405 lines from Raspberry Pi 5

[`FRANK.C]

Have looked up the DAC08 on Mouser. At €3.89 it is not expensive.
Although one variant DAC08RC/883C cost €85.18 and comes with the warning  "This product may require additional documentation to export from the United States."
Wonder why the need for extra documentation? Never had a problem with getting FPGA's etc from there.

Frank

[ppppenguin]

I really wouldn't bother with the DAC08 for video. An R-2R ladder is fine (probably better) and there's no problem with DC offsets. Choose the resistor values so it can be easily driven by the logic. This will need to be 74HC, 74HCT or a FPGA. (There's been a recent discussion going on elsewhere about deriving analogue video from logic. People are being a bit precious about it. For 3 or 4 bits you can do just about anything and it works reasonably. Though much better to use HC/HCT than LS etc. I know, I've done it)

The screenshot is the DAC in the Aurora. It's driven directly from a Xilinx 3 series FPGA. It shows very close tolerance resistors for the more significant bits. In practice resistors from the same batch of 1% parts will often be closely matched. Or match them with a DMM. The absolute value doesn't matter. You can use pairs of 1K resistors in series rather than separate 2K. Easier to get the match that way.

The odd looking values on the LSBs are a clever trick that Darryl used to make a 10 bit DAC using PWM for the low order bits. This technique, taken to its limits, gives you the 1 bit DACs commonly used for 16 bit audio. Not practical to go that far for video - the clock rate would be in the GHz range.

[DrStrangelove]

Have looked up the DAC08 on Mouser. At €3.89 it is not expensive.
Although one variant DAC08RC/883C cost €85.18 and comes with the warning  "This product may require additional documentation to export from the United States."
Wonder why the need for extra documentation? Never had a problem with getting FPGA's etc from there.

Frank

883 is a military spec.

That's why it's ridiculously expensive due to the processing it undergoes.

Back in the day Siliconix did lots & lots of that sort of thing.

[`FRANK.C]

That so would explain the need for extra documentation too.

Frank

[DrStrangelove]

That so would explain the need for extra documentation too.

Frank

Oh yes. The documentation for some of this stuff is "voluminous".

[Murphyv310]

Hi.
It wont conform to 405 line standards correctly additional broad pulses are needed. Those of us that use a PC with a graphics card add a little circuit with two IC's and this cures the interlace issues and other problems from PC generation.
See Peter Scotts website for more info, no doubt it'll cure the same problems on the raspberry Pi

[`FRANK.C]

Hi Trevor
I could be totally wrong but the impression I get from the thread is the the syncs are OK for 405.
I bereave that the chips in the Pi are more programmable compared to a graphic card if you know how.

Frank

[Murphyv310]

Hi Trevor
I could be totally wrong but the impression I get from the thread is the the syncs are OK for 405.
I bereave that the chips in the Pi are more programmable compared to a graphic card if you know how.

Frank

Thanks Frank.
I'd certainly like to see evidence of that with a scope trace.
If the Pi can do it then we certainly are on a winner.
The other thing is myself along with others is we are not computer programmers so this has to be an easy download and not hours pulling out the remaining few strands of hair left.

[Mike Watterson]

I don't think it involves actual programming, but simply settings in a config file defining the resolution, frame rate and timings etc.
I may have a look. First I'll figure how to enable or use the composite at all. At least for the Pi 4b I have it's a connector and I have two working cables. The one with RCA plugs to 4 pole has the ring 2 as 0V/Earth but mysteriously the plug colours wrong. Red goes to sleeve which is video. So I put red tape on the yellow plug and yellow tape on the red plug. It hardly matters if red & white (audio) are reversed. An old Nokia cable is sleeve as 0V/Earth and video on ring 2. A composite cable on an Edison Combo setbox seems similar to the Nokia cable. I think the 4 pole to RCA for Apple and Zune have ring 2 as 0V/Earth, but might not have the right colours. The working RCA plug cable came from a dead Redline Satbox. However the common €2 local 4 pole mic/headphone splitter adaptor seems to have the video on the mic socket. These also come in alternate pre-Apple wiring version with sleeve/ring 2 swapped. Ironically Apple dropped the socket after messing up the standard.

[`FRANK.C]

I had a quick play this evening.
And it looks good.
Syncs are good
I haven't got the aspect ratio quite correct. But that's most lightly down to me as I am not familiar with either Linux or raspberry Pi.

Not too difficult to install. I think it is an update.  I just followed the instructions in that thread I linked to in the first post. Some knowledge of Linux and Pi would be desirable.

Composite on the Pi5 is a 2 pin header. Which is left blank.

Frank