On Mon, 22 Dec 2003, Robert W. Fuller wrote: > 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 Yes, CCIR601 specifies a sampling rate of 13.5 MHz. 720 pixels / 13.5 MHz = 53.33 us. An entire line is 63.55 us long, but some of that is blanking and sync pulse. 53.33 us cuts out the blanking and sync, but there is still more of the line captured than would normally be displayed on a TV. You will be capturing some black space and/or garbage on the edges. Cut off that, and you get 704 pixels or 52.15 us. For NTSC, the bt8x8 captures at 14.32 MHz (and only 14.32 MHz), more than the 13.5 MHz specified in ccir601. A full NTSC line will have 63.55 us * 14.32 MHz ~= 910 pixels. Of course, some of that is blanking. If you captured the equilivent of a ccir601 720 pixel line, that is 53.33 us, you would have 764 pixels. A 704 pixel line (52.15 us), with overscan cropped, would be 746 pixels at the bt848's sampling frequency. The datasheet gives a value of 754 active pixels, which gives a captured length of 754 samples / 14.32 MHz = 52.65 us. That's halfway between CCIR601's "720 pixel" line length of 53.33 us and the "704 pixel" length of 52.15 us. In other words, the bt8x8 datasheet's idea of NTSC active line length captures less overscan than CCIR601, but more overscan than CCIR601 does when cropped to 704 pixels. When you request a certain horizontally sized image from the bt8x8 V4L driver, the driver must decide how to scale AND crop the image. The length of the image you capture could be 53.33 us, or 52.15 us, or 52.65 us, depending on how much overscan you want to capture. The length of the captured portion of a line, in microseconds, is hard coded into the driver. And historically, different versions of the driver have changed this around. But none of this changes the fact that when you request an image width of 720 pixels, that image is getting scaled by the bt8x8. The HSCALE register is not getting loaded with a value of 0, it's as simple as that. > 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. The 63.8 us scan of 913 pixels was an example of a longer than nominal line length, caused by the poor signals typical of a VCR or other low-quality source.
