ZX Spectrum Composite Video Mod
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- Опубликовано: 2 авг 2024
- Support Noel's Retro Lab on Patreon: / noelsretrolab
We learn the ins and outs of video generation in the Sinclair ZX Spectrum. Then we make a simple mod to output composite video out and improve the image quality dramatically.
Chapters:
00:00 Intro
00:29 YUV color space.
01:44 Looking at the YUV signals on the oscilloscope.
13:20 Making the composite video mod.
17:45 Comparing the results to RF and RGB.
Resources:
Spectrum Hardware Manual by Adrian Dickens. Melbourne House 1983
The ZX Spectrum ULA by Chris Smith 2010
Connect with Noel's Retro Lab:
Facebook ➤ / noelsretrolab
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Twitter ➤ / noel_retrolab 
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This is slowly but surely becoming one of my absolute favorite RUclips-channels. Really good stuff, well made videos, and interesting topics. :D Thanks!
Thank you! That means a lot!
Thanks for taking the time to go through the video signal explanation rather than just solder a capacitor and call it a day! Really informative video!!
You're welcome! I always try to get to the why of things, so I'm glad you're enjoying it.
Back in the far past I had a friend mod my Spectrum 48k to have both RF and composite in the original output connector. Both the RF and composite shoved no artifacts when I used them on different monitors and TV’s.
As far as I recall the mod was done using a single bipolar transistor as emitter follower and a few passives, to inject the composite signal “onto” the RF output. I do recall this was a theoretical stretch as the transistor barely supported the required bandwidth, but the monitors I used managed to display a fine picture with no distortion. The addition of the composite signal did not degrade the picture quality on connected TV’s. I assume a high pass or to in the RF antenna signal path attenuated the composite signal sufficiently as the integrated bandwidth limit / low pass filter in the monitors composite input cut the RF away.
Finally someone that has proper tools. Nice presentation thanks for sharing.
Many thanks, with your video, my old Spectrum start again ;)
This was very useful! Thanks for making awesome videos!
Having a luma (Y) and chroma (C) signals available on the board I guess that there is a possibility of adding an s-video jack for even greater video quality output.
Even better! As far as I know, YUV is the same as YPbPr but with different names. That means it may be possible to add component outputs!
Excellent video and I really enjoyed the analysis of the signal and explaining what was going on behind the scenes.
Thank you for explaining everything in such clear and detailed way!!! This is by far the best ZX Spectrum video I have watched so far. 18:23 Where can we get a t-shirt like that?
Another great teaching video, thanks!
My pleasure!
I learned a lot about how colour video is generated, thanks!
I'm glad. You're welcome!
I got my ZX Spectrum later in January of this year, the composite MOD was something I was more concerned about trying do given my lack of experience in soldering. After watching this video, it helped me out on how to solder a capacitor in the RF lead. I failed to solder it in the first time because I put the capacitor backwards but I managed to solder it in correctly the second time! If I ever introduced someone to the ZX Spectrum and that they have to do the composite MOD to it, I would recommend this video! Thanks for showing us how this composite MOD is done, very helpful video! :)
Thanks dude.
I remember going from RF to composite, the picture quality improves not only in its sharpness but also in the fact you don't need to tune the TV in.
Definitely! And even tuning in to RF is not even possible in a lot of modern TVs, but composite video usually is fine.
That’s nuts dood!
Just re-watched this to do the mod to my Spectrum 48K+. I can confirm that it works perfectly. The only difference I made was to put the capacitor inside the RF box, just for that more factory look :)
Nice! I take it you emptied the RF box then? Or did you manage to fit it inside? I've seen people who empty the RF box, but I'm always hesitant just in case one day I want it back (why I would want RF, I have no idea though 😃).
@@NoelsRetroLab I managed to squeeze it inside. It was actually quite easy. Seemed plenty of space. Just got it local yesterday. I have old cars, I don't like to modify them so they can't be reversed!
Brilliant
You make digital electronics easy, I wish I had teachers like you
Haces facil la electronica ojala hubiese tenido profes que explicaran como tu lo hacea
Thanks! :-)
Rewatching this video as I really need to composite mod my Speccy +2A - super helpful video as always - YUV still makes my brain hurt :)
My ZX Spectrum clones had video output as RGB and Sync out (4 different pins plus GND), so this could be connected to a TV via RGB input like SCART, if I had one back then. What I actually had was a vacuum lamp (tube? what I mean is not a CRT but what was instead of transistors there inside) based monochrome TV and I had to make a "brightness and sync" out of that 4 outputs. So I used one diode for sync and 3 different resistors for mixing RGB signals in special proportions into one signal that TV accepted as brightness with sync (Y-channel actually).
When you held up the back of the ZX spectrum to the light, for example 16:52 , it looks like the case is actually scored for another jack to be installed.
There are RF modulators that have the jack on the other side: NTSC modulators! Likely the idea was to make a case mold for both these regulators. But I do have a NTSC Spectrum that still has the jack on the same place! But my (NTSC) Timex Sinclair 1000 and 1500 do have a modular with the hole on the other side. Then there are these Argentinian Czerweny CZ Spectrum and CZ Spectrum Plus machines that I have which have two outputs on the same modulator! You've guessed it: one is RF and the other is composite video!
I'm not that interested in "The Speccy" but there's lots of useful info in here for poking into lots of old analogue video devices... thanks.
I did'nt put a cap on my comp conv. I'll do them tomorrow. Thanks for the tip. Loving your channel BTW. Subbed after watching 1st vid.
Thanks for all the positive comments. Glad you're enjoying it!
exellent explanation!!!!!!!! (i would expect a video for black burst , 100% modulation with a vector oscilloscope like we have for servicing camera equipment and beta vcr , dvc pro 50 !!!!!)
The expansion port (lower pin 15) also has the composite signal, the same one that goes to the rf modulator, so you can get composite signal without modding anything. I don't know why people aren't using this, instead of changing original hardware.
Iv done the mod on my old specky that never seemed to tune in correctly. As soon as I first plugged it in I was amazed at the jump in quality
There's a lot of prejudice against the RF connection, but it can have the same quality as the composite-video. All you have to do is to use the correct 50 Ohms cable for the connection.
The reason why RF usually looks horrible is because people use composite-video cables with it.
Surprisingly ... Composite looks better on my eyes than RGB, firstly the lines are sharper in the character, and the colours are brigth. Also I could tell the colour of the text down screen.... white/gray (on Composite) vs Yellow (on RGB).
I agree they look really good both of them. Composite has some edge issues (look around the lives of Manic Miner). On screen RGB looked sharper for sure (maybe too sharp?). I did notice the color difference after the fact. I wonder if I don't have the color pods properly calibrated because this is an Issue 2. I didn't notice anything else being clearly the wrong color though.
Hello, very nice video. I did this with my zx spectrum because i wanted to improve my video. RF was working. I used a 100μf capacitor 16volt because that was available. Strange thing is that i see a very nice picture but after a few minuts (sometimes seconds) he starts to disturb and eventually the image is completely gone. I also tried it without a capacitor. Same problem. Could it have to do with PAL? any idea?
Do you get color flickering (red purple) on composite modded Spectrum, connected to CRT?
I seemed to remember doing the mod on mine back in the 80's however I'm pretty sure I didn't use a cap. Now my monitor was a green screen so not sure what impact that had on color but it did work fine. What is the cap doing?
It blocks any DC current from being exchanged between the spectrum and television.
I'm a newcomer to ZX Spectrum and this video was very helpful for me. Many thanks. I will give something like this a try with my Sinclair ZX Spectrum+ 128K when I get tired of RF video. IMO this mod would be even better if there was a way of switching between RF video and composite video (or VGA), perhaps by using an external downconverter/receiver... Also, maybe there is a better mod for Sinclair 128K with its RGB output (I really don't want to use SCART cables though...)?
Hey. Loving this channel. I hope I can do something similar quality wise.
Just a question about the mod; how do you then get the sound to the TV? that was sent over the RF before was it not?
I did have the video on whilst I was doing other things, so I may have missed it if you covered it already.
Thanks! Well, that's one of the things that makes this mod so easy: There was no sound sent over to the TV before! The Speccy uses the internal speaker, so that part doesn't change.
@@NoelsRetroLab ah! that makes things much easier haha. I've never spent much time with a speccy. I prefer my Amstrads.
Thanks again!
Loving the channel Noel. I did this mod to mine a while ago but your video made me realise I needed to make a sticker to remind me it's now composite. Hooking up to a real TV isn't practical for me, any chance you could do a video about how to hook our retro machines up to modern monitors? Info I've found online makes it sound very hit and miss if the boxes you can buy will actually work with my 48K, 2600 or Amiga.
Thanks! That's actually a great idea for a video because I've had multiple people ask me about things like that. It might make for a nice change of pace.
I recently built a Spectrum 48k from a kit. All works okay with composite mod, but on one TV it doesn’t display colour (monochrome only) and the picture is slightly shifted down the screen. Other TV is fine. I’ve been comparing two signals to see what difference may exist. There is some variability in DC offset. Could this account for behaviour with the two TVs?
Do you know how to upgrade the memory of a zx81 from 1k to 64k ?
Where can I get the program? Thks
Is polarity of the capacitor important? Is the reason negative terminal connected to the video jack that in case there is direct current flow into the jack the capacitor won't blow up?
Good question. I don't remember. It could be that the video signal has a positive offset in the Spectrum, but I'm not 100% sure.
Noel I have a TV with Y/Pb/Pr input,so if i drive the signals from 15th ,16th and 17th ULA's legs to that port it gona give me a working screen?
Good question! I don't know the answer for sure, but there are a couple of things: First of all, I've read in a few places that component signals are not all the same and there are some variations. So maybe the Speccy one isn't the same as the one your TV expects. I'd love to find out more about it. The other problem is that, almost for sure, the signal out of the Spectrum isn't going to be strong enough to drive a TV (just a guess), so at the very least it will have to be bufferered with some transistors. If you do end up trying it, let me know!
@@NoelsRetroLab as a matter of act i tryed this today and all of the times my TV it was ''saying'' NO SIGNAL DETECT. So i think you are right that the signal is not strong enough. Because i don't have an oscilloscope it will be helpfull to be meassured the output signal after the ULA and after the ic14 and compare it. Next week i will try the transistor solution
Best wishes from Greece
@@sakisyoutuber It's not just a matter of measuring the voltage at the ULA, but finding out the resistance expected at the component inputs. That should be standard and easily available everywhere. One thing I did was to feed similar outputs (from a TMS9929A) through an OSSC and it managed to interpret the signal correctly and put out an image. But I know you want to do it directly, so you'll probably need to use those transistors to boost it.
where's the link to the basic program?
I'm totally useless at modding but I managed to do the composite mod on my 6 Spectrums
Noel, what scope are you using?
Keep up the good work!
Hi! It's a Hantek DSO5202P. Really liking it so far!
I know this is old but what did you do about the sound through the TV?
Nothing. The ZX Spectrum has a built-in speaker so I left that that way. Makes the composite mod almost trivial.
Where do I get that TShirt man ?
Would this mod work for the 128k +2a?
The short answer is no. You would have to route the signal directly from TEA2000 and basically replicate the circuit from 128K/+2. But the good news is that it's only three diodes, a transistor and a couple of resistors so it can be done. I created a similar mod on a small breadboard and placed it inside the modulator case. Some details can be also found here: spectrumforeveryone.com/technical/modifying-a-2a-or-3-to-output-composite-on-the-rgb-port/
Noel; I came with one question... Have you tried the different mods for composite video? with/without transistor? Does it make a chance that could be worthier than without to implement? Thanks
Good question. I've seen all those mods but I haven't tested them. You don't technically need anything, you could connect the composite vide out directly to the socket. The capacitor is just there to decouple the electronics in the Spectrum from whatever you plug in, but it doesn't actually affect the signal. The transistor supposedly boosts the signal, but I haven't seen the need for it in the TVs I've tried.
@@NoelsRetroLab you are right. There is not a clear reason to implement a transistor unless the cable were too long and high impedance or TV receptor in RCA connection with higher resistance than usual.
@@NoelsRetroLab Stupid question - doesn't the capacitor remove the DC part of the signal though? Couldn't this be an issue on some displays?
@@baze3SC Yes, the capacitor removes the DC part, but that's what you want anyway. That way the display could be biased in a different way and all it gets are the changes in voltage. Chances are the TV has its own capacitor at the input anyway, so this is just to be safe.
I find your explanation for the 64 microseconds of the Y, U & V signals awkward and confusing. You are right its the PAL video standard signal with 625 lines at 50Hz that is interlaced which is being used. So every second there should be 50 x 625 = 31,250 lines drawn on the screen (1/31,250 = 32 microseconds), but since it is INTERLACED, there are only half the number of lines that are drawn on the screen per frame, that is 15,625 lines which yields to 64 microseconds (1/15,625 = 64). Interlace is a technique that is used to draw only half the number lines for every frame due to electronic constraints that did not allow TV sets to do full scanning per frame or what is known as progressive scan nowadays. These lines are drawn/displayed in the following order, one frame does the ODD numbered scan lines and the other frame does the EVEN numbered scan lines. The peaks that are displayed in the oscilloscope at every 64 microseconds are for the retrace of the scan line.
Also the Y, U & V signals are present on the edge connector as well the composite signal at pin 15 underside.
Is it possible to remove the entire RF from the 'box'? I ask because I tried it and somehow I have a very terrible signal now. Very unstable. I also removed a piece of the metal casing so I can place a reset switch at that place coming out on the back. But I'm confused why suddenly the picture is so unstable and terrible. Checked for shorts etc but all seems fine. I thought the inner component weren't used anymore.
Yes, definitely! That's actually a great thing to do if you know you aren't going to use the RF out anymore. I love the idea of the switch there. The image quality shouldn't be affected. Make sure it's still getting ground (it was getting it from the box itself) and the composite video from the input.
@@NoelsRetroLab mm.. interesting...I makes ground quite well. There is an image (most of the time) but it is going all over the place, flickering, noise etc. Maybe I messed up something else on the board then.
I have an extension module also with a composite exit (directly connected to the edge-connector) and interestingly that shows the same garbage so I suspect the signal internally is already broken. Guess I'll have to dig up the schematics and see if I can understand enough to troubleshoot this.
@@dannybloe Check that you're getting 11-12V at pin 12 of the LM1889. You may have accidentally shorted something that caused TR4 to be damaged? That's also what you get when you have really bad electrolytic capactors (because they don't hold the 12V line steady).
@@NoelsRetroLab It's a steady 11.8V so that looks okay not? 12V pin on lower membanks is 12.xxV
would 100mf 16v cap work?
mF as in micro F (i.e. uF)? Sure. 16V is fine for that.
Is there a way to rgb mod it?
It's possible, but I've never done it. It should be pretty similar to what I did with the TI99 RGB out. But I really don't think there's a lot to be gained from it. Composite already looks pretty great.
Noel's Retro Lab I wonder how much better s video would be if possible
That the brighter colour has lower voltage seems counter intuitive. What have I missed?
Just that the signal is reversed. As a matter of fact, I've seen it listed as Y' in some of the technical manuals. It probably refers to that. Again, they probably did that to make the combining of the signals later easier.
@@NoelsRetroLab Doesn't Y' mean Luma, ie. gamma corrected Luminance (Y)?
Can we do this trick with the 48K model (non rubber keys)?
Absolutely! They're the exact same computer (different issues but it doesn't matter most of the time) and just different case/keyboard.
@@NoelsRetroLab Thanks!! :)
Hey again! :)
The negative of the capacitor is the short leg, right? And the short leg of the capacitor goes on the inside of the RF modulator box, right?
@@Quazatronn Correct! Good luck!
I've done it successfully! :)
Lol. I'm so happy I did it alone! Ah ah ah.
However, I still have a strange ghost over the general image. How can I take a pic abd show you? Are you on Facebook?
Capacitor type ?
Just about any electrolytic one is fine. I forget what I picked, probably 50 or 100 uF.
nice video but what about sound?
The ZX Spectrum has a built-in sound speaker, so there's no need for that. If you really wanted to send sound over the composite signal it would be significantly more work than just adding a capacitor though 😃
@@NoelsRetroLab ok thanks for your reply, I renembered you listened you spectrum in the tv speakers, maybe not
@@viditibi Good memory. That was probably the 128K or higher models (+2 and +3). All the 48K come with the built-in speaker so there's no need to do anything about sound there.
and a lot of ztx tranzistors why they dont use a bc547 or similar
Honestly, I don't know, but I know you can't use those to replace them. I think those transistors need pretty specific characteristics to get the kind of oscillations they need to generate those voltages. And they also need a fair bit of power since just about all the current for the computer goes through there.
Hey Noel.
I'm having troubles connecting a zx81 to an old and modern TV. Can you help me?
Thanks in advance.
Hi Paulo, I'm afraid I haven't worked on a ZX81 in a long while (I need to remedy that), but I seem to recall you can do a similar composite mod than the ZX Spectrum. If you can, I recommend you join the Discord server (discord.gg/ETcCh6J). There you can post pictures and I'm sure you'll find people who can help you. Good luck!
What if we don't modify it? Is it possible to connect it via a tv cable? Any specific better tv channel number to use?
RGB Mod?
It would be much more involved because the Spectrum produces YUV, so it would have to be converted to RGB, which is more complicated than putting a capacitor :-) And you'd also need to add a connector with at least 4-5 pins. Interestingly, next week I'll release a video showing how to do that with the TI99/4A, which has somewhat similar video signals. So in theory it would be possible to do something like that.
@@NoelsRetroLab Thanks for the reply. Looking forward to that one.
5 pins minimum including ground. Maybe removing the modulator and using something like a PS/2 connector mounted behind the hole it leaves?
@@dropdatabase8224 I have a story about that kind of thing. A while ago now, I made a USB extension system. It was two boxes, one (host box) had a boost converter in it that boosted the USB 5V up to 48V. In the other box (device box) was a buck converter that brought it back down to 5V. This allowed long lengths of cable because there was effectively no voltage drop, just used more current. I made the mistake of using RJ45 to link the boxes together. So what happened? Someone mistook the host box for a USB to Ethernet adapter. You can imagine how that turned out. That mod would open the door for some sap to plug a keyboard into it.
@@dropdatabase8224 Exactly. It would have to be something like that. It could be fun, but honestly, the image quality you get with composite out on the ZX Spectrum is so good, that it's hard to justify going the extra mile. It would also involve custom cables from that to SCART. But it would make for a fun project to be sure 😃
Can't we use the YUV and make a component output? Composite still sucks. Also you could compare the composite and RGB on and LCD monitor not a CRT.
It may be possible, but things get more complicated (need 3 output jacks, might have to boost each signal). And in the end, my go-to TV doesn't accept YUV, so I couldn't really use it.
As for composite, a good composite signal is really good. Not RGB good, but it can get pretty close (at those resolutions). LCD displays are usually horrible for retro devices, so I don't use those for anything other than testing on my workbench.
😂😂 cheapest and laziest composite soultion.
Hi Noel,, thanks for another excellent video! An obvious question: would picking up YUV signals work in making a real (albeit YUV) component port?? That would make things soooooo much easier...