Re: [Discuss-gnuradio] atsc_cpll *almost* works

[Brian Padalino]

On Tue, May 20, 2008 at 8:52 PM, Charles Swiger <address@hidden> wrote:
> After converting atsc_fpll to handle complex input (to eliminate one
> upconverter) it almost works, only the video out has problems.
>
> The problem starts when this:
>
>  float I = input.real() * a_sin;
>  float Q = input.real() * a_cos;
>
> is changed to this:
>
>  gr_complex IQ = input * gr_complex(a_cos,-a_sin);
>
> which would be necessary to handle negative frequencies I think.
> Everything else checks out ok, I even tried using gr_sincos.h in place
> of gr_nco.h like in gr_pll_carriertracking_cc and it behaves exectly the
> same.
>
>
> Working:
>
>  gr_complex input = agc.scale(in[k]);
>  gr_sincosf(d_phase,&a_sin,&a_cos);
>
>  float I = input.real() * a_sin;
>  float Q = input.real() * a_cos;
>
>  out[k] = I;
>  gr_complex filtered_IQ = afc.filter(gr_complex(I,Q));
>  float x = phase_detector(filtered_IQ,0);
>  static const float alpha = 0.001;
>  static const float beta = alpha * alpha / 4;
>  d_freq = d_freq + beta * x;
>  d_phase = mod_2pi(d_phase + d_freq + alpha * x);
>
>
> mpeg_packet_error_rate.py: 485526 errors out of 2165316 packets. 1679790
> good packets. Error rate = 0.224
>
> Almost working:
>
>  gr_complex input = agc.scale(in[k]);
>  gr_sincosf(d_phase,&a_sin,&a_cos);
>
>  gr_complex IQ = input * gr_complex(a_cos,-a_sin);
>
>  out[k] = IQ.real();
>  gr_complex filtered_IQ = afc.filter(gr_complex(I,Q));
>  float x = phase_detector(filtered_IQ,0);
>  static const float alpha = 0.001;
>  static const float beta = alpha * alpha / 4;
>  d_freq = d_freq + beta * x;
>  d_phase = mod_2pi(d_phase + d_freq + alpha * x);
>
> mpeg_packet_error_rate.py: 258539 errors out of 2165280 packets. 1906741
> good packets. Error rate = 0.119
>
> Looks like a 36 missing packets, even tho the error rate is less, as the
> video is certainly worse in mplayer.
>
> --Chuck

I don't know much about 8-VSB, but I was under the impression that
it's a single sideband modulation scheme that benefits from being real
only to avoid all the complex math associated with other modulation
schemes.

Do you know if this is true?  I suppose I am just a little ignorant on
the subject and looking to be enlightened.

Thanks in advance.

Brian