Let's start on a high note; with an apology. This feature is intended as a beginner's guide. It starts by making almost no assumptions whatsoever, except that the reader knows there is such a thing called a 'computer' and they can be used in the home to play 'audio'. This is not intended to patronize; there's a lot of misinformation circling about computer audio and its place (or even whether it has a place) in the system of an audiophile. So, we're starting at first principles; those who already feel well-versed in the subject might want to skim-read. Or move on.
At its most basic, computer audio and digital audio are almost interchangeable terms. A CD player could be considered a single-use digital music computer without too great a stretch, and the tracks on your CD can be copied, stored and replayed on a computer without any alteration to those tracks whatsoever. Files can also be reduced in size (compressed) for storage with no sonic degradation, or reduced still further to allow a vast number of tracks to be stored on a portable music player like the Apple iPod. A plethora of programs exist to control the process of copying, storing, organizing and replaying those music tracks.
The addition of the internet and music downloading websites means you no longer need to use a CD as the carrier for your music. An increasing number of these sites now offer music in CD quality and beyond. The addition of audiophile-quality digital processors (DACs) with computer-grade connections either as well as or instead of the traditional coaxial S/PDIF, optical Toslink, balanced AES/EBU or high-quality optical ST connections, means the computer can deliver digital audio potentially at least on a par with your existing digital audio player.
An exciting new twist in the computer audio tale has been the impact of "music discovery" sites like Last.fm and Spotify (opposite). These allow audio 'streaming', near instantaneous live access to any track or album on these companies libraries at the click of a mouse. Used wisely, these sites allow the listener to preview the music they intend to buy, before they buy.
Where to start
We're getting too far ahead too soon here. Let's start with the basics. Any home computer built in the 21st Century has the potential to be used as a source for computer audio. Of course, the newer the computer, the more computing power, memory and storage it's likely to have as standard, and it will be fully compatible with the latest software.
In most cases, buyers will have a choice of an Apple Mac or a Windows PC of some description, although a handful will prefer using the Linux operating system in place of Windows on the same basic PC architecture. Apple generally commands a premium, but is arguably the first choice for those who want an off-the-shelf music player solution, known as iTunes. Windows (by Microsoft) also has a default music player solution (known as Windows Media Player), but its principal strength is offering a vast array of alternative music software systems, and most are free. Linux is growing in popularity and comes in many guises, but the operating system is arguably more intimidating for the new user. Endless arguments have raged over the superiority of one format over the other, but the reality is the distinctions are fading as the systems converge.
For music use, there are potentially more important concerns than the choice of operating system. Such as, if the computer is to be used in the same room as the system, how quiet does it operate? If it's to be used elsewhere, will it form part of a network and if so, what kind of connections will be used. Today, typically many computer audio DACs (whether inside an amplifier or as a separate entity) assume you will be plugging a quiet computer or a laptop into your system and choose the USB connection. This has the advantage of being the easiest to set-up, but imposes a restriction on cable length between the USB port at the computer and the one at the DAC (the official limit of a USB cable is 5m). Firewire is a popular alternative to USB, especially in the music business, but this is predominantly an Apple digital path and this means more rare, more expensive products that support the format (Apogee and Weiss are supporters of Firewire).
A more permanent connection - and one that can allow computers around the home to be connected into the system involves home networking, usually using the CAT5 cabling system and a multi-pin Ethernet connector. At this time, Ethernet-enabled DACs are less commonplace (Linn being the most public supporter of Ethernet at this time), but as the technology permeates through the audio industry, so such products will form a key alternative to USB. A third option is to use WiFi (wireless fidelity) connections to link a remote computer to an audio system. There are already systems that exploit this pathway (often using Apple's Airport system, such as Resolution Audio and Micromega) and more are expected soon. The stumbling block for some of the networked options has been displaying the album or track info from a computer in one room to a system in another, but even that problem has been largely solved thanks to smart phones being able to double up as a remote control.
There are also systems that take over the whole audio front end. Music client systems like Sonos and Logitech's Squeezebox include the display and D/A conversion in a single box that becomes just another source for your preamplifier or amp, with all the file storage handled by a remote computer in another room. This computer becomes known as the 'music server'. More exotic versions of this arrangement exist specifically for audio (Qsonix, Meridian Sooloos) and for audio and video (Kaledescape).
Files of all shapes and sizes
The tracks on your CD are, in fact, individual data files, stored in a format called WAV in PC speak, or AIFF for Apple users (the difference between the two files is down to byte order, but although this might be of great importance to geeks, has no audible bearing to sound quality). These are uncompressed 16bit, 44.1kHz PCM files, exactly the same as those read by every CD player since the early 1980s. As each CD can store up to 650 megabytes of information, storing lots of CDs on a computer without any form of compression will soon eat through a lot of computer hard disk space. Data storage is cheap these days, but the amount of time it would take to archive and access all those gigabytes of information slows things down and kind of defeats the object of the exercise.
This is why compression is used. The word seems to automatically send shivers down the spine of an audiophile, but it need not be so scary in reality, because there's compression and then there's compression. The wholly benign form of compression is lossless. As the name suggests, the file is shrunk to take up less storage space but without any negative effects to the sound file itself. For the digital photographers out there, this is like taking a huge TIFF file and using ZIP compression to store and send it out. When the file is used, it's unpacked to its original size and no one's the wiser. Common lossless compression systems include ALAC (Apple Lossless Audio Codec), APE (Monkey's Audio own codec) and FLAC (Free Lossless Audio Codec) and typically shave the file size by anything between one third and one half.
The next stage is data compression, as opposed to file compression. This uses complex perceptual coding algorithms to determine not what data can be thrown away, but what musical instruments. As we fail to hear sounds that are masked by similar, louder sounds, so these quieter sounds can be removed to save space. There are many different types of this kind of 'lossy' compression (so called because once that data is lost, it cannot be retrieved) but the most well known is MP3. The result of all this space saving is that where FLAC can halve the size of a music file, a 128kbps MP3 file can shrink the file by as much as 11:1.
Different levels of compression can be applied to a piece of music, a trade-off between space saving and audio quality; a 320kbps (or kilobits per second) file is notionally indistinguishable from an uncompressed 16bit, 44.1kHz PCM track but results in a relatively large file, while a 96kbps file of the same piece of music will result in compression artifacts that undermine the performance on almost any kind of playback (these can manifest as swirling 'flanging' sounds). Somewhere between the two lies a comfort zone for most people, but for serious listening on large hi-fi systems, most audiophiles seem to prefer lossless files.
War, what war?
With different architectures and file types, this sounds complex and like the start of a format war, but the reality is completely different. Most of these systems are more similar than they are different, and they rely on the same basic protocols throughout. That means in most cases, if you buy one system and decide you want to change, your music comes with you without complaint or without bother on your behalf. In the case of the music player programs, they act independently to the music they play, because data is data. Some formats are proprietary (Apple's Lossless coding, for example) but transcoding to more universally used formats is possible and doesn't affect the music files. The days of the format war are hopefully over in computer audio.
There are decisions to be made over the choice of music player, but these are to do with picking out a system that suits your way of listening to music, not about influencing the sound. For many, music player means iTunes, Apple's almost ubiquitous media player software. It's the default choice of the Apple Mac user and the Apple iPod and iPhone user, which makes it incredibly popular. Other media players are available though, and some prefer the functionality or the simplicity of the alternatives.
For PC users, good choices include MediaMonkey, Foobar2000, WinAmp, and many more. Apple users get Songbird as an alternative to iTunes, with more to follow. Linux users get Rhythmbox and everyone can use VLC. Hardcore users also use a standalone 'ripper' software to transfer your music from CD with absolute clarity. Good rippers include Exact Audio Copy and dBpoweramp for PC, Asunder for Linux and Max for Mac, but unless you have a lot of very scratched discs, you'd struggle to hear the improvement. It's worth exploring the options here, but it's easy to get bogged down in minutiae, especially as each has its own set-up idiosyncrasies.
As with the choice of hardware, the software used to play can be considered a fit and forget, one-time choice, but if you decide to experiment with different software, the files will not be altered by the investigation process. Endless arguments rage over which software is best, but the differences are usually ones of taste and functionality rather than performance.
In every other way, a computer audio source behaves like just about any other source. The analogue output (ideally from a DAC) connects to analogue inputs on your amplifier or preamplifier. The change this computer-side system is having on the industry means sometimes even the DAC is built into the amplifier itself, and we expect this to become more commonplace.
Yes, but why?
The fact that you can do all this with a computer is wonderful, but what relevance does it have to good audio? One of the biggest misconceptions currently in hi-fi today is that computer audio means compromised audio. It's not hard to see why; many people consider computer audio to be synonymous with MP3 and those same people generally consider MP3 to be synonymous with low-quality MP3. While there are a lot of low quality MP3 tracks in circulation, this does not represent the entire story of computer audio; companies like Linn and HDtracks are now delivering music at master tape quality levels. But even with files that are as good as the original master tape available, the question remains; why should I bother?
There are many answers, but two of the most important are convenience and quality. Computer audio systems give you near instantaneous access to your whole music library. This has a fascinating effect on the way you listen to music. Although you still end up listening to complete works at a time, you also tend to investigate your music collection more thoroughly, as everything is to hand. On systems like the Sooloos for example, you can 'swim' through your record collection, picking out tracks connected to one another by drummer, producer, orchestra...you name it.
Then there's the quality issue. Many feel that CD files sound better when freed from being played direct from the CD, because you are not relying on on-the-fly error correction and because you aren't relying on an opto-electronic mechanism prone to vibration issues. But CD is only the start; high quality downloads offer potentially SACD and beyond sound quality in the home. This is something that only a few years ago was the stuff of dreams; music companies selling the Crown Jewels for not much of a premium. Now, it's becoming a reality. Again, expect to see such things blossom over the decade.
So maybe, the answer to the big computer audio question is not 'why?', but 'why not?' Computer audio will be a key theme in hi-fi of the second decade in the 21st century. It begins...now.