Homebrew Drum Computer

Gordon JC Pearce gordonjcp at gjcp.net
Fri Dec 14 08:42:34 CST 2007


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.

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.

Gordon



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