Making Music
  Computer Music Software and Hardware

Code breaker

If you've delved into some of the more detailed CD-R software you'll have noticed a range of codes and parameters that you can set and change:

	ISRC. The International Standard Recording Code enables information about the country of origin, year of issue and serial number to be burned to the CD. It is generally only used on commercial recordings.
	UPC. The Universal Product Code is a 13-digit catalogue number, again, mainly used on commercial recordings.
	Subcodes. This is where it gets interesting. There are eight subcode channels on each CD labelled P to W. Channels R to W are used for CD graphics while P and Q store information such as track start and sub-index information.

Mind your Ps & Qs

As PQ subcodes determine track times, you can change these times by changing the codes. The P channel indicates the start and end times of each track, while the Q channel provides timecode addresses. It also stores the state of the Protect or Lock flag which prevents the CD being digitally copied, and the Emphasis flag. Emphasis is a basic noise-reduction process but it's rarely used.

There is a host of rules about PQ codes such as having silent frames before each track, and sub-indexes should be early but most software that lets you adjust these settings does so in a friendly manner. Multi-purpose CD-R software doesn't usually let you get this far down and dirty which is why specialist audio software such as Sonic Foundry's CD Architect and Steinberg's WaveLab is a must for creating customised audio CDs.

Typical uses for messing with these settings include changing the gap between the tracks. The Red Book standard is two seconds but you can make it longer or shorter.

You can also record a 'live' session or join audio tracks together so they play continuously but insert track start positions in the data so you can skip to any of the songs. This is where dedicated software comes to the fore. With CD Architect, for example, you can crossfade between tracks - very professional!

Index linked

CDs can hold a maximum of 99 tracks. However, each track can be further marked with up to 99 index points. Again, with suitable software, you can drop an index into a track at any position.

Indices were first widely used by sample CD creators who would stuff 1000 samples onto one CD in 99 tracks, each track typically having 10 index points to make it easy to find individual samples. Another oft-touted use is in classical music to point out different sections in a piece.

Not all CD players recognise index points, however, and sometimes the hassle of using indices is more than it's worth. For whatever reason, indices are not in common use but it's nice to know that they're there should you need them.

You can quickly create audio CDs with 'ready-to-burn' software but a little knowledge of the audio CD format and how audio tracks are assembled on a CD will allow you to create highly-customised CDs.

For more info... There's more information about audio CDs and CD formats on the Web than you want to know. Here's an illustrated and animated guide to How CD writers work: http://computer.howstuffworks.com/cd-burner.htm