Destructive Imaging of DECTAPE II Media

Responding on-list to an off-list message, with permission... On Jan 27, 2015, at 09:30 , Lou Ernst <louis.ernst at verizon.net> wrote: Using the spindle motors to maintain tape tension seems like a better scheme than that danged drive belt in my opinion. I recall reading somewhere online about the heroic measures that some engineers (at HP? Or was it 3M?) had to go to to get a DC-100A-like cartridge design working, involving lots of hard work in the areas of lubricants, surface finish, and how to make that drive belt by stretching out a small disc of plastic. My interpretation is that they had to apply heroic efforts to make a crappy design barely work. ;) Cool! At least the real TU60 isn't hobbled by that dreadful cartridge design. That's what I'm thinking. While I could repeatedly use one or two good cartridge/belt assemblies with the guts of multiple tapes in order to image them in a TU58-XA transport, I don't relish the thought of repeatedly tinkering with the danged belt if I can come up with a better solution. 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_Sp… Also, I found a diagram showing track dimensions for various audio cassette formats here: http://richardhess.com/notes/formats/magnetic-media/magnetic-tapes/analog-a… 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. 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. 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. -- Mark J. Blair, NF6X <nf6x at nf6x.net> http://www.nf6x.net/