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From: touch@gradient.cis.upenn.edu (Joseph D. Touch)
Newsgroups: sci.electronics
Subject: VCR PLUS+ -- size of the code space
Summary: seems reasonable by THIS envelope-back
Keywords: encoding
Message-ID: <42693@netnews.upenn.edu>
Date: 6 May 91 20:13:06 GMT
References: <GUEST.91Apr29180522@geech.ai.mit.edu> <4764@osc.COM> <1991May6.165343.5041@msuinfo.cl.msu.edu>
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In article<<GUEST.91Apr29180522@geech.ai.mit.edu> <4764@osc.COM>

>>In article <GUEST.91Apr29180522@geech.ai.mit.edu> (Guest Account) writes:
>>For those of you who have heard of VCR PLUS+, does anyone know how the
>>date, channel, start time and stop time are encoded into the program
>>number?  Here are examples:

>Has anyone considered how much information is in this short code?  I've
>tried a few encoding schemes that I might use in designing a code and
>can't seem to figure out how they put so much info in so little space.

Let's see:

	Programs usually start/end on the half-hour, so there
	are 24*2 time-start indicators.

	Programs must be under 6 hours (vcrs can't tape longer),
	again stopping on the half-hour means 12 length indicators.

	There are 80 channels in my area of cable (not all channels 
	are encoded, in fact, only about 1 in 4 is) so there are
	about 20 channel numbers.

	Assume the date is a day of the week(7) / week indicator (week 1-2).
	Codes here are printed here only a week in advance, so 2 
	week-indicators is sufficient. So we need 7 * 2 day indicators.

	This means a total information space of 24*2 * 12 * 20 * 7*2 =
	161280 points in space, so I need 18 binary bits (17.2 
	actually) or 6 (5.2 more precisely) digits of decimal 
	information for the code.

	This is overly pessimistic - there are never two weeks of
	scheduling where all programs are 1/2 hour long, so that
	all start/stop times need to be encoded.

	Note that some VCR-PLUS codes are 4 digits, some are 5 or 6,
	but I don't remember seeing 7 anywhere. I assume the 4 and 5
	digit numbers are zero-prefixed (i.e. xxxx = 00xxxx, etc.)

	So it fits.  By the back-of-the-envelope method, anyway.
	Note that there doesn't appear to be much (if any) room for
	error-correction redundency, so this is apparently not a very 
	robust code. Typing a number in a digit off is likely to do 
	nasty things, rather than printing out a warning. 

	I'd be interested if someone with a VCR-PLUS would try a
	few 'random' codes, to see if the VCR gets programmed 
	incorrectly or if the error is detected.

	Joe Touch
	touch@cis.upenn.edu
	PhD Candidate
	Dept of Computer and Information Science

PS Anyone care to concur/disprove the above back-of-the-envelope?
   I'd like to know if it's on the mark or not!