while trying to find the root cause i rather accidentally noticed that setting the serial baudrate to 1 (=1200) actually produces correct telemetry output at 19200 baud, plus the issue really isn‘t limited to LTM telemetry, it occurs when using mavlink too.
so i‘m assuming some issue in AP_SerialManager‘s baudrate setting
while doing some more testing i found that the 1200 baud-issue is limited to UARTs 1 and 6 that use an individual APB2 clock divider, so likely an issue with 1200 baud being just beyond the limits of ChibiOS uart-driver’s BRR register.
Due to total fire bans here I will be unable to test fly the 4.0.3 release myself in the next few days. I’d really appreciate logs from anyone who can fly the beta. I will not release the stable version until I have a good set of test logs.
marc, i‘m not sure if using LTM on the DL‘s radio modem option really adds any benefit as compared to using the emulated dragonlink mavlink option on the DL‘s „internal“ telemetry. it‘d be worth testing that though.
still i think that when using a micro- or nano-rx, the DL telemetry downlink will always be the weakest link in your setup, due to the compareably low transmitted power, plus using a directive antenna on your RC transmitter usually adds quite some inconvenience.
that‘s why transmitting LTM over your AV audio is a neat option, as it allows adding telemetry onto an existing link without range or bandwidth limitations.
the above linked converter allows to connect a small „dongle“ containig a FSK modem and an ESP8266 to your video receiver‘s audio output, and have your telemetry downlink broadcasted wirelessly over wifi/UDP for visualisation on whatever mavlink-compatible GCS you prefer, like QGC on an iphone for example.
yes, it serves pretty well as a long range data downlink. back around 2012 when it came up, using the AV links’s audio channel was common procedure. today there’s a couple of good alternatives around and sourcing the modem components can be a pita. the low bandwidth it uses is still a big pro imho. works well right down to 1200 baud.
Actually the cost of making 6 modems is just $20. Just ordered all the parts. This is a nice addition to my long range smaller planes. Especially since I have made some QCZEK LRS receivers that do not have radio modem but are very long range capable. Thanks for this.