PROG ROCKS

Progressive scanning is a hot potato in the AV world. But what exactly is it, why is it so desirable and how do you get it? Danny Phillips explains all

Progressive scanning has been hyped as a miracle cure for poor-quality pictures. Several DVD players and TVs now sport the 'progressive scan' tag, the words emblazoned on their fascias. The technology has generated a huge amount of interest, not least from cinephiles keen to find out about the 'remarkable stability' and 'amazing precision' it lends to images.

But while prog scan may seem like a revolutionary, new-fangled technology, its implementation in AV equipment is the only new thing about it - the actual idea is as old as film itself.

INTERLACED SCANNING

In order to explain progressive scanning, we first need to discuss interlaced scanning. This is the usual method of displaying TV images, and was introduced in the 1930s to suit the best display technology available at the time - the cathode ray tube (CRT).

In a CRT display, a stream of electrons is generated by a gun, and is scanned across the face of the tube in scan lines, left to right and top to bottom. The face is coated in phosphors, which glow when hit by the electron stream. A method of scanning was required that would reduce the transmitted TV picture's bandwidth and work in accordance with the electricity supply frequency (50Hz in the UK and Europe and 60Hz in the US). The result was interlaced scanning.

The method involves splitting each TV frame into two parts, called 'fields'. Each field contains half of the scan lines of a frame. The first field consists of the odd lines, and the second is made up of even lines. The even lines are interlaced between the odd ones to 'fill in the gaps'. In the 50Hz PAL system, each TV frame lasts 1/25sec, with the electron gun scanning each field in 1/50sec.

Although interlaced scanning produces perfectly good pictures, there are some inherent problems with the system, many of which can be exaggerated on larger displays. The main drawbacks are visible line structure and flickering caused by rapid switching between the two fields. Another drawback is 'combing' (also called 'staggering' or 'jaggies'), which occurs during camera movement when objects are in a different position from field to field, lending them a staggered appearance.

With progressive scanning the frames are not split into two fields of odd and even lines. Instead, all of the image scan lines are drawn in one go from top to bottom. This method is sometimes referred to as 'sequential scanning' or 'non-interlaced'. The fact that frames are shown as a whole makes it similar in principle to the way film is shown at the cinema.

Compare the smoother progressive scan image (top) with the interlaced version beneath it

The type and number of displays which are able to show progressively scanned images is limited to non-interlaced devices such as LCD, DLP, D-ILA and plasma screens, although a number of CRT TVs and projectors will accept a progressively scanned signal via component video inputs (from a DVD player). These are not the same as progressive scan TVs (see later).

Progressive NTSC displays can show 60 frames in one second, rather than 60 fields. As a result, many of the artefacts caused by interlaced scanning are reduced or eliminated by progressive scanning. In short, images are more film-like, with smoother vertical resolution and less visible scan lines.

Many TVs offer 100Hz scanning, which is considered an alternative to progressive scan. Most TVs have a refresh rate of 50Hz and display each field in 1/50sec, a rate slow enough for flickering to be noticed. But TVs with 100Hz scanning can display both fields twice in the same period, effectively upping the refresh rate to 100Hz. These TVs digitise the two fields first and store them in a memory chip. The advantage is a complete absence of flicker, although interlaced scanning artefacts are still visible.

FILM-TO-VIDEO TRANSFER

To understand how progressive scanning improves the picture quality of movies on DVD, it's essential to know how film is transferred to video. Films made for the cinema are shot at a rate of 24 frames per second. When transferring film to the UK's 625-line PAL video system, the frame rate is increased to 25 frames per second. This incurs a barely noticeable four per cent increase in speed. Each film frame splits into two video fields, which fits perfectly with PAL's field rate of 50 fields per second.

The process gets more complicated with NTSC, which operates at a rate of 30 frames per second (or 60 interlaced fields per second). You don't need to be Carol Vorderman to know that 24 film frames into 30 video frames won't go, and speeding up the film to match the NTSC frame rate won't work, as the increase would be unbearably obvious. To solve the problem, what's known as '3:2 pulldown' is required. This describes the sequence that fields need to be shown in to make 24 frames fit into 60 fields.

The process of 3:2 pulldown involves showing one film frame using three fields, with a duplicate providing the extra field. The next frame comprises the usual two fields. This pattern is repeated, such that 24 film frames fit into 60 fields (or 30 video frames) every second.

Squeezing 24 film frames into 30 video frames is achieved by repeating fields in a 3:2 sequence

The problem is, when the image moves from the last field of a frame split into three and the first field of the next frame, any differences between the two frames will cause ugly artefacts such as twitching or feathery edges. The 3:2 pulldown process is not perfect, and the artefacts it creates are part of the reason why progressive scan DVD players are causing such a buzz.

DVD PLAYERS

The MPEG2 format on which DVDs are based can handle interlaced or progressive sequences, but most DVD players are only designed to output interlaced video. When a DVD is encoded from a movie source, a flag is inserted into the MPEG2 bitstream which instructs a normal DVD player to repeat certain fields.

Progressive scan DVD players take a different approach. The full frame is constructed from the even and odd fields, and this time the complete frames are repeated in a 3:2 sequence, using a duplicate of a full frame to make up the groups of three.

This irregular pattern creates a slight problem, though - every other frame lasts one 1/60sec longer than those in between, which causes a slight judder. The most important task the progressive scan player must perform is to detect and compensate for the 3:2 pulldown sequence of NTSC video, as outlined above. It has to identify which fields have been duplicated and included to make up the 60-field sequence.

Once this has been achieved recreating the frames to resemble their original film state is simple. The player ignores the extra fields, takes two fields from each film frame and compiles a full progressive frame from them.

Some (high-end) TVs which are described as progressive scan sets have a built-in deinterlacer (or line doubler). The progressive scan in a TV performs the same function as that in a DVD player - it scans each frame in one single sweep instead of two separate fields. Prog scan TVs are not as effective as DVD decks because one, they are not equipped to process in a 3:2 sequence, and two, all the 3:2 pulldown processing is performed digitally in a DVD deck - there is no chance of any analogue jaggies affecting the image.

PROGRESSIVE SCAN IN THE UK

There are a number of DVD players in this country that are capable of outputting NTSC progressive scan video. Arcam's FMJ DV27 and Denon's DVD-2800 are two recent examples, but Arcam's player is unique in that it can also process PAL video signals progressively. However, concerns over copy protection mean that the technology is yet to be licensed for use with the European 50Hz/ 625-line signals.

But it can only be a matter of time before progressive scanning universally supersedes the antiquated interlaced method. Its benefits are clear, and the proliferation of progressive scan products is an indication that we're in for a flicker-free future.

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