TI8168 DVRRDK V4.0 VGA display shift

John,

This is actually a simple issue to fix, and is in fact a monitor problem not a DM issue.

With digital video such as HDMI there is an embedded signal called DE (data enable) which qualifies exactly where the active part of the video image stream exists.

Unfortunately for analog VGA there is no such signal so the monitor has to 'guess' where in the analog signal stream they actual displayed part of the image really is. Now, VESA specifies where this image SHOULD be, but many sources do not follow the VESA standard fully (note, the DM816x should).

So, what does this mean? Well, all monitors are designed to be able to 'find' the real video by executing a search phase on a video signal to look for the left, right, top and bottom most active pixels. This is usually initiated on the monitor by pressing a button marked "auto" or something similar. Sometimes this is in one of the menus. When executing this procedure you will see the displayed image move around on the screen, then snap in to position correctly. During this process the monitor will basically look at the left edge of the image and modify its internal sample timing until it 'sees' a pixel, then it knows where the left most pixel is in the video stream. Then it repeats this for the right edge, top edge and bottom edge. Finally it will also adjust the sampling phase so that it samples each pixel in the optimal position to give the sharpest image.

Why would the DM show to be wrong? So, most monitors actually store the results of this search for each detected source resolution (determined by monitoring the H sync and V sync frequencies) so if a similar resolution has previously been presented to the monitor which was not exactly VESA compliant then the monitor will have remembered these 'bad' settings. Since there is no DE signal the monitor cannot determine that it is actually a different source with a different alignment.

The solution therefore is to simply find the "auto" button for your monitor and press it.

Note, however, that this algorithm does require the image being displayed to have non-black pixels extending to the left most, right most, top and bottom extremities of the display. Pressing auto on a black/blank screen will confuse the monitor.

Hope tis helps.

BR,

Steve

Hi Steve,

    Thanks for your patient and detailed explanation.

    I'll proceed to try your suggestion right away and feedback the result.

    However, I have three questions in the mean time I give it a try:

    1. From your description, it means that there is no need to modify the VGA output setting & timings on DM8168 side? right?

    2. Does DM816x VGA display fully follow the VESA standard?

    3. What's to be done for those projector (or monitor) which don't have "auto"  button or menu setting on it?

best regards,

- John

John,

The DM8168 timings should follow the VESA standard for enumerated modes but the exact timings are fully configurable so you can generate any desired mode/timing configuration. See the "timings" sysfs entries in the HDVPSS software documentation for full details.

For monitors which don't have auto set, I have never come across a monitor/projector that does not have this. It is really a requirement for the reasons mentioned previously. Some monitors simply do it automatically whenever they detect a mode change.

BR,

Steve