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Re: Recording digital data to analog tape... revisited
On 4 Oct, in article
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Even to do it as an OSD thingy on a camcorder (integral unit) is not
guaranteed from unit to unit. To do either method of VBI data (SMPTE timecoe
or similar) or OSD type, requires disrupting the signal between the
camera and the recorder part. So I say wise choice, laying down on sound
track is a good choice.

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Despite its startup glitches seems a sensible solution.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Recording digital data to analog tape... revisited
On Monday 04 October 2004 11:59 am, Paul Carpenter did deign to grace us
with the following:

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And, of course, having seen the light, I have to mention that all in the
days of seeing data put on cassette, I've never seen one that used side A
for clock and side B for data, which I'm guessing is because they wanted
as much density as they could get.

But given that, I think that the technology for getting data onto a
cassette reliably is fairly mature, like 8" floppy disks, ;-p

Cheers!
Rich



Re: Recording digital data to analog tape... revisited
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[...]
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In case you haven't done so yet, you may want to investigate the way
long-term (24h) electrocardiograms are recorded.  My dad had to have
one a while ago.  It was a device that he had to carry around with him
during an entire 24-hour day at home, and it did put the data on a
single audio cassette.  It actually looked a bit like a converted
walkman.

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What kind of tape?  One serious design flaw of VHS, IIRC, is that it
lacks any standardized mechanism for in-band time-synching.  Most VCRs
have invented their own system, and so did camcorders, I suspect, but
they are independent and incompatible with each other.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: Recording digital data to analog tape... revisited
In newsgroup: comp.arch.embedded
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If all you've got is a bathtub, sometimes using the bathtub will work
just fine.

    -hpa


Re: Recording digital data to analog tape... revisited
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How about a onboard pc or dsp that compress any arbitrary numbers of video and
audio streams + telemetry data ?

It all depends on the amount of telemetry data and budget I guess =)


Re: Recording digital data to analog tape... revisited
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A PC is out. I just removed an x86 SBC from the design; I am moving
the functionality into small micros. There is a power budget issue
here. Also, hard disks are not robust enough for this environment
(underwater vehicle). Flash media are not good value, comparatively
speaking.

Trust me, I want to do this on magnetic tape. The camcorder is in many
ways ideal.

Re: Recording digital data to analog tape... revisited
On Wednesday 13 October 2004 12:35 pm, Lewin A.R.W. Edwards did deign to
grace us with the following:

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It seems whenever there's a home video on TV they've got the date and
time. But you want other stuff. So, what if you took, say, a 40-char
alpha display, and put it in the field of view of the camera? Take a
picture of your telemetry! Scroll it across like a little marquee. :-)

Cheers!
Rich


Re: Recording digital data to analog tape... revisited
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Depth of field problems. My camera is inside a submarine and the
display must be within an inch or so of the lens. The objects I'm
interested in seeing are at infinity.

This was discussed way back in my earlier thread. I asked for ideas on
either putting a sync mark in the video stream (an LED was considered)
so that I could establish correspondence points between separate
telemetry and video streams, or ideas for putting my data directly
onto the camcorder without modifying it at all (if possible).

Re: Recording digital data to analog tape... revisited
By author:     snipped-for-privacy@larwe.com (Lewin A.R.W. Edwards)
In newsgroup: comp.arch.embedded
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Not to mention that the bandwidth of scrolling text across the screen
wouldn't be anywhere close to what you can extract from the audio
tracks (and it wouldn't easily be machine readable.)

    -hpa



Re: Recording digital data to analog tape... revisited
IIRC nasa does something like this, with a data stream on a couple of lines
of the video. They float it across the screen, so that it never consistently
obscures any part of the screen. Looks reasonable to encode, but decoding
could be a bear.

--
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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It's not really related at all - the OP was talking about putting an
LCD in the field of view, which is very different from replacing video
lines with data lines. The latter method is impossible given my design
constraint - don't modify the camera. It would be impossible to
document a generic solution to do anything involving insertion of data
into the video stream, because the video path inside camcorders isn't
standardized (esp. not with respect to what parts of it are accessible
and what parts are buried in an ASIC).

Re: Recording digital data to analog tape... revisited
snipped-for-privacy@larwe.com says...
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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


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

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

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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.

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

Re: Recording digital data to analog tape... revisited
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... snip ...
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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 ( snipped-for-privacy@yahoo.com) ( snipped-for-privacy@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
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Re: Recording digital data to analog tape... revisited
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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.
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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


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

Re: Recording digital data to analog tape... revisited
In newsgroup: comp.arch.embedded
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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

Re: Recording digital data to analog tape... revisited
snipped-for-privacy@spamnuke.ludd.luthdelete.se.invalid

A 44-character screen name with no spaces.
Congratulations. You get the booby prize.
Ever thought about just using "pb"?

Re: Recording digital data to analog tape... revisited
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Was your wife mean to you this morning?  The dog?  Kids fail to
show the proper respect?

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