Sync video/audio with Arduino program (led show)

I'm trying to build a little led show with an Arduino. The idea is that I time and plan changes in the lightning for a specific video and later play it on a screen in sync with the show I previously programmed on the Arduino.
I've managed to achieve the result with a photoresistor taped in a corner of the screen. By flashing the pixels underneath it black-white-black at the beginning of the video the Arduino knows when to start the pre-programmed sequence and plays the show in sync with the video/audio.
Even though the solution works I'd prefer to have something less visible, so I thought of adding at the start of the audio track a high-pitched noise. My speakers can't play that sound but it could be detected by the Arduino if somehow I feed it a copy of the audio signal. The problem is that I have no idea how to approach something like that.
Maybe there's something simpler I could do.
Any ideas?


I think I get what you're doing.

Waayy back when film ruled the visual world, projectionists and TV stations manually listened to the countdown beeps on the leader of the film. The last second was always silent, and the beep at the two-second mark was different than the 5 or 10 prior marks. When the operator heard the two second mark, he/she would start the Telecine or open the shutter to the screen.

So, first to do this correctly, you need an oscilloscope. (Because you can't hear the high frequency signaling tone you want to use). Make a test video and see how high in frequency that you can reproduce at the speakers of the monitor.

Then build a tone decoder circuit using the LM567 tome decoder chip:

It looks great but I don't have access to an oscilloscope :frowning:

Certainly doable! So many years ago, a guy tried to hire me to help him develop a similar approach to syncing a slide projector to a sound track on a tape recording. Nothing ever went beyond talking. Would be a lot easier today!

It looks great but I don't have access to an oscilloscope :frowning:

...and you call yourself an experimenter? A maker??

OK, do it the brute force hard way.

Build a tone decoder (or two or three) for different frequencies then make a test video with those test frequencies to see if the decoder will respond to one of them.

I like, the approach of a high frequency in not new but has been around for a long time. You can find a lot of information on this. A LM567 as suggested by SteveMann was one of the parts used for the tone decoder. From the data sheet: The LM567 and LM567C are general purpose tone decoders designed to provide a saturated transistor switch to ground when an input signal is present within the passband. The circuit consists of an I and Q detector driven by a voltage controlled oscillator which determines the center frequency of the decoder. External components are used to independently set center frequency, bandwidth and output delay. You can generate your signal and tweek the circuit without a scope, just a LEd and resistor. This part has been around for a long time and there is much published on it. These can be very sensitive so you levels for the tone can be low, even though your speaker cannot produce it it will passed through as heat.

A basic scope is not expensive at all. I'm very happy with mine, it served me really well answering the basic questions "do I have a signal, and does it look like the way I expect it to?". Indeed for a project like this you really need one or you're basically working blind.

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