[Discuss-gnuradio] symbol_sync_cc stuck when error too large (resulting d_avg_period goes negative)

[WarMonkey]

Bug

When input amplitude is too large, symbol_sync_cc get stuck.

Screenshot

Tracing

• in file symbol_sync_cc_impl.cc line 537:
  error = -488.4;
  advance_loop(error);
• in file clock_tracking_loop.cc line 128 function advance_loop(float error) :
  before: d_avg_period=10.771521, d_beta=0.0090558, error=-488.4;
  d_avg_period = d_avg_period + d_beta * error;
  after: d_avg_period = -0.417
  out of range [d_min_avg_period, d_max_avg_period]
• back to file symbol_sync_cc_impl.cc line 540:
  d_clock->phase_wrap();
• in file clock_tracking_loop.cc line 145 function phase_wrap()

phase = -87.227, limit=-0.417/2=-0.20895, now we stuck in these while loops;
ps: i think period should be d_avg_period, because avg period is estimated symbol period. when loop bandwidth relatively larger ( 0.05~0.25 ), limiting d_inst_period can make tracking error larger, even loop unlock. i'll benchmark both later.

Workaround

apply limit to d_avg_period immediately after d_avg_period changed

• in file clock_tracking_loop.cc line 127

Ultimate solution

• check every input, state and output in range when calculate control loop