Tings were so much simpler just a few years ago. There was just the one DVD player in the living room, and the output format was the only obvious one: MPEG. Today, DVD players play AVI files and MP3 songs as well, and for those who do not have Blu-ray but still want to enjoy high definition movies, there are players that support the Matroska (MKV) format.

Add to that the mobile devices that play videos-iPhone, PlayStation Portable (PSP) and the current generation of gaming consoles such as PlayStation 3. All these devices can play MPEG-4 files with fixed resolutions, frame rates and bit rates (see visual on the right).

Only experts can finc:l' their way about in this dense jungle of formats. Users who don't have much knowledge about converting videos and songs have to use tailor-made software to get the specified formats and profiles right. We have tested six media conversion tools that promise to perfectly convert videos and music just at the press of a button.

In the format jungle
We expect a lot from an all-round media converter. It should be capable of handling the four currently-popular container formats: MPEG, AVI, MP4 and Matroska (MKV). Each media file is embedded in one such container which manages the video and audio tracks. The track formats can however vary. You can see what lies within only by using analysis tools.

The MPEG format for DVDs is a relatively easy job: MPEG-2 video and Dolby AC3 audio are standard. On the other hand, AVI, the most popular film container for PCs, is extremely flexible. DivX-compatible DVD-players that play AVI files have rigid parameters, on the other hand. The video track should be as per the Advanced Simple Profile of the MPEG-4 standard (MPEG4-ASP) and have a resolution no larger than a DVD's (DivX Home Theater profile). DivX and XviD encoders master this format. Although XviD can do a little more than that, anyone who selects a suitable profile in the encoder and leaves out the 'Global Motion Compensation (GMC)' option will have a compatible video track.

MP3 and AC3 codecs are the options for audio tracks in the AVI container. MP4: Standard on mobile devices Complications start creeping in with MP4 which is a very common video container for mobile players. The container allows audio and video formats that are usually also a part of the MPEG-4 standard. The video track must be in regular MPEG4 or in MPEG-4 part 10 (H.264 format). Devices such as the iPod and the PSP can play both variants, however H.264 offers better image quality and should therefore be given the first preference by the converter.



We kept the encoder parameters for mobile players very strict. For MPEG-4, the software must stick to the Simple Profile, and for H.264 to the Baseline Profile. Both profiles are very trim and have no resource-intensive features such as B-frames. Since B-frames are calculated from the previous as well as the following frames, it is more difficult to encode these. They also require more processing power for playback. On the other hand, they do not take up too much space.

This means that at the same bit rate, they provide better picture quality. If devices are capable of playing back such quality, an all-round converter should also use B-frames. These devices include DivX-compatible DVD players and even the PlayStation 3 which also plays high definition videos in MP4 format. Although DivX also has a profile for high definition for resolutions greater than 1280x720, H.264 in High profile is the standard- after all, it is also used in Blu-ray movies. The audio track for MP4 file can only be AAC or MP3 format. It's only since August 2008 that MP4 also plays AC3 (Dolby Digital). But this has always been optional on the Pc.

Based on the restrictions of MP4, dedicated users of online communities have developed another container called Matroska (MKV). It has no boundaries and manages any combination of audio and video tracks and subtitles-in short, it's ideal. However, since it's not an official standard, hardware manufacturers hesitated for a long time before they included support for MKV in their players. Meanwhile, there are a few players such as the Ellion HMR-350H which make full use of MKV for high definition playback.

The combination of H.264 and AC3 is no problem here as opposed to MP4. Even the makers of DivX think so and have included an exclusive H.264 encoder with the new version 7. In addition to this, with the DivX Plus HD format, it will urge DVD player manufacturers to add this certification to the list. DivX Plus HD comprises an MKV file with an H.264 video track and will be seen in upcoming devices very soon in the near future.

MP3 and others: Popular audio formats While the number of video formats is increasing consistently, there's not much happening on the audio front. Beside the technologically mature audio CD, the good old MP3 format is still going very strong; the new addition AAC is practically its only successor. This is because the iPod is restricted to only these two formats. This puts everything else out of place. MP3 files are embedded in the same MPEG container, but the AAC track of an MPEG-4 file is a discrete file with the extension M4A. Nowadays, the HE-AC (High Efficiency) format is picking up. The format uses AAC with another compression technology, Spectral Band Replication (SBR) to enhance the compression efficiency. With tois, the high frequencies can be done away with because these are later reconstructed by the decoder during playback. A bit rate of 64 kbps is normal in this case, which is equivalent to 128 kbps in MP3 format or 96 kbps in AAC format. HE-AAC is mainly used on the Internet as a space saving audio format for Flash videos.