adding noise to a signal I have an audio file. I took an extract of the file called variable "CALL", and I took another extract from the file which is just filled with background noise, called "BACKGROUND".
Using matlab, I found the signal to noise ratio of CALL using:
snr(CALL,BACKGROUND); % both variables are of the same length

the signal to noise outputted was     5.3949

I want to add noise to CALL, and lower this signal to noise to 3 for example. How would I go about programming this?

I've only seen examples where people add noise to a perfect signal (i.e. they just create some kind of sinewave, then add noise to that), I have a signal which already has noise, and I want to add more noise, and know its signal to noise value. 

Also, is the above method for how I found my first signal to noise value correct? (I'm assuming the background noise is somewhat constant throughout the audio file)
maybe the info at the following web page will be of some use to you.  http://www.dsprelated.com/showcode/263.php  [-Rick-]