an XVID around the 640x480 size,

Full NTSC TV on a PC gives 52.6555 µs * 12.3064 MHz = 648 pixels, ending with 648x480. Your card/driver combo has a capture window of 52.80 µs. So, how many PC sized pixels fit in there? 52.80 µs * 12.3064 MHz = 650 pixels.

So you have two options (if you can't capture at arbitrary sizes you only have one option) here:

a) Cap pixels that are at or close at the target size. So, try capping at 650x480 and remove 2 pixels to get 648x480. Add or remove 8 pixels to obtain a width which is divisible by 16.

b) Capping at another size and resize to the correct pixel size afterwards. If you want that, cap high, say 720x480.
However, your card caps only 52.80 µs, so you need less pixels to make up the difference with the 52.6555 µs a PC needs for correct AR. How many?
(52.6555 / 52.80) * 720 = 718 in total.

So, remove 2 black pixels of your 720x480 cap. You now have 52.6555 µs of info in 718 pixels. Resize the resulting 718x480 to 648x480 or a scaling of it.

2) make a DVD with 720x480 size.

NTSC DVD gives 53.333 µs * 13.5 MHz = 720 pixels, ending with 720x480.
Your card has a capture window of 52.80 µs. So, how many DVD sized pixels fit in there? 52.80 µs * 13.5 MHz = 713 pixels. That is not a nice number to cap. Depending on the codec, you should always cap mod 2, 4 or 8.

So you have two options (if you can't capture at arbitrary size you only have one option) here:

a) Cap pixels that are at or close at the target size. So, try capping at 712x480 and pad to 720x480.

b) Capping at an other size and resize to the correct pixel size afterwards. If you want that, cap high, say 720x480.
However, your card caps only 52.80 µs, so you need extra pixels to make up the difference with the 53.333 µs a DVD needs for correct AR. How many?
(53.333 / 52.80) * 720 = 728 in total.

So, add 4 extra black pixels to each side of your 720x480 cap. You now have 53.333 µs of info in 728 pixels. Resize the resulting 728x480 horizontally to 720x480 and again, you have DVD with correct AR.

I hope it's a bit clear :)

[Gav]

Thank you, Wilbert, for a very thorough answer. It is much appreciated. I believe it is much clearer now. I hope this also might help others who may have been a bit confused. These concepts and calculations seem so much different from your previous capture guide so I guess I needed an extra lesson to grasp it.

[jggimi]

Yes, they can be confusing. I read through both sections 4 and 5, and am thinking that I should have stayed in section 4. ;)