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Know It All: 625i and 1125i -- Entertai1n1n6 Numb3rs

Published in Geare magazine, Issue #56, 2009

Some numbers have become second nature to those interested in home entertainment technology. Numbers such as 576 and 1080, for example. These are, respectively, the vertical resolutions of standard definition and high definition TV. The first is also the vertical resolution of most Australian DVDs.

From time to time you will see another number: 480. That is, of course, the US standard definition/DVD vertical resolution.

But on rare occasions other numbers pop up. Confusingly, they pop up as alternatives to the above numbers. Sometimes our TV standard is not referred to as a 576 pixels system, but a 625 line one. High definition can be called 1125 rather than 1080. And the US TV system is 525, not 480. What gives?

Well, it all comes from the days of analogue TV, but it still retains its relevance.

In fact: 576i ('i' is for interlaced) is the same as 625i. 1080i is the same as 1125i. 480i is the same as 525i. Of those display devices which report the figures at all, most report the former ones, but some report the latter.

Why the two numbers? Let's consider 625 and 576. And, to complicate matters, 585!

With an analogue CRT TV, the picture is created by 'painting' the end of a cathode ray tube with a sequence of 25 image frames in each second. The 'paintbrush' is a beam of electrons and it does the painting by drawing a series of horizontal lines across the screen. The speed at which the beam of electrons proceeds is that necessary to allow 625 of those lines to be painted in that 25th of a second.

However, not all that time is used in the actual painting. The beam traces a horizontal line from one side of the screen to the other. At the end of the line the beam of electrons has to be switched off very briefly as the electron gun is swung back (not mechanically, but using electro-magnetic fields generated by coils) to the other side of the screen, and down a notch, to paint the next line. This period in which the beam is switched off is called the 'horizontal blanking interval'.

Each line occupies in total 64 microseconds of time, of which 52 is spent drawing the line, and 12 in returning the beam to the other side of the screen ... and other activities. Also within this 12us, the TV signal includes a sharp 'pulse' for synchronisation and, in colour systems, a colour burst (ten cycles at 4.43MHz in our system). The details aren't important here. What is important is that of the time allocated for each line to be drawn, nearly one fifth is used up in doing other stuff.

Now each frame, which takes one 25th of a second to be transmitted, consists of two fields. The electron beam traces a series of horizontal lines, starting at the top of the screen and working its way down to the bottom. Once it gets to the bottom it has to jump back up to the top to start the next field. The lines are drawn at the rate necessary for 312.5 (half of 625) of them to be produced in one fiftieth of a second, but not all are in fact drawn. In analogue TV systems, typically only 292.5 are produced. The other 20 lines worth of time is consumed by the beam jumping back up to the start point, plus doing other stuff. That other stuff includes another synchronisation pulse (different to the horizontal one so that the TV knows what's going on), plus any Macrovision copy protection signals and any coded subtitles or teletext information.

So, per field, we have 292.5 lines of picture information followed by 20 non-lines of housekeeping (called the 'vertical blanking interval'). Per frame we have 585 lines of picture information and 40 lines non-picture. Total: 625.

Where does 576 come in? I'm not certain but I can speculate. Obviously that is to do with DVDs. When engineers were developing the DVD specification they would have been aware that even though 585 lines were typically broadcast in PAL analogue TV, not all of them were seen. The TVs of the day always had significant amounts of 'overscan', in that the edges, top and bottom of the picture would be pushed off the viewable area of the screen. So for efficiency they probably decided that they could lose 9 lines from a frame, half from each of the top and bottom, within it even being noticed. That reduced the compression factor required a little. Every bit helps.

576 also turns out to be quite useful as a number. It relates far more easily to common digital video numbers. For example, the XGA computer monitor resolution is 1,024 by 768 pixels. If you display a 576 line image anamorphically (horizontally stretched by one third) on such a monitor, the scan lines of the image match perfectly the rows of pixels.

Of course, as far as the content of a DVD goes -- and digital TV as well -- blanking intervals and the like don't come into it. They are simply a sequence of still digital images. However, the DVD player had to present the video to a TV in a format the TV could understand (and likewise for digital TV): as an analogue video signal at the right frequency, complete with sync pulses and blanking intervals.

As for 1080 and 1125, HD TV was first developed in analogue format. I believe it was actually rolled out in Japan in analogue. So there were blanking intervals inherent in that system. Digital versions were designed to be backwards compatible.

Incidentally, for US-style NTSC TV, even though we talk about 480i from DVDs, their analogue broadcast system is a 525 line system, of which probably around 485 lines were used.

© 2002-2009, Stephen Dawson