PAL/SECAM and NTSC systems


Introduction
There are two major television recording, broadcasting and viewing systems in the world: PAL/SECAM and NTSC. Video tapes produced with the PAL/SECAM system cannot simply be played back on a NTSC system, and vice versa. The difference between the two systems is to do with scanning lines, frame and colour frequencies.

Field and Frame
While recording with a 2 head video drum, one video head records only all the odd lines from the scanning lines from a complete frame (image). The result is an image with half of the scanning lines, which is called field-1. The second video head only records the even lines from a complete frame and that is called field-2. The resolution of one field is of course half the resolution of a complete frame (image). Therefore an image on the screen appears less sharp when a still frame or freeze is performed from only one field.

PAL/SECAM
The system uses 25 frames (images) per second. Each of these frames consists of 625 horizontal lines. If one video head is used, it produces all 625 lines with every rotation to form one frame. If two heads are used, the first one records all odd lines of the image (line 1-3-5-7-…-625); these odd lines together form the first field. The second head records the even lines (2-4-6-8…-624) which form the second field.
This means that each second 50 fields of half the amount of scanning lines are produced. Field one and two together form one frame or image.

NTSC
This system uses 30 frames per second. Each of these frames consists of 525 horizontal lines. The two video heads record the image on the same principle as with PAL/SECAM: one head takes care of all the odd lines, the other all the even ones. Every second 60 fields of half the amount of scanning lines are produced. Field one and two together form one frame or image.

25 versus 30 frames per second
The difference in frame frequencies (25 versus 30 frames per second) originated from different frequencies of the electric mains in various countries (50 Hertz in Europe against 60 Hertz in the US and Japan). In TV sets the mains frequency was easily divided by 2 to get the right frame frequency to synchronise the video signal.

To ensure the video signal from transmitted by broadcasting companies remained stable on the receiving television sets, the frequency was synchronised with the frequency of the mains supply. In the past, when the frequency of the mains from the studio deviated from that of the mains at home, the image started to roll vertically over the television screen. By adjusting the television set the image could then be stabilised.

Nowadays high quality quartz crystals are used for electric generators for synchronisation with extremely accurate and stable frequency values so these problems no longer occur. Exactly the same high quality quartz crystals are employed in all modern equipment for recording and playback, for studios and for consumer electronics such as video cameras, MD, CD, DVD etc. This is called ‘Absolute Time’.

PAL/SECAM and NTSC playback
The differences in image frequencies also explain why video signals recorded in one system cannot be played back by the other. When a NTSC video signal with 30 frames per second is played back on a PAL/SECAM machine, the machine will try to synchronise the video tape at 25 frames, with the result that the tape is played back too slowly for faithful reproduction. This can be noticed immediately by the audio because of the low speed. Conversely when a PAL/SECAM tape is played back on a NTSC machine, the machine will try to speed up the tape resulting in distorted audio as the track is played too quickly.

Because an NTSC video signal has 30 frames per second, there will be 5 frames per second that cannot be ‘fitted in’ in a PAL/SECAM system. When an NTSC video signal is converted to a PAL/SECAM system, the number of frames per second is reduced by leaving out one frame every 6th image. As such the image becomes a little jerky , particularly when there is a camera movement in the picture, as with panning or zooming. When the credit titles are shown scrolling over the screen this effect is somewhat annoying when viewed on a PAL/SECAM screen.

Conversely, when a PAL/SECAM video signal is converted to a NTSC video signal, 30 frames have to be created from the original 25 frames. This is done by duplicating every 5th frame, which similarly results in a jerky image when there is a camera movement in the picture when viewed on a NTSC screen.

The frame, colour frequencies and amount of image lines are different between PAL, SECAM and NTSC. When playing back different video systems viewing problems can be avoided by using a Multi-System player and screen because they can switch between PAL/SECAM and NTSC. The colour frequency of the NTSC signal is not fixed as with the PAL/SECAM system, when viewing NTSC you can adjust the colour of the screen by using the hue adjustment knob.

An Amusing Anecdote (from the beginning of the 1960)

From a technical room in the television studio in Bussum, Netherlands, technicians could see into the living room of the butcher who had his shop and lived opposite the studio.
The butcher and his family were watching television and one of the technicians had a funny idea. If he changed the speed of the studio sync. generator a bit, the image on the television set would start rolling vertically. The result was that after a while someone must get up to correct the tv set.
The plan was to adjust the studio sync. back, just before they arrived at the tv set so they returned to their seat again.
One evening the technician start to change the studio sync. and across the street they could see the image on the tv set start rolling. After seconds someone got up and walked to the tv set. Just before they arrived the technician turned back the studio sync. so they sat down again.
The technician repeated this several times knowing that in the whole country everyone would be getting up to fix the tv for nothing. Lots of fun and hilarity for all the technicians.