32x64 LED monitor

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32x64 LED monitor

Postby dominicbeesley » Thu Nov 05, 2015 12:54 am

I bought a few 16x32 LED matrix boards off ebay a while back and have been messing around with them. I've finally got round to having a go with them. Here's my initial results.

The camera is a tad over-exposed, in real life it is quite bright and clear but could do with a diffuser as in normal room lighting it looks like a lot of very bright dots close up but resolves to a nice picture a distance away.

Problems:
- one of the 4 boards is from a different manufacturer and has a different (better) white balance but worse purity
- the scan code (Arduino due, SAM3x ARM processor at 84MHz) is taking up 1/3 of the processor time (it has to clock out 2048 pixels 400 times a second for PWM)
- no colour of frame sync yet
- its a bit fussy about levels and syncs

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Re: 32x64 LED monitor

Postby Harry Dalek » Thu Nov 05, 2015 8:53 pm

Impressive work Dom i have a few of those led blocks wishful thinking to make a low def scope one day with them but great to see good use of them making this nbtv...i wasn't sure what the gap effect between the leds would be like but its not to bad .
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Re: 32x64 LED monitor

Postby dominicbeesley » Thu Nov 05, 2015 10:40 pm

Thanks Harry,

The gap effect is actually worse in real life, the camera flatters it a little! For instance a picture of a human face just look like a load of random dots unless you go a long way away from it, or look at it askance or force your eyes to defocus. A sheet of paper over the top helps and is even better if held a couple of millimetres above the surface to defocus the spots. I suspect that this is some psycho-optical effect where the eye / brain are trying to distinguish and process the individual dots rather than the picture.

I've got some sheets of 3mm frosted and smoked acrylic on order and will try adding those to see if they make an improvement without removing too much light. In direct sunlight the display is still quite bright and with a bit of tweaking I should be able to rearrange my PWM code timings to get another 50% brightness.

The real challenge is going to be working out how to get the colour processing working in the processor time available...

D
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Re: 32x64 LED monitor

Postby Harry Dalek » Fri Nov 06, 2015 2:21 pm

So your going to make it a colour monitor ,think that would be a first for some thing like this ? be the largest size wise i have seen for b/w as is .
Good luck with the rest !
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Re: 32x64 LED monitor

Postby AncientBrit » Fri Nov 06, 2015 7:20 pm

Hi Dom,

Impressive work indeed. Well done.
Are you intending to write it up for the Newsletter?

I made a diffuser for a light box using polystyrene sheet from B & Q and 'graining' it in one direction by using fine wet and dry abrasive cloth.

Cheers,

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Re: 32x64 LED monitor

Postby dominicbeesley » Fri Nov 06, 2015 8:18 pm

Thanks Graham,

I may write it up for the newsletter though I'm a bit wary of that having spent a long time writing up an article (along with Klaas) on apertures that still hasn't been published in the newsletter four years later, I'm not sure why. I'm glad you prompted me to look for it in searching my emails I found the latest newsletter. I'd not realised that they were being sent by email - are we no longer getting them in print?

I may try your tip and "frost" the reverse side of the smoked Perspex as an alternative to the sandwich idea. I'm hoping that the frosted sheet will diffuse the pixels and the smoked sheet will provide "blacker" background - the leds in the matrix are white so the blacks aren't very deep in daylight.

If this gets beyond the point of a work-displacement activity on my desk I will build this onto a back-board and make the spacing of the sheets adjustable for subjective experiments on the effectiveness of blurring on the ability to resolve pictures.

The replacement for the differently coloured panel arrived yesterday and I'm hoping the acrylic sheets will arrive today. If they do I'll post up some more pictures.

Cheers,

D
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Re: 32x64 LED monitor

Postby AncientBrit » Fri Nov 06, 2015 9:07 pm

Hi Dom,

Newsletters are still being sent in print, although members may elect to receive them electronically,

Cheers,

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Re: 32x64 LED monitor

Postby dominicbeesley » Sat Nov 07, 2015 1:39 am

Thanks Graham, I'm missing the last paper issue but that may have got mislaid with all the junk mail we get

This afternoon the perspex sheets came. Also a replacement panel for the "odd one out". Unfortunately this new one is even more blue than the one it replaced even though it is from the same manufacturer. I'm guessing this is from a different batch or they are sold as matched sets I'll have to discuss it with the seller.

The smoked glass makes the picture appreciably dimmer but with better contrast as it hides the white led dies more than it attenuates the whites. Not really shown up by the camera.

With the frosted sheet flat up on the displays behind the smoked sheet the dots are softened but not blurred. Testcards look good but I still find faces hard to recognize.

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syliva - no spacing
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With the frosted sheet about 2mm spaced I find best

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sylvia - 2mm spacing
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testcard - 2mm spacing
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4mm and its just a bit too blurry!

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sylvia - 4mm spacing
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I've got a first go at colour working. This needs hand synchronising (i.e. I have to keep stopping and starting the input until it settles with all the right colours in the right places) but at least I've proved that it is possible with this processor to get all the processing done in time!

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bars
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(153.69 KiB) Not downloaded yet
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Re: 32x64 LED monitor

Postby dominicbeesley » Tue Nov 17, 2015 3:34 am

I've got a little further. I've now switched from using FIR filters to IIR filters. This took a lot of working out as I didn't fully understand how to convert the numbers in my filter calculation program to numbers that the DSP functions on the microcontroller could understand. After a LOT of reading it turned out that I'd made a simple mistake in my pen and paper workings out and had a minus sign missing!

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The IIR filters behave more like "real world" L/C/R filters and I think give a far more pleasant result with less ringing. I will be changing the NBSC software to use these at some point.

The problem now is that I can't quite get the Arduino Due to do all the calculations and all the pushing and shoving of signals fast enough so it is only showing every other frame.

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(199.09 KiB) Not downloaded yet


So next up I'm going to try and port it all to a "Teensy" which is a tiny board with a Freescale Cortex M4 chip which can be clocked at 120MHz and has more built-in DSP capabilities which should be good for getting everything into shape and adding some brightness/hue/saturation/constrast/etc menus.

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(214.84 KiB) Not downloaded yet


Still to do:
- port to Teensy
- get 4 matching panels!
- frame lock
- automatic colour control
- power supply - this thing is a bit thirsty
- bluetooth to deliver the signal wirelessly

D
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Re: 32x64 LED monitor

Postby gary » Mon Nov 23, 2015 5:54 pm

If you are using IIR filters won't you just be exchanging the ringing for frequency dependent phase shifts? I would have thought the latter would be worse. I say that without actually knowing what you are using the filters for...
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Re: 32x64 LED monitor

Postby dominicbeesley » Mon Nov 23, 2015 9:23 pm

Yes, that is true but they actually look better as the phase/delay errors are outside the pass bands and the results look a lot nicer whereas with the FIR's I'd used in the past they tended to cause ringing in the pass-band unless they were quite long filters and that requires too much memory on this uController. The filters are used for separating the chrominance and luminance signals - a band pass and band stop and a low pass filter for filtering the decoded colours. I'd say I prefer the IIRs as they seem to use a lot less processing power to do the same job and the results look better subjectively.

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Re: 32x64 LED monitor

Postby gary » Mon Nov 23, 2015 9:48 pm

Good oh - empirical evidence ALWAYS beats theory! Nice one.
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Re: 32x64 LED monitor

Postby dominicbeesley » Mon Nov 23, 2015 10:20 pm

Thankyou Gary, I was certainly surprised how well they worked. I'd never got IIRs to work before. I now know that I was just doing the simple maths of converting coefficients from the old DOS DISPRO25 design software to numbers ready to plug into DSP libraries incorrectly. No matter how many times I work it out on paper I come up with the same answer but randomly swapping two of the signs in the equations makes it all work!

At the moment I'm mainly faffing about with the PLLs that control the line sync and the colour burst sub carrier. I took a leaf out of Karen O's book and rather than dropping / doubling the odd sample to make the pixels fit the incoming signal I vary the sample rate. This works really well when I have detailed control over the ADC and I get nice line sync even on noisy signals. It is causing the colours to "wander" a little though as the colour PLL has to cope with noise, the signal and the effects of the line-pll causing more phase errors. Varying the sample rate certainly makes the decoding code simpler!

The main problem I now have is that there is quite a lot of noise on the ADC input - I need to investigate where that is coming from, I suspect it is a mixture of the microcontroller, the USB port and the sound card that is feeding the thing.

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I've also put up a you tube of the thing showing a testcard - about 2/3 of the way through I stop and restart the signal to show the line and colour PLLs (slowly) locking: https://youtu.be/n3XGlI-I054

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Re: 32x64 LED monitor

Postby dominicbeesley » Sat Nov 28, 2015 3:18 am

I may be getting carried away with this...instead of ordering 4 replacement panels from the same batch, I now have 6 32x32 panels for a 96x64 televisor...

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Still lots of work to do on:
- better filters in recording software
- automatic black level adjustment
- automatic colour control
- 64 line mode

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Re: 32x64 LED monitor

Postby dominicbeesley » Thu Dec 10, 2015 2:43 am

I've got a little bit further. The line hold is now very robust, frame syncs are less upset by noise and the black level is clamped on syncs (after some filtering) which means that even signals that have had their low frequencies cur come through relatively unscathed. The new larger acrylic sheets give a nicer picture too.

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I'm hoping to get it mounted up on the wall soon so I can have my desk back...
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