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Forum Index : Microcontroller and PC projects : PicoMite - composite video output
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Nimue Guru  Joined: 06/08/2020 Location: United KingdomPosts: 367 |
Hi all It's been a long time since I posted (Covid multiple times, other stuff which led to 18months or so of enforced "rest" -- all good now). I've been using the PicoMite to introduce Pi's and MMBasic to teachers in the area to to the idea of using such devices both creatively and to support the transition from Scratch to "a proper" language -- usually that's Python, but in my personal opinion MMBasic has more "out of the box" potential and I can position it as an "ecosystem" (speaking like a true consultant now)  A question arose the other day - to paraphrase: Can we connect the PicoMite to composite input? I know connecting to VGA is "easy" and just requires creation of a resistor ladder / DAC and appropriate connections. I also know that the DAC for composite is even simplier. BUT the creation of the video signal (porches, sync etc etc) will be different. Question: Is it possible to modify the VGA installation to work with composite or does that require changes to the firmware? Use case: Most schools I work with have VGA -- but that will be connected. It's like asking for seven generations of unborn children to get things unplugged / re-plugged -- but most (especially if using interactive white boards) also have composite input. Cheers N Entropy is not what it used to be |
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Rickard5 Guru Joined: 31/03/2022 Location: United StatesPosts: 328 |
I Don't know about in the UK but in the US NTSC it's really just a Cable adaptor and they can be Cheep as Chips! This one here is a little more speedy one @$15.50, I've used $2 cables that worked fine and no firmware changes needed, but the real Cagey thing of it if you took the $15 one I bet you could hack the Pico into it and have a Real neat all in one solution with out a bunch of mess and cables I turned the volume on the monitor to max and could hear sound. Thanks Stanleyella |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
Yea I know -- I have one already, but it seems somewhat "hacky" and would prefer to output native PAL Composite. Schools are very reluctant to plug anything into anything else -- I can manage with one cable -- plug this into there -- but there will be resistance to "plug this into that, find another power supply and then plug this into that also". Not sure if PAL / NTSC needs some form of tweak, but that's probably just a voltage / timing thing. Seems to me that VGA is more complex ;-) As you say, what I really want is an all in one solution but based on the Pi and a custom 3D case. But if your solution is the only way, I'll give it a go. Cheers N Entropy is not what it used to be |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3008 |
Welcome back, Nimue. Glad you're better. This claims to do VGA to composite for $2.77Canadian, including shipping. There are also circuits here (with the suggestion that you won't get color--which may also apply to the cable above). There's also this and this (googling for "RP2040 composite video output"). ~ Edited 2022-08-15 22:54 by lizby PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
Had seen that link - so know it can be done -- of course I am looking for an MMBasic solution  so that I can stay within that environment -- going to "C" or programming the PIO directly will loose what audience I have left in the teaching community.Have purchased a cable, but am not overly hopeful.... N ps - glad to be back, feel like I've missed 18months of fun! Edited 2022-08-15 22:56 by Nimue Entropy is not what it used to be |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 3490 |
Hi Nimue, For composite video you have to buy an external device (such as the one Rick showed). Generating composite video is rather complex, especially when there is color involved. This is mainly because of the modulation of the color signal onto the luminance (brightness) signal. That is why the original MAXIMITE had a composite video option (it did not have color) and the color maximite (CMM1) did not have this feature anymore. Regards, Volhout P.S. the VGA standard support a variety of frequencies and frame rates. The cheaper cable lizby suggested may only work with one of these that accidentally aligns with the TV framerate. And it is most likely VGA-YC (S-video) and not composite video. Not guarantied that it works with CMM2 or picomiteVGA, since these have a fixed subset of the VGA possibilities. P.P.S PAL and NTSC are very different. Not only in framerate, but also in bandwith and modulation method for color. You are UK based, so your TV's are probably PAL (not NTSC which is an American standard). PicomiteVGA PETSCII ROBOTS |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 8566 |
Won't work Basically the answer is No: the PicoMite can't do composite video. There is a simple passive circuit that MIGHT give a crap mono output, otherwise use an AD724 chip with a handful of passives. This can be powered by the Pico and could be designed onto a simple Pico motherboard |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 1769 |
The $2.77 unit isn't a converter, just a straight through adapter. Some Laptops can be configured to output composite or S-Video through the VGA socket to avoid having extra sockets on the back. A real colour converter will cost a a lot more. Colour composite is much more complicated than VGA. |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5705 |
I'm pretty sure this has come up before and Peter said no for some reason. It's quite a while ago. Probably around the time the PicoMite VGA was developed. I'm not sure why most people would want CV now. VGA is simply the colour values pumped out on three lines instead of having to be processed again to combine them and mixed with the synch signals. I would guess that VGA is actually much simpler. CV had it's place when you had to feed an RF modulator, but those days have gone. VGA will also work on any VGA compatible TV or monitor anywhere in the world. PAL & NTSC are different systems. Schools already have to cope with HDMI and VGA and power supplies. They have done for years. Nothing new there. I bet most of them haven't got any monitors with only CV inputs now - they went out with the BBC computers. The only big operational difference is that HDMI can carry audio, CV and VGA can't. Edit: Sheesh! I'm getting slow at posting in my old age.... :( Edited 2022-08-15 23:10 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
Nice - quick Goolge shows that's not too complex -- must be if I can understand it. For now, either getting them to cable swap or a more expensive signal convertor seems to be the way foward. Dare I mention RF modulation out put for those schools that still have TVs in the classroom (only semi joking  )Thanks all -- good to be back. N Entropy is not what it used to be |
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Martin H. Guru Joined: 04/06/2022 Location: GermanyPosts: 890 |
as the Composide is a combination off all signals you will need additional Hardware to transform the RGB+Syncs to FBAS. This might work for an Grayscale Signal, but if you want Color, you'll have to produce a 4.43361875 MHz (PAL) colour carrier Signal in which the Colors are coded by Phase displacement. Not realy trivial in the DIY way. https://www.hisour.com/pal-color-system-25921/ it might work with an intigrated RGB to NTSC/PAL Encoder https://www.analog.com/en/products/ad724.html#product-overview Edited 2022-08-15 23:13 by Martin H. 'no comment |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 3490 |
The issue here is that picomite VGA is 60Hz. By only doing the RGB-composite conversion (like the AD724 circuit in lizby's link) you will get a composite video signal, at 60Hz framerate and most likely NTSC modulated. But UK TV's expect 50Hz and PAL modulation. So you need to convert framerate also, which requires a sh*tload in digital electronics and memory. Frame grabber, up-down convertor (240/480/600 - to 525 lines) and video (re-) generator. Basically an the opposite of your TV electronics, since the TV translates composite video to RBG and drives the tube or LCD. Maybe Nimue is better served with a KVM switch.... Volhout PicomiteVGA PETSCII ROBOTS |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
I work with 400 ish schools in the south of Wales and many have CV in -- especially those with interactive blackboards and old visualisers. I came across a school the other day still using VHS via SCART as the video they wanted to watch was no longer available in any other format. Cable swapping is a real issue especially in Primary - I regularly come across VGA cables that are so tightly screwed in by their paramilitary IT team that we don't even bother attempting to unplug them. That said, PicoMite via serial is a real boon as it just works. My only remaining challenge is getting it to work via Chromebook as I cant seem to get a decent terminal software for them. N Edited 2022-08-15 23:14 by Nimue Entropy is not what it used to be |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
^^^ great explanation - thank you. I'll stick with VGA and continue to fight the cable guys in schools ;-) Thanks all. N Entropy is not what it used to be |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 8566 |
The AD724 handles PAL or NTSC depending on the Xtal used and position of a jumper. The chip is now expensive from the likes of RS/Mouser (18GBP) but is available from China at cGBP2. Nimue, if it would definitely be used in schools I'd be happy to do a simple PCB layout for a convertor motherboard. |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
That's hugely generous. I work for what is called "regional Consortia" what we the school improvement service, here in Wales. We serve 400+ primary / secondary schools. I am the STEM lead and a current significant line of work is to upskill our teachers to teach coding, including accessing physical hardware. Two years ago I wanted the CMM2 to be that vehicle, but Covid and illness stopped me working on things. Now I'm "back" and the PicoMite fits the bill more completely. As said above, one persistent issue is cable swapping. Image a reluctant teacher (primary) not with a CS background running a session in a class of 25 10yo's - and before they even begin, they need to get them all to unplug and replug cables -- it breaks the lesson before they begin. The PicoMite fixes that - tether to a PC and use the serial terminal and off you go. My foray into PicoMite VGA was prompted by a couple of schools at the end of last term wanting a more "stand alone" version, no PC involved. This led to VGA cable shuffle etc. A couple of schools then wanted the same but for teacher demos and it so happened that their interactive whiteboards (new) had composite in - so whilst the VGA and HDMI was currently occupied I thought it would be opportune to use CV. Now I know how complex it is, I don't think its worth it -- as I got to in the end, if I could nail Chromebook access via serial terminal then I think the PicoMite would be a "Microbit killer" (are you sensing my dislike there?) -- that or MMBasic for the Microbit......( ) (After all these are tethered and schools manage ok with them eventually)I totally appreciate the offer, but dont think its worth doing other than for the learning experience (on my behalf). ;-) Cheers N Edited 2022-08-15 23:43 by Nimue Entropy is not what it used to be |
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thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 3830 |
Hi Nimue, glad that you are up and running again. This would seem like the best to me too, coupled with a standard "library" to simplify the use of ANSI escape codes / block graphics. What have you tried on the Chromebook and how is it failing ? Best wishes, Tom Edited 2022-08-15 23:51 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5705 |
I've never even seen a Chromebook, but is this of interest? Serial Term Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 3830 |
[ASIDE] If you are using PicoMites via serial terminal in schools then I invite you to take a look at "kingdom_pico.bas" here: https://github.com/thwill1000/cmm2-kingdom. If you have the background I believe you to have then you may appreciate that after almost 40 years things have come full-circle. [/ASIDE] Best wishes, Tom Edited 2022-08-16 00:01 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Nimue Guru Joined: 06/08/2020 Location: United KingdomPosts: 367 |
I've tried all the freely available apps to use with Chromebooks. Enabled "Linux" but gave up on that as I cant get teachers to do that for the same reasons as above. Some don't allow connection via USB, some connect to the PicoMite, but the screen updating just doesent seem to work after the first screen - almost like its just sending the updates - the screen get unreadable almost instantly. Others, the function keys dont work. I imagine that they are not emulating VT100 correctly. I really need to pick this up again on Chromebook -- an activity for tonight I think. It seems to be the bidirectional nature of the terminal that is the "issue". I can use http://bipes.net.br/ide/ to connect Chromebook to the Pi using Micropython, but it just uses the serial connection to push the code to the Pi. Glad to be here in more ways than one. N Entropy is not what it used to be |
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