Destructive Imaging of DECTAPE II Media

Tue Jan 27 16:36:38 CST 2015

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:
>    This is not silly. I think it can work. I had a similar idea about TU60 decassettes. My task was easier there though since the TU60 uses standard Philips compact cassette shells. The TU60 however has no capstan and uses the spindle motors to maintain tape temsion. It was designed by the same guy who designed dectape (TU55/56).

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. ;)

>     Long story short, I was able to play TU60 cassettes in a good audio cassette deck and digitize the audio on a PC. I manipulated the audio file in Audacity. I was able to cut single data blocks and dump the blocks to a long .CSV file. I opened these in Excel and created intelligence in the spreadsheet to digest the audio samples back into the block data, even properly decoding the header and checking the checksum. The hardest work in the spreadsheet was tracking the bit boundaries due to the temporally varying bit-rate (no capstan in the real TU60).

Cool!

>     At last I did not have to use this means of recovering data from TU60 cassettes. My real TU60/TA11 worked well enough to recover the data on the cassettes I was provided to recover. I developed that tape deck/digitizing method in case there were dropouts unrecoverable by the real TU60.

At least the real TU60 isn't hobbled by that dreadful cartridge design.

>     Perhaps you can build a sort of open frame reel-to-reel deck to play and digitize the tu58 tapes.

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_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. 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/