NTSC/PAL through speaker channel possible on Pi 4?

[NotSellingEth]

I'm super new to the Raspberry Pi. I am using the 3.5mm a/v jack to send NTSC to a pair of CRT tv's with a simple RCA splitter.

I want to know if possible to send a video signal through one of the audio channels so I can treat each screen as seperate without using the HDMI port.

This is just a curiosity really and I'd like to know if it is theoretically possible or if there is some kind of hardware limitation before I dive too far into the idea.

Thanks!

[mahjongg]

No!

The PI (any model) only has ONE composite output driver, not two. The two A/V audio channels are driven by PWM outputs designed for audio only.

Its also NOT possible to simultaneously output composite and HDMI. Or LCD screen or VGA through the GERT666VGA adapter.
the only duals screen exception are the two HDMI ports.

[NotSellingEth]

Thanks mahjongg. That makes sense.

Assuming infinite computing power, would it be theoretically possible to emulate a video signal through PWM audio? Again, this is purely a curiousity; I can always do HDMI to RCA if I am that desperate for dual CRTs haha; I'm just a novice that finally jumped from Arduino micro-controllers.

[cleverca22]

Assuming infinite computing power, would it be theoretically possible to emulate a video signal through PWM audio? Again, this is purely a curiousity; I can always do HDMI to RCA if I am that desperate for dual CRTs haha; I'm just a novice that finally jumped from Arduino micro-controllers.

grey-scale composite, just maybe? with current peripherals (but not the headphone jack on the pi)
color, that would likely be impossible

i dont know the exact rules on the pi4, but on older models, hdmi+composite conflicted, so an HDMI->RCA adapter wouldnt solve anything for you

all currently released models have 3 pixelvalves, so they could potentially run 3 displays at once, just not sure what the rules are on which combinations are valid
but the official firmware only does 2 at once i believe

DPI->composite might be plausible, if you have a dedicated RGB888->NTSC chip to deal with color burst&junk
DSI is basically just serial DPI with some NDA's sprinkled ontop to keep you from making your own chips

[drgeoff]

Assuming infinite computing power, would it be theoretically possible to emulate a video signal through PWM audio?

No, even with infinite computing power as computing power is not the only problem. For starters there are low pass filters on the audio outputs which would smear video horribly. Text would be a mess. There are also high pass filters which would likely upset the vertical sync part of the composite waveform. And the output impedance of the audio channels is so high that the resulting very low signal level into a standard 75 ohm composite video input would give very washed out pictures even if the display managed to lock to the weak and distorted (by the above filters) sync pulses.

[Burngate]

I disagree (just for the sake of).
It very much depends on how fast you can allow your video to change, and how long you can wait for the result.

Starting with a standard PAL frame, turn it into a JPEG file then send that as fast as you think you can get away with.
Once it's reached the far end, reconstruct the frame and display it.

Just a matter of bit-rates, and how much information you can do without.

[drgeoff]

I disagree (just for the sake of).
It very much depends on how fast you can allow your video to change, and how long you can wait for the result.

Starting with a standard PAL frame, turn it into a JPEG file then send that as fast as you think you can get away with.
Once it's reached the far end, reconstruct the frame and display it.

Just a matter of bit-rates, and how much information you can do without.

That doesn't come anywhere near my understanding of the OP's quest - to connect an audio channel on the 3.5 mm AV socket directly to the composite video input of a CRT display.

[cleverca22]

thinking about it more, you could probably make a DPI->grey-scale composite adapter with just a couple resistors

and DPI can generate all of the timings without any cpu load at all

but the problematic part, is that DPI expects color output, and mushing all of the channels together into grey-scale would be a bit tricky with dumb resistors