EBU Timecode/Broadcast History Question

[citysoundman]

Hi Everyone,

I've started to teach post production recently at IAR. I introduced the 29.97 frame rate, and after explaining why NTSC color required a change of reference from 60Hz to 59.94, an excellent question was asked that I couldn't answer. The question is why didn't EBU need to change their reference when color broadcast began in Europe?

I framed my NTSC explanation in this context: Black & White broadcast was using a 60 Hz reference, and when color was ready to broadcast a new reference was required - the same 60 Hz ref could not be used for both B & W and Color broadcast. So a student then asked why didn't the EBU need to adjust their 50 Hz reference for color broadcast in Europe? Why are those guys so lucky as to have only one frame rate for everything??

Thanks In Advance!

[Sonsey]

Hi Bob,

The PAL system used in Europe, carries the colour (sorry, I'm Canadian, I insist on the "u") information in a different manner then the NTSC used here in North America. So the same POTENTIAL problem wasn't an issue when colour was introduced. Of course there is still some debate as to whether or not there would have actually BEEN a problem, but the potential was there, hence 29.97

Here's a good Wikipedia on it:

http://en.wikipedia.org/wiki/PAL

[citysoundman]

Thanks Sonsey.

[Richard Barrie]

This is actually a very subtle question. The reason that there was a potential problem was that it was decided that colour broadcasts should still be backward compatable with black and white.

I don't have my engineering textbooks to hand but have cut and pasted some snippets I have Googled :-

The North American NTSC system added color to an existing black-and-white TV standard as follows: a subcarrier frequency was chosen which was half of an odd multiple of the horizontal frequency; and then, this subcarrier was modulated so that its phase indicated hue and its amplitude indicated chrominance.

The choice of subcarrier frequency was made so that in any area of uniform color, the peaks and troughs of the color signal, when seen on a black-and-white TV set, would occur at opposite locations on successive lines of the picture, thus minimizing its obtrusiveness. It was chosen to be 227 1/2 times the horizontal frequency which is 3.579545MHz. To keep it from interfering with the audio carrier, which was 4.5 MHz from the video carrier, the field rate in the NTSC system was changed from 60 Hz to 59.94 Hz.

This changed the horizontal scanning frequency to 1/286th of 4.5 MHz, so that the audio carrier would be located in a trough of the spectrum of the signal produced by the color portion of the scene if it repeated from one horizontal line to the next, since the Fourier transform of a signal that repeats in every line would consist of multiples of the horizontal scanning frequency; added to the color subcarrier at 227 1/2 times the horizontal frequency, then, the spectrum would include 285 1/2 times the horizontal frequency and 286 1/2 times the horizontal frequency. Since the audio spectrum had no particular relationship to horizontal scanning, this was not done to protect the sound from interference; instead, it was done to protect the color component of the picture: just as, in NTSC, the effect of the color subcarrier on the black and white picture tended to cancel out from one line to the next, so the effect of the audio carrier on the color picture would tend to cancel out from one line to the next.

Fortunately with PAL the numbers work out exactly so a change in frame rate was not required :-

The PAL subcarrier frequency is 4.43361875 MHz. This is derived from 284 subcarrier cycles, or dots, in each line period. This would give about 231 visible dots on each picture line - the active line period. They would all be stationary (in the same position in each frame and field) ,and all in the same position on each horizontal picture scanning line, forming a highly visible static pattern of vertical stripes. Offsets are added to make these lines move, and tilt them over diagonally. The quarter line offset means 283.75 cycles per line, one quarter of a cycle less on each line. The only other offset is the 25Hz offset, which means one more cycle per frame of 625 lines, i.e. 0.0016 extra cycles per line. Hence the actual number of subcarrier cycles per line 283.7516 This is why the PAL subcarrier frequency is what it is. Multiply the number by 625, the number of lines in a frame, and then by 25, the number of frames in a second, and it comes to the number of subcarrier cycles per second, 4,433,618.75

You should check for a more rigorous explanation in any of the standard textbooks. The choice of sub-carrier frequency is also the reason why, in the good old days of analogue video editing, a perfect field match was only possible once every eight fields in PAL, but four fields in NTSC.

[citysoundman]

Hey Richard,

Thanks for the wonderful reply! I honestly don't know if I could find a MORE rigorous explanation .

Now if I can only memorize this and shoot it off to my students - wouldn't they be impressed!

But seriously, this is the kind of info that no matter how many times you learn it, you need to refresh yourself in order to teach it.

Best Regards,
Bob