MPEG-2 or MPEG-4: the best format for HD

MPEG-2 is a standard designed for compressing video signals that require a wide bandwidth. MPEG-4 improved on the performance of MPEG-2, but for lower bit rates. The thing is, for some time now MPEG-4 has proven more efficient and far better than MPEG-2. Should MPEG-4 be the one and only format? When it comes to high definition, MPEG-4 is undoubtedly the best choice.

We are sure that many of you have heard of MPEG-4 at some point. This format is a fundamental part of a number of other versions that have reached us under different names: AVC, H.264... We have seen it championing the recent versions of QuickTime and, above all, heralding the arrival of Blu-ray and HD DVD. It is also available for digital terrestrial television (DTT). Even so, after two long years of existence, MPEG-4 is still battling MPEG-2 for the leading role in high-definition content. Information is scarce, but the question, however blurry, needs an answer: does MPEG-4 improve high definition so much that we should abandon MPEG-2? Absolutely.

MPEG-2 deserves considerable credit, since it was partly responsible for the success of DVD-Video, the first standard format to bring digitized video images into our homes. Digitization avoids most of the degradation problems inherent to analog technologies, and also offers numerous advantages that are difficult to implement in an analog system. One of them, if not the most important, is compression. It is an absolutely necessary method that allows us, today, to watch a movie in excellent SD quality stored on an optical disc 12 cm in diameter, to receive high-quality SD signals over the air, or even to send our own video productions using any e-mail account. Something that seems so simple hides a complex technological process which, precisely now, relegates MPEG-2 to a level far below what MPEG-4 can offer. One of the most valuable advantages of MPEG-4 is its better compression ratio.

When we analyze an image on any display, we look for the defects and artifacts that may appear: we look closely at the lines and the pixels, we analyze resolution, motion... Aspects that point directly to the codec (the system that encodes and decodes the original signal).



ON TO THE VIDEO



As far as video is concerned, we are interested in two specific standard formats: MPEG-2 and MPEG-4, although, as you can see in the corresponding sidebar, MPEG-4 was not designed for this purpose (that is what MPEG-2 is for; and this is where the debate begins). The reason MPEG-4 enters this debate is that experts have discovered that this format is much more efficient than the one that is supposed to be used.

Faced with the need to compress digital video (and audio) data, the engineer can choose one format or the other depending on the bandwidth required. For example, MPEG-2 allows bandwidths of 6 MB/s for DTT, cable, video on demand or standard-definition DVD-Video. With MPEG-4, it is possible to send recognizable video images with a bandwidth of just 10 kb/s (perfect for mobile services or the Internet) and, depending on the final quality, increase it up to 8 MB/s for high definition. The first thing that comes to mind is that MPEG-4 makes it possible, using the same codec, to work across different platforms, whereas MPEG-2 is very much limited to a few fields. But there are more factors that point to MPEG-4.

Some are practical, such as the faster processing speed of MPEG-4, but there are others that are more decisive, such as its greater intelligence, its superior adaptability or its better intelligence. It includes important tools and algorithms for encoding, scaling and compressing various objects (elements that are not video). It can compress not only audio and video, but also animations, text, still images, textures, buttons... It is even superior at error detection and recovery, an extremely important feature, above all, in the field of broadcasting. For a time it was believed that MPEG-2 should be the standard format for content requiring high data rates, while MPEG-4 would handle content for lower bandwidths. Precisely for this reason, MPEG-4 is no stranger to those who usually work in the computer world. Nero, RealPlayer and QuickTime have long used MPEG-4 as their standard basis. But, little by little, new MPEG-4-based applications appeared that have been displacing MPEG-2, even where it was supposed to be strongest. H.264 and AVC are two clear examples.

These systems are capable of delivering superior video quality using the same bandwidth as MPEG-2. In other words, without having to increase the available bandwidth (and here you can include any of the known distribution systems: terrestrial, cable, satellite, packaged media...), we can significantly increase picture quality.

The satellite broadcaster DirectTV was one of the first to use MPEG-4 AVC as the compression system for its high-definition broadcasts, a step that many other similar companies have been taking over the past year.



THE CASE OF BLU-RAY AND HD DVD



What concerns us users directly and on a large scale (allow us to set satellite digital platforms aside for now) is what has been decided for Blu-ray and HD DVD, the two high-definition formats currently available to us. Both accept the MPEG-4 AVC compression system, basically because it allows them to hold, with the same (or even better) video quality, 9 to 15 hours of HD video instead of the 4-8 hours that would be possible using MPEG-2. Even so, both formats also allow MPEG-2-encoded video, since DTT broadcasting standards (and DVD-Video) mostly use this format. Curiously, they are also compatible with a "competitor" format, SMPTE VC-1, whose main backer is Microsoft.

For users, the use of one format or the other should not be a headache, but now we know that it is. Especially when we use projectors as our display systems and large-diagonal screens. Under these conditions, the "defects" of a format conceived years ago for a bandwidth that was probably lower than the one we use now become evident. Problems in the reproduction of lines and pixels, or motion artifacts, become apparent, since, ultimately, Blu-ray and HD DVD have a limited capacity. A capacity that MPEG-4 virtually increases.



BETTING ON MPEG-4



Since April 23, Televisió de Catalunya S. A. has been broadcasting a high-definition channel on a trial basis via DTT. It is the first initiative of its kind using the DVB-T standard. Televisió de Catalunya has decided to opt for the MPEG-4 format (we have detected MPEG-4 AVC streams at 10 MB/s at 1,400 x 1,020 pixels using the H.264 codec). Unfortunately, MPEG-4-compatible DTT decoders are not widely available. Just a few products (such as a Loewe television we will present in our next issue) and little else. But it is a good start. The industry expects MPEG-4 to gain ground. This would allow this format to be adopted as a true standard, which, in addition to giving us better video quality using the same bandwidth, would (potentially) make it possible to offer new interactive services that are not possible with MPEG-2.

From a commercial point of view, MPEG-4 means added value for broadcasters. And for users, a completely improved audiovisual experience. But this means that, as users, all we can do is wait for companies to react and provide us with everything we need to enjoy MPEG-4, which is no small task.

HOW MUCH DOES MPEG-4 COST US?

When the Blu-ray and HD DVD formats were developed, they included both MPEG-4 and MPEG-2 (and, as we have seen in the article, also the VC-1 format proposed by Microsoft). This means that all licensed Blu-ray and/or HD DVD players include a decoder compatible with each of the announced formats. If a disc is in MPEG-2, we can watch it; if it is in MPEG-4, we can too.

The trials launched by Televisió de Catalunya have opened Pandora's box. If these trials eventually become daily broadcasts (as we sincerely hope), given the encoding system used, it means we will have to buy an MPEG-4-compatible DTT decoder. Even those who have bought a television with a built-in DTT decoder will, in the vast majority of cases, have to do so. In the short term, once they become a reality, all the advantages of MPEG-4 will prove expensive and even inconvenient for us. But we are convinced that the change is well worth it, and we are firmly in favor of it.