[Discuss-gnuradio] atsc speed boost

[Charles Swiger]

This works a bit more effeciently in matching a usrp to the atsc code
(either all 0.9, or part 0.9 and part 2.x):

Instead of: collect data with usrp decim = 8, then interp by 5 (8Msps
to 40) then decim by 2 (40Msps to 20) while upshifting to 5.75MHz -

just collect data with decim = 10, then interp by 3 (6.4Msps to
19.2Msps), then upshift.   The 0.9 code need two changes, one to
atsc_rx.cc, set input_rate to 19.2e6 (IR_20 = 19.2 instead of 20e6), and
there is also needed a little tweak in GrBitTimingLoop3.cc and
atsc_sssr.cc to get  past the sanity checks for ratio of receiver clock
to symbol rate, change the 1.8 to 1.78, it's *just* under the limit ;)
( 19.2 / 10.76 ).

This puts the bottleneck back on the modified lt-atsc_rx process, which
includes bit_timing_loop, fs_checker, equalizer and field_sync_demux. I
should be able to split those up and manually redistribute the load
a little better still.

With that it's up from 1MiB / 6 seconds to about 1.2 or 1.3, or at least
20% faster, plus it takes 20% less disk space recording the signal
( before, a two hour movie would *just* fit on two striped 220GB disks
using decim=8, with 5GB left ;)

--Chuck