Destructive Imaging of DECTAPE II Media

Tue Jan 27 20:27:02 CST 2015

> On Jan 27, 2015, at 17:22 , Brent Hilpert <hilpert at cs.ubc.ca> wrote:
> 
> 
> 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
[...]
>> 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

Hmm, you're right of course, but that's how it is drawn in Figure 5 of the spec linked to above. Maybe the dimensions were accidentally reversed in the drawing?

> 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.

I have never seen one that moved the heads. I'm pretty sure that I've seen ones that simply have four coils/gaps, just like the head in a 4-track audio recorder would have.

> 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.

I'm also blindly assuming that the coil/gap orientations are the same. Well, I could always avoid the problem by using a real TU58 head; then I have to guide the tape externally to the head (perhaps using tape guide spools harvested from a sacrificial tape cart) instead of using the sheet metal guide notch found on typical audio tape heads, and I need to get that epoxied-in head out of the transport and provide some means to mount and adjust it. In contrast, my recollection of audio tape heads that I've seen is that they're usually screwed in place (not epoxied), and in better drives there's even an alignment adjustment.

-- 
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/