12-05-2019, 12:06 PM
(01-05-2019, 04:00 PM)`FRANK.C Wrote: I suspect the reason for the horizontal shift in the picture is due to the need to stop reading from the time redistribution memory for one line during the frame blanking period.
I cant think of the reason right now but the line chosen is critical or horizontal shift or other problems will occur.
The reason is:
The interpolater delays the video which pushes the picture down the screen.
To counteract that, on the 405 side FRAME_RESET resets to 403 rather than 0. Which has the effect of pulling the picture back up the screen.
The TR (time redistribution) memory needs to be cleared at the start of each frame.
Clearing the TR memory with FRAME_RESET would mean losing the last line or two of the field. So another point needs to be found during field blanking period to clear the TR memory.
At the time the TR memory is cleared it must not be either written to or read from or there will be an incomplete line stored in the TR memory. Which will have the effect of shifting the picture horizontally. For that reason the start of 625 line 12 was chosen as the TR memory clear point.
On the 405 side after the TR memory is cleared the memory is not read for one line to allow the memory to fill a little.
As the 567 lines are shorter in duration than the 405 lines more of them are required to get passed the point the TR memory is cleared.
resetting to 563 with FRAME_RESET as opposed to 565 is one way of achieving this.
Frank
