Recording digital data to analog tape... revisited - Page 4

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Re: Recording digital data to analog tape... revisited

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It doesn't make sense it looks bloody awful.

The recorder is broken or you have some gross (ac and/or dc) overload of
the front end or something strange with the way you have connected the
output.

Is the trace you referenced even showing playback? Unless one of them is
stored how can you show the recorded input and played back output on the
same screen?

If the trace is monitoring the recorder output while recording then the
spikes might be capacitive coupling from the fast edges on the input.

Are you just stuffing 5v TTL square waves into the recorder mic (or even
line) input? that isn't gong to work.


Re: Recording digital data to analog tape... revisited
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It does look like you need WAY less signal to the mic input.
I'd expect a distorted square wave, but not the pulse per edge.

Mic level is something like 20mV IIRC.

--
KC6ETE  Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR



Re: Recording digital data to analog tape... revisited

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If you had looked at the picture closely enough you would probably have
noticed a possible DUT behind the keyboard. ;>

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Re: Recording digital data to analog tape... revisited
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Most of my really cool stuff - such as my original PDP-11/23 and my CRDS
11/23 clone (with 14" hard disk!) had to stay behind in Australia :( I
have however built up something of a collection of old friends here. The
only thing I really miss is a ZX Spectrum.

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Yes!! Some brands are much, much worse than others though. I'm currently
working on a vpr Matrix 120-180B5 (Samsung P10 in the rest of the
world). I was given this laptop due to a heat problem, and in fixing it
I picked up at least half a dozen such units on eBay.. every single one
of them has partial or complete hinge failures.

Toshiba's new swiveling hinge arrangement looks good.


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I go down the street every week on trash day looking for computers and
printers mainly for the motors, but in some cases
 
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Note I did say it was "part"... that's about 1/3rd of one wall, and
three walls are occupied with work materials in that room.

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The scopes serve different purposes to me. The TDS210 is great for
debugging serial communications where I need to freeze the signal and
count bits. The Tek 2445 is great for debugging odd timing issues where
I need more bandwidth than the TDS210. It's also my weapon of choice
when debugging strange analog issues, investigating ringing on signals,
etc. The Nicolet 3091 is strictly for long-term analysis of circuits
with very slow operation (battery chargers). It has a built-in RS232
interface, and that was an optional extra on the TDS210.

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Ah, the easiest way is for me to wait until my workplace throws some
out. I don't need much bandwidth. We have some VERY nice equipment in
our FCC Part 15 open-air test site, but that's at least $50k worth. Oh
well. It's not something I _need_, it's just something I know I would
occasionally use if I owned it.

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I'll take a look. I was recently asked by my manager to research capital
expenses in a certain price range, as apparently my division has been
obscenely profitable and management is looking to plough some of those
profits back into test equipment.


Re: Recording digital data to analog tape... revisited
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I haven't posted new photos of myself in a while, but they're not
incredibly out of date. I turn 30 in a few weeks (i.e. I had my last
birthday last year, and there will never be another).

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I really enjoyed the BBC B. Did you ever see that Amstrad Z80-based
portable machine (very small and light) which included BBC BASIC?

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Exactly so. Imagine that you were looking at a channel that showed
5-minute stills of fine art. You wouldn't notice the freeze-frame.


Re: Recording digital data to analog tape... revisited
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Hi Lewin,

You are using Manchester coding (biphase'M' I think).
The problem is that the cassette recorder introduces a lot of phase
shift.
If you put an all-pass filter after it with a pot to adjust the
turnover frequency (I can't remember if you need lead or lag) you
should be able to get the waveform a lot better.

With the Spectrum I wrote routines that used a similar coding scheme
to record at 4800 baud. The nice thing about this coing scheme is that
it is self clocking.

The only difference in the method I used was that I sent a lot of
zeroes (the low frequency) so that the clock edge sync occurred as
quickly as possible and then sent a '1' to indicate start of data.

I used to regularly save & load up to 48k of data error free.

Good luck

Robert

Re: Recording digital data to analog tape... revisited
Sorry to post a late addition to this thread, but I did run into a
1995 paper with some interesting results on using more complex
encoding techniques on conventional audio cassette tape.  If you're
willing to do some DSP on the receiving end (the transmitting side is
still quite simple) then it might be interesting:

http://kabuki.eecs.berkeley.edu/~abo/papers/224/224_report.pdf

This is one of the few places where I've seen discussed dealing with
the time jitter problem that's inherent in the use of tape for the
more advanced modulation techniques.

Note, though, that the best results for magnetic media are achieved
when the media is always magnetized to saturation, and the data is
encoded in binary flux transitions.  Time-modulation techniques like
RLL can be used to increase the amount of data encoded per flux
transition.  This is of course hard to achieve with analog recording
devices, of course, since they are designed exactly to avoid
saturation with the resulting distortion.

    -hpa


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