On Fri, May 12, 2006 at 01:27:14PM -0700, John Gilmore wrote: > > What you're describing is currently tough with the basic tx since it's > > effectively "always on" once you fire it up. If you underrun, the > > FPGA will continue transmitting the same value to the DAC (which has > > the upconverter in it), thus even if you're "not transmitting", it's > > highly likely that you really *are* still transmitting. > > ... > > * modify the FPGA code such that when the TX fifo is empty, you > > ramp the value fed to the DAC down to zero over say 8 clocks. > > Why would it take 8 clocks? Surely the hardware could do the hard > part (jam it to zero immediately) and the software could do the rest > (if it needs to taper off over 8 clocks, end with the right samples).
Jamming it to zero creates a square wave, which in the frequency domain is really ugly (infinite harmonics... Like bad CW key shaping). Ramping it down avoids the problem. If we right shift by 2 (with proper handling of the -1 and -2 cases) every clock, after 8 clocks it'll be zero. > Hmm, in fact, why is this a problem at all? Why can't the software > ensure that the final sample before underrun is a zero? The hardware > will then keep repeating this final sample forever. With the current flow graph, it's *really* hard to do this. I would have done it already otherwise ;) The input to the flow graph is data to be modulated, the output is the stream of modulated samples. There's no way simple/sane way to get the last samples to be zero -- it's FSK. This is part of the motivation for the m-block stuff. It allows us to run the existing flow graph and then tweak/append to the output. > (I suppose this would be possible if you aren't using any of the > upconverters -- but if the FPGA or the DAC is upconverting, then you > have to jam a real zero, not an upconverted zero? Is this even > possible?) An upconverted zero is still a zero. The complex mag is zero, so even though we're using the upconverter in the DAC, sending zeros will work. Eric _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
