Voice Recorder

When a gateway has the Voice Recorder licensable feature, the Voice Recorder tab becomes visible in the ICU

RTP audio packets can be sent to 1 destination.

Multicast vs UDP
The 1 destination address in the Voice Recorder setup in ICU is either a UDP or Multicast address.
Multicast is IP Addresses are in the range 224.0.0.0 through 239.255.255.255

We send an RTP audio packet for each received audio packet from a PC client, and for each recorded audio that comes from the radio.

Before we send each RTP audio packet, we take the raw audio bytes and encode them to one of the following audio specifications that can be chosen in the ICU:

  • GSM @ 8000 Hz
  • G722 @ 8000 Hz
  • PCM Linear @ 8000 Hz <-- Switched from little-endian to big-endian 2/11/2021
  • PCM Linear @ 11025 Hz <-- Switched from little-endian to big-endian 2/11/2021
  • PCM uLaw @ 8000 Hz
  • PCM uLaw @ 11025 Hz

Each audio packet has an RTP Header. Within that RTP Header, we have inserted our own custom header information.

The RTP Protocol include a "Header Extension" that we are using to add additional information for the audio stream.

https://en.wikipedia.org/wiki/Real-time_Transport_Protocol#:~:text=Header%20extension%3A%20

We include the following custom data:

  • TransmittingUser
          Name (Radio ID)
          Type Of User (0=Radio, 1=Group, 2=PC)
          GPS Location (Lat, Lon, Speed)
  • ReceivingUser
          Name (Radio ID)
          Type Of User (0=Radio, 1=Group, 2=PC)
          GPS Location (Lat, Lon, Speed)
  • GatewayInfo
          Gateway Name
          Channel Info (Zone, Channel)

A PC user and Group User will not have GPS information.