I'm trying to learn about analog video signals like PAL,VGA and S-video.

I recently read about PAL signal It says

Each PAL frame consist of 625 lines, 25 frames per second

However It says nothing about length of the lines.

I am more familiar with digital graphics where image is made of discrete pixels and each image has a well specified resolution which defines the no of pixel in each horizontal and vertical line like 800x600

So does image resolution mean anything in PAL standard? If so what could be the maximum resolution supported! or if it is dependent on something else how is it related?

closed as off-topic by Dave, Shevek, a CVn, James P, Tog Dec 14 '13 at 13:38

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  • 3
    @Chenmunka: We do not need pixels to talk about resolution. – pabouk Dec 13 '13 at 13:08

Yes, the PAL standard does have a vertical resolution and we can estimate the horizontal one. PAL is analogue but in most cases we can safely use the standard digital resolution 704×576 to represent a PAL picture. The details are below.

The vertical resolution is truly discrete

PAL is an analogue signal however it is defined to have 625 discrete lines in a frame because the lines are transferred and drawn discretely one by one. The visible part of the picture is defined to have 576 visible lines. The invisible part is needed for the beam in CRT to return back to the beginning to start drawing a new frame (the picture is drawn from the top left corner line by line).

The horizontal signal is fully analogue

As Hoppo wrote, in the horizontal direction there is no distinction of discrete pixels because this signal is analogue so the actual resolution depends on the bandwidth of the signal. For PAL the allowed bandwidth is defined to be 4.2 - 6 MHz (depending on the PAL variant). The whole line lasts 64 µs but the visible part is 51.9 µs. Again the rest is used for the beam in CRT to return from the right to the left. The bandwidth in the course of the duration of the visible part could allow up to about 622 "pixels" per line. Note also that the colour information has a lower bandwidth so its horizontal resolution is lower in analogue PAL.

For more information see PAL.

The corresponding digital resolution

There is also a corresponding digital resolution called 576i (or one variant of standard definition television) which is defined to have 576 lines with 704 (sometimes 720) pixels. The same frame frequency 25 Hz as in PAL is being used. See 576i and standard-definition television. In most cases it is safe to use this corresponding digital resolution 704×576 or 720×576 to represent a PAL picture.

The "i" means that the picture is displayed as interlaced. One frame (takes 1/25th of a second) is in fact displayed as two fields (50 per second). One field displays the even lines the second one the odd lines of the single frame. The full name of the resolution including the field rate is 576i50 or 576i/25 when referring to the frame rate.

  • 2
    How does it happen that both 720x576 and 704x576 are "valid" PAL resolutions if the bandwidth is constant? Or is the resolution tied to specific standard/bandwidth? – gronostaj Dec 13 '13 at 13:53
  • @gronostaj: There is no single constant bandwidth. In fact there are several variants of PAL with bandwidth of 4.2 to 6 MHz for the picture signal. See PAL. The horizontal resolution of 576i does not correspond to PAL exactly. It is just a choice. In fact it seems that the choice of 704 or 720 horizontal pixels was arbitrary one above the rough maximum horizontal resolution of 622 pixels. See 576i. I think that the value is divisible by 8 because of certain hardware design limitations. – pabouk Dec 13 '13 at 14:10
  • @gronostaj: The additional 16 pixels in 720 should not convey the picture. See Standard-definition television so the real standard horizontal resolution should be 704. --- It seems that only 704×576 should be used for the picture. I do not know why a modern hardware should need additional 16 off-screen pixels for blanking (invisible CRT beam return movement). – pabouk Dec 13 '13 at 14:11

The PAL system is an analogue system, therefore as such there is 'effectively' infinite resolution, or resolution down to the controlability of an electron beam. The line timing of a PAL video system is 64uS, which is worked out by calculating the reciprocal of frequency multiplied by the frame rate i.e. 1/(625*25) Therefore each horizontal line takes 64uS to move from left to right on the screen. So in answer to your question. No. image resolution in PAL is set at a standard and will not change. If you want to sample it and digitise it, then the sample rate can be set to any figure you choose, and of course the greater the sample rate then the higher the quality, up to the original PAL resolution, or to the point where the human eye cannot detect a difference !

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