I need to extract some data which is coming in Sub 20hz Audio Signal. Please advise how can I do this. As I am not Audio expert, so simply pointing for the devices or a readymade solution would help a lot. The backend to which the extracted data would be transferred can be connected via any of the Interface like SPI/I2C/Serial Pls help.

First of all, I am not sure if this question belongs here, but I will give some information regardless.

I need to extract some data which is coming in Sub 20hz Audio Signal

This sounds like it could belong on Signal Processing


TL;DR

  • Capture the signal
  • Find out encoding and speed
  • Find a decoder for the encoding and speed
  • "the extracted data would be transferred can be connected via any of the Interface like SPI/I2C/Serial"

Challenge 1:

Your first problem you are facing is How to record a sub 20 Hz signal

Challenge 2:

Your second task is to decode the signal to extract the data. In order to do so you need to know what encoding is used. For signals in this frequency range; a range of encodings are in use. Here some examples:

There are ofcourse others more exotic protocols, you will need to find out how it is encoded.

Besides the list above and others, on sub 20 Hz signals it is common that the normal "speed" of data encoding is decreased. This is to ensure a reliable encoding despite the low frequency "carrier".

This is called QRSS While this is in use for Amateur Radio, QRSS refers to transmitting extremely slowly. To compensate for the decreased signal-to-noise ratio. QRSS derives from the standard Q code used in radio communications, where "QRS?" asks "Shall I send more slowly?" and "QRS" requests "Send more slowly". QRSS (the second S) is for "really slow"

Example: To receive single elements of a QRSS Morse signal may take several tens of seconds. Minutes for a word/byte. Hours for any meaning full "message".

Furthermore, multiple modes can be combined, such as:

  • FSKCW (Morse code with frequency-shift keying), where "key up" is one frequency, "key down" is another frequency
  • DFCW or Dual Frequency CW, where "dit" is one frequency, "dah" is a different frequency, and spaces have no carrier or a third carrier frequency. With DFCW, the "dah" time is typically shortened to the same length as the "dit" time, and the short space between "dit" and "dah" in a letter is often eliminated.

The list is now endless in regards to encoding or combinations of encoding, and "speed" of encoding.

Challenge 3:

Once you have Challenge 1 and Challenge 2 out of the way, you need to find a decoder which can reliably decode your data from your sub 20 Hz signal and turn it into bits and bytes. As we don't know what the answers of the above are, there is little advice which can be given here.

Challenge 4:

The backend to which the extracted data would be transferred can be connected via any of the Interface like SPI/I2C/Serial

Yep, but this depends on how the decoder of Challenge 3 is outputing the stream of data decoded. YMMV

Summary/Conclusion:

  • Capture the signal
  • Find out encoding and speed
  • Find a decoder for the encoding and speed
  • "the extracted data would be transferred can be connected via any of the Interface like SPI/I2C/Serial"

Final thoughts:

As I am not Audio expert, so simply pointing for the devices or a readymade solution would help a lot

I am afraid that "ready made solutions" are probably few-and-far-in-between, if they exist at all for this type of capturing and decoding.

Signal decoding can be tricky at best for signals in "normal" frequency ranges, and get exponentially more difficult when the frequency drops.

With respect to your skills and experience: in this case a signal in the sub 20 Hz range will be classified as ELF if it were of a electro-magnetic nature (which is still technically analog). Even when your signal is pure audio/analog, many ELF technology and techniques would be, or could be, applicable for your quest.

Decoding such is certainly not a task I would see a novice perform easily.

HTH.

  • Thanks Edwin, I would take some time to digest this and I will revert back for any further queries..Thanks again. – Harish Goswami Oct 13 at 7:02

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