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?
I also notice that the Irish plug type specified is "A" (Europlug). I think that's something to do with our TV system too though. Irish TVs have to ship with UHF and VHF & cable ready hyperband tuners as most of the country uses wide band cable for analogue tv and RTE uses PAL I (PalPlus widescreen & Nicam digital stereo) on a mix of UHF and VHF to broadcast RTE 1, Network 2, TV3 and TG4. Apparently VHF still gives better overall coverage in very hilly and mountanous areas (e.g. South west of ireland). RTE have been switching off VHF transmitters and adding extra UHF stations though to make frequencies available for DTT (Digital TV) and Digital Radio.
Yes, even though many countries throughout Western Europe use the 625-line PAL standard, there are still quite a few variations of it. As I mentioned before, the vision/sound spacing is 5.5MHz in system B (West), 6.5MHz in system D (East), and 6MHz in system I (UK/Ireland). Other differences include the overall video bandwidth, bandwidth of the vestigial sideband, and so on.
Channel allocations are another matter entirely. UHF allocations are the same thoughout Europe, although the sound carrier frequencies vary because of the spacing mentioned above. For VHF, most of Western Europe uses a common set of channels. That includes Germany, Austria, Switzerland, Netherlands, Sweden, Norway, Spain, etc. France went its own way as usual, with its 819-line system and unique channels, and as you noted, Italy adopted its own channels, identified with letters rather than numbers.
Similarly, most of Eastern Europe uses a common set of (different) channels, the OIRT allocation referred to in an earlier post.
Britain's VHF used its own channels 1 through 13, but broadcasts were to a different standard entirely - The old 405-line monochrome network (system A), which was finally shut down in 1985.
The allocation of VHF channels in the Republic of Ireland is more complex because there were two sets of channels sharing the same spectrum.
TV broadcasting didn't start in Ireland until the end of 1961. By that time, most countries in Europe, including Ireland, had decided that they would all ultimately adopt the 625-line standard.
However, many people in the RoI were already receiving British transmissions from across the border using imported 405-line receivers. So to keep them happy, Telefis Eireann (later RTE) opened 405-line transmitters to serve those areas where TV receivers were already in use (north & east), while going straight to 625 in other areas (e.g. the southwest). The 405-line transmitters, quite naturally, followed the British channel numbering 1 through 13, while the 625-line service was allocated the unique Irish channels A through J, which you still use today.
[This message has been edited by pauluk (edited 01-15-2003).]
Trumpy, Each of the systems has various merits and drawbacks. You also have to look at the combinations used, for example throughout almost all of the world the PAL color system is coupled with 625-lines and the NTSC color system is used with 525-lines. The inherent difference in resolution between 525 and 625 can't really be attributed to the difference between NTSC and PAL, of course (and there are one or two exceptions to the PAL-625/NTSC-525 rule).
As I mentioned before, NTSC vertical color resolution is better than that of PAL or SECAM, even with its fewer lines to start with. On the other hand, PAL and SECAM survive phase errors much more happily and don't need the phase angle adjusted with a separate hue control.
Whatever the technical specifications, given a good signal path and properly adjusted equipment all three can give an excellent picture. Once you start talking about a poor signal, be it due to multipath distortion on a broadcast signal or due to a poor cable network, the picture can degrade fast on all three.
Believe me Paul, you don't need to tell me about faults in Coaxial cables!. I've seen the whole spectrum of them, from a simple fault caused by a bad connection at the Aerial, to a complex fault in a Multi-point MATV system, caused by the person owning the house, looping all of the points in the house together, in a Daisy-Chain fashion, to save using a splitter in the roof void, these people always(and I mean always!) complain, when the Bill comes in for the job, to fix it all up.
[This message has been edited by Trumpy (edited 01-16-2003).]
RTE has pretty much replaced the VHF network bar in one or two areas with UHF transmission sites. It doesn't seem to be anything to do with improving quality it's simply to free VHF spectrum up for other services. TG4 and TV3 are only available on UHF. Channels A-J are pretty much gone.
Irish cable systems use Band I, II, III and UHF though and as nearly 2/3 of the population recieves analogue tv from cable networks tvs sold here must come with Band I/II/III and UHF tuners (hyperband) otherwise you can't watch the full analogue cable service (typically about 18 channels with no decoder) As digital TV is becoming more and more common I doubt that it'll make much difference as long as a TV can accept PAL or RGB via a SCART lead.
There are also spots that use VHF transmission for RTE 1 and Net 2 so UK TV's won't necessarily work.