Downsampling pcm/wav audio from 22khz to 8khz

Question

My android application needs to convert PCM(22khz) to AMR , but the API AmrInputStream only supports with pcm of 8khz.

How can i downsample the pcm from 22 khz to 8 khz?

Cozmo

Answer 1

The sample rate is hard coded in AmrInputStream.java.

// frame is 20 msec at 8.000 khz
private final static int SAMPLES_PER_FRAME = 8000 * 20 / 1000;

So you have to convert the PCM to AMR first.

InputStream inStream;
inStream = new FileInputStream(wavFilename);
AmrInputStream aStream = new AmrInputStream(inStream);

File file = new File(amrFilename);        
file.createNewFile();
OutputStream out = new FileOutputStream(file); 

//adding tag #!AMR\n
out.write(0x23);
out.write(0x21);
out.write(0x41);
out.write(0x4D);
out.write(0x52);
out.write(0x0A);    

byte[] x = new byte[1024];
int len;
while ((len=aStream.read(x)) > 0) {
    out.write(x,0,len);
}

out.close();

For Downsampling, You can try the Mary API.

Jughead

i have done this already, what i want to do is downsample the pcm before converting to amr. – Cozmo

Find a mary api to downsample audio file. – Jughead

this api doesn't run on android. :( – Cozmo

Yes, but I'm pretty sure there is no other API in Android that can downsample an audio file unless you implement one or use an existing library. – Jughead

do you know any implementation of downsampling in c or c++? – Cozmo

Yeah, here you are, and this link contains a guide to use ffmpeg. But do you want to use native code to do this in Android? – Jughead

Androd allow you to use native library. see Android NDK – Jughead

Answer 2

I find a java downsample lib :https://github.com/hutm/JSSRC there is also a c version could be used by jni

Incoherent