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A Look at the Axia Livewire AoIP Protocol
a-look-at-livewire
What is Axia Livewire?
Axia Livewire is an IP networked audio protocol used in broadcast. Among other things, it allows for 32767 channels of raw multicast PCM audio distributed over a standard IP network. Axia nodes are connected to the network and provide audio I/O.
What Does a Livewire Stream Consist of?
Raw PCM (big endian, 24-bit signed, 48000 kHz, stereo interleaved) delivered in the form of multicast IP/UDP/RTP packets.
Receiving a Stream
To recieve an Axia Livewire stream, your client needs to join the multicast group associated with the Livewire channel. Add the Axia channel number to the base ip (239.192.0.0) to find its multicast address. Axia channel 27 has a multicast address of 239.192.0.27 and Axia channel 1212 is 239.192.4.188. See: this gist.
*If you are using a backfeed stream, the multicast base IP is 239.193.0.0. *If you are using a backfeed livestream, the multicast base IP is 239.195.0.0. *If you are using a surround stream, the multicast base IP is 239.196.0.0.
livewireStreamMulticastAddr = 0xEFC00000 + channelNumber;
A Look At The Packet
There are three different types of Livewire streams:
- Standard
- Livestream
- 5.1 Surround
Standard Livewire Packet
| Function | Bytes | Notes |
|---|---|---|
| Interpacket Delay | 12 | This is not actually transmitted, but is an Ethernet requirement and must be taken into account for bandwidth purposes. |
| Ethernet Header | 26 | Includes the VLAN/priority fields |
| IP Header | 20 | |
| UDP Header | 8 | |
| RTP Header | 12 | |
| Audio Payload | 1440 or 720 | Either 240 Samples or 120 Samples at 48 kHz, 24 bits, stereo. Configurable. |
Total bytes per packet: 1440, with core delay = 5ms (respective values of 720 bytes and 2.5ms using the variant format)
Livestream Livewire Packet
| Function | Bytes | Notes |
|---|---|---|
| Interpacket Delay | 12 | This is not actually transmitted, but is an Ethernet requirement and must be taken into account for bandwidth purposes. |
| Ethernet Header | 26 | Includes the VLAN/priority fields |
| IP Header | 20 | |
| UDP Header | 8 | |
| RTP Header | 12 | |
| Audio Payload | 72 | 12 Samples at 48 kHz, 24 bits, stereo |
Total bytes per packet = 72, with core delay 2.5ms
Surround Livewire Packet
| Function | Bytes | Notes |
|---|---|---|
| Interpacket Delay | 12 | This is not actually transmitted, but is an Ethernet requirement and must be taken into account for bandwidth purposes. |
| Ethernet Header | 26 | Includes the VLAN/priority fields |
| IP Header | 20 | |
| UDP Header | 8 | |
| RTP Header | 12 | |
| Audio Payload | 1440 | 60 Samples at 48 kHz, 24 bits, stereo + 5.1 (eight channels) |
Total bytes per packet = 1440, with core delay 1.25ms
Surround channels are in the following order: front left, front right, center, low-frequency enhancement (LFE), back left, back right, stereo left, and stereo right.
Packet information taken from the book "Audio Over IP: Building Pro AoIP Systems with Livewire" by Steve Church & Skip Pizzi (2010).
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