[aprssig] Interesting Findings - 300 Baud AX.25 on VHF-FM APRS
Stephen H. Smith
wa8lmf2 at aol.com
Sat Apr 2 21:24:45 CDT 2016
On 4/2/2016 7:30 AM, Ev Tupis via aprssig wrote:
> In regard to the findings below...you said that you were surprised that the
> difference played out on FM as it does on SSB but didn't go on to say why you
> thought that it would not.
> It's going to be tough for anyone to offer comments until they know what you
> are unclear about, exactly. ;-)
This is due to my previous experience with the very non-linear curve of
S/N-vs-signal-level on FM as you start falling off the fully-quieted "cliff"
and quite abruptly start losing quieting over a very small change of RF level.
I.e. decreasing the RF level by 1dB at the "cliff" doesn't decrease the S/N by
1dB; it may decrease by 5 or 6 dB.
This is very different from the "gradually getting noisier" behavior one
experiences on SSB as the signal drops. I.e. where a decrease of 1dB in RF
level yields more-or-less proportionally a one dB reduction in S/N. In my
experience, the loss-of-quieting cliff far out-weighed any subtle differences
due to audio modulation mode, bits-per-second, pre-emphasis or not, ultimate
demodulator performance, etc.
A couple of years ago, I did a controlled "shoot-out" comparing various
hardware and software TNCs. The test setup was similar to the setup I
described in the first post of this thread. The difference in that test was
that I used a high-quality digital off-the-air recording of Mic-E packet bursts
from a Kenwood D700 to modulate the IFR's RF generator. (It's actually one of
the tracks on my test CD.)
I tested an MFJ-1270, a KPC3+, an original KAM (VHF port), an AEA PK-88, the
AGW packet engine, MixW in packet mode, and the UZ7HO Soundmodem. All of them
failed within 1 dB RF level of each other except for the Soundmodem which was
dramatically 4 dB better. Later, I discovered that the Soundmodem had
implemented a missing-bit recovery algorithm that the other hardware and
All those previous tests had been done at 1200 baud only. At that time, any
subtle differences in audio demodulator performance were overwhelmed by the
rather abrupt loss-of-quieting from the decreasing carrier level.
(I later tried a direct audio-audio connection between the test signal source
and the TNCs under test. Gradually decreasing the audio level did show a wide
range of differences, but it's inconclusive since the optimum audio input level
for the various devices may be sharply different.
Some day, I will repeat these tests, but mix varying amounts of white or pink
noise into constant-level test audio, to truly characterize the different
devices' performance with noisy signals, unbiased by the FM discriminator
quieting threshold effect.
This time around, I was contrasting 1200 vs 300 baud on the same software (the
UZ7HO Soundmodem) and saw dramatic differences in the decode performance. It
turns out the Soundmodem has DSP audio bandpass filtering before the
demodulator. The filter bandwidth automatically varies for the different baud
rates. Note that these are pre-detection audio filters; not the post-detection
low-pass filters with a cutoff just above the bit rate common in many hardware
and software modems.
Stephen H. Smith wa8lmf (at) aol.com
EchoLink: Node # 14400 [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net
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