MP3 files on DECtape [was RE: 1" paper tape buy ?]

[Rich Alderson]

> From: bfranchuk at jetnet.ab.ca
> Sent: Thursday, February 26, 2009 1:19 PM

> Mike Loewen wrote:
>> On Thu, 26 Feb 2009, Gordon JC Pearce wrote:

>>> Didn't someone once work out that to store a single MP3 you'd need a
>>> stack of 80-column cards ten miles high?

>>    Assuming a 6MB MP3 file and 40 bytes per card:

>>    6 * 1048576 = 6291456 bytes
>>    / 40 bytes per card = 157286.4 (157285) cards
>>    * .178mm card thickness = 27996.73 mm high
>>    / 25.4mm per inch = 1102.233465 inches
>>    / 12 =  91.85278871 (91.85) feet

> Umm if you store binary ... that is 80 bytes per card .. 46 feet per file.
> Now how many dec tapes is that?

We'll assume a PDP-11 DECtape since we're discussing 8-bit bytes.  Depending on
how we map those, the numbers can come out the same for PDP-1/4/7/9/15 and
PDP-10 tapes; PDP-5/8/12 tapes *will* be different.

A PDP-11 DECtape consists of 577 sectors of 256 words (= 512 bytes) each.  If
we assume a non-directory tape (that is, written with a user program instead of
OS services), we can put (* 578 512) = 295936 bytes on a tape, so we will need
(/ 6291456 295936) = 22 DECtapes; there will be only (- 6291456 6214656) =
76800 bytes on the last tape.

If we instead choose to use a DECtape initialized by one of the various OSes,
we lose different numbers of blocks per tape depending on OS.  For example, if
we use a Files-11 layout (RSX/IAS), we lose at least 10 blocks to filesystem
overhead (5120 bytes) per DECtape.


Rich Alderson
Vintage Computing Server Engineer
Vulcan, Inc.
505 5th Avenue S, Suite 900
Seattle, WA 98104

mailto:RichA at vulcan.com
(206) 342-2239
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