How karaoke CDG disks work!

If you've ever searched the internet trying to find out how CDG works then you'd be suprised as to how little information there actually is out there. I know! I invested quite a lot of my time a few years back doing just that when I set out to create EditCDG, an MP3+G CDG file editor, so I know how rare such information is.
The best piece of specification I found comes from Jim Bumgardners excellent CDG Revealed document which has been around now for quite some time, although unless you have a bit of a technical/coding background it will probably look like gobbledigook to you, and that's what this post is for, to try to explain it in way that you can understand! :)

This first picture to the left (click to enlarge) is something that you're all familiar with: the graphics screen that displays our words to sing to. This screen is made up of a set of pixels (or tiny squares if you prefer) and there are 300 pixels across and 216 pixels down. Not all tv screens can show the entire picture so there is a 'safe' area defined inside this set that measures 294 pixels across and 204 pixels down which can be displayed by all. This is where all our graphics take place and the outside border left is used as just that, a border.
This graphical area is then split down again further into blocks of 6 pixels across by 12 pixels down to give a grid of 49 by 17 blocks. In computing terms these are strange numbers as we usually keep our figures at least divisible by 2 or as a power of 2 e.g. 48 by 16, but hey ho, ours is not to question!

So, knowing what the screen is made up of we now need to figure out where the data comes from to display the graphics. The data of course, comes from the CD! But how come we can play the CD on a normal CD player and it sounds perfectly fine and not like ye olde ZX Spectrum tape data loader? The answer lies in that a CD is made up of sectors and each of these sectors contains 2448 bytes (pieces) of data. Of these 2448 bytes, only 2352 are used to store audio information which leaves us with a remaining 96 bytes per sector to store any additional data known as the Subcode. The most common data stored on audio disks is CD-Text which is how some CD players can tell you what the current track and artist is. It is also used to provide the player with information such as what the current track number is and how far into the cd we are etc... and finally, it is where we can store our CDG data that can be converted to displaying graphics on the screen. Standard CDs read 75 sectors per second and we can fit 4 CDG graphical instructions into each sector's subcode as they only take up 24 bytes each so this gives us 75 * 4 = 300 CDG instructions a second.

Although we only have just 24 bytes in which to get our graphical instruction to screen, the CDG specification does a pretty good job of squeezing everything it can out. The image to the left shows how the data in each 24 byte block is represented. The first byte is used to tell the player whether this is a CDG command or not and if it is, then the 2nd byte will tell you what kind of instruction to carry out. In display terms these instructions range from painting a single 6x12 block, defining the current colour palette or simply clearing the screen. Once the player knows what the instruction it then uses the values in the data section to determine how to carry out that instruction. For example with a Clear Screen command, the data in the first two data bytes will determine what colour to clear the screen to. A Paint Block command will use the first two bytes to determine the background colour, the next two bytes for the forecolour and the remaining 12 bytes to say whether each pixel in the 6x12 block should be set to the foreground or background colour. CDG can only have a maximum of 16 colors on the screen at any one time although those colors can be changed at any time. This is how Easy Hits disks manage the fade in/out on their karaoke disks, they simply change the colors to be progressively lighter or darker dependant on the fade direction.

And in laymans terms, that's pretty much all there is too it. Hope you found it interesting and feel free to ask any questions or point out any inaccuracies!