How Do PAL and NTSC Really Work?
Many projects on these pages do clever things with video. Whether it’s digital or analogue, it’s certain our community can push a humble microcontroller to the limit of its capability. But sometimes the terminology is a little casually applied, and in particular with video there’s an obvious example. We say “PAL”, or “NTSC” to refer to any composite video signal, and perhaps it’s time to delve beyond that into the colour systems those letters convey.
Know Your Sub-carriers From Your Sync Pulses
A video system of the type we’re used to is dot-sequential. It splits an image into pixels and transmits them sequentially, pixel by pixel and line by line. This is the same for an analogue video system as it is for many digital bitmap formats. In the case of a fully analogue TV system there is no individual pixel counting, instead the camera scans across each line in a continuous movement to generate an analogue waveform representing the intensity of light. If you add in a synchronisation pulse at the end of each line and another at the end of each frame you have a video signal.
But crucially it’s not a composite video signal, because it contains only luminance information. It’s a black-and-white image. The first broadcast TV systems as for example the British 405 line and American 525 line systems worked in exactly this way, with the addition of a separate carrier for their accompanying sound.
The story of the NTSC colour TV standard’s gestation in the late 1940s is well known, and the scale of their achievement remains impressive today. NTSC, and PAL after it, are both compatible standards, which means they transmit the colour information alongside that black-and-white video, such that it doesn’t interfere with the experience of a viewer watching on a black-and-white receiver. They do this by adding a sub-carrier modulated with the colour information, at a frequency high enough to minimise its visibility on-screen. for NTSC this is 3.578MHz, while for PAL it’s 4.433MHz. These frequencies are chosen to fall between harmonics of the line frequency. It’s this combined signal which can justifiably be called composite video, and in the past we’ve descended into some of the complexities of its waveform.
It’s Your SDR’s I and Q, But Sixty Years Earlier
An analogue colour TV camera produces three video signals, one for each of the red, green, and blue components of the picture. Should you combine all three you arrive at that black-and-white video waveform, referred to as the luminance, or as Y. The colour information is then reduced to two further signals by computing the difference between the red and the luminance, or R-Y, and the blue and the luminance, or B-Y. These are then phase modulated as I-Q vectors onto the colour sub-carrier in the same way as happens in a software-defined radio.
At the receiver end, the decoder isolates the sub-carrier, I-Q demodulates it, and then rebuilds the R, G, and B, with a summing matrix. To successfully I-Q demodulate the sub-carrier it’s necessary to have a phase synchronised crystal oscillator, this synchronisation is achieved by sending out a short burst of the colour sub-carrier on its own at the start of the line. The decoder has a phase-locked-loop in order to perform the synchronisation.
So, Why The PAL Delay Line?
There in a few paragraphs, is the essence of NTSC colour television. How is PAL different? In essence, PAL is NTSC, with some improvements to correct phase errors in the resulting picture. PAL stands for Phase Alternate Line, and means that the phase of those I and Q modulated signals swaps every line. The decoder is similar to an NTSC one and indeed an NTSC decoder set to that 4.433MHz sub-carrier could do a job of decoding it, but a fully-kitted out PAL decoder includes a one-line delay line to cancel out phase differences between adjacent lines. Nowadays the whole thing is done in the digital domain in an integrated circuit that probably also decodes other standards such as the French SECAM, but back in the day a PAL decoder was a foot-square analogue board covered in juicy parts highly prized by the teenage me. Since it was under a Telefunken patent there were manufacturers, in particular those from Japan, who would try to make decoders that didn’t infringe on that IP. Their usual approach was to create two NTSC decoders, one for each phase-swapped line.
So if you use “NTSC” to mean “525-line” and “PAL” to mean “625-line”, then everyone will understand what you mean. But make sure you’re including that colour sub-carrier, or you might be misleading someone.