Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!srhqla!quad1!ttidca!schear
From: schear@ttidca.TTI.COM (Steve Schear)
Newsgroups: comp.graphics
Subject: Re: Re: Workstations that can record/play realtime video
Message-ID: <7645@ttidca.TTI.COM>
Date: 14 Nov 89 18:42:28 GMT
References: <6509@portia.Stanford.EDU> <1360003@hpspcoi.HP.COM>
Reply-To: schear@ttidca.tti.com (Steve Schear)
Organization: Citicorp/TTI, Santa Monica
Lines: 22

In article <1360003@hpspcoi.HP.COM> jchristy@hpspcoi.HP.COM (Jim Christy) writes:
>
>> VHS quality 24 bits x 640 pixels x 480 lines x 30 Hertz x 3600 seconds
>> is 100 gigabytes uncompressed.
>
>This would imply that a standard VHS cassette tape has nearly 200 GB of 
>storage capacity.  Your multiplication is OK, but I think that's a little
>high, else we would all be using these as mass storage backup devices.
>Granted, this signal is stored in analog not digital form in the typical
>recorder.  
>
Well, not quite.  A standard NTSC image does not contain the digital
equivalent of 24 bits per pixel.  The color coding scheme of NTSC provides
high spatial frequency image data only to the luminance component of the 
picture.  The color information is "smeared" atop this grayscale data
at a much lower spatial frequency.  The eye appears to notice little of
this psychophysical magic due to, among other things, the larger size and 
wider spacing of the cones (as opposed to the rods) in the fovea of the retna.

Another reason is the efficiency of analog coding, from a media standpoint.
This difference in efficiency is one reason behind all the digital image
coding efforts (e.g., CD-I, fractal compression).