Here's a very brief and somewhat simplified outline of the basic difference between NTSC, PAL, and SECAM.

All three systems work by deriving color difference signals for transmission. The overall luminance signal is called Y, so the color difference signals are R-Y, B-Y, and G-Y. In all three systems only the red and blue color diff. signals are transmitted (G-Y can be reconstructed at the receiver from R-Y, B-Y, and Y).

Both NTSC and PAL use a principle called quadrature modulation, which is basically a way of putting two distinct signals on one carrier by making sure that there is a 90-degree phase difference between them. NTSC puts each signal "as is" onto the color sub-carrier (to be accurate, they are actually both phase shifted a little to get NTSC's I and Q signals, but let's not get into that!).

Somebody mentioned earlier that PAL was designed as an improved version of NTSC. In simple terms, that's true. PAL uses the same basic modulation principle as NTSC (although without the I/Q phase shifting). The PAL signals after adjustment are called U and V. Where PAL adds its twist is that the V signal is phase inverted on every other line. (Hence the name PAL = Phase Alternation by Line.)

The 180-degree phase difference on alternate lines means that any phase error which causes a shift in color on one line will cause a shift in the other direction on the next line. The errors cancel out on the screen, or in a delay line, as I mentioned in an earlier post.

SECAM works on a different concept which doesn't involve quadrature modulation. In SECAM, the R-Y signal is transmitted on one line and the B-Y signal is transmitted on the next. A delay line stores each signal in turn, so that when a line with the R-Y signal is being transmitted, the receiver gets B-Y from the delay line (i.e. from the previous line), and vice versa. The delay line acts like a memory. Sequential Color To Memory -- Get it? [Linked Image]