[UFO Chicago] Compression??

[Jack Beglinger]

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.... 
  IS "COMPRESSION" A FORM OF "ENCRYPTION"?
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.

jackb



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