Recording digital data to analog tape... revisited

I agree wholeheartedly with your desire to keep the camcorder intact. This is as a result of a few attempts to repair VCRs----which are mechanically much larger. The idea of loosening a screw on the camcorder and hearing "click, click, sproing!" gives me the shivers! Replacing the whirring noise of the motors in the submarine with a telemtry stream seems very sensible.

I've managed to get 230KB async from a UART through a kilometer of intercom wire (simulating an oceanographic cable) by putting out full cycles of pseudo sine waves for 1 bits and nothing at all for zero bits. The signals were transformer coupled for impedance matching and DC isolated so the cable could be used for HV power.

I think a similar approach would work on an audio recorder---but probably at a lower baud rate. I used an SX chip from UBICOM as a modulator/demodulator. For lower baud rates, a PIC would probably do the job with less power dissipation.

Mark Borgerson

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Mark Borgerson
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Followup to: By author: Rich Grise In newsgroup: comp.arch.embedded

The nice thing with using the audio tracks presumably is that the telemetry and picture get synchronized. Otherwise I'd say use solid-state storage... much more reliable.

These days, if I'd do something similar I'd use a (good) webcam and record everything including video to hard disk.

-hpa

Reply to
H. Peter Anvin

Back in the 80s I did a small project that would record data onto a regular stereo tape deck. Put the data on the left channel and the clock on the right channel. If I remember correctly all I did was use a resistor divider and capacitor to go directly from TTL to audio line level for recording. For playback, I had a pair of op-amps for converting from line level back to TTL. I had to massage the data a little bit before recording in order to keep the DC balance near 0 but otherwise it worked fine for low bitrates, it was pretty immune to tape stretch and dropouts.

Obviously if your camcorder isn't stereo then this won't work.

--Tom.

Reply to
Tom

Guys! Gals! Disguised eel-monkeys! This is a dead issue!!

1) The project has two forks. Fork A is to record a telemetry stream on the audio track of the camcorder. Fork B is to use the same analog output to record the same stream on a regular cassette recorder for applications where the user is using this same module to control a DSC or electrically-operated automatic film camera. Fork A already worked nicely when I started this discussion. My problem with Fork B turned out to be a stupid interconnect issue (albeit with really weird symptoms). 2) I have a hardware and firmware solution that works well, thanks for all the responses. I am doing the decode on a PC at the moment but will eventually build a little box that feeds the playback signal through a slicer and passes it to a digital input on a micro driving an LCD.

You missed a few steps, such as "buy a 9600 BPS modem chip that is readily available to everybody who might read about this project, and will remain so for some time" and "build the support circuitry for a

9600 BPS modem chip" (and then "work out how to decode the result easily"). Everything I need to do can be done in an 8-bit micro with plenty of horsepower to spare, and the resulting on-tape format is really, really easy to decode. Plus it uses off-the-shelf general-purpose parts.

No. This is not a throughput issue and never was.

Reply to
Lewin A.R.W. Edwards

... snip ...

You should be able to go a lot further and faster with balanced pairs, provided you avoid DC effects. The basic driver is a differential pair, with a current source 2I in the emitter circuit. The normal current mirroring methods can be used to generate currents I from the positive rail into the collector circuits of that pair, so that each output line now switches a net

+-I. There is only one switch, avoiding nasty crossover distortions, which can make the receiver get slowly varying signals just at its most sensitive point. The DC component of injected signal is zero over the differential line pair. Other methods can ensure the average on each half of the line is also at zero net DC.

The drive is then high impedance, so the line conditions depend solely on the line termination. In particular any biases set for receivers are set for the overall line, in just one place, with current injection. Most cables have well defined impedances, in particular almost all twisted pair telephone cable looks like 100 ohms or so at 1 Mhz up. With the above configuration the single line is a buss, and you can fairly freely add transmitters and receivers, provided you can compensate for the varying point to point propagation times. There are ways to handle this. One is by defining a transmission direction and adding a master clock line. Now all you have to do is put all transmitters to the left of all receivers, assuming direction is left to right. No tuning.

--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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CBFalconer

I'll look into that idea if I work with long cables again and have actual twisted pairs to work with. I was looking at some of the techniques used for DSL communications. I would love to find a pair of DSL modems that accepted serial from the microcontroller and produced DSL signals for coupling to the cable. Didn't seem to be anything workable when I was looking about three years ago.

My application was for an oceanographic cable, which can be quite different from a telephone cable. In particular, I had to cope with the fact that one of the wires might have about 300V DC to ground (or the other wire). I've also heard, but not been able to verify, that the cable characteristics change as the cable is reeled out into the ocean. I do know that we had be be able to cope with slip ring noise also--which was handled with ACK/NAK and packet repeats.

Mark Borgerson

Reply to
Mark Borgerson

Yes, power is often moved along the same copper. When towing multiple instrument packages at the end of a kilometer of cable, you can quickly run out of conductors. The package I worked up was designed to allow 4 or 5 instruments with modest data rates to mux their data together over the same pair of conductors.

"installed line" is probably not the proper term for an armored cable that gets winched in and out several times a day. They do use TDR to locate breaks and pinches on the line, though. However repairing a problem is extremely difficult---particularly 400 miles off the coast of nowhere! Oceanographic cables often grow shorter over their lifetime as new terminations are made or you get kinks over the sheaves---particularly as you bring the package aboard. The worst--- and very expensive case---is a problem in the middle of a 1 or 2KM cable. You will often see large spools of cable sitting in the shore facilities. I sometimes wonder whether they are too short, too few conductors, or simply too expensive to repair.

Mark Borgerson

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Mark Borgerson

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