CODEWITHSUNDEEP
javaaudiosignal-processingdownsampling
Samer Makary
Samer Makary

Reputation: 1915

Audio files down-sample

I am facing a problem while working with audio files. I am implementing an algorithm that deals with audio files, and the algorithm requires the input to be a 5 KHz mono audio file.

Most of the audio files I have are PCM 44.1 KHz 16-bit stereo, so my problem is how to convert 44.1 KHz stereo files to 5 KHz mono files?

I would be grateful if anyone could provide a tutorial that explain the basics of DSP behind the idea or any JAVA libraries.

Upvotes: 1

Views: 2782

Answers (3)

Matt Montag
Matt Montag

Reputation: 7471

Just to augment what was already said by Prasad, you should low-pass filter the signal at 2.5 kHz before downsampling to prevent aliasing in the result. If there is some 4 kHz tone in the original signal, it can't possibly be represented by a 5 kHz sample rate, and will be folded back across the 2.5 kHz nyquist limit, creating a false ("aliased") tone at 1.5 kHz.

See related: How to implement low pass filter using java

Also, if you're downsampling from 44100 to 5000 hz, you'll be saving one for every 8.82 original samples; not a nice integer division. This means you should also employ some type of interpolation since you'll be sampling non-integer values from the original signal.

Upvotes: 2

Malz
Malz

Reputation: 173

With the stereo PCM I have handled usually every other 16-bit value in the pcm bytearray is a data point corresponding to a particular stereo channel, this is called interleaving. So first grab every other value in the stereo channel to extract a mono PCM bytearray.

As for the frequency downsampling, if you were to play a 44100 Hz audio file as if it were a 5000hz audio file, you'll have too much data, which will make it sound slowed down. So take samples in increments of int(44100/5000) to downsample it to a 5khz signal.

Upvotes: 1

Mohammad Najar
Mohammad Najar

Reputation: 2009

Java Sound API (javax.sound.*) contains a lot of useful functions to manipulate sounds.

http://download.oracle.com/javase/tutorial/sound/index.html

You could find the already implemented java codes to easily down sample your audio file HERE.

Upvotes: 1

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