Transmit music using light

Hi-Fi stereo sound

Roman Rubanovich, NENCOM

We recently used a silicon solar cell instead of a microphone, with which we obtained a flicker detector. We «listened» light bulbs, computer monitors and television screens, detecting the presence of harmful pulsations of light.

As a «side» effect the ability was noticed of photovoltaic cells to receive information transmitted through light. We generated different frequencies by sending the received electrical signal to the LED, and with the help of the solar cell and headphones we received the corresponding sounds. For example, the flicker of the LED with frequency 440 Hz (440 timers per second), completely unnoticeable to the eye, through listeners sounds like a note «la» in the first octave.

After playing around with simple electronic tunes, we decided to test the ability of such a system to transmit «live» high quality stereo sound. To do this, we have put together the simplest two-channel scheme: one LED, a resistor and a capacitor per channel. We decided to use a 5 volt USB power supply. A stereo sound-to-light converter was obtained:

For receivers, we used two solar panels from old garden lanterns, plugging them into a listening headset or a portable sound recorder.

At first the sound came out awful: quiet, snoring, and absolutely no bass. But by trying out about 100 combinations with different denominations of electronic components, we were able to get a clean sound, almost indistinguishable from the original. We recommend listening to quality headphones:

Selection of components

The brightness of the LEDs indirectly affects the volume. Making loud noise when using ordinary indicator LEDs did not work, so we switched to more powerful LEDs.

Good sound is obtained when operating the LEDs in range 5÷50% of their maximum power. The less current we gave them — the more pulsations of light at low frequencies were observed, the lower the sound quality. At maximum power, the sound became flat. With the help of the resistors, we limited the current flowing through the LEDs and, accordingly, their brightness to a minimum, with the sound still loud and clear.

The capacitors allowed us to run power from the sound source to our device, but not to turn it back on. In addition, capacitors affect the ratio of low and high frequencies: as capacity increases, the bass becomes more powerful.

We selected components by ear, listening to music in different genres through studio headsets. We settled on such denominations:

In this configuration, in each LED passes a current 85 mA at a voltage of about 3 V. This means that the actual power of the LEDs is 0.25 W — about 8% of the maximum. In this mode, the LEDs do not heat more than +50 °C even in a closed housing, so we decided not to put the radiators that came in the kit.

The whole device consumes only 0.9 W without connecting to the line output of the computer and 0.7 W by including it:

Such a system is resistant to outdoor lighting. Even in bright sunshine, the sound is still loud and clear. Disturbances can only be caused by a flickering light source.

Historical background

Alexander Bell, one of the founders of telephony and the creator of National Geographic magazine, was able to transmit the first telephone message without cable in 1880. He used reflected sunlight, modulated by voice. Bell called his invention «Photophone», and considered it the most important achievement in his life. He even wanted to name his second daughter that way, but his wife wouldn’t allow him. Marian had to name her.

Today, light-assisted data technology is called VLC (Visible Light Communication). It is completely invisible to the eyes, and allows information to be transmitted at speeds up to 10 Gbps or more. One of the VLC variants is Li-Fi (Light Fidelity) — Wi-Fi light analogue.