Re: [Discuss-gnuradio] ATSC Magic coupling constant

[Brian Padalino]

On Tue, Jul 30, 2013 at 5:55 PM, Andrew Davis <address@hidden> wrote:

Hello all,

I'm working on fixing up gr-atsc and I have been working on a little problem for a while now, there is a constant ( FPLL_BTLOOP_COUPLING_CONST ) that sets the reference for an AGC, the value is ( 2.5 * 3.125 ) and is literally defined as "Magic", so it seems to be a value that "just works". With that value the AGC filters the real input and then this is multiplied by a NCO part of a PLL. This puts the value of the +/- 5 timing sync levels at about +/- 5. The problem is when I pass samples though the AGC and then do 'complex' multiplication on them the +/- 5 values end up at about +/- 7. I'm not sure why but it seems like the values coming out of the complex multiply are not the same as the real multiply. The original code is: 

nco.sincos (&a_sin, &a_cos);  // compute cos and sin

float I = input * a_sin;

float Q = input * a_cos;

My code is:

nco.sincos (&a_sin, &a_cos); // compute cos and sin

gr_complex result = (gr_complex(input_real, input_imag) * gr_complex(a_cos, a_sin));

float I = result.real();

float Q = result.imag();

I is larger in my version and so the equalizer and slicer fail downstream. I built a coherent AGC into the sync timing loop but it still fails with large gain differences. 

My question is whether there is anyone around who worked on gr-atsc who could give me a hint as to how the "Magic coupling constant" was derived in the first place so I can build a new one so I don't have to rebuild the equilizer.

I can't help you with your MAGIC, but if you say the real signal is mixed then filtered in the original code that works, whereas it is just mixed with your code - maybe it's just off by a sqrt(2) since you're filtering off your image after the NCO happens and losing 1/2 power?

I did notice that sqrt(2)*5 = 7 - so maybe making your NCO a little less powerful might bring you to the correct power levels since I am assuming you aren't filtering in your chain since there is no image to filter out?

Brian