Simple Video Mod for 210ULA

Discussions about Sinclair ZX80 and ZX81 Hardware
zx81user
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Simple Video Mod for 210ULA

Post by zx81user » Fri Apr 26, 2013 4:28 am

I received another TS1000 but I couldn't get my TV card to tune in to the modulator RF channel. So I thought I would take the board out of the modulator and feed the video signal directly into the TV. Although this worked, the video was very dark and I had to increase the brightness and contrast to almost maximum levels. By analyzing the video signal I saw that the back porch was there but the sync level was way below normal CVBS levels. I thought that I could probably rectify that with a few series diodes, so I took some ordinary 1N4148 diodes, soldered them in series and voila, a beautiful crisp picture! It's a very simple mod that works well for a 210 ULA with back porch but not for ULA that hasn't got this back porch. The diodes also protect the ULA from ESD, so it's a quick simple mod.

Michel
series diodes inside modified RF modulator
series diodes inside modified RF modulator
VideoDiodes1.jpg (200.1 KiB) Viewed 5710 times
Video signal without and with series diodes
Video signal without and with series diodes
VideoDiodes2.jpg (114.88 KiB) Viewed 5710 times

sirmorris
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Re: Simple Video Mod for 210ULA

Post by sirmorris » Fri Apr 26, 2013 9:28 am

Nice!

gozzo
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Re: Simple Video Mod for 210ULA

Post by gozzo » Fri Apr 26, 2013 11:04 am

Even simpler if you leave the modulators innards there, no need to remove them, the modulators output circuit connects to the phono socket through a very small value capacitor, it won't interfere with the composite signal, and you then have a ZX81 which will still work with RF out if you need to ! (you will need a capacitively isolated lead, though or you will short out the composite signal if using RF connection )

However I don't really like the idea of not buffering the video, and isolating it with a capacitor, but if it seems to work ok and the ULA isn't being overloaded, then its great !

gozzo
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Re: Simple Video Mod for 210ULA

Post by gozzo » Fri Apr 26, 2013 11:11 am

Not using a buffer could be the reason why you had a dark picture , the video input of your TV was loading the ULA's output too much, if it was buffered with an emitter follower OR a video op-amp/buffer I've come across, the AD817AN , (not yet tried it, though), it would probably be ok.

zx81user
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Re: Simple Video Mod for 210ULA

Post by zx81user » Fri Apr 26, 2013 11:41 am

gozzo wrote:Even simpler if you leave the modulators innards there, no need to remove them. However I don't really like the idea of not buffering the video, and isolating it with a capacitor, but if it seems to work ok and the ULA isn't being overloaded, then its great !
I didn't think about that as I had first taken out the board and then found out I had a dark picture. In my modulator that trick wouldn't have worked because the RF output also has a small inductor to ground that acts as a high pass filter, so it would have shorted out the video signal.

Buffering the video signal is not really required, the modulator input impedance is also quite low, not any different from my diode circuit.

Michel

zx81user
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Re: Simple Video Mod for 210ULA

Post by zx81user » Fri Apr 26, 2013 12:15 pm

gozzo wrote:Not using a buffer could be the reason why you had a dark picture , the video input of your TV was loading the ULA's output too much, if it was buffered with an emitter follower OR a video op-amp/buffer I've come across, the AD817AN , (not yet tried it, though), it would probably be ok.
Not so, the reason for the dark picture is that the back porch level was much too high compared to the white level. As you can see from the 2 scope screens, the top one is actually higher in amplitude (about 1Vpp) but much darker than the signal in the bottom picture which is only about 400mVpp. It is not the amplitude that matters, it is the ratio between black and white that matters and this is defined by the (back porch voltage - sync signal voltage). If you take the top signal (without diodes) and would amplify that, the picture will still be just as dark, the only way to get it brighter is by lowering the back porch level. Both these signals have been measured with a TV connected to it b.t.w., so the load resistance is in both cases the same.

Michel

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PokeMon
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Re: Simple Video Mod for 210ULA

Post by PokeMon » Fri Apr 26, 2013 6:29 pm

Can not see in your scope pictures very good but I think the same reason gozzo meant here.
The white level voltage from ULA breaks down due to a low impedance connected.
This moves the adapted signal in a way with not much white level.
The ULA normally puts out a range of up to 5V which is modulated correctly inside the modulator.

In my eyes you solved a problem with your diodes, which you self created by connecting low impedance to ULA output.
The CVBS / composite video impedance is technically defined with 75R to reduce noise through connecting cables.
75R is a very low resistor and tries to get about 50mA (peak) out of the ULA.
Anyway I would advise to use a discoupling transistor (simple emitter follower) because there is a risk that over the time the ULA will be damaged internally.
It may work now but who knows how long ?

Or you wait till I ship you the ZX81CCB boards we talked about. ;)

zx81user
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Re: Simple Video Mod for 210ULA

Post by zx81user » Fri Apr 26, 2013 11:51 pm

PokeMon wrote:Can not see in your scope pictures very good but I think the same reason gozzo meant here.
The white level voltage from ULA breaks down due to a low impedance connected.
This moves the adapted signal in a way with not much white level.
The ULA normally puts out a range of up to 5V which is modulated correctly inside the modulator.

In my eyes you solved a problem with your diodes, which you self created by connecting low impedance to ULA output.
The CVBS / composite video impedance is technically defined with 75R to reduce noise through connecting cables.
75R is a very low resistor and tries to get about 50mA (peak) out of the ULA.
Anyway I would advise to use a discoupling transistor (simple emitter follower) because there is a risk that over the time the ULA will be damaged internally.
It may work now but who knows how long ?

Or you wait till I ship you the ZX81CCB boards we talked about. ;)
Please see attached picture that might explain it a bit better. The sync level is defined as -40IRE and pure white as 100IRE. So the peak-peak voltage of CVBS signal is 140IRE in total. The signal without diodes has a 40IRE level of 650mV which means that the TV will regard a 2.25V as a pure white level. However, the signal is only 1.0V, so that is the reason it is very dark, that makes perfect sense because it is less than half of what pure white should be. With the diodes however, the 40IRE level is 100mV and the TV therefore assumes pure white is 350mV, which happens to be exactly that! Perfect!
VideoDiodes3.jpg
VideoDiodes3.jpg (108.37 KiB) Viewed 5663 times
There is no need to buffer this signal, the current drawn from the ULA is not 50mA, it is 350mV/75 ohm = 4.67mA

I still need the ZX81CCB board for the ZX81's that don't have the back porch ;-)

Michel

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PokeMon
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Re: Simple Video Mod for 210ULA

Post by PokeMon » Sat Apr 27, 2013 2:05 am

Sorry but I have to correct you. ;)
Your TV may work with this signal due to amplifying it internally but it doesn't meet the standard.
What is correct is the "proportion" between sync, black level and white level. (0% - 30% - 100%).

One "IRE" is 7.2 mV.
Sync level is about -40 IRE = -288mV
Black level is about 7.5 IRE = 54mV
White level is about 100 IRE = 720mV

So the correct level is about 1Vpp.
The unit of measure for the amplitude is in terms of an IRE unit. IRE is an arbitrary unit where 140 IRE = 1Vp-p. From the figure, you can see that the voltage during the active video portion would yield a bright-white picture for this horizontal scan line, whereas the horizontal blanking portion would be displayed as black and therefore not seen on the screen. Please refer back to Figure 1 for a pictorial explanation. Some video systems (NTSC only) use something called "setup," which places reference black at a point equal to 7.5 IRE or about 54mV above the blanking level.
http://www.maximintegrated.com/app-note ... mvp/id/734


There is no need to buffer this signal, the current drawn from the ULA is not 50mA, it is 350mV/75 ohm = 4.67mA
It is just because the ULA can not deliver more current on this output and due to reducing voltage with the diodes. Anyway the ULA is getting hot and 5mA is a kind of stress for it. Due it's rarity status of the ULA I wouldn't recommend doing this. It can significantly reduce the lifetime of this logic array. I had some burned out ULAs which work in general and give even sync level at the output but no more white level. They seem to internally burned out.

But there is another reason why I wouldn't recommend this solution. It's about the noise level. Main reason for developing the ZX81CCB was to reduce the noise which come already through the ULA at #16 output. I found noise in the video signal of about 80mV which is about 10% of white level. You can see this noise in the white background on a TV. It does help a little bit to block the power supply at the ULA with an additional capacitor but has anyway about 40mV noise level. This does come internally due to architecture of the logic array and the power consumption.

What is done with ZX81CCB is following: The video signal is digitized in sync signal and pixel signal and recreated with a clean double blocked power supply with nearly no noise at all. This can not be reduced with the simple transistor solution (emitter follower) because the noise follows same way as signal. A voltage devider will keep the same proportion of noise. A voltage reducer with the diodes is problematic in this way because it does reduce the voltage by full keeping the noise level. I guess the noise level higher due to dropping it down while taking more current through pin 16.


So you maybe take a look at my pictures in this thread (german forum):
http://forum.tlienhard.com/phpBB3/viewt ... =572#p3906

gozzo
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Re: Simple Video Mod for 210ULA

Post by gozzo » Sat Apr 27, 2013 2:39 am

ZX81 user - "Not so, the reason for the dark picture is that the back porch level was much too high compared to the white level" ... yes,because loading the ULA output directly is most likely cramping the white level - the higher the voltage, the more it is loaded by the TV's input !(if you was to check the waveform on an oscilloscope with the modulator still connected, it would probably look more 'correct', but many computers do not generate a strictly correct video signal)- The modulators input circuitry is not what you might think, it is not driving a load referenced to ground, such as a base input circuit, but is actually the emitter circuit of a pair of push-pull transistors, so the ULA is sinking current to ground more than feeding it out.

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