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Ideas? Detecting a narrow frequency within an audio signal

Former Member

I am looking for a way to detect the presence of a single musical note that lies within a real-world audio signal. For example, from a piano recording, I want to detect when Middle C (440 hz) is sounded while ignoring all the other notes.

 

Perhaps some kind of tunable bandpass filter would do the job? If so, it would have to be a very narrow bandpass filter because musical notes are separated in frequency by a ratio of about 1.06, which is quite close.

 

The thing might also be thought of as an AM receiver that operates with a carrier frequency not in the RF but in the audio frequency range.

 

In my application, the centre (detected) frequency will be controlled by microprocessor. The receiver will frequency-hop, quickly changing the detection frequency. I am imagining that the microprocessor might do this by supplying a clock frequency or by directly writing a digital value.

 

It does not need to be a precision circuit — simplicity and low cost are more important. The application can tolerate a fair degree of error.

 

Any ideas?

 

Thank-you!

 

Gordon Hicks

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  • 0

    Can you describe the application a little more - to offer good advice I need to know how many different tones you want to detect, how fast, how many others present at the same time etc. Is it to be low cost for one or low cost to make lots ?

     

    MK

  • 0 in reply to michaelkellett

    Hi Michael,

     

    Thanks for your questions. The application is an investigation into using audio tones for communication over open air – say, across a large room. There will be people and ambient sounds that introduce noise into the communication channel, but for my purposes reliability is not a critical requirement. A 10% failure rate would be acceptable.

     

    • How many different notes?

     

    As a starting point I plan to use the tones from the chromatic musical scale, probably an octave on either side of A-440. So, maybe 25 different notes. Notes on the chromatic scale are spaced fairly closely in frequency... here, for example, are the five notes on around of A-440:

     

    ... 391.9954 hz, 415.3047 hz, 440.0000 hz, 466.1638 hz, 493.8833 hz, ...

     

    • how many others present at the same time ?

     

    Up to five notes/tones may be present when trying to detect the target note, and they may be notes adjacent on the musical scale.

     

    • How fast?

     

    Not fast, at least, not in electronic terms. The note duration is at least 100 ms long, so the rate is 10 hz or slower.

     

    • Is it to be low cost for one or low cost to make lots ?

     

    At this experimental stage there will be 25 to 100 circuit boards — so, I need to minimize cost for a small multiples.

     

    cheers,

    Gordon

Reply
  • 0 in reply to michaelkellett

    Hi Michael,

     

    Thanks for your questions. The application is an investigation into using audio tones for communication over open air – say, across a large room. There will be people and ambient sounds that introduce noise into the communication channel, but for my purposes reliability is not a critical requirement. A 10% failure rate would be acceptable.

     

    • How many different notes?

     

    As a starting point I plan to use the tones from the chromatic musical scale, probably an octave on either side of A-440. So, maybe 25 different notes. Notes on the chromatic scale are spaced fairly closely in frequency... here, for example, are the five notes on around of A-440:

     

    ... 391.9954 hz, 415.3047 hz, 440.0000 hz, 466.1638 hz, 493.8833 hz, ...

     

    • how many others present at the same time ?

     

    Up to five notes/tones may be present when trying to detect the target note, and they may be notes adjacent on the musical scale.

     

    • How fast?

     

    Not fast, at least, not in electronic terms. The note duration is at least 100 ms long, so the rate is 10 hz or slower.

     

    • Is it to be low cost for one or low cost to make lots ?

     

    At this experimental stage there will be 25 to 100 circuit boards — so, I need to minimize cost for a small multiples.

     

    cheers,

    Gordon

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