On Sun, Jan 14, 2007 at 12:45:48PM -0800, Matt Ettus wrote: > Eric Blossom wrote: > > > > > > This could work, with two caveats: > > > > (1) you're stuffing 0's into the input of the signal processing > > chain, instead of handing the underrun at the output of the > > signal processing chain. With the existing code, you're > > probably OK, but in general, if the idea is to zero the output > > when there's no input, I'd prefer to do it closer to the DACs. > > [ Don't let this comment stop you from trying this ;) ] > > > > Have to disagree with you here. The best place to do this is before the > signal processing chain, for exactly the reasons you state below... > > > (2) You're going to get the equivalent of a "key click". That is, > > wide band harmonics in the freq domain from the discontinuity in the > > time domain (step function). It would be better to ramp the last > > value down, say be doing the equivalent of an arithmetic shift right > > by 2. This really ought to be done by the code that feeds the DACs. > > > > The key clicks will get filtered if you do this before the upconverter. > They'll still be there, but they'll only be as wide as the baseband > bandwidth instead of the full 32 MHz. > > > I think a better place to fix this would be to filter the output of > > tx_a_a, tx_b_a, tx_a_b and tx_b_b in usrp_std.v based on their > > previous values and the delayed value of tx_empty. > > You don't need this filter if you do it before the interpolators. > > > I believe that a > > delay on the order of 12 clocks is about right. You'll need to count > > pipeline stages through the tx path to get it right. > > > > The delay through the interpolators is variable -- it depends on the > intepolation ratio. Again, this is why this should be done before the > interpolators.
Agreed as long as the input being pushed into the signal processing pipeline is interpreted as samples. I think we're going to need to visit this problem again when somebody put a modulator into the FPGA. Eric _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
