Composite Video Signal
On my quest for the perfect composite video mod for retro consoles, I inevitably ended up making my own (aptly named the CleanComp). I started with the Atari 2600 and the perfect test candidate game... E.T. 🤫
After a lot of research, some real-world scoping, and many trial PCB designs I finally understood the signal completely.
A nice side-effect to this was I naturally discovered exactly how S-Video, RGB and RF signals are formed as well, along with the quirks of the earlier years when standards were not yet agreed upon. I now fully understood how and why ghosting happens, blown out brightness, shifting colours, checkerboards and many other visual artefacts on the signal appeared, and how to solve them.
Composite video is a form of signal designed to carry one of several video standards - primarily 525 line (American 60Hz), or 625 line (European 50Hz) as the most common signals. It is a single-wire interface meaning all the video data is carried on a single cable of ground and video (the yellow phono lead on your video RCA composite cable, with the red and white being the audio data.
We will focus on the 525/625 line video standard for now as it is the most common type. They are both pretty much identical and once you understand one you will understand the other.
Let's start by looking at probably the first image you will discover if googling for composite video signal.
This is a great start to understanding how the signal is formed. Let's begin with what we are seeing here. We are seeing a single line of video data in this screenshot. This is a single scanned horizontal line of data that would be shown on a CRT back in the day.
A single line data line the image above runs 50 times per second for 50Hz or 60 times per second for 60Hz. The output image can be progressive (each frame contains all the lines of the video signal) or interlaced (each frame goes between odd then even lines, forming a double vertical resolution image every 2 frames, running at 25/30 frames per second.
Interlaced meaning each full frame drawn to the screen is all the odd lines (line 1, 3, 5, 7 etc...) then the next frame is the even lines in between (line 2, 4, 6, 8 etc...). Typically, this is shown in resolutions as an i, for example 480i (meaning 480 vertical lines, interlaced). On the other hand, 480p (meaning 480 lines progressive) means the entire frame is drawn in a single pass. All modern forms of display are now progressive, so you no longer see the p at the end of resolutions these days.
Disconnected Video Output
A very important thing to remember when making a composite video signal is that the signal is expected to output at 75 ohm impedance, as the TV / receiver has a 75 ohm pull-down resistor, meaning the final signal once the cable is connected to the TV / receiver will get halved.
This means the output for a disconnected composite video pin on a device like an Atari 2600 should be double the voltage mentioned here. For example, the color information mentions 300mV amplitude, so the Atari should generate 600mV signal that has a 75 ohm impedance.
A great reference for all the video standards and expected voltages (the expected voltage when connected to receiver) can be found here.
The AC pulses in the image above labelled Color Burst is used to to synchronize the 3.58 / 4.43 MHz color oscillator in the television set or receiver with the transmitted signal's color reference. This pulse should be 300mV in amplitude (+150mV and -150mV around the blanking level).
The AC pulses in the image above labelled Chrominance contains the chroma (color) information in the form of pulses whose phase (the offset in time horizontally to a reference pulse) define their hue (color such as red, blue, green etc...) and the amplitude (the voltage level from top to bottom) of the pulse indicates the saturation (amount of color, so 0 amplitude with red hue is black, 100 amplitude is pure red).
Although most specifications state the colorburst is 300mV but the color data that gets super-imposed on the luminance is 600mV for 100% saturation, from testing on VCRs, Megadrives, Ataris and many other consoles I have seen that the color information seems to always ben 300mV, not 600mV. This is also reinforced by the fact the Atarti 2600 TIA outputs the same amplitude signal for the colorburst and the color signals.
The "nothing" part of the signal also known as the blanking level, when nothing is happening (between lines and frame data) is referenced in the image above as 0 IRE and is where the signal comes in from the left side in the image above. 0 IRE is 0.3V in this case.
The lowest part at 0V (the first low pulse) is the Sync pulse which indicates the start of a new line and can indicate the start of a new frame (full screen image). This is labelled H-Sync Pulse in the image above.
The brightness information about the signal, also called the luminance, is provided in the form of an analog voltage level that comes after the blanking signal and color burst.
The color information is also super-imposed on top of the brightness signal as shown in the image above where the brightness level is in purple underneath the AC color signal that rides on top of it.
The IRE Scale
This IRE level is a scale invented by the Institute of Radio Engineers and was designed specifically for measuring composite video signals. In short, 0 IRE refers to 0.3 volt and is called the blanking level because it forms the "blank" part of the signal.
Then -40 IRE is 0V, and 100 IRE (for pure white) is 1V (700mV above 0 IRE).
That is all you will need to know in terms of the voltage levels, and that the IRE is roughly 7.14mV per unit. So 55 IRE would be 55 x 7.14mV = 392.7mV.
From the above information, you can see the expected voltage levels of the signals.
Blanking Signal: 0.3V Sync Signal: 0V (down -0.3V from Blanking) Color Burst: 0.3V Luma Signal: 0.3-1V (700mV range)
Common issues with composite video signals can usually be attributed to specific issues in the signal voltage levels, impedance or timing. Below are some of the most common causes of specific video atrifacts.
Ghosting / Cross Hatching
The repeating signal that appears to repeat the actual video information at a lower intensity to the side of the original is called ghosting.
Cross hatching/checkboard is another issue where you often see a chessboard style repeating ghost pattern.
Both of these issues are usually caused by one of two things:
Chroma/luma or overall composite video signal is too high. So instead of being 1.1V peaks they are too high.
The other cause is impedance of the final signal is not matched to the TV receiving the signal. It should be 75-ohm output impedance. If this is not balanced, or the monitor, or cable is changing the resistance then you can get reflected signals causing ghosting. This can be fixed by changing the impendance of the output signal through a BJT transistor for example.