By Anthony Fordham

Here’s a question you may have asked: why can a CD only hold about 75 minutes of music, while a DVD can hold a three-hour movie? Unlike a long-play VHS tape, which has more tape than a short one, CDs and DVDs are the same size. So what’s the secret?

CDs

The answer is compression. A CD carries digital information encoded in a series of pits on the disc surface. A laser reads the pits and converts them into music.

The pits are encoded in what’s called a ‘raw’ format. Each pit corresponds to a discrete piece of digital information. So because of the physical size of a CD, only a certain number of pits will fit on the surface, and this happens to equal about 75 minutes worth of music.

DVDs

A DVD, on the other hand, is a much more complex piece of silicon. Even though it’s still 12 cm in diameter like a CD, it uses a laser with a shorter wavelength and so the individual pits are smaller and there are many more of them.

This means a DVD can hold more raw data – 4.7GB on a single-layer DVD versus 700MB for a CD – but even this increased physical capacity isn’t enough to store a whole movie in a raw format comparable to CD audio – for that you’d need ten times as much space.

So how does a movie fit on a DVD? Using a file compression system called MPEG2. By examining the data in a stream of video, an algorithm is used to reduce overall file size. When the DVD movie is played back, the process runs in reverse, turning an encoded compressed file into a video stream.

MP3

The only reason your new MP3 player can fit 1,000 songs is because it uses compression. The only reason your mobile phone can store all those polyphonic ringtones, SMSs and photos is because of compression. And, as we have discussed, the only way you can get a whole season of Sex and the City on four discs is because of compression.

Data is compressed using one of a number of different formats. These formats all do the same job: they make data take up less space. But they go about it in slightly different ways.

The most popular and best known audio format is MP3. It was designed to be a universal standard for compressing music, and it has succeeded pretty convincingly. After all, an entire market of devices is colloquially named after MP3.

But the reason very few companies refer to their music players as ‘MP3 players’ is because these devices can play several different formats, and in many situations you can even choose which format you want to use.

The choice is made when you rip a CD into your music browsing program. Somewhere in the options menu, you’ll be able to choose which format to compress the music into. There are a large number of options, including to what degree you want the music compressed, but there are three things to consider when choosing a format: What effect it will have on the sound or image  quality, how much storage capacity it will take up, and how much battery life it will use.

The more you compress a piece of music or video, the bigger the impact on sound or image quality. Compression works by throwing out what the computer thinks is redundant data. Unfortunately it can’t restore that data perfectly when it decompresses it. Throw out too much data (ie. compress too hard) and the restored quality will be lower.

However, if you don’t compress enough, you may find yourself unable to store as many tracks or movies on a device as you wanted.

Other compression formats

Finally, some compression formats are more power intensive to decode. MP3, for instance, is notorious for this. Because the main processing chip of your device has to work harder to decode the data, it uses more power and so batteries drain more quickly.

While some formats, such as Sony’s ATRAC3, are designed to minimise power usage, in practice most devices still run for more than 10 hours even decoding MP3, so this is unlikely to be something you’ll have to worry about.

There are many factors that will determine which compression format is best for you. Power drain, audio and video quality, even whether or not you want to support a giant corporation by indirectly paying royalties (you can avoid this by choosing ‘open-source’ formats such as OGG).

Fortunately for the end user, the majority of hardware can handle the majority of popular compression formats. The sheer variety of discs that a half-decent DVD player can handle often means the whole front of the unit is festooned with different compression format logos.

What this means for you is that compression formats will rarely be something you have to worry about. You can improve the performance of your devices slightly by choosing different formats or compression rates, but mostly you can just sit back and enjoy carrying an entire music collection around in a player the size of a matchbox.