Is a series cap needed when converting old crt tv for composite input? - Page 1
Forum section: electronics / beginners

author: Cykar
author_rank: Contributor
timestamp: February 15, 2021, 08:03:16 am
content: I have an old crt tv which i have converted to take a composite input. Many people do this, and it wasn't a problem. But i remember watching a 12voltvids video on youtube when he did exactly the same. The difference was he put a cap in series with the composite signal to block any possible dc from going back into, in my case, the dvd player. Makes sense i think. So i tried that and the result was that the signal got messed up.I am wondering what is going on here and the only difference i can see is that 12voltvids broke the track going to the transistor where the signal was put in thereby cutting off any signal from the antenna. Is this the culprit? I just want to understand what's going on here and if i really need the cap.If i remember correctly he used a 47u electrolytic with the negative towards the inputted signal, tho he said the value was not crytical as it was just for blocking any dc voltage. I tried this, plus various other caps both polarised and not, and always got the same bad result. Interestingly, when i put a 470u cap in, the signal worked initially and got worse and worse as the cap charged until the same result occurred, the signal being destroyed and no picture.I hope someone can help me understand this and whether or not i need worry about this dc blocking cap.

author: gnuarm
author_rank: Super Contributor
timestamp: February 15, 2021, 08:52:15 am
content: The first question is why does the DC need to be blocked? That is what is the purpose of the cap without saying, "to block DC".The second is to ask how does he expect the DC level to be restored? Composite video is a DC level signal. It has specific levels for black, white and sync. DC is inherent in it's function. That said, I seem to recall the DC level being taken out by something, perhaps the demodulation process not being DC accurate. So the DC would need to be "restored". I want to say this was done by a clipping circuit that would charge a cap so the negative level of the signal would no longer clip (other than the very brief sync pulse) and the black level established. While the sync pulse is very brief, the video signal is black around the sync for a longer time (retrace).It's been too long for me to recall the details of any of this. I built a composite input for a small TV. It's still rattling around a house somewhere with a BNC jack sticking out the back. For safety it used an opto coupler, so the video was only black or white. I don't recall how the sync was conveyed. Perhaps the video and the sync were separated first, then two optos used to control the TV. That was perhaps 30 or 40 years ago.A bit of Google should turn up some circuits.

author: MIS42N
author_rank: Frequent Contributor
timestamp: February 15, 2021, 08:52:56 am
content: AFAIK composite video is DC and shouldn't go through a cap. 0 volt is used as a sync pulse, something +ve is used as black level, the picture signal lives above the black level. If the signal goes through a cap then a clamp circuit is needed to stop the output signal going negative.

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2021, 12:20:08 pm
content: CVBS is indeed DC blocked, which you'll see if you look in a service manual for a VCR or a TV with SCART input. But 47 uF is too small, I usually see two 470 uF back-to-back in series.The TV should have DC level restoration based on the sync extraction circuit, but this in only true if it's already prepared for CVBS input.

author: gnuarm
author_rank: Super Contributor
timestamp: February 15, 2021, 12:53:03 pm
content: Quote from: Benta on February 15, 2021, 12:20:08 pmCVBS is not the same thing as composite video even if they are similar. I don't think composite video is expected to be other than the standard DC levels. So DC blocking is not required. It is when the video signal comes from other circuits like an RF demodulator that the DC needs to be restored. Apparently this is also true for CVBS signals. The OP didn't mention this interface, but if that is what he is using, then I guess he will need the cap.

author: Cykar
author_rank: Contributor
timestamp: February 15, 2021, 04:46:01 pm
content: Thanks all.Quote from: gnuarm on February 15, 2021, 12:53:03 pmNo, I'm not using an RF demodulator or anything else, just an RCA plug to connect to a dvd player.

author: Benta
author_rank: Super Contributor
timestamp: February 16, 2021, 12:30:29 am
content: Quote from: gnuarm on February 15, 2021, 12:53:03 pmThis is new to me, and I'm intrigued.Please explain the difference between "composite video" and CVBS. Please list the voltage levels and differences in signals and transmission as well as other differences.I've only worked with analog video a couple of decades, so I'm open to learn.Go ahead! Tell us!Hint: Composite video and CVBS are two terms for exactly the same thing.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 12:45:53 am
content: Hmmm,CompositeVideoBasebandSignal aka composite video.

author: helius
author_rank: Super Contributor
timestamp: February 16, 2021, 12:57:45 am
content: Well if you want to gettechnicalyou can have digital composite, the format used by the D-2 and D-3 VTRs, defined by SMPTE 259M-A and -B.But that is not the subject that was being discussed.

author: helius
author_rank: Super Contributor
timestamp: February 16, 2021, 01:01:01 am
content: Quote from: JohnnyMalaria on February 16, 2021, 12:45:53 amNo, that is an ahistorical and incorrect name made up by some marketroid or wikiidiot.CVBS is a mixture of four video signal parts, theColor subcarrier,Video,Blanking, andSync and that is why it is called that.

author: BrianHG
author_rank: Super Contributor
timestamp: February 16, 2021, 01:08:13 am
content: Whether color or monochrome, the 15.7Khz composite signal supposed to be 'DC Restored', clamped during the color burst signal. The amplitude between the bottom of the H-Sync and middle of the color-burst is usually used by VCRs to judge the video level to see if any +or- gain is required on the picture signal.Make sure you use at least 470uf if not 1000uf when sending CVBS to a 75 ohm load. If you are driving the input pin of a color decoding IC, they usually expect their input to be DC filtered by a 0,1uf cap which that color decoding IC or an NTSC/PAL sampling/decoding IC will clamp the reference black level during the color-burst timing.If you are driving that video with a class A transistor buffer to the 75 ohm load, make sure that you have enough current drive to supply an un-crushed video signal even if the cap had a DC voltage of 0v up to 2v. (Unlike the first old RCA & Sony satellite dishes which occasionally crushed their video output when jumping back and forth from a white to a black picture with the right speed, especially when Macrovision was enabled with those 2v peaks (under load, otherwise it was 4v peaks) around the vertical sync.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: February 16, 2021, 01:46:53 am
content: Well,CVBS Composite Video Baseband SignalCVBS Composite Video Blanking and SynchronizationCVBS Composite Video Broadcast SignalCVBS Composite Video, Burst, SyncCVBS Color, Vision, Blanking, SynchronisationCVBS Color subcarrier, Video, Blanking, and SyncI'd like to see the official, original definition.

author: gnuarm
author_rank: Super Contributor
timestamp: February 16, 2021, 06:04:18 am
content: Quote from: Benta on February 16, 2021, 12:30:29 amWhen I looked it up yesterday the page I found said it could convey DC power over the same pair, but I don't see that now. I guess that was someone's adaptation.Why come up with a new name for an existing signal? "Composite video" was fine for decades. I've never seen the abbreviation CVBS until this conversation. Maybe people got "composite" mixed up with "component" video.

author: Gyro
author_rank: Super Contributor
timestamp: February 16, 2021, 10:32:26 am
content: There needs to be a capacitor somewhere in the CVBS signal chain so that DC restoration can take place internal to the TV, based on the sync pulse magnitude.The Video input needs to be terminated by a parallel 75R resistor (hopefully part of the conversion) to get the correct video signal level. The DVD player will include a 75R series resistor in its output which is driven by twice the video voltage swing (2V p-p peak white to sync tip), so if your conversion doesn't have a 75R input resistance, you won't get the correct 1V p-p video level.Regarding the capacitor. There are two standard practices in operation. In Europe, the coupling capacitor is located in the video input circuitryafterthe 75R terminating resistor (is there one included in your conversion?), and the Video output in the sending device is DC coupled. This minimises the size of the capacitor (100nF is common), because it only has to couple into high impedance circuitry.The US practice is to AC couple the Video outputs. This means that Video sources must include a large electrolytic coupling capacitor to be able to drive a 75R load without distortion. This is pretty wasteful of board space. The two systems aren't incompatible - worst case, you just get an additional capacitor in the chain.Can you provide more detail of your CVBS input conversion? As long as it already has 75R termination and an internal coupling capacitor, there's no need to include an additional one.P.S. I'm hoping that the TV isn't old enough to be a live chassis type. If it is, then the conversion isn't safe.