Homebrew Drum Computer (magnetic heads)

Roy J. Tellason rtellason at verizon.net
Fri Dec 14 13:11:11 CST 2007

On Friday 14 December 2007 13:55, Mark Tapley wrote:
> 	...don't you also need to *write* at the same speed? I'll bet
> there's something in the design of the audio cassette heads that
> makes it hard to do that.
> 	I'm pretty firmly in the camp of (at least initially) abusing
> technology by changing as few parameters as possible. That'd mean try
> the experiment with a (or 8, but since it's a serial computer .... )
> head mounted over a drum (or disc) that pulls tape past it at the
> design speed for that head, 1 7/8 ips (?) for a cassette tape head.
> That also implies the max. data rate will be something like the max.
> bandwidth of the tape recorder, maybe 10 kHz. (Hey! Stereo recorders
> will give you 2 bits parallel, at close to 18 kHz....)
> 	Once you get it running at that clock rate and get the
> distributed.net client compiled and running ( :-) ), then work on
> speeding it up. I'd say the kewl factor on that machine will so blow
> away anything I've ever done that you need not also have a MHz
> advantage... Good luck!

This whole idea strikes me as just plain crazy...

But!  For some reason the above reminded me of something that I was 
considering early on,  probably early 1970s or so.  Instead of a drum just 
use a loop of tape.  You can start with an old reel-to-reel deck,  ignoring 
the reel transport portion of it,  and putting as long of a tape loop as you 
want to tolerate in some sort of an enclosure (two sheets of clear plastic 
might be kinda cool) and having a metal strip on one spot on the tape as 
an "index pulse" or similar.

Funny thing about that idea was that the slower speed does give you somewhat 
less frequency response,  yeah,  but cutting the speed in half does _not_ cut 
your frequency response in half -- it just cuts it down some,  so you can 
actually get more bits on the tape at the slower speed.  Of course your 
access time does get longer,  but...

Just one of those weird ideas I was kicking around back when,  and never did 
anything with.   :-)

> At 9:43 -0600 12/14/07, cctalk-request at classiccmp.org wrote:
> >  > Very interesting project.  Won't standard tape heads only work
> >  > reliably if
> >>
> >>  the magnetic material's passing by at quite a narrow range of speeds,
> >>  though? Google suggests that's 1 7/8" per second, which isn't very fast
> >> at all - a drum that can do a few tens of RPM seems possible, but 6000??
> >
> >I don't think it makes a difference.  The higher the speed, the larger the
> >voltage from the flux transition, but that shouldn't be a problem (within
> >reason).  The head gap and medium speed dictates the "resolution" of the
> >system.  You can think of it as being like trying to write with different
> >sizes of pen nib.
> >
> >Look at reel-to-reel audio recorders - they may run at a variety of
> > speeds, giving a tradeoff between audio quality and recording time.  You
> > can use a higher flux density with a larger head gap (and a
> > correspondingly larger drive signal), but you need to haul the tape
> > through faster to maintain the bandwidth.
> >
> >On playback, you get the problem that higher frequencies produce a higher
> >voltage, hence the need for equalisation (not unlike the RIAA curve for
> >magnetic record pickups).
> >
> >In this case you probably just want to detect the presence or absence (or
> >possibly polarity) of a pulse.  Equalisation won't be a worry.

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