June 14, 2011 3 Comments
The comments by Julian and Marc on my previous post on APRS have been extremely useful. Julian pointed me to a writeup by John Ackermann that explains that APRS decoder do not work well when the 2200Hz tone is much weaker than the 1200Hz tone. I recorded a few packets and looked at the signal in Audacity, an open-source sound editor. Sure enough, the amplitude of the 2200Hz tone was much smaller than that of the 1200Hz tone:
John Ackermann’s article discusses one reason for this phenomenon: a modem that overdrives the transmitter. APRS packets are sent on VHF using audio tone that modulates an FM transmitter. FM voice transmitters pre-emphasize high audio frequencies and then clip the audio to limit deviation. If the modem overdrives the transmitter, low and high frequencies have the same amplitude. This seems fine, but the receiver de-emphasizes high frequencies, leading to the 2200Hz tone being too weak.
Initially I suspected that this problem caused my problems. I asked Avishay Ginzburg to send a few packets at his normal setup and then to reduce the drive from the modem. With a reduced drive, the overall amplitude of the received audio on my side was lower (both by ear and in Audacity), but the 2200Hz tone was still much weaker than the 1200Hz tone. So overdrive was not the problem.
Avishay’s signal is very strong in my home, so the signal-to-noise ratio that I initially suspected was not the issue either. Time to look for other suspects.
Marc’s comment on my earlier post suggested that the same problem could be caused on the receive side, by the interface to the modem or by the modem itself. I use a soundcard, not a hardware modem, but the sound-card interface certainly could be the problem. For the APRS experiments (or at least for most of them), I used a galvanically-isolated soundcard interface, not the simple interface that I described here a while ago. I switched to the simple interface; problem solved. I could now decode packets from a couple of stations, both stationary and mobile.
I also switched from the FM mode of the FT-857D transceiver to the packet mode. That may have made a difference too. The reason that I did not use the packet mode earlier is that the Windows software modems switch from receive to transmit using the RTS or DTS signals of a serial port, which is a mode that I don’t like to use. The packet mode on the FT-857D does not support sound-activated transmission (VOX), so I used the FM mode. Soundmodem, the Linux soundcard modem, supports receive/transmit switching using serial CAT commands, so I could use the packet mode.
Why was I able to decode ISS packets but not local packets? Perhaps I used the simple interface and not the isolated one, but perhaps I used the packet mode. The main conclusion here is that I should have kept notes on the setups that did work and on the setups that did not work, but I didn’t keep any notes. Big mistake.
Why does the galvanically-isolated interface act as a low-pass filter? I still don’t know. I should probably test the frequency response of the audio isolation transformers. The interface also includes resistors to drop the signal to a level that’s good for the microphone input of a sound card; perhaps the resistors formed an RC filter with some capacitance, or an LC filter with the transformers. The interface works fine for narrowband modes like PSK31 and JT65A, so I assumed it would also work for APRS, but this assumption is wrong; an interface with a non-flat frequency response may work fine for narrow-band modes but may fail for wider-bandwidth modes.