When to use 704x480 vs 720x480 for video capture
 

I've come across a curious thing.

While working on a Toshiba RD-XS54 DVD/HDD recorder and using VirtualDub 1.9.11

If I leave a capture device on its default of 720x480 and attempt to capture the composite or s-video output signal the image is black and white or monochrome.

If I send the same signal to a field monitor it displays the same image as color.

If I change the capture device to 704x480 and attempt to capture the composite or s-video output signal the image is color.

I've noticed this with several video output signal sources before but didn't understand what was going on.

Changing to 704x480 as a work around is a new discovery.

I've also checked this with about eight different brands and types of video capture devices.

So its a pretty robust work around.

I thought the information might help someone else out if they found they were getting a monochrome image from their video capture device.

Does the image appear monochromatic from the moment you connect your recording and capture devices, or does the image become black and white slowly over a period of time?

The change is instant.

From VirtualDub I can change the Video Capture Pin (Output Size) from 720x480 to 702x480 and it immediately goes from B&W to Color and vice versa when going from the other direction.

I was willing to assume the RD-XS54 DVR had a bad video output circuit and move on.. when I happen to accidentally connect the field monitor and the signal was displayed in color.. so then I started going back and changing things randomly.. like crop settings, interlace ect.. ultimately it was the 704x480 setting that changed VirtualDub to displaying the signal as color.

I realized that it might not be unique to this DVR or other sources..

Further experimenting.. even if VirtualDub is 720x480.. if I go Video > Cropping (in preparation to set the Crop settings) a popup window show the B&W image as color

I'm not sure if its a bug in VirtualDub.. or something more insidious in the way the overscan in a signal is examined to find the color burst part of the scan line. If its "overlooked" or "missed" for all scan lines then the entire frame will be interpreted as a B&W image.

I tried changing the Crop.. and that did not change the Preview Crop window from color to B&W.. but also it made no difference in the main view area.

The reason this is important.. is it may occur for VCRs, other brands of DVRs or ATSC tuners.. the symptom (getting a B&W image) may be the result of the capture device trying to interpret a video scan line that is too short.. and over looking the color burst portion of the scan line for all scan lines.

It might be for some other reason but I'm really thinking it may be the length of the scan line.

A TBC may correct for this.. I may try inserting a TBC to (stretch) the scan lines back to a traditional length and see if this brings the 740x480 back to a color image.. but I have not done this yet.

The problem would then be, most people would not have a TBC.. and would have to resort to 704x480 as a "workaround" in order to capture in color.

I should state I am doing this in NTSC.. but I suspect it might occur for PAL and SECAM as well.