440Hz

[quote name='opticfibre' timestamp='1470311916' post='3104986'] The frequency of the LED isn't changing just the brightness. Some people will say AM amplitude as its the amplitude of the light. people assume we are transmitting a frequency i.e 100khz and changing the intensity of the 100khz and modulating the signal on that.. but we don't transmit a frequency. The 660nm is just the frequency of RED light, but i see your point about 660nm being the carrier frequency and us changing the amplitude of it. Its similar to when we say optical, people assume its digital, Peter Jones assumed we were encoding and decoding. [/quote] To avoid confusion 660nm is the wavelength of the light not the frequency, as the wavelength gets shorter the colour changes by the time it has roughly reached 2/3 of it original length it is BLUE rather than RED. Wavelength multiplied by Frequency = the speed of light The speed of light is around 300 000 000 metres per second Thus the original BBC Light Programme* (No Pun intended) was on a wavelength of 1500m dividing this into the speed of light gives 200 000Hz or 200kHz Similarly for 660nm we get 454,545,454,545,455 Hz or 454THz which is the carrier frequency which is then Intensity or Amplitude Modulated. Since LEDs and Photo-diodes combinations result in a photo-diode current proportional to the LED current then if the LED current is proportional to the amplitude of the signal voltage the system will be fairly linear. Note that the speed of light through a fibre optic cable is slower than the speed of light through a vacuum because the light does not travel straight but is constantly being refracted and bounced off the internal edges of the fibre thus taking a longer path at the actual speed of light giving an apparent speed which is lower. In practice, it is around 200 000 000 metres per second. The speed of light divided by this apparent speed is known as the refractive index of the cable and is around 1.5. * Transmitter now changed to 198kHz (to meet new regulations for 9kHz channel spacing) and used for BBC Radio 4.

I was watching Dragon's Den on catch up and thought what's new? I Googled and landed on this forum which, as a retired electronics technician working in education, I found interesting. I agree that varying the brightness of the light entering a fibre optic cable is amplitude modulation of an extremely high-frequency carrier. (light is just an extremely high frequency radiation travelling at the same speed as radiowaves in free space) I remember years ago (probably late 60's) putting a 15W pigmy lamp on the output of a valve audio amplifier via a step up transformer to match it. The audio was effectively modulating the light. However if you consider a single note say an A at 220 Hertz this will light up the lamp not 220 times a second but 440 times a second an octave higher(since the lamp lights up the same irrespective of the direction of the current). A photocell (I actually used a cell from a photographic light meter) which sees this light will thus produce a voltage at 440Hz and if connected to an amplifier and speaker will sound very distorted as it is mainly second harmonic distortion. A very simple trick was then applied; a 90 volt battery (from an old portable valve radio) was put in series with the lamp and with:[list] [*]no signal the lamp glowed [*]on the positive parts of the signal the lamp became brighter [*]on the negative parts of the signal the lamp became dimmer. [/list] The complete 220 Hertz signal was causing 220 fluctuations of brightness each second. The resulting audio via the photocell was then reasonably undistorted and with the pigmy lamp (thin single non-coiled filament) the frequency response was sufficient for intelligible speech to be carried across the link. Coiled coil lamps had too much thermal storage for a good frequency response. Jump about 40 years[list] [*]Replace the valve amplifier with a guitar pickup. [*]Replace the pigmy lamp with an LED, The 90 volt battery with about 1.4 volts bias on the LED or better still drive it with a constant current around 1mA, vary the current source up and down with the audio signal [*]Pass the light through a fibre optic cable [*]Demodulate the light at the other end with a photo-diode or photo-transistor and you have a link with a frequency response extending well beyond audio. [/list] The quality of the link is dependent on the linearity of the devices. More signal less noise but greater distortion and vice-versa pick a good compromise and you have a good quality analogue AM link. If you were to use FM modulation you could achieve a greater dynamic range and would not get the distortion caused by non-linearity in the diodes. FM does not introduce a delay as encoding a digital signal would and the system would still be sort of analogue as an FM signal is a continuous analogy of the original signal and not broken up into samples as it is with a digital a to d and d to a system. For an AM system performance could be improved by adding negative feedback over a separate fibre optical cable using matched transducers but this would complicate things. Anyway why not use an FM wireless link and do away with all the cables on stage?