[RTTY] Re: external DSP audio filters
Kok Chen
chen at mac.com
Sat Oct 2 14:47:03 EDT 2004
On Oct 2, 2004, at 5:54 AM, Ian White, G3SEK wrote:
> Or do the outboard DSP boxes really perform so much better than the
> DSP options in the radios and in MMTTY?
IMHO, they don't. It depends on the hardware, and as they say, "the
rest is just software/firmware."
On the hardware end, you need to look out for the A/D converters that
are in the DSP boxes and how well the box is shielded, whether they
have balanced inputs, what is the dynamic range (not just how many bits
it has, but the true dynamic range) and how clean the power supply is,
etc.
The same exact thing can be said of the "sound card" that you are using
with computer based DSP programs.
A good DSP box will beat a lousy sound card, and a good sound card will
beat a lousy DSP box. The rest truly is just software/firmware, and
amateurs can crank out equally good algorithms as commercial companies
do. Often better, since presumably they do it out of the love of it.
A good filter algorithm cannot make up for a crummy A/D converter.
Analog garbage in, digital garbage out.
Naturally, the software in the computer or the firmware in the DSP
boxes also have to also be able to handle the dynamic range of a good
A/D converter -- using a 105 dB dynamic range A/D converter with 16-bit
processing is a waste, for example. Modern operating systems have
floating point audio chains that accept 24-bit A/D converter inputs and
enough horsepower to use it.
> As far as I'm aware, the twin peak filter is the only feature
> mentioned that the MP doesn't have at all (don't know about the
> Pro-II) but both bandpass and twin-peak filtering are also available
> in MMTTY.
At 170 Hz shift and 45 baud data, properly designed set of matched
filters for Mark and Space tones will implicitly create a "double peak"
response.
I usually don't specifically design a double peak filter ahead of the
demodulator, they only screw up the matched filtering, and you would
have to compensate for the envelope waveform if you already have an
ideal matched filter. Instead, I use a symmetrical flat FIR bandpass
for the RTTY signal for the "roofing filter" of the RTTY signal that is
about 350 Hz but with very steep skirts. I then let proper matched
filters do their stuff; I use separate matched filters for Mark and
Space ahead of the adaptive slicer. The resulting system response is a
double peak for the design data rate.
That said, all else being equal, for noise reasons I prefer that my A/D
converter is outside of the computer. I write my own DSP software, but
I use external USB-based A/D converter boxes instead of PCI sound
cards. I use a handful of Griffin iMic converters, an M-Audio Transit
and an M-Audio Quattro; these are 20- to 24-bit converters of different
ages and dynamic ranges.
73
Chen, W7AY
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