Java GSM encoding from mic

Hi

I'm trying to encode a microphone stream into GSM using
org.tritonus.lowlevel.gsm.Encoder but I am receiving just static noise -
I've checked the raw PCM output and it's fine. Is there anything obvious
that's missing here?
Thanks

__________________________
The mic is opened as:

AudioInputStream ais=null;

 DataLine.Info info_mic = new DataLine.Info(TargetDataLine.class,
 new AudioFormat(AudioFormat.Encoding.PCM_SIGNED, 8000.0f, 16, 1, 2,
8000.0f, false)
   );

  TargetDataLine mic = (TargetDataLine) AudioSystem.getLine(info_mic);
  mic.open();
  mic.start();

  ais = new AudioInputStream(mic);

_____________________

And the GSM frame encoder is:

/*------*/
byte [] gsmframe(AudioInputStream ais){

 byte [] encd = new byte[33];

 try{

    byte [] b = new byte[320];

    ais.read(b,0,b.length);

    DataInputStream di = new DataInputStream(new ByteArrayInputStream(b));

    short [] buf = new short[160];

    short j=0;

    for(int k=0; k<buf.length; k++){
     j=di.readShort();
     buf[k]=j;
    }

    org.tritonus.lowlevel.gsm.Encoder enc = new
org.tritonus.lowlevel.gsm.Encoder();
    enc.encode(buf, encd);

 }catch(Exception e){
  System.out.println("error with microphone reading "+e);
  return null;
 }

 return encd;

 }
/*-------*/