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Recording audio on a raspberry Pi
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Recording audio on a raspberry Pi

by benjgorman on Wed Feb 05, 2014 2:01 pm

Hello,

I'm trying to record audio on Raspberry Pi using the adafruit microphone breakout board and the 12 bit adc breakout board over the PI's i2c.

I can read in the values from the adc using python and plotting them on a graph shows it's a basic audio wave format. I'm sampling at 1000hz.

My question is, how am I supposed to take these values and make them into an WAV file for instance?

I've looked at pyAudio but I'm still a bit lost as it refers to a stream generally.

Thanks!

Ben
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Re: Recording audio on a raspberry Pi

by adafruit_support_mike on Wed Feb 05, 2014 6:48 pm

In general, a WAV file is just a list of the values you'd get from the ADC. You can find more information about the file format here: http://en.wikipedia.org/wiki/WAV

PyAudio handles the file encoding for you, but does work with streams, which are a programming IO abstraction with a 'medium' challenge rating. Basically a stream is anything that produces or receives data, but the devil is in the details.

Your biggest challenge for recording audio on a RasPi will be the hardware itself though. A 1kHz sampling rate will only record sounds at 500Hz (slightly higher than a dialtone) and lower. Sounds at higher ferquencies move faster than the sampling system can catch them, so you get what's called an 'aliased' component of the signal. To sample audio for the human hearing spectrum (about 22Hz to 20kHz), your sample rate needs to be at least 40kHz. The industry standard for CDs is 44.1kHz.

In general, you can't do that with a RasPi.

Linux is a time-sliced operating system, meaning the OS gives each program a certain amount of time on the CPU (usually 10 milliseconds) then suspends it and gives some other program a turn. That's fine for plug-and-chug number crunching, and for human-speed things like word processors, but not so good for machine-speed stuff like audio recording. You end up with randomly-spaced gaps in the data.

To make Linux do time-critical work, you need a kernel module which runs outside the OS's time-slicing mechanism.
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Re: Recording audio on a raspberry Pi

by benjgorman on Thu Feb 06, 2014 1:45 pm

Right that all makes sense. However I'm fine with the low sampling rate I have, but from the ADS1x15 I just have an output of voltages. I'm not sure where to even start turning this into a wav file, even a raw audio file.

Would it simply be a list of binary values which represent these voltages?

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Re: Recording audio on a raspberry Pi

by pburgess on Thu Feb 06, 2014 3:25 pm

A different approach would be to use a USB audio adapter like this one:
http://www.adafruit.com/products/1475
WIth the addition of a regular 1/8" recording mic, this is compatible with the 'arecord' utility to produce a WAV file.

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Re: Recording audio on a raspberry Pi

by adafruit_support_mike on Thu Feb 06, 2014 4:09 pm

benjgorman wrote:Would it simply be a list of binary values which represent these voltages?

Pretty much.. that's the core of what would be called a 'Pulse Code Modulated' (PCM) audio file. The Wikipedia link above explains more about the information the WAV format adds to the PCM section.
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