sound card output to matlab giving funny noise

Hi,

I posted this on in the matlab forums but got no response, thought I
might find somebody with more relevant knowledge here.

Basically. Whilst playing music
on Itunes or windows media player, I ran the following code to get
and
plot 20 seconds of audio data:


ai = analoginput('winsound');
Fs=8000;
time=20;
set(ai,'SamplesPerTrigger',Fs*time)
addchannel(ai, 1);
tic; start(ai); toc
pause(time)
data = getdata(ai);
data=data/max(data);


stop(ai)
pause(5)
sound(data, Fs)


plot((1:length(data))/Fs,data)


I put a picture of the output at
http://personalpages.manchester.ac.uk/student/Adam.Chapman/soundcard_data.fig


As you can see, every now and again the data seems to oscilate around
-1. I would have thought that the data couldn't take values <-1,
because the "addchannel" command shows:


Index:  ChannelName:  HwChannel:
   1       'Mono'             1


InputRange:  SensorRange:  UnitsRange:  Units:
[-1 1]            [-1 1]               [-1 1]           'Volts'


I wouldn't think it's a hardware problem, as the sound from my
speakers is fine.

I've also ran the same code with speakers outputting no sound at all
and still get the noise. The data I aquired before did have the music
data in it, although when playing back it was distorted by this funny
noise signal.

Here's what I get from right-clicking the object "ai" in my workspace
and selecting "display hardware info":


   AdaptorName: 'winsound'
                       Bits: 16
                   Coupling: {'AC Coupled'}
                 DeviceName: 'Microphone (SoundMAX Integrated'
            DifferentialIDs: []
                      Gains: []
                         ID: '0'
                InputRanges: [-1 1]
              MaxSampleRate: 96000
              MinSampleRate: 5000
             NativeDataType: 'int16'
                   Polarity: {'Bipolar'}
                 SampleType: 'SimultaneousSample'
             SingleEndedIDs: [1 2]
              SubsystemType: 'AnalogInput'
              TotalChannels: 2
    VendorDriverDescription: 'Windows Multimedia Driver'
        VendorDriverVersion: '6.0'


I'm not sure about the fact that the device name is "microphone",
maybe that's default for an audio device. Also I dont know the
hardware info is aquired from the device drivers or if they are
merely
default values.


I've also put online the data plot with no sound at:
http://personalpages.manchester.ac.uk/student/Adam.Chapman/no_sound_playing.fig
Also plotted the data in uint16 format after seeing above the native
data type for the sound card is uint16. Again, not sure if this is
actual or default.



Thanks for any help offered.
Adam