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Newsgroups: net.periphs
Subject: Re: Magtape capacities vs densities ?
Message-ID: <377@abnjh.UUCP>
Date: Wed, 4-Jan-84 13:00:10 EST
Article-I.D.: abnjh.377
Posted: Wed Jan  4 13:00:10 1984
Date-Received: Fri, 6-Jan-84 04:21:58 EST
References: <190@rna.UUCP>
Organization: ATTIS, NJ
Lines: 34

The facts you need to calculate the capacity of a given length of tape
are as follows:
	'nominal' inter-record-gap:
			0.3 inch for 6250 density
			0.6 inch for 1600 density
		actual irg *may* be very large (~100 feet, I think)
		but almost never is significantly more than the 
		'nominal' value.  And (exact value depending on error
		recovery strategy used by the software) should never
		be more than a few times the length of the physical record.
	Actual number of 8-bit frames of user data (ie non-overhead
	data; that is, CRC frames and other redundancy subtracted
	out) per inch recorded on the tape.  
			6250 for 6250 density
			1600 for 1600 density
		The actual number of flux changes per inch are greater
		because of redundancy and/or details of recording
		mode.  But for your purposes, this is irrelevant.

Thus, for 8Kilo-byte records at 6250 density, a record (including the
trailing inter-record-gap) is 8000/6250+0.3=1.58 inches long.
A 2400 foot reel of tape has space for 12*2400/1.58=18227 such records,
or 146 MegaBytes bytes.  This should be treated as an upper limit, but
I have found it to be reasonably accurate (to within 20% say).

One other number you may find useful:  A 'tape mark' (EOF for people
who don't care about formal standards) is minimum 3 inches long, and
(again) *may* be very long depending on software error recovery
strategys).

Rick Thomas
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