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Music on a Light Beam

How far can we transmit high-fidelity audio with a cheap laser pointer
(and other cheap equipment)?

Max Carter

These photos show some DX testing with the cheap laser modulator, receiver/demodulator, and associated (cheap) optical equipment (click the links above). Tests were made at 80' (24m), 500' (150m), 1200' (365m), 1 mile (1600m), 6.5 miles (10.5km) and 22 miles (35km). The tests were generally successful to the 6.5 mile test distance, though at 6.5 miles the sound was less than broadcast quality (future hardware enhancements may improve that). The photos are grouped by distance.



Reference Test at 1 Foot (30cm)

The cheap laser pointer is on the tripod, the photo-detector is in the foreground.

The transmitted audio is a 1 kHz tone from a function generator.
The scope shows the received audio data.





Test at 80 Feet (24m):

Here's the transmit end of the beam.

The laser pointer is installed in the mount at the left end of this rig.
Laser light shines out through the red muzzle to the right. See laser shotgun.
The audio source is the sound card of a computer, playing an audio file.



Receive end.

For this test the receiver was co-located with the transmitter.
The beam is being reflected from a mirror 40 feet away (80 feet round trip).





500 Feet (150m):

Here's the transmit end of the beam.

As you can see, it was dark outside (about 11 PM) though there was a nearly full moon.
The portable radio is the audio source.



The receive end.

The concentrator (see laser beam concentrator) is pointed,
through the open back door of the shop,
at the laser, 500 feet away in the middle of a prairie dog town.



Another view of the concentrator.



The laser beam as it shines in through the open door.



A view of the laser beam through the concentrator.

listen






1200 Feet (365m):

The transmit end.

In broad daylight this time.
The square blue/grey thing is a 12-volt battery.
The audio source is the MP3 player on the ground.
The laser modulator is taped to the tripod.





One Mile (1600m):

The Transmit end.

The transmitter is set up in a cemetery.



An Entrepid Experimenter guards the transmitter during the test.



Here's the receive end.

This was the receiving location for both the 1200' and 1-mile tests.
The demodulator is taped to one of the mount arms.
The black things taped to the tripod are speakers.
A 12-volt battery on the ground powers the demodulator.





6.5 Miles (10.5km):

The transmitting end.

The laser shotgun is in a barn.
The beam is shooting out the barn door.





The receiving end.




The received audio quality is mainly a function of the continuity of the laser beam. The biggest variable in this test seemed to be the ability of the operator/listener to keep the concentrator fixed on the beam. The sound was also probably affected by variations in air temperature ('heat waves' could be seen) and maybe the laser transmitter wobbling in the breeze (though the transmitter was protected in a barn). Higher quality optics and more stable mounts might greatly improve results.

Whatever.. This guy seems to be enjoying the music. Listen here (recorder over-driven, turn volume down).





22 Miles (35km):

Transmit end.




Receive end.

A Big Gun is shown here, gunning for a laser beam.




Here's a view through the concentrator.

The laser pointer, 22 miles away, is the little red spot just to the left of the concentrator on the horizon. The camera imperfectly captures the effect. A 25-watt flashing strobe light co-located with the transmitting laser was totally invisible to the intreped observers. Only an occasional faint whisper of music could be heard through blasts of noise.



The concentrator and sunset.

A place called Pine Ridge, 22 miles distant from the transmitting laser. The receiving system seems to work as well (or poorly) in daylight as in darkness.

Related Links

laser beam concentrator (photos)

laser beam concentrator (schematic)

laser shotgun

laser audio modulator

music on a light beam - the system tested

Sewer-Pipe LED Projector

Range Calculator/Estimator



Related External Link

Using Laser Pointers for Free-Space Optical Communications


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