[TenTec] Pegasus DSP Project

[Mark Erbaugh]

Carl,

> Your 1.05 filter shape still won't be in the pegasus AGC loop.  This will
> make the filter the same as can be found on any good add on audio DSP
unit.
> In order for the steep skirts to really help, the AGC loop has to go
through
> the filters.  Otherwise, AGC pumping on signals outside your external
> passband will ruin the effect of the steep skirts.

I haven't gotten that far with the design, but I was planning on
implementing an AGC loop within the DSP that would reduce the RF gain as
needed. That probably won't be as good as what the Pegasus DSP chip can do
as I'm betting it has some sort of direct feedback to the RF gain stage.

I realize that I'm not going to get Orion level performance, but I'm hoping
to improve over the Pegasus filtering.

> Also, 2000 taps will cause a significant time delay and ruin any hope for
QSK.

You're probably right about QSK. If it comes to that, I may resort to
something like you do in your Pegasus / RX340 program and switch back to the
original Pegasus audio or to a faster filter while in Tx.  However, I am
planning on using a 'trick' that I learned from the FlexRadio SDR-1000
project. When you apply filter taps in the time domain you are convolving
the filter taps with the signal. However, convolution in the time domain is
equivalent to simple multiplication in the frequency domain. By working in
the frequency domain, I can drastically speed up the filtering. The FFT is
already being done get the data into the frequency domain for spectrum
display. I've played with this technique on the SDR-1000 and its using less
that 20% of my 1.6 MHz P4 with around 2000 taps.

Mark