PS: Also the video data's timing is spot on. Silly question: The Pi runs
at 3.3V, the display expects TTL (5V). Could this lead to these
distortions? I have not yet added a proper level shifter.
On 2022-02-25 11:34 p.m., jb-electronics wrote:
Hey David, I had a few minutes spare time tonight and I verified that
both the hsync and vsync signals appear at exactly the right frequency
(19.2kHz and 60Hz, respectively) and have the correct duration. Yet,
the image is still garbage. How can that be, any ideas?
Jens
On 2022-02-21 7:59 p.m., David Forbes wrote:
Jens,
Connect your oscilloscope to the H Drive signal and verify that it is
19.2 kHz. If your oscilloscope cannot do this job, then do yourself a
favor and buy a better one. It will be worth it.
I say this because I did a lot of video work in the 1980s, and a
Tektronix 465 scope or its equivalent was essential for this work.
You should also be able to see the video on the oscilloscope. It may
not be displayed in full resolution, but the text portion should be a
higher voltage than the black portions at the sides of the screen.
You can change the settings in the Pi and you should be able to see
the signal timing change to follow the settings. If you cannot do
this, it will never work correctly.
Good luck!
On Mon, Feb 21, 2022, 3:44 PM jb-electronics
<[email protected]> wrote:
David,
Thanks for your pointers! In the Raspberry Pi's DPI configuration
I set the amount of horizontal pixels to 844px, which at a pixel
clock of 20MHz should result in 19.2kHz if I am not mistaken.
Sorry, I forgot to mention this part.
Regardless of how I think I configured it, you are of course
right that it looks like there is something wrong with the
horizontal timing. But the absoltue value of the pixel clock
should not play any role here, should it, assuming that I
calculated the correct horizontal timing?
Best wishes
Jens
On 2022-02-20 8:58 p.m., David Forbes wrote:
Jens,
The horizontal line timing is incorrect. This is why the image
is diagonal.
The numbers that you provided do not specify the horizontal line
total time or frequency.
The monitor requires 52 microseconds or 19.2 kHz.
You need to find a way to specify this, perhaps it is the
character count or pixel count and the pixel frequency.
On Sun, Feb 20, 2022, 6:26 PM jb-electronics
<[email protected]> wrote:
Dear all,
Apologies for the off topic question, but I know there are
some CRT experts in our group and I have been banging my
head on this one for a while.
I picked up a cute 6" monochrome CRT module which accepts
TTL video. This is the timing chart:
My goal is to run this thing using the DPI module of the
Raspberry Pi. But for all of that to work I have to specify
certain parameters and pass them along to the DPI module,
and here is what I read off the sheet:
* h sync: 4.615 us
* h front porch: 2.637 us
* h back porch: 2.637 us
* v sync: 0.208 ms
* v front porch: 0.209 ms
* v back porch: 2.917 ms
* active lines: 256
I have verified that at least the vertical signal timing is
implemented correctly, my scope is a tad too slow to resolve
the horizontal timing. But there appears only gibberish on
the screen, highly distorted text. Here is how it looks
like: http://jb-electronics.de/tmp/screen01.jpg
And this is what it should be (composite video output on my
Apple IIc monitor) for the same "scene":
http://jb-electronics.de/tmp/screen02.jpg
Can somebody tell me if I am on the right track here, or if
there is a fundamental mistake somewhere in the numbers I
read off? Your help is very much appreciated.
Best wishes
Jens
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