How the PAL Colour Video System Works
This is a bit of background for those interested in the workings of my Commodore 64 PAL video decoder.
PAL video encoding process and
Luminance and Colour Difference
For backwards compatibility with monochrome television, instead of
encoding R, G and B directly, three other signals are derived:
luminance (Y), and two colour
difference signals R-Y and B-Y. These are weighted so that for a
monochrome signal, R-Y and B-Y are both zero.
Colour information is transmitted by adding a colour subcarrier to the luminance.
The subcarrier is modulated with R-Y and B-Y using quadrature amplitude modulation.
This means that R-Y and B-Y are used to amplitude modulate two carriers
of the same frequency but with a 90 degree phase difference. Balanced
modulation is used, so that the carrier is suppressed. Mathematically,
the chrominance signal C is
(R-Y) sin 2πft + (B-Y) cos 2πft
where f is the colour
subcarrier frequency (about 4.43MHz for PAL). Note that if R-Y and B-Y are both zero, then C is
zero, and the composite video signal consists only of luminance.
The following diagram depicts C as a vector, whose projections onto the
axes represent R-Y and B-Y. You can also think of the phase of the
vector as representing the hue of the colour, and its length the
To demodulate the subcarrier, the receiver needs a phase reference.
This is provided by inserting a sample of subcarrier having a known
phase just after the horizontal sync pulse. This is known as the colour burst.
We're not quite done yet. There is one more twist, designed to reduce
the problem of phase errors in the received chrominance signal. This
can happen if, for example, you're getting a signal directly from the
transmitter and also bouncing off a nearby building. The reflected
signal takes slightly longer to arrive, so it's shifted in phase with
respect to the line-of-sight signal. Adding them together gives a
vector in a slightly different direction, resulting in a hue error.
(This problem is why some people say that NTSC stands for "Never Twice
the Same Colour".)
In the PAL system, the phase of the R-Y component is inverted on
alternate lines (hence "Phase Alternating Line"). This has the effect
of reflecting the chrominance vector about the B-Y axis. Then, in the
receiver, the chrominance for the current line is averaged with a copy
of the chrominance from the previous line with R-Y inverted again. This
cancels out the phase error, at the expense of a slight change in
saturation, which is much less noticeable.
So that the receiver can tell whether an odd or even line is being
received (known as "ident"), the phase of the colour burst alternates
between 135º and -135º relative to B-Y (this is called a "swinging
|Cancelling a phase error ø. Here c1 is the intended chrominance vector, c1' is the received one. Averaging c1' and the reflected c2' gives a vector with the same phase as c1.
Here's a diagram of what goes on in the receiver to recover R, G and B from the composite video signal.
First, filters are used to separate the luminance and chrominance. (If
separate luminance and chrominance are already available, as they are
in my C64 video decoder, this step is omitted.)
A phase-locked loop keeps a local subcarrier oscillator aligned with
the average phase of the colour burst. The local oscillator produces
two reference carriers 90 degrees apart for demodulating the R-Y and
G-Y components. The phase of the colour burst is also used to detect
whether an odd or even line is being received. A flip flop stores this
information and uses it to switch the phase of the input to the R-Y
Chrominance from the previous line is added to the current line at the
input of the B-Y demodulator, and subtracted at the R-Y demodulator,
thereby mirroring the delayed chrominance about the B-Y axis as
required to achieve phase error cancellation as described above.
The luminance is usually passed through a short delay line to
compensate for the fact that the chrominance is delayed by having to go
through more processing.
Some arithmetic is performed on R-Y and B-Y to recover G-Y, and finally Y is added to all of them to produce R, G and B.