Optical analog audio intercom, LED based...

[Externet]

What is more convenient for voice band, short range of about 10 metres ? Direct modulation of a LED emitter in AM; or FM with a carrier ? No interference is expected.
Which phototransistor would be convenient for blue light reception ?
'Omnidirectional' (hemispheric) blue LEDs, do they exist ?

I think I saw long ago projects, tutorials and schematics somewhere. Cannot find them now. ¿?

 

[audioguru]

Conference halls use infrared in large conference halls for voice translation. They use FM to avoid POPs. Your idea needs the lights to be turned off.
My toy helicopters, quadcopters and drone also use IR.

I think the Navy used to use visible light doing Morse Code.

 

[schmitt trigger]

Why would you want to use blue leds?

Most, if not all commercial photodetectors are optimized for 940 nm IR.
The sensitivity at the shorter 470 nm blue wavelengths would be extremely low.

To your question, use FM.

 

[AnalogKid]

10 meters - yell?

 

[audioguru]

With my new hearing aids in the "very sensitive" mode I can hear a whisper at 100m. I tried it in the mall. A woman behind me said, "Hi Audioguru" so I turned around and said Hello. But the woman was 300m away talking to another Audioguru on her phone.
No, they were not talking about me.

 

[AnalogKid]

No, they were not talking about me.

And how does that make you feel?

 

[Externet]

Found the leds... the same used in christmas, concave top
---->https://www.totaloutdoorlighting.co.../led-christmas-lights-5mm-blue-green-wire.jpg

The phototransistor could be a solar cell... so far.
----> **broken link removed**
Seen them in TO-5 package... cannot find those now
In wide angle:

 

[crutschow]

Conference halls use infrared in large conference halls for voice translation. They use FM to avoid POPs. ..................

How do you do FM with IR?
I would think they would use some form of pulse modulation.

 

[audioguru]

How do you do FM with IR?
I would think they would use some form of pulse modulation.

1) AM modulate the IR with an ultrasound frequency.
2) Modulate the ultrasound with FM.
Some systems use an RF frequency of 6MHz to 10MHz instead of ultrasound.

 

[Mikebits]

And how does that make you feel?

lol

 

[crutschow]

1) AM modulate the IR with an ultrasound frequency.
2) Modulate the ultrasound with FM.
Some systems use an RF frequency of 6MHz to 10MHz instead of ultrasound.

Interesting.
Seems like it would be easier to generate and demodulate an audio PWM of the IR then doing the AM and FM modulation.

 

[audioguru]

Analog FM ultrasounded IR was used for wireless stereo headphones before Wifi.

 

[AnalogKid]

How do you do FM with IR?
I would think they would use some form of pulse modulation.

It is not direct modulation of the light frequency, but more like this:
200 kHz squarewave VCO + audio on DC pedestal as the control voltage > 200 kHz +/- 0 to 50 kHz squarewave > IR emitter driver

ak

 

[crutschow]

It is not direct modulation of the light frequency, but more like this:
200 kHz squarewave VCO + audio on DC pedestal as the control voltage > 200 kHz +/- 0 to 50 kHz squarewave > IR emitter driver

That makes more sense to me than AM plus FM modulation.
So it's square-wave FM modulation.

One easy way to generate this FM signal is to use the VCO section of a CD4046 phase-locked-loop.
---------------
Below is another option which uses a simple PWM circuit to modulate the LED.
It consists of a CMOS relaxation oscillator whose pulse width (and somewhat the frequency) is modulated by the input audio signal.
The output PWM varies from about 50kHz to 70kHz with the component values shown.

The audio is easy to recover with a simple low-pass filter as shown in the bottom right of the schematic, which is a 3rd order, Sallen-Key active filter with a 10kHz corner.
The PWM input would normally be the output of the photo-detector circuit (not shown).
The simulation is shown for input frequencies of 1kHz, 4kHz, and 8Khz.
The linearity is more than adequate for voice, with the input to output distortion being about 1%.
The output has a slight 0.5dB peak at 8kHz due to the 1dB ripple Chebyshev filter.

 

[audioguru]

I think I smell a lousy old LM324 in an audio circuit? It is noisy, has crossover distortion and its slew rate is so low that it has trouble with high level frequencies more than only 2kHz.
The PWM is the AM carrier of the IR.

 

[Nigel Goodwin]

I think I smell a lousy old LM324 in an audio circuit? It is noisy, has crossover distortion and its slew rate is so low that it has trouble with high level frequencies more than only 2kHz.
The PWM is the AM carrier of the IR.

You need to get over this silly obsession AG - the LM324 has been used in vast numbers of perfectly good sounding PA amplifiers.

 

[audioguru]

With so many inexpensive low noise, low distortion and wideband audio opamps available why would anyone use an LM324?

 

[AnalogKid]

Because it is there.

 

[crutschow]

I think I smell a lousy old LM324 in an audio circuit? It is noisy, has crossover distortion and its slew rate is so low that it has trouble with high level frequencies more than only 2kHz.
The PWM is the AM carrier of the IR.

Feel free to substitute the single power supply op amp of your choice.

How can PWM be called Amplitude Modulation?
There's no change in amplitude of the carrier.
It's a form of pulse modulation, not amplitude modulation.

 

[audioguru]

The IR amplitude is modulated by the carrier frequency of the PWM. Then the PWM Frequency Modulates the carrier frequency.