61352Re: BEAM colony trails idea
- Jun 2, 2014
Here are some numbers for mag tracking Herbies:
Cassette tape is 0.144—0.150” (3.66mm) wide.
Mono tape heads record tracks 0.060” (1.52mm) wide, with a centre guard band of 0.027” (0.69mm) width. This gives two tracks on a mono tape, one for each “side”.
Stereo tape head tracks are reduced to 0.0235” (0.597mm) wide, with a pair guard band 0.012” (0.30mm) wide between L & R tracks. Similar to the mono format, there are two “sides” on which stereo tracks can be read, so there is another guard band in the centre 0.0275” (0.699mm) wide to separate the two sets of stereo tracks. Note that adding up the widths of two track with the pair guard band width gives a total width of 0.59” (1.50mm), just a bit narrower than a mono track.
A mag tracker always records with a mono head (MH) and reads back track signals with a stereo head (SH).
Now, if the mag tracker SH is travelling centred and aligned on a previously laid mono track, both L and R elements of SH should see roughly the same signal. If SH is centred but the direction of travel deviates slightly from the track — azimuth error — L & R should still produce similar signal levels relative to each other. After further travel in this direction one of the SH pickups will eventually leave the mono track while the other pickup will remain on the track, thus creating an imbalance in the signal levels. It’s this which a mag tracker uses to servo back toward alignment with the mono track.
I’m supposing that we can use saturation recording so we won’t have to muck around with an ultrasonic carrier. Since each pulse would then consists of both a positive and a negative going spike, the read circuit needs some means of clipping or maybe just ignoring negative polarity spikes.
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