Homebrew Drum Computer

[Allison]

>
>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:
>
>> 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.

However the inductance of the head windings are a significant factor in 
how high you can go for 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