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

author: maginnovision
author_rank: Super Contributor
timestamp: January 12, 2019, 07:32:59 am
content: I'm not very familiar with composite video. These are two NTSC composite waveforms, the orange is the output, the yellow is the input. My question is what is causing the orange waveform to skew like it does? Could this be caused by excess capacitance? I seem to get more vertical lines in the output than are visible on the input. I'm also attaching the schematic for the circuit. The input to the circuit is the output of an RF modulator. The output is to the TV. If you need any questions answered I will answer them just looking to see what I can do to improve this before finalizing the board.Edit: Just thought I'd add. In these waveforms the input/output was a black box in the middle with a yellow border all around. So dark colors the levels are rising, light they are dropping. However the input waveform is more accurate to how it should look having proper and level peaks across the frame. One other thing... The transistors used are PZT3904 from ON semi.

author: soldar
author_rank: Super Contributor
timestamp: January 12, 2019, 09:46:09 am
content: Looks like poor low frequency (and DC) response to me. Have you tried increasing cap values?

author: Benta
author_rank: Super Contributor
timestamp: January 12, 2019, 10:25:28 am
content: The output confuguration is wrong for a video signal. Increase R3 to 2.2 kohm and place 75 ohms in series with the output. 220 uF is normally OK for 75 ohms, but the 10 uF might be too small. Do you have 75 ohms across the input?Note that the 75 ohms at the output will attenuate your video signal, so your buffer should have a voltage gain of 2. Using an integrated video amp is a better solution.

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2019, 02:11:04 pm
content: Input needs to be 47 or 100uF, and the pot should be 1k or less, around 100R works well. C1 polarity is also wrong, it will fail eventually, positive side must be to the base of the transistors. R5 change to 150R and it will improve things, though as this is just a unity gain stage it will not do much other than DC restoration to the signal.

author: maginnovision
author_rank: Super Contributor
timestamp: January 12, 2019, 04:22:40 pm
content: Quote from: soldar on January 12, 2019, 09:46:09 amI have not, I will try that.Quote from: Benta on January 12, 2019, 10:25:28 amThe 75 ohms in series with the input doesn't exist, I'm not sure if it's needed though. I'd actually have to take the modulator apart to check for sure. I think the circuit designer had more experience with monitors and things. Some research showed this output config is usually used there and the output devices have the 75ohm in series so I will definitely do that and the R3 change along with a larger value cap at the pot.Quote from: SeanB on January 12, 2019, 02:11:04 pmC1 polarity makes sense since the waveforms are on opposite sides of 0 when viewed DC coupled. However I'm not sure which you're referring to as R5.Thanks for the input, I'll be back when I have tried these things(or switched to an actual amp).

author: Ultron81
author_rank: Contributor
timestamp: January 12, 2019, 04:47:03 pm
content: I would suggest using an amp like the THS7314 for this. It’s the proper amp for this application. I’ve used it before for a similar application, and it works perfect.http://www.ti.com/lit/ds/symlink/ths7314.pdfThe TV will have 75 ohm termination, so you need to have a 75 ohm resistor in series with the output as was stated above, and a 2x gain amp (the 2 75 ohm resistors form a voltage divider).

author: Benta
author_rank: Super Contributor
timestamp: January 12, 2019, 05:00:06 pm
content: Just to clarify: a general impedance-matched video buffer looks like the attached picture (x2 voltage amplifier). Whether you actually need impedance matching is a question of distances. If you just have short traces, you can mostly skip it. But if you're going through coax cables you'll need it.I've had very good results with the LM6171 with no measurable phase distortion.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 13, 2019, 03:02:53 am
content: Analog video signals refer the luma "black level" to zero volts, hence they have a DC component.Ideally, all video amplifiers should be "DC coupled", but that is not always possible.Although we refer to a "DC component", the lowest frequency we need to pass is 60Hz(NTSC) or 50Hz( PAL,SECAM,etc).The RC coupling of the amplifier input constitutes a differentiating network.Such networks, operating at frequencies which are fairly close to their time constant will not exhibit the classic "positive spike, then a drop to a relatively low level, followed by a negative spike".Instead, a square or rectangular wave will start off at its correct level, then "droop"or "ramp up", depending upon the transition.In television, this is called "tilt"( in this case, field tilt)This manifests itself as luma "shading", where the resultant picture is lighter at (usually) the top, & gets darker as it progresses to the bottom.It is possible to "cheat", by making the RC time constant of the coupling components very long, so the "shading"over each TV frame is so small as to be imperceptible.This is probably what you will be able to do.Some cheap BW TVs did this in the early days, but the more expensive ones used "DC restorers" which did "restore"the black level , but took a whole field to do so, which made it a poor choice for studio work.In Broadcasting, where video signals are "piped" around studios, or even, around countries, if the signal does need to be "ac coupled" at any stage, devices called "keyed clampers" are used to restore the black level.( restore the DC component).These "clamp"the black level every line.Near the top of this forum, you will find a thread called "Level shifting" containing a discussion which gives a good idea of how these things work.By the way, your RTB 2004 is really battling to resolve field rate video--- I thought a modern DSO would do better.

author: maginnovision
author_rank: Super Contributor
timestamp: January 13, 2019, 03:42:10 am
content: Quote from: vk6zgo on January 13, 2019, 03:02:53 amI'm not sure what to say, it could be the RF modulator, the 4" ground leads, the scope or the fact that the board I demo'd my issue with leaks 60hz so bad it's always visible and occasionally causes malfunctions. Sorry it's not up to your standards and thanks for the info.I swapped the 10uF input cap for a 47uF and reversed the polarity. I also added the 75ohm series resistor. There is still noticeable tilt but slightly less. I have the THS7314 on order and will try them in a couple days when they arrive.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 13, 2019, 03:50:22 am
content: Quote from: maginnovision on January 13, 2019, 03:42:10 amTry looking at the video signal at line (15kHz) rate .If it is RF from the modulator, it should still be there.My suspicion is that your sample rate is very low when looking at field rate, so you are getting aliasing with the line rate & higher frequency components.This was the big failure of early DSOs---- but they didn't just "look noisy" they were unusable!

author: maginnovision
author_rank: Super Contributor
timestamp: January 13, 2019, 03:57:36 am
content: I'm not sure what you're looking for but here is a SS triggering on lines instead of frames.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 13, 2019, 04:32:29 am
content: Quote from: maginnovision on January 13, 2019, 03:57:36 amI think you may be right about the RF getting into your video.Analog video signals should look like the ones in these links.http://www.radioradar.net/en/measurements_technics/oscilloscope_display.htmlhttps://electronics.stackexchange.com/questions/66588/how-do-analog-video-signals-work

author: maginnovision
author_rank: Super Contributor
timestamp: January 13, 2019, 04:40:31 am
content: Quote from: vk6zgo on January 13, 2019, 04:32:29 amI'm not sure, it's pretty far outside my wheelhouse. The modulator is based on LM1889 and this unit doesn't properly ground the box(ground spring is missing). Here is one more triggering on lines without the RF modulator. The problem is without the modulator I don't get color information so it isn't really something I'm worried about. The other test unit looks much better but I prefer to test with the worst case.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 14, 2019, 03:04:41 am
content: Quote from: maginnovision on January 13, 2019, 04:40:31 amThis is the second comment you made which rings alarm bells.The first was in the OP,where you saidQuoteTheoutputof the RF modulator should be amodulated RF envelopeas shown in this link:-https://www.britannica.com/technology/television-technology/Principles-of-television-systemsThere should not be a recognisable baseband video signal at the RF output of the LM1889, unless it is faulty, wrongly wired, or perhaps, unpowered.Look at this link now:-http://rbsfm.org/Downloads/Datasheet/LM/LM1889.PDFThe LM1889 is intended, as far as I can determine, to combine a Luma signal (on pin 12) with two separate Colour Difference signals (R-Y on pin 2) & (B-Y on pin 4), to produce a Composite NTSC colour video signal,prior to modulation.Also,as far as I can see, this signal, once produced, does not appear as an output on any of the pins of the IC, but passes directly to the RF modulation section where it amplitude modulates a VHF RF carrier, giving a signal which can be received by a standard analog TV receiver via its antenna input.Your last waveform was, I assume, on Pin 12.( for some unfathomable reason, the manufacturers just label that pin "video")It is a horrible Luma signal, which raises the spectre of your source of these video drive signals being faulty.

author: maginnovision
author_rank: Super Contributor
timestamp: January 14, 2019, 03:50:28 am
content: Quote from: vk6zgo on January 14, 2019, 03:04:41 amActually no, it is from pin 13 of the LM1889, all the wavefoms are from pin 13 or the output of the circuit I posted. When I say it's the output of the modulator it's simply because the signal is modified by the modulator and it's another thing entirely when it's not connected. I can't modify the modulator, I can't modify the board or chips that feed it either those products are already done and not mine. I'll attach a picture of the modulator. I'm not going to pretend the waveforms look good or great though, they don't and if it were my project I would make sure it looked good. However they do produce acceptable video aside from seeing the 60hz 5 times on the screen. This board again has about 100mV 60hz AC riding along the power rails and the modulator gets 10V, 2.5V and -5V. I haven't even bothered checking the vertical and horizontal drive signals but the chroma clock looks fine.Honestly if I had more time and was more knowledgeable on the subject I would probably replace the modulator with a different circuit altogether to get cleaner video from the get go. The way I have to do it now relies on the modulator or just gives B & W. I don't have the time or knowledge though.

author: Ultron81
author_rank: Contributor
timestamp: January 14, 2019, 05:13:38 am
content: OK, I was a little confused when you originally asked the question and thought maybe you were talking about the output from an RF demodulator.So you are looking at pin 13 (chroma carrier) right? What are you trying to do? Use the RF modulator (LM1889 in particular) as a video encoder for composite video? There are tons of ICs that would be better in that case, and may give you a better signal as well. In fact, if you already have access to the chroma and luma signals, you can buffer the 2 signals with the THS7314, then connect the chroma to the luma with a 470pF cap and use that as your composite video.Well, anyway, if you plan on using pin 13, the only way you are getting composite video is if the luma is applied to that pin as well. Look at the application circuit I attached (from the LM1889 data sheet).Can you tell us what the RF modulator is from (i.e. video game console)? There may be other ways to get the composite signal than going from the RF modulator.EDIT: Oh, just saw getting the signal another way would be no dice. Still, would be good to know where it is from so I can see the rest of the video circuit.

author: maginnovision
author_rank: Super Contributor
timestamp: January 14, 2019, 05:31:26 am
content: Quote from: Ultron81 on January 14, 2019, 05:13:38 amIt's a little complicated. It is from a video game console and I don't actually have direct access to pin 13 of the LM1889 but it is routed to a pin on a 50 pin expansion port. I have access to both the luma and the composite signal encoded by the modulator. What I don't have is the chroma clock but I do have it 180 inverted and hor/vert drive signals. I also do NOT have b-y or r-y. That's the reason it's not ideal but I can't do much about it. It's also still better than the only other built-in option, RF.EDIT: Almost all of the video is done in some custom IC's(no good data) that generate r-y, b-y, luma, hor/vert drive. Chroma is just split off from clock circuit.EDIT 2: The RF modulator gets -5V, +10V, +2.5V, audio, chroma, luma, b-y and r-y.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 14, 2019, 08:59:48 am
content: Quote from: Ultron81 on January 14, 2019, 05:13:38 amYou & I have got "caught out" by mistakes in the drawings in the data sheet.On page (1), pin 12 is shown as "video input", & pin 13 as "chroma subcarrier".On page (4), pin 12 is labelled "video bias" & pin 13 is "video input".On page (6), pin 12 becomes "sound carrier input" & pin 13 is still "video input"---"chroma subcarrier" seems to have disappeared!"Moving right along" to page (7), the "TV Game Schematic", pin 12 goes back to being "video input".Pin 13 has no identification, but is connected to the -9v supply via a 2k resistor!Finally, page (9), in fig (1) shows the connection you posted in his last posting.Pin 12 is now "DC reference voltage", pin 13 is "luminance & sync input".No explanatory notes for that one, at all!Aaaaarrrrgggghhh!,

author: vk6zgo
author_rank: Super Contributor
timestamp: January 14, 2019, 09:41:51 am
content: Thinking over it, there is a vague hint of an explanation on page 8.It seems that the luma & chroma are passively added in the 2k resistor.It does seem to look as if there is no blanking of the subcarrier during the blanking interval, or, it seems, any provision of colour burst, so the TV or monitor relies upon locking to the, always present, chroma subcarrier.This would account for the presence of "noise" upon the syncs in the screenshots from the OP.I really expected these chips to be able to produce standard video, rather than a rather primitive kluge.The circuits in VCRs seem to be able to do so!

author: Benta
author_rank: Super Contributor
timestamp: January 14, 2019, 10:50:04 am
content: Be very careful with the old video chips from National (TI). There are deliberate mistakes in the data sheets, both LM1881 and LM1889. I believe it stems from the VCR times, where Macrovision exerted pressure to keep amateurs from using the chips for copy protection removal. "Legitimate" users would get an amended data sheet under NDA.

author: maginnovision
author_rank: Super Contributor
timestamp: January 15, 2019, 12:47:42 am
content: Yea, I don't doubt something isn't right. I'm attaching the schematic for the modulator for anyone interested.

author: Ultron81
author_rank: Contributor
timestamp: January 15, 2019, 03:01:14 am
content: OK, so can you clarify some stuff for me?1 - You can't make any modifications to the main board (I'm guessing this is a Bally Astrocade, right?)2 - You don't have access to the RF modulatorIn reality, you have the 3 signals that make up component video (Y, R-Y, B-Y) going to the RF modulator. If you have a TV with component inputs, you can just take these signals off the RF modulator connector and pass them through the THS7314 amp and into your TV, should work.I'm not sure how easy it will be to get composite with passive components.EDIT: Looking at it again, yeah grab the signal from pin 13. That should be your composite video. Send that into the buffer and should be good to go. The chroma is on pin 13, the luma is added to pin 13 with a 3.3k resistor inside the Astrocade.

author: maginnovision
author_rank: Super Contributor
timestamp: January 15, 2019, 04:47:44 am
content: Yea, it's not the cleanest signal but it is a valid composite signal. The THS7314's should show up tomorrow or the next day. After swapping to a bigger cap and reversing polarity and adding the proper series term resistor the output looks pretty good, but the THS7314 being a proper amp with the integrated low pass filter may help clean up a little of the noise and fix the remaining tilt; At the very least it's cheaper than the transistors. This is just a small part of the project(value add because it's cheap) but as it's close to finished I'm trying to polish some of this stuff as much as I can.

author: vk6zgo
author_rank: Super Contributor
timestamp: January 17, 2019, 02:17:32 am
content: Quote from: maginnovision on January 15, 2019, 04:47:44 amSorry, but it does not meet any of the specifications for a "valid composite signal", & nobody in Television Broadcasting would accept it as such.Those of us "greybeards" who learnt the characteristics of a "standard composite video signal" by heart, & in our day-to-day work for decades, tested equipment & systems relying upon those standards,tend to be very critical.In your case, it works OK for the purpose, & incidentally, shows how forgiving the decoding circuitry is in the later generations of analog TV receivers.Something like a iPhilips K9 would probably produce rubbish from a signal that looked like that.QuoteI'm sure you have heard the saying about polishing some things!

author: maginnovision
author_rank: Super Contributor
timestamp: January 17, 2019, 07:17:13 am
content: Quote from: vk6zgo on January 17, 2019, 02:17:32 amI really couldn't say for sure, not being a greybeard, but the maxim pdf on cvbs seems to say it's going to be similar to what I'm seeing although they have no actual waveforms.The good news is the waveform is much improved with the ths7314. Yellow is from the unit, orange is to the TV. Also the video output looks very good.EDIT:At your behest(kidding), I compared the output to that of a Sega Dreamcast. I'll admit it is a cleaner signal, but I don't think the output is any less valid and easily explains the noise on screen and the more limited color space.