search

TLeconte / vdlm2dec

vdl mode 2 SDR decoder
GNU General Public License v2.0
71 stars 23 forks source link

IQ values in rtl.c #39

Closed BradWalker closed 5 years ago

BradWalker commented 5 years ago

Inside of in_callback() the IQ values get adjusted by 127.37..

How did you determine this is a correct value to adjust them?

Thanks.

static void in_callback(unsigned char *rtlinbuff, uint32_t nread, void *ctx)
{
..
    for (i = 0; i < RTLINBUFSZ;) {
        float r, g;
        r = (float)rtlinbuff[i] - **(float)127.37;**
        i++;
        g = (float)rtlinbuff[i] - **(float)127.37;**
        i++;
        Cbuff[i / 2] = r + g * I;
    }
..
}
TLeconte commented 5 years ago

If you average the value of IQ over a long time on a dummy load, you'll find a value a little less than 127.5. It could depend of each device and temperature. I don't remember why I choose .37, perhaps it the result of a measurement or a value commonly used in other codes (perhaps from osmosdr ) It's an "experimentaly good value" :-)

BradWalker commented 5 years ago

Thanks! Makes sense..