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[dsp] Packet via flashing light? Was PWM/phase relationship

Robert Donnell kd7nm at pugetsound.net
Thu Jan 26 00:06:17 UTC 2006


With regard to your application, if you were to use more than one capacitor bank, you should be able achieve a rather higher data rate, since you would be able to make two flashes (at least) much closer together in time.  I've got a vehicle strobe (safety beacon) that is able to produce four flashes in rapid succession (200 ms between flashes?) then repeat that after a second or so, as long as I can keep it powered.  Or for some redundancy, just use two independent flash tubes - redundancy when it comes to trying to find the balloon in the dark, is what I'm thinking of.  If power budget isn't a big issue (since you didn't mention wether these balloons have people onboard, for example, a scheme where you run an inverter to charge a large capacitor, then use an SCR/FET/Bipolar switch to connect that power supply to the capacitor used to supply the charge for the xenon tube, and with multiple switches and capacitors, you should be able to fire one or more tubes in rapid successi
 on.  Could be quite a light show!

Now here's a scary thought - if you use a flash detection to flip a D-flipflop, you could then feed the data to an NRZI-configured HDLC decoder chip, and have clock recovery and framing in the bargain.  Does that make it packet "radio"?  :-)

73, Bob, KD7NM

---------- Original Message ----------------------------------
From: "Scott Miller" <scott at opentrac.org>
Reply-To: TAPR DSP Mailing List <dsp at lists.tapr.org>
Date: Wed, 25 Jan 2006 12:59:27 -0800

>I'd like to experiment with pulse position modulation.  On a high-altitude
>ballooning list we were talking about the possibility of using a xenon
>strobe to send position and telemetry data.  The same thing should work with
>a pulsed laser.  what makes it interesting is the very high ratio of peak
>power to average power.  Data rate's not very high, though.  Say the strobe
>has a half-second recharge time, and you have millisecond timing accuracy.
>You'd start with a reference pulse, and the window starts .5 seconds later.
>The next .5 seconds are divided into 256 slots, so the slot during which the
>pulse arrives determines the value of an 8-bit byte.  That gives you a data
>rate of 1 byte per second.  Not good for voice, but you can get position and
>altitude in 10 seconds.

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