01 - Composite Video
Composite video is called such because it is a combination, or composite, of the sync signals and the image information, all into one signal that can be transmitted over a TV transmitter. In North America and Japan, composite is known as NTSC and is 60 fields/sec. In the UK and other places it known as PAL and has 50 fields/sec. A few places in Europe have another standard called SECAM.
This is also known as baseband video. This reference is from the TV transmission world, where a TV receiver will convert the signal from the RF channel, down through multiple stages, until it gets to the baseband signal. This is what comes out of the single RCA jack, usually colored yellow.
To reduce flicker on the CRT TV screen, a 60Hz refresh rate was chosen for NTSC. However to reduce the overall system bandwidth, only half the image is sent with each refresh. On the next refresh, the lines in between the previous refresh are sent. The important thing to know is that the image is shot in the camera in two passes, first the odd lines, the even lines are captured. At the CRT displaying the image, they are presented in the same order in time, all the odds, then the evens. Understanding this is the key to understanding one of the more difficult tasks of a deinterlacer / scaler. More on that later.
There are a total of 525 horizontal lines that make up a complete frame (odd and even fields), however only about 480 of them are displayed. The rest are either bloomed off the screen slightly, or are part of the blanking intervals needed to get the CRT beam back in position to draw another line or frame.
Composite video can also be modulated on an RF carrier channel. Doing so can allow for long distance transmission over standard coax cable. Modulated video will typically use an F connector as shown here. This is no different from what a cable TV system does. They just use many channels over the same coax.
Composite video is a low voltage signal, usually below 1 volt. Looking at it with a scope, it should be about 1 volt peak to peak, depending on the image being sent. The actual DC level may vary, and that is acceptable. Most equipment includes DC restoration to set a proper DC level internally. Using a AC Voltmeter will be misleading since they will typically measure a average AC voltage level, not peak to peak.
Composite video needs about 12 MHz of bandwidth in a cable or through an amplifier, which is pretty easy to achieve with the cheapest of equipment. Almost any 75 ohm coax will be fine here, including any RG59 or RG6 type.
Sidebar to an important issue of Impedance. Impedance is the resistance to an AC signal. Since a multimeter is basically a DC tester, you can’t measure impedance with one, so don’t worry that your RG6 cable (75 ohm impedance) doesn’t measure 75 ohms in resistance.
One bad practice you have probably seen is the use of a “Y” cable to split or combine two video signals between several devices. Impedance is the reason this is bad. In a proper video link (called an transmission line) from one box to another, the source device has a output impedance of 75 ohms, the cable is 75 ohms, and the sink device (receiving end) is 75 ohms. If there are two 75 ohm sinks as would happen with a “Y” cable, the equivalent resistance is half, or 37.5 ohms, which means a half level signal goes to each sink device. It might produce a picture, but not a very good one.
on January 11th, 2007 at 2:56 pm
How can I check for a 1 volt peak to peak with a conventional analog volt meter or a digital volt meter? I don’t want to use a scope in the field. I constantly run into problems with no video going into a DVR. They want to see at least 1 volt peak to peak. I can get a picture on my hand held monitor but apparently it is not enough to drive the DVR because the screen is blue ( or blank ). There must be a way to take this peak to peak reading. Also I assume the 1 volt PP isan ac signal and not a dc.
Thanks,
Joe
on January 11th, 2007 at 3:24 pm
The most you can do with a volt meter on a video signal is to determine that there is voltage activty, or not. With your meter in AC volts mode, it’s usually looking at the mean AC voltage, rather than peak to peak.
With your type of problem, you really need to see the signal to determine why the DVR doesn’t like it. Could be sync levels, DC restoration issues, who knows.
Fluke and others make some ’scope-meters’ that can display like a scope, but are ruggedized, handheld and battery powered, so are good for just this type of use. You might look into those.
on May 1st, 2007 at 1:01 am
Hi
I’m currently researching a project to digitise a PAL composite signal from a CCTV camera. I’m going to keep it simple. leaving all syncs in as it’s only a project to see if I can do it The signal is -0.3V to 0.7V but most ADCs need an input from 0V upwards. What would be the best way to scale it up?
Dave
on May 1st, 2007 at 8:47 am
You’re going to need a chip or circuit to perform DC restoration or clamping. There are chips out there that are designed to be the front end of of an ADC to handle this problem. they will maintain everything at an even level, and shift the signal up so that the entire signal is within the 0-5V range of the ADC.
Here is a good article by Maxim that explains the issues better than I can:
http://www.maxim-ic.com/appnotes.cfm/an_pk/3303
on May 3rd, 2007 at 7:16 am
Thanks, you cleared that one up.
Another question if you don’t mind. The composite input into a TV, is normally designated yellow and using an RCA adapter. Is the composite signal supplied here just the regular 1 Vp-p signal, without any modulation or other modification ?
Thanks
on May 4th, 2007 at 1:52 am
Hi
The composite TV input ( yellow RCA) isn’t a modulated input. Therefore, is it just the nominal 1Vp-p composite signal that it inputted here
Dave
on July 2nd, 2007 at 2:28 pm
Bill,
I am connecting a c-arm with composite output to a transcoder to convert to s-video, and then convert to RGBHV thru a scaler. My transcoder has a NTSC and PAL selector switch. I get flickering in the picture which suggest sync problems and by switching to Pal it cleans up the image except every 5-10 seconds it gliches. Any suggestions?
on July 2nd, 2007 at 3:57 pm
The composite to S-video transcoder has a switch to handle the color decoding differently, as that is the primary difference between NTSC and PAL. There is also an ever so slight difference in the sync levels as well. It’s possible the transcoder is shrinking the sync pulses a little, causing the instability. Can the scaler accept composite video directly? Might be a better solution.
on July 9th, 2007 at 1:25 am
There is “flagging effect” on the upper left corner of the screen and some rolling.I think this is about sync levels.How can we correct this?
on July 9th, 2007 at 9:24 am
Your flagging effect is most likely caused by problems in the DC restoration of the signal. The device outputting the signal to your TV is most likely using AC coupling, which causes effects like this. You can either address it at the source device by replacing it with something that has a DC couple output, or possibly find a device to sit in the middle such as a video sync stabilizer. A sync stabilizer will take the AC couple signal and properly restore the DC levels.