Composite video issue - Page 1
Forum section: electronics / repair

author: mayor
author_rank: Regular Contributor
timestamp: September 05, 2019, 01:37:29 am
content: Hi,I am bringing back my childhood ColecoVision to life. It's pretty crusty. After replacing the 7.15909 MHz crystal (unknown load capacitance!), a 7404 and a broken ceramic disc cap, I managed to get composite video out (hijacked from the RF modulator input). Only thing is, there's a lot of horizontal bleed. Not too sure what causes that, exactly. Amplitude, timing? The composite signal is about 700mV p/p, with DC of around 5V.Any help appreciated! Pic attached.

author: Renate
author_rank: Super Contributor
timestamp: September 05, 2019, 02:59:12 am
content: You're stealing the video from a high impedance source.It's not designed to drive a 75 ohm input.Also, whatever you are plugging it into may not like 5V right in its ear.(It also probably doesn't need the 4.5 MHz sound carrier that you are probably feeding it.)You need a buffer.

author: mayor
author_rank: Regular Contributor
timestamp: September 05, 2019, 08:30:53 am
content: I am using a transistor to drive the input, so I don't think current is the issue. The DC level is as intended by the CV designers. I should have added this info in my original post.

author: PKTKS
author_rank: Super Contributor
timestamp: September 05, 2019, 01:36:15 pm
content: I would also guess you have an unmatched impedance causingghosts on the video signal.at least distortion is low enough to still have SYNC finePaul

author: vk6zgo
author_rank: Super Contributor
timestamp: September 05, 2019, 02:51:23 pm
content: Quote from: mayor on September 05, 2019, 08:30:53 amThe actual video waveform including syncs should be 1 volt peak to peak for a normal standard video signal.This implies that your composite video level is low.Your device may, however use some other levels, as it seems to already use a non-standard DC offset.Have you looked carefully at your signal to check that it is not "smeary" going into the monitor?A screenshot of what your Oscilloscope sees may be helpful for us to help debug this.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 05, 2019, 03:12:30 pm
content: Scope waveforms of your composite baseband baseband signal would go a long way towards troubleshooting your problems.It could be that the black setup is too low, or that the video transitions are causing ringing into the blacker-than-black areas (the realm of sync pulses).Or perhaps the DC level is too high. Or the sync pulses too short.Also, your statement that "I am using a transistor to drive the input, so I don't think current is the issue" is pretty vague, don't you agree? Where is the schematic?

author: Renate
author_rank: Super Contributor
timestamp: September 05, 2019, 05:02:34 pm
content: Quote from: mayor on September 05, 2019, 08:30:53 amSo, I'm betting that you are using an NPN as an emitter follower and that your emitter resistor is too high a value.That means that the transistor can easily (and quickly) pull to white while decay to black is slow.So just replace the resistor with half the value and notice the improvement.(Not that that is the complete solution.)

author: mayor
author_rank: Regular Contributor
timestamp: September 05, 2019, 11:52:06 pm
content: Thanks all!PKTKS: not too sure how I'd go about matching it, TBH.vk6zgo: Yeah, I've probed around the video chips, and the signals are lower than 1V p/p, and don't appear very clean at the source, so yeah, it could be considered "smeary". Not too sure what to look for, or what the cause might be. I'll try to find a usb key for some captures.schmitt trigger: Agreed. Essentially :12V -> 1K POT -> CollectorComposite video -> BaseGND -> 1K -> Emitter-- will provide some traces for what I find on the board...Renate: will try lower resistor value on emitter. Thanks for the suggestion.

author: mayor
author_rank: Regular Contributor
timestamp: September 06, 2019, 12:44:31 am
content: Halving the emitter resistor hasn't produced a significant improvement.That being said, I feel a little shame at not having paid more attention to the output of the graphics chip -- or, maybe more importantly, one of its inputs. Here are some traces.1. Main clock (looks fairly clean)2. Video chip clock (derived from main clock)3. B-Y signal (ouch!)So I guess the next step would be to figure out why the clock for that chip is so bad, huh?

author: vk6zgo
author_rank: Super Contributor
timestamp: September 06, 2019, 04:54:52 am
content: Quote from: mayor on September 06, 2019, 12:44:31 amI googled for Colecovision, & found this:-https://en.m.wikipedia.org/wiki/ColecoVisionwhich further lead to this:-http://www.cs.columbia.edu/~sedwards/papers/TMS9918.pdfIt appears that the TMS 9918A chip doesn't present Y, R-Y, & B-Y to the external circuit, but produces composite NTSC video.The TMS 9928A doesn't do composite, but supplies 525 line format, Y, R-Y, & B-Y to the external circuitry .This chip requires a separate encoder to produce NTSC or PAL composite video (therewasa 525 line PAL system in one country).The TMS 9929A doesn't do composite, but supplies625 lineformat, Y, R-Y, & B-Y to the external circuitry.This chip requires a separate encoder to produce SECAM or PAL composite video.Two rather interesting things arise with your 'scope screenshot.(1)You seem to be looking at the video signal at field rate or thereabouts, so it puzzles me that the display looks as if it is suffering from aliasing, but your sample rate seems quite adequate to correctly show the signal.(2)Strangely, you have a box at the top of your pic which reads15625Hz.*For NTSC it should be15750Hzor thereabouts.Can you show us a pix at line rate?Edit---*Sorry, I read that wrong--- that said, the frequency it does show makes no sense at all.On the other hand, I don't have a DS1054Z, so maybe that information is nothing to do with the actual frequency components of the signal.

author: mayor
author_rank: Regular Contributor
timestamp: September 06, 2019, 09:50:30 am
content: Hi,I think the frequency doesn't make sense in part because I am unable to trigger on that signal (ie, it's not very constant, even with a static image). I've found the following:https://rigol.desk.com/customer/en/portal/articles/2311798-ds1000z-counter-technical-details, which is interesting. I'll experiment a little later with the vertical scale, since it impacts the measurement.That being said, the B-Y signal looks pretty bad. All the drops to 0 between syncs is unexpected to me, but then, so are many other things :-) I'm not familiar enough with electronics to know for sure, but doesn't the input clock look bad to you, which might explain the bad output?

author: nali
author_rank: Frequent Contributor
timestamp: September 06, 2019, 10:19:35 am
content: There's a bunch of info including reverse-engineered schematicshere, they show that the RF modulator is a LM1889. According to the app notedataheetit's possible to hack a composite video from pin 13 via an emitter follower and decoupling cap.

author: PKTKS
author_rank: Super Contributor
timestamp: September 06, 2019, 10:23:20 am
content: Quote from: mayor on September 05, 2019, 11:52:06 pmThat depends on the circuit you have chosen to couple yourcomposite output.Essentially you "REFLECT" the output back to the base of thetransistor calculated to be 75 Ohms.75 Ohms is expected in any input as say if you have 10.000 Ohmswould be too high causing harmonics to bounce in group delay ofhigh components.. thus explaining those late line phantoms...Top list first thing to check by design.Sure there are other methods to measure with a good scope thereflected signal.. but far too details to put here..Paul

author: mayor
author_rank: Regular Contributor
timestamp: September 06, 2019, 10:24:13 am
content: Quote from: nali on September 06, 2019, 10:19:35 amHi, yeah, that's what I essentially did. But the signal isn't very clean at the source, if I'm interpreting it correctly.

author: mayor
author_rank: Regular Contributor
timestamp: September 06, 2019, 10:29:08 am
content: Quote from: mayor on September 06, 2019, 10:24:13 amAh but I will review: they use a series resistor (75 ohm) and a DC blocking cap. OK, that'll be an experiment for tonight!

author: PKTKS
author_rank: Super Contributor
timestamp: September 06, 2019, 10:46:01 am
content: Quote from: mayor on September 06, 2019, 10:29:08 amThat only fits if your reflected impedance (the HFE reflection)if low enough compared with the series resistor e.g. it doesnot add enough not causing harmonics delay.BTW the transistor you choose is also sensitive as low BWgates tend to introduce group delays as well (harmonics delay)Paul

author: Renate
author_rank: Super Contributor
timestamp: September 06, 2019, 01:11:18 pm
content: Reflections are a considerations, but we're nowhere near that level yet.If somebody were to recommend a unity gain IC buffer with near zero output impedance then talk of a 75 ohm series build-out resistor would make sense.The current buffer doesn't need any collector resistor and the emitter resistor is still too big.You're also shoving 5V down its throat.While the attached schematic is not the last word in design, it should get you decent video.(If you take any photos of the video signal, please show 10uS/div sweep.)

author: PKTKS
author_rank: Super Contributor
timestamp: September 06, 2019, 02:54:14 pm
content: The color sub carrier contains harmonicsmore or less necessary for proper color definitionIn 3.5MHz at least 8MHz full band pass windowIn 4.4MHz at least 10MHz full band pass windowGood color definition requires a careful chosentransistor so we do not cut off higher order harmonicswhich will eventually lead to poor colors.so assume a proper safety margin of at least 10MHz bandwithout reflections whatsoever. (or phantom colors appears)PAL and NTSC demodulation resides in the higher orderharmonics of the sub carrier.Paul

author: Renate
author_rank: Super Contributor
timestamp: September 07, 2019, 06:33:13 pm
content: Quote from: PKTKS on September 06, 2019, 02:54:14 pmErm, not really.The color subcarrier certainly has sidebands.The second harmonic of NTSC color subcarrier would be 7.16 MHzNTSC ran on a 6 MHz bandwidth channel for decades with a 3.58 color (and that was already offset 1.25 MHz).Moreover, if you really want things to look good you should just buffer the Y, R-Y, B-Y out of the Coleco and avoid the whole encode/decode process.

author: PKTKS
author_rank: Super Contributor
timestamp: September 08, 2019, 10:09:01 am
content: Quote from: Renate on September 07, 2019, 06:33:13 pmYep, I am sorry about the crude terms.. but the real thingis only put in proper terms with a not trivial math.The first correction is that what I meant by "harmonics"actually are the vestigial bands of the HORIZONTAL componentwhich actually fall shortly on the primer sub carrier (by figure)The vestigial bands are different for PAL and NTSC thusexplaining the different precise XTALs used.Any further understanding unfortunately goes through theproper Fourier expansion which including the periodic squarewave adds a plethora of things not visible at first glanceAttached figure is the result of Laplace and Fourier plottedwith proper math done and proper vestigial band (for NTSC-M only).That I meant for "harmonics" ... (actually vestigial bands)Just in case you would like to see formal equation I can add tooPaul

author: vk6zgo
author_rank: Super Contributor
timestamp: September 08, 2019, 12:32:12 pm
content: https://en.m.wikipedia.org/wiki/PALThe PAL wiki above is a good resource, but I will attempt to provide my own description.As I am more familiar with PAL(B), I will use it as an example:-The RF channel allocated for transmission in this system is 7MHz wide.This must allow for "guard bands" at the upper & lower extremities of the channel width.The video signal,which is limited to 5MHz bandwidth by specification in PAL (B), modulates the Vision Carrier (Vc) using Vestigial Sideband Modulation(VSB)'with an Upper Sideband extending to 5MHz above Vc.There is a "Guard band", then the first sound subcarrier at 5.5MHz above Vc, then another sound carrier a5.75MHz above Vc.(for the stereo system employed in Australia).As the term "Vestigial Sideband" implies, the lower Vision sideband is partially supressed, only extending out to 1.25MHz below Vc.All the above goes to point out that it is aband limited system.Video signals in PAL(B) are rolled off quite rapidly after 5MHz.The band occupied by the video signal is often drawn, as if it continuously occupies the whole spectrum, but this is not so.The (black& white) Luma video information appears at multiples of line frequency, like a lot of "carriers", with field rate sidebands, & areas of spectrum between these "carriers" carrying little or no energy.Providently, this allows a method of transmitting colour information.The human eye has relatively poor resolution for colour, compared to black & white, so it is possible to reduce the required colour signal (chroma) bandwidth to around 1.3 MHz.Instead of having to transmit R, G, & B signals, colour difference signals R-Y & B-Y are used.These are Quadrature amplitude modulated onto a 4.43361875MHz subcarrier.The reason for the strange subcarrier value is that the video sidebands from this modulation fall in the "empty" spaces between the luma "carriers" ----so calledfrequency interlace.The colour signal is placed at the high end of the video passband because the average signal power at that end is quite low.Very early Colour receivers used Low pass filters to separate the Chroma & Luma components of the received signal, but this caused a loss of Luma resolution.A later iteration was to use "notch"filters, but the definitive method employed "comb" filters, which caused much less loss of Luma resolution.The PAL wiki page shows a diagram of the frequency components of a transmitted TV RF signal, followed by a spectral display of the same signalOP--- your pix of the B-Y signal at field rate looks a lot more like a composite signal.The colour difference signal will not normally have syncs attached.In any case, you need to drop vour volt@/div control so the video signal takes up most of the screen height.The "scrunched up" version you provided makes it very difficult to get a good idea of the signal's appearance.Ohh, & a line rate screenshot would be nice, as I suggested earlier

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 08, 2019, 10:37:55 pm
content: Indeed!As a fact: a line rate screen shot is the only way to determine whether the active video setup is correct.

author: mayor
author_rank: Regular Contributor
timestamp: September 08, 2019, 10:44:13 pm
content: Here's some more info. With the same circuit I described above, I have managed to get a good (albeit a little dark) image. What I had to do was trim the pot down initially so the TV picked up the signal, and once it did, I could increase the resistance until I got a decent pic. I measured the signal with this adjustment, and it looks like the TV "blows out" above 6V. I'll give Renate's circuit a try after this to lower the DC component. Hopefully, this will produce the desired result, while keeping signal amplitude high enough.Here are some captures of the original signal, and the output with my current circuit. The image signal is somewhat "skewed", which seems to happen with lower resistance on the emitter (500 ohm here).

author: mayor
author_rank: Regular Contributor
timestamp: September 08, 2019, 10:53:41 pm
content: Quote from: schmitt trigger on September 08, 2019, 10:37:55 pmSorry about the noob question, but not sure what the "line rate" is!

author: mayor
author_rank: Regular Contributor
timestamp: September 08, 2019, 11:15:06 pm
content: Input and output with 1K resistor from emitter to ground.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 09, 2019, 12:22:55 am
content: Line rate is exactly what you have done: set the timebase to display a waveform from the start of a horizontal sync to the start of the next horizontal sync.In other words, you are displaying a single video line.And as you can see from the cursor readings (which I may add, you also have set correctly), the frequency reading is now very close to the actual NTSC horizontal frequency of 15734 Hz.A terminated video waveform should be 1V, from sync tip to peak white, of which about 0.28V belong to the sync (blacker than black), and the remaining is the active video.Your waveforms look fine, other than the levels appear to be a little different from the NTSC specs, but that may be the cause that your black background is not as black as it should be.The color burst appears OK, too.

author: mayor
author_rank: Regular Contributor
timestamp: September 09, 2019, 08:46:38 am
content: Cool, thanks for the explanation. I'll bring the DC voltage down and see where that leads!

author: mikerj
author_rank: Super Contributor
timestamp: September 09, 2019, 09:06:41 am
content: The output waveform looks horribly messy, assuming you were displaying the same screen as the input (though presumably not the same line). The input looks exactly as I'd expect from low resolution computer generated video with a limited colour palette; there are four discrete luminance levels clearly visible.Displaying the same video line on input and output simultaneously, with the timebase speeded up to see some detail would be very helpful in diagnosing this. I'm wondering if your emitter follower is oscillating, if so a ferrite bead on base wire (assuming through hole components) or a lowish value resistor (typically in the range 47-470 ohm) in series with the base may fix this.

author: Renate
author_rank: Super Contributor
timestamp: September 09, 2019, 03:37:47 pm
content: Off hand, I don't think that it's oscillating.If you were using my circuit that has no collector resistor, oscillation would be impossible anyway.Are those last scope shots with the monitor connected or not?

author: vk6zgo
author_rank: Super Contributor
timestamp: September 09, 2019, 04:25:08 pm
content: Quote from: mikerj on September 09, 2019, 09:06:41 amThe video waveform does look better than that in the previous postings, but it is still a bit "rough around the edges"A good composite video waveform should look pretty much like the one in this link.https://hackaday.com/2018/01/18/know-your-video-waveform/Note, however,that the "Raspberry Pi" video in this screenshot doesn't show "setup"between blanking & black level,which a standard NTSC signal should include.The burst peak to peak amplitude should be about the same as the sync amplitude--- anything less points to loss of high frequency response in the transmission system.With the waveform shown in the OP's most recent posting, the burst amplitude is low, & the noise level is high, both at blanking,& sync level either of these conditions would be seriously "out of spec" in TV Broadcasting.

author: StillTrying
author_rank: Super Contributor
timestamp: September 09, 2019, 04:29:25 pm
content: Quote from: mayor on September 05, 2019, 01:37:29 amAs your video signal is still riding on top of 5V DC, I'd try just AC coupling it to the monitor with a 10 to 100uF, +ve end to your TR's emitter.

author: mayor
author_rank: Regular Contributor
timestamp: September 09, 2019, 05:22:17 pm
content: Quote from: mikerj on September 09, 2019, 09:06:41 amAlright, I'll capture the same line and post back! I think it was the same screen, but can't be 100% sure.

author: mikerj
author_rank: Super Contributor
timestamp: September 09, 2019, 07:07:49 pm
content: Quote from: Renate on September 09, 2019, 03:37:47 pmNo true, emitter followers are notorious for oscillating.

author: Renate
author_rank: Super Contributor
timestamp: September 09, 2019, 07:28:18 pm
content: Quote from: mikerj on September 09, 2019, 07:07:49 pmThanks, I learn something new every day.I was thinking of collector-base feedback.This is lead inductance/base capacitance resonance.The frequencies involved would be pretty high.Still, although the scope view still looks cruddy, I'm not convinced that it's oscillating.You'd have to crank up the scope timebase and see a nice ringing on an edge.

author: mayor
author_rank: Regular Contributor
timestamp: September 09, 2019, 07:46:52 pm
content: Quote from: Renate on September 09, 2019, 07:28:18 pmWill do that too.

author: mayor
author_rank: Regular Contributor
timestamp: September 10, 2019, 08:41:01 am
content: Quote from: Renate on September 09, 2019, 03:37:47 pmNone of my captures are with tv attached. Haven't had time to experiment yesterday, but hopefully will tonight.I will def try the DC blocking cap, and if there are suggestions on how to limit noise, that'd be great. Feels like this might be a little more involved, though...

author: mayor
author_rank: Regular Contributor
timestamp: September 11, 2019, 12:29:14 am
content: Some more pics from input & output for same line + close ups of source. Output doesn't really seem worse than input, but input looks bad -- at least, to my untrained eye! (input on ch1, out on ch3)

author: Renate
author_rank: Super Contributor
timestamp: September 11, 2019, 08:33:17 pm
content: All the scope waveforms look halfway decent for that era.They don't correspond with the smear that you showed on the TV.You have to either scope it with the TV connected or try out a more serious buffer.

author: nali
author_rank: Frequent Contributor
timestamp: September 12, 2019, 05:50:48 am
content: I'm not so sure, the screen shotimage-data-closeup.pnglooks like the white levels might be driven into saturation somewhere along the line?That exponential decay from the white levels is the ghosting effect seen on screen.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 12, 2019, 06:12:17 am
content: Quote from: Renate on September 11, 2019, 08:33:17 pmComing from a TV Broadcasting background, I would say it looks unacceptable.Were video signals from Games machines always that bloody awful?

author: maginnovision
author_rank: Super Contributor
timestamp: September 12, 2019, 06:51:09 am
content: Quote from: vk6zgo on September 12, 2019, 06:12:17 amThat era? Absolutely. It's ok because the screens were also garbage.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 12, 2019, 10:16:50 am
content: Quote from: maginnovision on September 12, 2019, 06:51:09 amTry to tell that to the licencing Authority back in the day-------they could pull your station licence if you consistently did not make specs.Somehow I don't think "The screens are crap, so we can transmit non standard signals" would have flown as an answer to ACMA(or their predecessor), or the FCC!Normal TV CRTs were designed to display proper analog video signals, not video games, & were very good in their intended usage.One of my activities in a past life was fixing, replacing CRTs & calibrating Studio picture monitors to specs.The Studio units were capable of displaying the whole resolution capacity of the PAL system, when fed with Composite video.They could do even better fed with RGB.Even domestic TVs were not much worse.The pix in the original posting is really pretty dire.I seem to recall the Commodore 64 doing considerably better on a little 13" Sanyo ------RF modulator & all!I wish I still had the C64, so I could look at its video signal with a 'scope.

author: mikerj
author_rank: Super Contributor
timestamp: September 12, 2019, 11:26:29 am
content: Quote from: nali on September 12, 2019, 05:50:48 amI'd agree with this, it's taking around 3us to fully return to black level which is an awful long time when a whole line is only ~64us. OP: is that scope trace taken with or without your buffer connected? If with, does it look the same when it's disconnected?

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2019, 11:43:32 am
content: Quote from: nali on September 12, 2019, 05:50:48 amThe rigol draws straight lines when the signal reaches the top or bottom of the screen, those completely flat tops will be due to the waveform hitting the top of the scope's screen and then being brought down 1/3 div. after being STOPed.

author: mayor
author_rank: Regular Contributor
timestamp: September 13, 2019, 09:27:41 am
content: Quote from: mikerj on September 12, 2019, 11:26:29 amHey mikerj, I'll do a comparison, but iirc, those were without the buffer.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 13, 2019, 03:12:15 pm
content: Quote from: mikerj on September 12, 2019, 11:26:29 amThe OP has "zoomed in", in the process changing the time/div to 1us/div, so that return to black level isvery much lessthan 3us.This is obvious if you compare the previous pictures which are 10us/ div.The H.A.D. of a horizontal sync pulse is around 5us, so you have a known reference to work from.

author: mayor
author_rank: Regular Contributor
timestamp: September 14, 2019, 07:36:59 pm
content: Hi,finally tried lowering the DC bias. I am sure it could be further improved, but I'm happy with it. The actual image is better than the attached picture shows, the bleeding is subtle, colors are saturated and the background is truly black.Thanks to all, this has been fun and educational!

author: mayor
author_rank: Regular Contributor
timestamp: September 15, 2019, 10:55:53 am
content: Hi, one thing I want to go back to. The clock on the video chip looked jittery to me. Did you consider it to be normal?

author: StillTrying
author_rank: Super Contributor
timestamp: September 15, 2019, 11:42:47 am
content: "The clock on the video chip looked jittery to me."Yes, maybe the 3.5795Mhz is getting mixed in with it somehow, I'd try to see if there are only 3 traces of jitter."Did you consider it to be normal?"Well I dunno.Years ago when I was using video emitter followers I used 220R for the emitter resistor with 12-15mA flowing though it.

author: mayor
author_rank: Regular Contributor
timestamp: September 15, 2019, 01:57:43 pm
content: Quote from: StillTrying on September 15, 2019, 11:42:47 amInteresting, not too sure how to go about that?

author: Renate
author_rank: Super Contributor
timestamp: September 17, 2019, 04:00:18 pm
content: My (lousy) scope doesn't have variable hold-off, but that's what I'd play with to see if you can get a single trace.You can also sync the scope to the 3.58You could also use an external VFO/DDS at sub-multiple of 10.7 for sync.

author: mayor
author_rank: Regular Contributor
timestamp: September 18, 2019, 08:56:40 am
content: Thanks Renate, will experiment with variable hold-off. Only option available to me.

author: StillTrying
author_rank: Super Contributor
timestamp: September 19, 2019, 12:38:33 pm
content: I'd just move the trigger from the center to the far left so that you can see 5 or 6 cyclesafterthe trigger.