I've been thinking about this and reading specs on NTSC and the BT878.
A 720 wide pixel line is typically achieved using a scanning line period
of 53.33 microseconds. This is considered an overscan. A 704 wide
pixel line is normally achieved with a scanning line period of 52.15
microseconds and is considered an exact scan. At least in the NTSC
realm, I think that if you ask the BT878 for more than 704 pixels, your
mileage may vary, perhaps including black borders. This seems
consistent with page 26 of the spec which talks about the BT878 using a
63.8 microsecond scan to read 913 pixels. Surely with a 63.8
microsecond scan period, you will be outside the normal picture borders.
Thus, I think if you ask for more than 768 active PAL square pixels or
720 CCIR601 pixels, you're likely to see black borders. Similarly, for
NTSC, asking for more than 640 square pixels or 720 CCIR601 pixels is
likely to produce black borders.
Tuukka Toivonen wrote:
On Fri, 19 Dec 2003, Trent Piepho wrote:
resolution do you have to capture in with a BT8x8 to ensure no hardware
down scaling is done?
With my BT878: 768x576. I live in PALland, I don't know about others.
No, that's wrong. For NTSC it is 754, for PAL it's 922. See page 26 in
the datasheet.
I don't care about the datasheet, I care what happens in reality. If I tell
xawtv or other V4L programs to capture in more than 768x576 all I get is a
768x576 picture with black borders. If I ask only 768x576 capture, the
black borders are gone but the actual image is still the same. And even
that is still much more than what a real TV would display.
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