14 Dec
2007
14 Dec
'07
10:41 a.m.
Subject: Re: Homebrew Drum Computer
From: Gordon JC Pearce <gordonjcp at gjcp.net>
Date: Fri, 14 Dec 2007 14:42:34 +0000
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
On Friday 14 December 2007 14:19:06 Jules Richardson wrote:
However the inductance of the head windings are a significant factor in
how high you can go for bandwidth.
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.
At low bit densities it's not an issue otherwise you get into peak shift.
Low being around 300FCPI (Flux Changes Per Inch) based on testing I did
30 years ago. I was using the best ferrite heads I could get my hands on
from top of the line analog casette decks. Those heads
were very limited
and heads from 1/4" machines performed far better and still
limited me
to under 600FCPI at 15ips (~4800 bits sec) for NRZI and I could hit 9.6k
using phase encoding. Most audio heads are not so hot for saturation
work.
Allison
Gordon