Non-audible control tone on a CD

[tgsimmons]

For a Halloween prop I'm working on, I need to have two seperate audio tracks on a CD, but also need a control track to trigger a prop. I was thinking of recording a non-audible tone on one of the tracks, maybe 22-25khz (something that would still be within the range of a standard audio CD player), then using a bandpass filter to detect that tone and trigger the prop. Any thoughts on whether this will work. How about a schematic for the bandpass filter.

[dacflyer]

sounds like it would work,.i rember as a kid in school...they had a tape in conjunction with a slide projector...when the unit made a tone.the projector whould change the slide to the next foto
it wasn't a inaudiable tone but it worked,,and kept us awake in class..lol
this tape and projector was all one unit...

[cato]

I thing you're going to need a very sharp filter to avoid false triggers from the regular audio track. You might want to look into NOTCH filters instead of the band pass. It is a similar concept of course, but notch filters are much narrower and can be designed to react to your exact frequency and will not pass the harmonics of a high pitched scream or symbol crash that might be on your cd.

[jollyrgr]

The sampling frequency of a CD player is 44.1KHz. The highest frequency "theoretcially" you can have on an audio CD, therefore, is only 22.05KHz. Most CD players will not go that high. But on the lower end you can get down to about 5Hz. <p>I'd suggest using a 25Hz tone and detector. You can make a detector easily using a 567 IC. Why this tone and this IC? There is logic behind all of this. First, I know the 25Hz tone is NEARLY below the human hearing range. Plus I know it worked fine in the automation system of a radio station I worked at many years ago. But the main reason I suggest the 25Hz tone is because it is used as the pilot tone in AM stereo broadcasts. Thus there are a number of circuits already designed to detect this signal. I did a quick check and came up with this site:<p>http://home.netcom.com/~wa2ise/radios/amstmod.html<p>Look for the graphic "AM STEREO PILOT TONE INDICATOR CIRCUITS (SIC)"<p>
I have not tested this circuit but the 567 is a straight forward chip. Do use the isolation OP AM as suggested. You will feed the audio signal from the CD player to this circuit. Since this may be a higher level signal, you may not need the transistor and related components.

[Chris Smith]

A 567 Tone decoder can filter and decode any frequency within reason. Simple to use, and very few parts.

[Mike]

a little story,<p>i took a sub speaker, set a pc to output 10hz, and turned it on. i felt shaking, saw the speaker moving, yet heard nothing<p>pretty cool!!<p>but your thing sound like it should work.<p>you don't need a bandpass, a high-pass set to 22khz or whatever since no sound is going to go over that normally.<p>-Mike

[tgsimmons]

Thanks for all the great suggestions. It sounds like the LM567 is what I'm looking for. I've read over the data sheets and I think it's perfect for that application. Now it's time to order some and try it out. I'll let you know how it turns out.