Destructive Imaging of DECTAPE II Media

[Brent Hilpert]

On 2015-Jan-27, at 2:36 PM, Mark J. Blair wrote:
> 
> I found a specification that includes the magnetic track dimensions of the DECTAPE II on Al's site:
> 
>    http://bitsavers.trailing-edge.com/pdf/dec/dectape/tu58/TU58_Engineering_Specification_Jun78.pdf
> 
> Also, I found a diagram showing track dimensions for various audio cassette formats here:
> 
>    http://richardhess.com/notes/formats/magnetic-media/magnetic-tapes/analog-audio/015-cassettes/
> 
> in particular:
> 
>    http://www.richardhess.com/tape/cass_trk_lrg.gif
> 
> Both the DECTAPE II media and standard audio cassette media are 0.15" (3.81mm) wide. DECTAPE II has two .057" (1.448mm) wide tracks, centered 0.046" (1.168mm) apart.

Something's funny there, the C-to-C distance between two tracks has to be greater than (with separation gap as shown in the manual) or equal to the track width, that is:
	C-to-C = TW/2 + SEPGAP + TW/2
	C-to-C >=  2*(TW/2)
but:
	.046 < .057


> Data density is 800 BPI, with 2400 flux reversals per inch. At standard read speed of 30 ips, that turns into bits in 41.7us increments and flux changes in 13.9us increments. The reels inside the cartridge will spin at around 380 to 800 RPM depending on how much tape is on them, if I still know how to do math. Or twice that at the scanning speed of 60 ips.
> 
> Just based on track geometry, it seems to me that it may be quite possible to read DECTAPE II media with the inner two tracks of a 4-track recorder head or auto-reversing stereo audio deck head.

Do some auto-reversing stereo decks have 4-transducer heads? The ones I've seen (not many, so I don't know) mechanically flip a 2-transducer head around 180 degrees.


> Or even with both tracks of a 2-track 2-channel head as shown in the diagram I found, though I don't know how common those are. 4-track recorders and auto-reversing stereo decks are pretty common, though.
> 
> Running the tape at a stable speed near 30 ips instead of 1-7/8 ips would be the hardest part, I think. My gut feeling is that a normal audio cassette capstan and pinch roller assembly may not work well at 16x normal speed, and I haven't seen references to any audio cassette tape applications that run the tape at controlled speeds that fast (rewind and fast-forward speeds may be that fast or faster, but those run the tape at unregulated speeds with the pinch roller disengaged).
> 
> I don't know if audio tape heads are electrically suitable for this application, but I think they might be since the flux change period is in the same ballpark as the AC bias frequency used in cassette recorders, so the head coils ought to respond ok at those higher frequencies (?).
> 
> I'm curious about what folks with deeper magnetic media experience than I have might think about this.

I think the question would be what are the head-gap widths. It's got to be small enough to adequately discern the linear-rate-of-change of the flux variation on tape. I don't think that's necessarily going to correspond to the AC bias F in audio recording, but yes, it's still in the ballpark of 20KHz hi-fi audio.