[UFO Chicago] Compression??

Jack Beglinger jackb_guppy@yahoo.com
Sat, 26 Jan 2002 08:36:04 -0600

Compression - first requires a frame of reference.

In the case of CAV / CLV of laser disks there are two reference points. (most 
likely more, but lets stick with two :-)

1) The amount of space (volume) that the disks take on the shelf.

2) The Video Signal coming form the "read head".

Case 1: Is compressed.

CAV was the original form of laser disks - the "plater" rotated at a fixed rate, 
just like LPs (yes I am old!).  When CLV technology came along where the 
"plater" was rotated at constantly changing rate - the amount of video to be 
recorded on a side went from 1/2 hour to 1 hour.  So the number of disks 
required to play a 90 minute movie went from 2 (3 sides) to 1 (2 sides).

Case 2: It is not compressed.

Since the data is analog.  The single coming from the "read head" is sent 
directly to the TV to be shown.  No processing is made - so the rate of data 
is constant is under both encoding methods, so no compression.

So by these two example compression requires a frame of reference...  In 
the above example is it MEDIA or SIGNAL? Once that reference is defined a 
comparison can be made.

In one thread a comparison was made between 5 1/4 and 3 1/2 floppys.

If we look at the MEDIA.  a  5 1/4 Double Side FAT12 formatted disk - would 
hold a maximum of 720K of data.   3 1/2 Double Side Fat12 formatted disk - 
would hold a maximum of 1.4M of data.  -- Basically 2 5 1/4 disks worth of 
data would fit on a single 3 1/2 disk. Also the volume required to store two 5 
1/4 disk (and remain usable) is almost 2 twice that of storing a single 3 1/2 
disk.  So the MEDIA is compressed.

Now look at the data.  If the data was made of a series of 1k text files.  The 3 
1/2 would hold about twice as many then the 5 1/4 (directories and other 
media overhead :-)  But sending the file to the printer - both would print the 
same.  So no SIGNAL compression. 

So inclusion we where all correct... It is all based on our frame of reference.

To continue the debate.... 
Are gzip, gif, jpeg, mpg -- are they encryption?

For me the answer is yes - they are.  Just because the key is known does 
not make them any less an encryption scheme.


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