Broadcast video to be used as a wireless comm. link?

Philldapill

If I understand the Propeller's video hardware correctly, it has the ability to transmit low quality video over the air for a short distance. Is this correct?

If this is indeed correct, can't this be exploited, with a few external components, to transmit data over a short range? Heck, 5 feet would be terrific!

So, is this at all possible? If so, has it been worked on?

heater

Do we need to use the video hardware? The counters can generate some nice R.F. simply enough.
On the receiving end there is already a published quadrature demodulator/spectrum analyser.
Looks like frequency modulation might be the way to go.
What would be a good carrier frequency?
What about all those spurious harmonics from the transmitter.
Do we need a couple of transistors for an R.F. amplifier on the receiver?

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ericball

Broadcast is mostly amplitude modulated baseband (ignoring the aural bit).· I remember seeing a post where Chip(?) described the actual process but I haven't been able to find it.· Baseband is 3 bit PCM with 4 bit PSK (with wrap around at PCM 0 & 7).· IIRC, broadcast then modulates this to 7 or 8 levels so there's probably some quantization loss.

So if this fits your receiver requirements, then it may be feasible to use broadcast.



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HollyMinkowski

It seems unlikely that you could transmit anything better than unusable rf noise from the propeller chip
unless you add external rf hardware parts. If it is just data and not an image you wish to transmit for
short distances it would be cheap to use an IR LED and an IR phototransistor. You could easily modulate the
LED using the propeller

If it is video you wish to transmit I suggest an old surplus modulator from a defunct video game
or computer that accepts composit input and outputs rf on channel 3 or 4. These were meant to
connect directly to the antenna input on a tv but if you connect the output to a simple dipole cut
for the proper frequency it should transmit video at least a few feet with perfect clarity

Phil Pilgrim (PhiPi)

The Propeller chip alone, with a simple 1/4-wave vertical antenna, does a rather good job of transmitting video on channel 3 over short distances — no external modulator needed. Other channels are not so good, since the PLL generates more jitter at other frequencies. The idea is to pick a frequency such that frqa consists of one or two "one" bits followed by all zeroes.

I have tried to ameliorate the jitter and the spectral spurs it produces by dithering the phase of the PLL counter, but with only limited success.

-Phil

Beau Schwabe

Phil Pilgrim (PhiPi),

How well does a simple LC tank immediately off of the I/O pin tuned to the appropriate frequency improve spectral spurs?

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

Beau,

Hmm, I haven't tried that, so I don't know. It will probably have to be highly selective, since the worst spurs occur in the same band, on either side of the carrier.

-Phil

hinv

TV broadcast was covered in this thread:
http://forums.parallax.com/forums/default.aspx?f=25&m=229130

Checkout the picture of Paul Bakers's protoboard with three resisters and a wire antenna! Quite impressive! I will have to try it.

Receiving, on the other hand may not be so easy. This is a bit over my head, admittedly.

ericball: I was able to catch PCM and PSK, but is IIRC? Any reference to the post by Chip?

PhiPi: Have you tried a 6mhz crystal?

Phil Pilgrim (PhiPi)

No, I haven't tried that, but I'm sure it would help on channels 2 and 4.

-Phil

hinv

Hey Phil. I was searching for the AM radio receiver, then I noticed it was from you.
http://forums.parallax.com/showthread.php?p=743002
How difficult would it be to make a transmitter? This receiver seems pretty simple hardware wise.

Here is some good information on protocol over the air.
http://forums.parallax.com/showthread.php?p=770046

Here is a project with some cheap tranceivers from RC cars:
http://forums.parallax.com/showthread.php?p=789947

Hopefully these help. This forum has such a wealth of information because of the sharp people on it. It is just a matter of digging in the right places.
To find the AM radio receiver, I had to do a search for "oklahoma radio" because I just ran into the wav file in my propeller directory. It was a recording of WKY Oklahoma city.
Maybe there would have been a better way.

Doug

Post Edited (hinv) : 5/31/2009 8:44:56 PM GMT

ericball

hinv said...
ericball: I was able to catch PCM and PSK, but is IIRC? Any reference to the post by Chip?

If I Recall Correctly


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Leon

Doug:

Using a Propeller for transmitting RF might get you into serious trouble. The output wouldn't be clean enough, even if you were authorized to transmit on a particular band.

Leon

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Post Edited (Leon) : 6/1/2009 1:43:33 PM GMT

Phil Pilgrim (PhiPi)

That's certainly a concern. Even operating under Part 15 of the FCC rules, it would be too easy to cause interference to another service at a frequency you didn't even know you were emitting on.

-Phil

Leon

Using a spectrum analyzer (expensive, but they can be hired) is the only way to be sure that transmissions are clean.

Leon

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