根據 ONVIF 規範，我們將其分成兩部分來討論
一、聲音的設定，例如 audio codec 可以是 AAC, G.711 或 G.726，full duplex or half duplex
此部份可參考 ONVIF-Media-Service-Spec-v221.pdf Chapter 5.14 Audio channel modes，摘錄如下：
1. full duplex operation, no special handling is required
2. half duplex, should assign an optional Send-Primacy Parameter in AudioOutputConfiguration
Acoustic echo cancellation is out of ONVIF scope.
- www.onvif.org/ver20/HalfDuplex/Server The server is allowed to send audio data to the client. The client shall not send audio data via the backchannel to the device in this mode.
- www.onvif.org/ver20/HalfDuplex/Client The client is allowed to send audio data via the backchannel to the server. The device shall not send audio data to the client in this mode.
- www.onvif.org/ver20/HalfDuplex/Auto It is up to the device how to deal with sending and receiving audio data.
若要支援 audio backchannel，則應該實作下列對應的 ONVIF API
- GetAudioOutputsRequest
- GetAudioOutputConfigurationsRequest
- GetAudioOutputConfigurationRequest
- GetCompatibleAudioOutputConfigurationsRequest
- GetAudioOutputConfigurationOptionsRequest
- SetAudioOutputConfiguration-Request
- GetAudioDecoderConfigurationsRequest
- GetAudioDecoderConfigurationRequest
- GetCompatibleAudioDecoderConfigurationsRequest
- GetAudioDecoderConfigurationOptionsRequest
- SetAudioDecoderConfigurationRequest
二、聲音的傳送
此部份可參考 ONVIF-Streaming-Spec-v221.pdf chapter 5.3 Back Channel Connection
其實作是使用 RTSP [RFC 2326]的一個擴充tag "Require"，將其設定為 "www.onvif.org/ver20/backchannel"。
當 NVR 發出 RTSP Request時，若帶有此tag，表示NVR想要使用 backchannel功能
- 若IPCAM支援，則回應的SDP 內會描述對應的 audio 設定，例如："a=sendonly"。
- 若IPCAM不支援，則應該回覆 "551 Option not supported"。
註1：NVR 應該先使用 ONVIF API 詢問 IPCAM 能力，確認IPCAM支援 backchannel之後，才能夠在 RTSP Request 中加入 "www.onvif.org/ver20/backchannel" tag。
註2： ONVIF 還定義了另一個 Require tag "onvif-replay"，用來做 replay功能。
以下摘錄 spec 中針對 backchannel 的 RTSP 範例
Example 1: Server without backchannel support:
Client – Server: DESCRIBE rtsp://192.168.0.1 RTSP/1.0
CSeq: 1
User-Agent: ONVIF Rtsp client
Accept: application/sdp
Require: www.onvif.org/ver20/backchannel
Server – Client: RTSP/1.0 551 Option not supported
CSeq: 1
Unsupported: www.onvif.org/ver20/backchannel
Example 2: Server with Onvif backchannel support:
Client – Server: DESCRIBE rtsp://192.168.0.1 RTSP/1.0
CSeq: 1
User-Agent: ONVIF Rtsp client
Accept: application/sdp
Require: www.onvif.org/ver20/backchannel
Server – Client: RTSP/1.0 200 OK
CSeq: 1
Content-Type: application/sdp
Content-Length: xxx
v=0
o= 2890842807 IN IP4 192.168.0.1
s=RTSP Session with audiobackchannel
m=video 0 RTP/AVP 26
a=control:rtsp://192.168.0.1/video
a=recvonly
m=audio 0 RTP/AVP 0
a=control:rtsp://192.168.0.1/audio
a=recvonly
m=audio 0 RTP/AVP 0
a=control:rtsp://192.168.0.1/audioback
a=rtpmap:0 PCMU/8000
a=sendonly
// setup the sessions
Client – Server: SETUP rtsp://192.168.0.1/video RTSP/1.0
CSeq: 2
Transport: RTP/AVP;unicast;client_port=4588-4589
Server – Client: RTSP/1.0 200 OK
CSeq: 2
Session: 123124;timeout=60
Transport:RTP/AVP;unicast;client_port=4588-4589; server_port=6256-6257
Client – Server: SETUP rtsp://192.168.0.1/audio RTSP/1.0
CSeq: 3
Session: 123124
Transport: RTP/AVP;unicast;client_port=4578-4579
Server – Client: RTSP/1.0 200 OK
CSeq: 3
Session: 123124;timeout=60
Transport:RTP/AVP;unicast;client_port=4578-4579; server_port=6276-6277
Client – Server: SETUP rtsp://192.168.0.1/audioback RTSP/1.0
CSeq: 4
Session: 123124
Transport: RTP/AVP;unicast;client_port=6296-6297
Require: www.onvif.org/ver20/backchannel
Server – Client: RTSP/1.0 200 OK
CSeq: 4
Session: 123124;timeout=60
Transport:RTP/AVP;unicast;client_port=6296-6297; server_port=2346-2347
// PLAY request
Client – Server: PLAY rtsp://192.168.0.1 RTSP/1.0
CSeq: 5
Session: 123124
Require: www.onvif.org/ver20/backchannel
Server – Client: RTSP/1.0 200 OK
CSeq: 5
Session: 123124;timeout=60
// TEARDOWN request.
Client – NVT: TEARDOWN rtsp://192.168.0.1 RTSP/1.0
CSeq: 6
Session: 123124
Require: www.onvif.org/ver20/backchannel
NVT – Client: RTSP/1.0 200 OK
CSeq: 6
Session: 123124
