Saturday, January 08, 2011

A matter of timing

I'm finding it hard to love the new Kenwood TH-D72. Despite the fact that it has a more sensitive GPS, a proper TNC that you can interface to computer APRS or packet software and is firmware upgradeable, I'm steadily coming round to the opinion that the VX-8GR is the better performing, more usable radio.

Things I don't like about the TH-D72 is that it is bigger and heavier, has a screen that gives far less information at a glance than the corresponding screens on the Yaesu and has poorer ergonomics. I have also been harbouring a suspicion that its packet modem was less sensitive. Today I think I discovered the reason.

I recently built a Fox Delta weather station that outputs AFSK packet directly into a radio. I noticed that although my Kenwood TM-D710 and my VX-8GR decode it's S9+ packet bursts the TH-D72 didn't. I thought it might just be a case of adjusting the deviation but I tried the weather station on two different radios adjusting the audio level from nothing to definitely clipping and could not find a setting at which the D72 would decode anything.

Recently I set up a low power APRS repeater in the shack. It is a sound card TNC (TrueTTY) driving a low power UHF radio (the FT-817ND) running into a dummy load, which is connected to the aprsg gateway software. This gates everything that is going on in APRS within a specified radius to UHF so that I can monitor activity and reply to messages using an APRS HT anywhere I am in the house. This has been working fine with the VX-8GR but last night I forgot to switch it off and the battery was dead so I tried monitoring using the TH-D72 instead. Nothing was copied!

Again I tried an entire range of audio levels into the radio but while the VX-8GR and the TH-D710 both decoded the packets over a wide range of settings the D72 didn't decode anything. I was using TrueTTY into my USBlink home-made VOX-based digital interface. I wanted to try different software (AGWPE) and a different sound card but Windows got confused having different USB sound devices connected to it and it is also a dog at handling serial ports. I have real serial ports occupying COM2 to COM5, a pair of virtual ports mapped between COM8 and COM9, and other USB serial devices I have used in the past have been assigned to COM1,6 and 7. AGWPE can only use COM1 to COM9 and trying to change the USB serial device to use one of the three currently unused ports in this range resulted either in Windows complaining that the port was in use even though it didn't show in Device Manager or the application saying that the port did not exist even though it did show in Device Manager. Eventually things seemed so screwed that I restored back to this morning and gave up.

Having restored the system and checked that everything worked again one more idea occurred to me. TrueTTY allows you to specify the exact sample rate used by the sound card, to compensate for timing errors. Instead of 11025Hz I tried 11000Hz and while the D710 and the VX-8 still decoded the packets the D72 still didn't. I then tried 11050Hz and lo and behold, the D72 started decoding!

It's impossible to make a suggestion that there is something wrong with a radio in the owners' groups on Yahoo as so many people can't bear to consider the fact that something they bought is anything less than perfect and will come up with any alternative explanation they can think of. So I'm sure that the problem I have described will be blamed on the AFSK modulation being slightly off-frequency which, of course, it is.

However in the real world a radio will be used to receive transmissions from people whose modulation is off and don't know it or may not even have any way of adjusting it. A modem that is more tolerant of these deviations from the precisely correct will decode more signals than one that expects the modulation to be spot on and in that respect the VX-8GR is by far the most easy-going and most sensitive of all the APRS radios.

It's just frustrating to hear braaaps and not see them decoded, so I think the Yaesu is going to be the one of these two APRS hand-helds that I hang on to.
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