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.
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.