The meaning of Composite
If Y’CbCr is component video (which sends these components via three different cables), then composite video is everything combined into one cable.
There are many mathematical ways (all of them complex) to mash up the luma-chroma signal into one signal. The only thing that need concerns us is to know that composite video is the last mode of analog data transfer we should consider, if no other option is available to us.
The modern digital equivalents of the ‘composite video’ way of doing things is HDMI and Displayport. The idea is to do everything in just one cable, but the method is totally different (HDMI can separate Luma-chroma information).
A composite video signal is measured in IRE (Institute of Radio Engineers). It is a percentage of the total voltage. This is how it looks, if you have the courage to look, that is:
Let me make it simple:
- 0 IRE is 0 volts is black.
- 100 IRE is white.
- There is also a sync pulse of 40 IRE that is sneaked in below 0.
- Therefore, the total IRE of a composite signal is 140 IRE, and the total voltage is about 1 V.
Signals are measured in percentages compared to 140 IRE. Since the total voltage is 1V, the actual image is contained within approximately 0.7 V or 700mV (milli Volts).
You can raise the amplitude of this signal, but the IRE must remain constant. PAL, NTSC and SECAM are all transferred this way.
For PAL, black is at 0 IRE and white is at 100 IRE.
For NTSC, black is at 7.25 IRE (US), and 0 IRE (Japan); and white is at 100 IRE.
Why is this important? IRE is like quality control, and it is one of those properties that directly measure the strength of a signal. Low IRE values could mean the signal isn’t strong enough, or it doesn’t contain enough information. In either case, poor IRE values are a cause for concern.
What it means for HDTV
Digital information no longer has to be encoded this way, but the signal still needs to run electrically. This is called the Video Payload. The HDTV payload specifies a peak-to-peak voltage of 800 mV.
For HD-SDI 4:2:2 Y’CbCr:
- Y has a maximum of 700mV (encoded from 0 to 700) for luma, while
- Cb and Cr each have 700 mV (encoded from -350mV to +350mV), for chroma
For HD-SDI 4:4:4 RGB, all three channels have 700 mV each, and are encoded from 0mV to 700mV, similar to Y’. As you can see, even in the digital age, signals still have the old ‘genes’ running in its veins. This is what allows HDTV to be broadcast on the old systems even today.
Since these signals are encoded digitally, in hexadecimal notation, you could embed data outside these limits – just like you can write billions on your bank check even if you can’t cash it.