I/Q samples frequency shifting

[Masood_786]

Hi
I have downloaded I/Q samples of AM modulated signals (Radio broadcast) that contains 2 stations. one centered at 0 Khz and other at 8 Khz.
I am able to play 0 Khz stations by demodulation. now i want to shift (down-conversion) the 8 Khz signal to 0 Khz. I understand the process and studied various books but by applying what i have studied i am unable to shift frequency. I have done the following
Fs=22050
Fc=-8000 // 8Khz to be shifted to 0 Khz

ph_inc=(2*PI*FC/Fs) for NCO
LO_ph_acc=0

fptr[1024] //it contains I and Q samples interleaved

then in loop i do following
for(int a=0; a<samples_count*2;a+=2)
{
i = fptr[a];
q = fptr[a+1];

LO_ph_acc+=LO_Ph_Inc;
if(LO_ph_acc> (2 * PI) )
LO_ph_acc-=(2 * PI);

i=(i * cos(LO_ph_acc)) - (q* (sin(LO_ph_acc) ) );
q=(q* cos(LO_ph_acc))- (i* (sin(LO_ph_acc) )) +

newfrq[a]=i; //still the frequency is not shifted
newfrq[a+1]=q;
}

In SDR applications like SDRSharp, two stations can be seen and even 8 Khz station shifted to 0 Khz when selected for shifting. Can any one tell mistake in the code.

 

[Dan Mills]

You modify i then use the modified value of i in the q calculation, I don't think this is correct.
Also should not the q line be something more like (q* sin(LO_ph_acc)) - (i * cos....

Please use cut and paste (and [ code ] tags) when copying code into a post, the above has numerous typos.

73 Dan.