soundmodem won't decode 300 baud signals?

There was a very similar question posted here back in November 2005 which never got an answer, so I guess I will have to ask again. I've been trying to receive 300 baud packet signals via HF with soundmodem, and have had no luck thus far. I have a channel configured with these settings on the Demodulator tab:

Mode: AFSK
Bits/s: 300
Frequency 0: 900
Frequency 1: 1100
Differential Decoding checked

When receiving a signal, the diagnostic screen shows that the modem is just detecting a solid string of '1's. The app seems to be able to decode 300 baud packets sent over FM from another computer running soundmodem with the same settings.

Is this supposed to work? If so, what am I doing wrong?

-Joe, KD8ATU

[..]

At a glance at the source code, 300 baud isn't supported, so I don't know how you got 300 baud from another computer to be detected. Was that running soundmodem also, or some other implementation?

Hamish

Hamish Moffatt VK3SB <[email protected]> <[email protected]>

The code at the beginning of the "demodconfig" function in afsk/modem.c suggests to me that it'll at least try to work at rates of between 100 and 9600 bits/second.

If it works correctly using FM, and not on HF, then my guess would be that it may be an issue of tuning accuracy.

I suspect that the soundmodem demodulator probably uses fairly sharp (high-Q) digital filters, in order to improve noise immunity. If that's the case, then the tuning of your HF rig would become quite critical. You're trying to distinguish between two frequencies which are only 200 Hz apart... and if the filters are actually sharp enough to distinguish these frequencies, then a tuning error of as little as 50 Hz might be enough to move the carriers too far away from the filters' center-of-passband points.

You might want to fiddle with your rig's RIT function, while trying to receive/demodulate an HF packet signal. Adjust the tuning in increments of as small as you can - 10 Hz or so at a time - and see if there's a point at which the demodulator manages to lock onto the signal and give you a recognizable bitstream.

Commercially-built HF packet modems usually have some sort of tuning indicator on the front panel, which shows the frequency of the audio carrier in some way - this allows the receiver tuning to be adjusted to allow proper decoding of the tones.

Digital-mode software for HF (e.g. PSK31 and similar modes) often incorporates a software-driven fine-tuning feature, to allow the software to lock onto the signal in the face of slight tuning errors, frequency drift in the transmitter and/or receiver, etc. This fine-tuning is done by changing the configuration of the software demodulator (e.g. recalculating filter coefficients), not by actually re-tuning the radio.

As far as I can tell, the soundmodem demodulators don't have this capability. I imagine it could be added, at some investment of effort.

I tried tuning 100 Hz up and down in 10 Hz steps from what should have put the bounds of the signal at 900 and 1100 Hz and was not able to decode even 10db+S9 300 baud signals at any point.

I would speculate that soundmodem's decoder is unable to cope with the background noise present in sideband mode, but it decodes both 300 and 1200 baud signals just fine through a microphone held up to the speaker of another computer running the same software, and that's quite a bit noisier than a strong sideband signal.

Anyone have any other ideas?

-Joe, KD8ATU