0
\$\begingroup\$

I have a DCMI peripheral (STM32H7) that receives video data from TVP5150 with discrete VSYNC/HSYNC.

I have 306 (it should be 305, but there is a chance that this is because of the way I count number of interrupts) active video full line interrupts in one frame, with one line duration of 64 us. It's all sounds good but it's seems that I have 800 Y values received during one line (monochrome video).

That's seems odd to me, Is there any explanation why it is that way?

\$\endgroup\$

2 Answers 2

1
\$\begingroup\$

That's what you get when you capture everything except for the HSYNC pulse itself using a standard 13.5 MHz sample rate — back porch (5.7 µs = 77 pixels) + active video (700 pixels) + front porch (1.65 µs = 23 pixels) = 800 pixels total.

The HSYNC pulse is 4.7 µs = 64 pixels, giving you the total of 864 pixels for the line period.

\$\endgroup\$
2
  • \$\begingroup\$ Thank you so much! Do I understand correctly that this is bt601 standart? What type of documentation describe what pixels I should use to draw a picture? Or is it as simple as you say just use pixels from 77 pixel to 800-23? \$\endgroup\$ Commented Oct 27, 2018 at 19:17
  • \$\begingroup\$ That does not add up. It's 720 pixels active, so the blank area is 144 pixels. \$\endgroup\$
    – Justme
    Commented Oct 27, 2018 at 20:43
4
\$\begingroup\$

Most likely you have set the video decoder to output HSYNC; but have set the STM32H7 to expect HACTIVE, so it samples everything but the sync. By default TVP5150 HSYNC pulse is 64 pixels or 128 clocks, so 864-64 is 800, and you are left with 800 pixels per line, so it contains back porch and front porch blanking too instead of just the 720 pixels you really want.

\$\endgroup\$
2
  • \$\begingroup\$ What pixels in this 800 pixels line are video pixels, what standard describe it? Where I can get the information how I should draw this pixels? \$\endgroup\$ Commented Oct 27, 2018 at 20:55
  • \$\begingroup\$ The TVP5150 datasheet will tell you what it outputs. It is another matter if it conforms to a standard or not, but the relevant standard is ITU-R BT.601. The Y pixels are the grayscale luminance. Cb and Cr must be upsampled and YCbCr converted to RGB if you want to display a color picture. \$\endgroup\$
    – Justme
    Commented Oct 27, 2018 at 21:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.