FT 991A as an APRS Digipeater

I was able to configure my FT-991A as an APRS digipeater - but not without some caveats.

The starting point was this excellent video by KM4ACK. While the video explains the procedure for a Raspberry Pi and a Baofeng - and my setup is a Windows desktop plus the FT991A, this video hits the nail in the head on all the settings needed for properly configuring Direwolf as a Digipeater. I stored the APRS frequency in a memory slot, set it as DATA-FM, ensured that there are no tone/code encoding (TONE/DCS set to OFF) and using the Standard COM port as the PTT control, using the RTS signal - and also requiring to set the configuration clause 076 - FM PKT PTT SELECT to RTS.

However, I stumbled upon an issue where my HT was sending the APRS messages to the digipeater (991A), properly decoded by Direwolf, sent to the internet - but the VX8 almost always failed to decode the packets originated by the 991A, such as Digipeater ID, packet ACKs, message from other stations and such.

After lots of troubleshooting, I found the culprit to be the audio lag in my computer. The PTT was dropping too early - enough to cut off some data that was being still transmitted, thus corrupting the packet - and hindering the decode of the packet.

I found no Direwolf configuration for an extra PTT hold time to cope with the audio delay - but was able to work around this reasonably well by using the VOX. Set the config 142 VOX SELECT to DATA, trim down the config 147 DATA VOX DELAY to 30msec and enable the VOX mode. So while there's audio being generated, the VOX will hold the PTT on, dropping the TX shortly after the sound actually ceases.