Voice Alert 136,5 Hz!
Voice alert is a system, that allows "living" users (those who are present at the station) of APRS to announce themselves and have a possibility to communicate to each other and not take into account, that the second VFO of the radio is on the different frequency.
How does Voice Alert (VA) works
While we travel by car, we use frequency 144.800MHz with 136.5Hz CTCSS (both RX and TX). Since the "dead" APRS stations (DIGIs WX stations, unattended stations) don’t transmit the CTCSS, we can hear APRS packets only in case, that there is another station producing 136.5Hz nearby. I can take a microphone and call him then. Thanks to fact, that my signal has 136.5Hz on the background, the oposit station can hear me, can respond and we both can move to another frequency on the second VFO or second radio and continue the conversation.
Note.: APRS Bible recommends to use CTCSS 100Hz, but thanks the 50Hz mains in Europe, this tone frequency is in practice unusable. Therefore we use 136.5 Hz !!!
Use Voice Alert (transmiting and recieving CTCSS 136.5Hz) really just in case when you are physicaly present at the radio, have a volume enough to hear and you’re ready to respond to possible request! With radios, such as Kenwood TM-D700 a TM-D710 is possible to use the PM function, where, for example, under PM1 we can have VA set and under PM2 not. Switching between them is a matter of pushing two buttons.