[Coco] hard drive questions

[Mark McDougall]

Bob Devries wrote:

> Some SCSI and IDE drives are quoted as using RLL encoding
> *internally*. RLL is a compression technique which is also used in
> some picture storage methods. The "2,7" refers to the amount of
> compression IIRC.

I think you're getting confused between RLE and RLL.

RLL (run length limited) is, as you've correctly pointed out, a 
"compression" method used internally in old disk drives to increase the 
amount of storage on a disk. However, rather than compressing the actual 
data, it allows the controller to eliminate certain clock bits when 
writing to the media, so that more room is left on the track for data. 
The numbers, eg. "2,7" refer to the minimum and maximum number of 0 
symbols which may occur between 1 symbols (inclusive of clock pulses).

RLE (run length encoding) is used in image compression. Here the data is 
compressed by replacing a sequence of identical pixel values with the 
token value and a count of how many pixels of that value should follow. 
This compression scheme is quite basic and works well only for images 
with fewer colours and/or low complexity.

Regards,
Mark