Acquiring Data with a Sound Card

Hello,

I have successfully followed the following tutorial:

http://www.mathworks.co.uk/products/daq/code-examples.html?file=/products/demos/daq/acquiring_data/acquiring_data.html

I output a frequency of 523Hz using NCH tone generator, follow this tutorial exactly... but then end up with a fundemental frequency of 1048Hz (nearly bang on double what I was expecting).

I'm just wondering if anyone here has a suggestion as to why I am getting double the frequency I was expecting? Everything else seems to be working well.

Thanks a lot,

Dan