1
\$\begingroup\$

I am looking for a single-chip implementation of a bandpass filter for audio frequencies, but unsure if they even exist. All the filters I've seen seem to be at least in hundred KHz range. So my question is, do such filters exists?

An ideal filter would be something like:

  • A single-chip implementation with minimum extra elements added;
  • Reasonably priced (let's say under $10 a piece in bulk);
  • Narrow enough (+/-500 Hz is fine for my purposes)
  • Offer the binary output - i.e. when the input frequency is 0-350Hz or 450+Hz it outputs logical "0"; otherwise outputs logical "1".

The ultimate goal is to control a block of relays from the soundcard audio output by outputting a specific frequency.

Thank you for reading, and sorry if the question sounds lame - my EE degree is really dusted.

UPDATE: Thanks everyone for answering, but it only took an extra hour of thinking to understand that I can use DTMF. Extra five minutes to find about MT8870. Feeling really dumb now; you guys are awesome!

\$\endgroup\$
  • \$\begingroup\$ So, if you do have a signal in the desired band of frequencies to generate a logical 1, it may not work if the signal level is too low. For instance broadband low-level noise will contain remnants of every frequency yet I expect you would not want that type of signal to trigger a 1. You must decide not only of frequencies but amplitudes as well. \$\endgroup\$ – Andy aka Jun 2 '16 at 9:28
  • 1
    \$\begingroup\$ Probably not an option for your application, but worth knowing about MSGEQ07, which splits into seven bands: 63Hz, 160Hz, 400Hz, 1kHz, 2.5kHz, 6.25kHz and 16kHz. It gives a DC output representing the amplitude of each band. \$\endgroup\$ – Roger Rowland Jun 2 '16 at 10:45
  • \$\begingroup\$ There are also the old-fashioned PLL-based tone decoder chips such as the LM567. \$\endgroup\$ – Dave Tweed Jun 2 '16 at 11:01
2
\$\begingroup\$

I would probably use a switched capacitor filter.

There are numerous offerings available and they are very easy to drive.

There is also a universal filter - I am sure more types are available.

\$\endgroup\$
2
\$\begingroup\$

You can build a pretty good narrow bandpass filter from a typical quad op-amp package and some resistors and capacitors. If you want to use fewer components than that, you could look at one of the specialised filter ICs on the market, such as TI's UAF42. Opamps will probably work out cheaper though.

The requirement for binary output might be harder. You'll probably need to add an envelope detector and a comparator.

\$\endgroup\$
  • \$\begingroup\$ Thank you for the UAF42 hint, this might be actually useful! \$\endgroup\$ – George Y. Jun 2 '16 at 9:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.