Adding two signals at a certain SNR

Hello,

I need to add two signals (one is the noise, the other is the speech) at a certain SNR.

How can I do this?

I found on Mathworks but I do not understand it and I am reluctant to using it

noise = noise / norm(noise) * norm(signal) / 10.0^(0.05*snr);
noisy = signal + noise;

Could you please explain it to me?

Regards,

Dan

[Preethi]

hi,

may not be exact, but something along the same lines

I think the step uses the standard formula used to calculate SNR..like SNR =10*log(signalpower/noisepower). reverse it to obtain the noise value in order to achieve the required SNR

[Petru-Daniel Tudosiu]

If I am to reverse the equation of SNR ( SNR = 10*log(sp/np) ) I will obtain sp = np * 10 ^ (0.1*SNR) which I cant "match" to the equation from my question.

[David Goodmanson]

Hello P-D, the norms of the signal and noise waveforms are are linear quantities, i.e. amplitudes and not squared amplitudes like power would be. For linear quantities the SNR is

SNR = 20*log10(norm(signal)/norm(noise))

which is exactly the same quantity as with the power expression

SNR = 10*log10(norm(signal)^2/norm(noise)^2)

The first expression inverts to the equation you have. That's my take on all this anyway. As you mentioned, it's a good idea to be reluctant to use something you do not understand and not rely overly much on outside commentary that you have not verified, including this one.