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From: kev@hpcpbla.HP.COM (Kevin Jones)
Newsgroups: comp.periphs.scsi
Subject: Re: Random Access DAT
Message-ID: <9850030@hpcpbla.HP.COM>
Date: 18 Jun 91 08:18:41 GMT
References: <1991Jun14.211246.4340@newshost.anu.edu.au>
Organization: HP Computer Peripherals Bristol, UK
Lines: 78

All conclusions drawn in your response to my posting are correct.

Some specific points:

>  that you are unaware of any software that uses the
>  DDS facilities available,

(ie. w.r.t fast search etc....) On PC's, applications like SYTOS
allow selective store/recovery. On workstations, I am not aware of any
good backup (lets say "data archiving") applications (that's not
saying none exist).

> with a "stacker" you cannot randomly select the next cassette

Correct.

> Can you please provide any details you have available on who
> is marketing "stackers" and at what price?

I'm not sure about what stackers are currently on the market.
I've seen lots of "demo" type stuff that "looses" the occasional
cassette. Many stackers have SCSI and/or other "computer" interfaces
and claim to be able to do random access. Unless you've got a device
driver for such a machine then you still have no random access ability,
ie. you've got a stacker that might be a juke box if someone wrote
    a driver for it.

> I don't understand this answer. My understanding was that the audio DAT
> format is fully digital and the DDS format is just a particular form of

Correct.

> it that defines block numbers etc. If so, why should only the mechanism

DDS is "built on top" of the audio format. It defines a further level
of blocking (23 audio "frames" form a single 126Kbyte DDS "group").
A DDS "group" contains blocks of user data (multiple blocks can be
packed into a group, a block can span one or more groups, no space
is wasted irrespective of block size) and a "block access table"
that is, in effect, an INDEX for locations of blocks in the group and
their sizes. The "block access table" also contains information on
the prescence and quantity of filemarks/setmarks. The drives embedded
software is responsible for the creation of block access tables
on writes, and their use in "unpacking" groups on reads.

> be the same and not also the digital electronics (including read/write
> amplifiers)? If there is a digital signal being recorded and played
> back, why should accessing that for audio be any more difficult than
>recording an audio CD (digitally) on a DAT? I can understand there
> might be extra stuff for fast searches etc but surely more than the
> mechanism is the same.

A lot of the DAT digital electronics can be "re used" in a computer
product. Specifically, circuitry to do the 1st and 2nd levels of
error correction, and servo circuitry.

There are many functions not performed in audio dat though:
Read After Write, Buffering of Data (typically 0.5 Mbytes),
3rd level error correction, SCSI (not an insignificant part!!!),
DDS format (built on top of DAT audio format), extensive quantities
of performance/data-integrity oriented embedded software.
These functions contribute more to the cost of a
product than the "audio" functions (including the mechanism).


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Kevin Jones.                       | Hewlett Packard Ltd,
                                   | Computer Peripherals Bristol,
kev%hpcpbla@hplb.hpl.hp.com        | Filton Road,
                                   | Stoke Gifford,
Tel: 011 44 272 799910 (ext 22351) | Bristol.   BS12 6QZ.
                                   | ENGLAND.
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