But the other thing you have to consider is that your digital mode
program
also has its own DSP filters. It probably isn't necessary to have
brick-wall
filters in the radio. All I can say is that you should experiment.
Yes, and all the digital mode programs I am aware of use FFT's. So
the ultimate filter is the width of 1 FFT bin (longer FFT's imply
narrower filters), and the kind of pre-windowing function used (Hann,
Blackman, etc). So no matter what kind of IF and audio filtering you
use, it makes no difference -- as long as those IF and audio filters
aren't distorting the passband to any great degree.
In fact, an argument could be made that you should strive for the
widest reasonable bandwidth on the incoming audio and IF, so that you
can avoid noise aliasing artifacts that might increase the noise
level within each FFT bin. In other words, take the full bandwidth
that your digital mode program can handle, and don't do any
filtering, apart from the FFT itself.
eh?
- de Dave, N7AIG
Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ 85750
email: d...@refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com
On Oct 12, 2010, at 14:20, Brian Lloyd wrote:
On Tue, Oct 12, 2010 at 12:57 PM, Brian Lloyd <br...@lloyd.com> wrote:
On Tue, Oct 12, 2010 at 9:57 AM, Drax Felton
<draxfel...@gmail.com> wrote:
So what sample rate / buffer sizes would be best for someone
interested in
weak signal digital mode operation?
I'd assume matching the 48000 VAC sample rate because
a. Latency isn't that big of a deal to the digi mode software.
(with JT65
maybe not)
b. Only need to see a small bandwidth on the panadapter at high
resolution.
Any others? Am I correct?
Well, there is no real advantage to matching the 48kHz sampling
rate out to
VAC. At each point where you change sample rates you have to do
sample-rate
conversion. When the new sample rate is a power-of-two submultiple
(divide
by 2, 4, 8, 16, 32, etc.) then resampling is no issue. When the
new rate is
not related to the old rate then you can have problems. So going from
192kHz, 96kHz, or 48kHz to 48kHz is not a problem.
But you have to remember that there is yet another sample rate
conversion
that is going to happen in your digital mode program as most of
them use
either 8kHz or 11.025kHz internally, depending on the modem.
(11.025 kHz is
the standard 44.1kHz used by CDs and most sound cards divided by 4.)
Resampling to 8kHz is not a problem (power of 2 again) but
resampling to
11.025kHz can produce distortion products depending on the quality
of the
interpolator code. You might want to try to match the digital modem's
internal sample rate in VAC to eliminate one resampling.
So if you don't care about pan/waterfall span, 48kHz sample rate
is good.
Do that with a 4096 sample buffer (in the DSP RX buffer settings)
will net
you some *seriously* sharp filters.
But the other thing you have to consider is that your digital mode
program
also has its own DSP filters. It probably isn't necessary to have
brick-wall
filters in the radio. All I can say is that you should experiment.
--
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
br...@lloyd.com
+1.767.617.1365 (Dominica)
+1.931.492.6776 (USA)
(+1.931.4.WB6RQN)
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