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Audio Backchannel (ONVIF / RTSP)

Overview​

The Audio Backchannel feature allows an RTSP / ONVIF client to send audio from the client to ITVDesk IPCam. This is typically used for:

  • Two-way audio (talk-back)
  • Intercom and PA systems
  • Operator announcements
  • Audio injection into surveillance environments

ITVDesk implements audio backchannel according to ONVIF specifications using standard RTSP extensions and SDP direction attributes.

AudiocallBack channel ITVDesk

Standards & Compatibility​

  • RTSP: RFC 2326
  • ONVIF Feature Tag:
    www.onvif.org/ver20/backchannel
  • ONVIF Profiles: Profile S / T
  • RTP-based audio transport (client → server)

ITVDesk supports all standard ONVIF audio codecs, including:

  • G.711 (PCMU / PCMA)
  • AAC
  • OPUS
  • G.726 variants

Codec negotiation is performed via SDP.

RTSP Require Header​

An RTSP client that wants to establish an audio backchannel must include the following Require header:

Require: www.onvif.org/ver20/backchannel

If the server does not support backchannel, it will respond with:

RTSP/1.0 551 Option not supported
Unsupported: www.onvif.org/ver20/backchannel
tip

ITVDesk fully supports this Require tag.

Bi-Directional RTSP Session Setup​

The client establishes a standard RTSP session with an additional audio stream used for backchannel transmission.

Session Flow​

  1. DESCRIBE (with Require header)
  2. Server returns SDP with backchannel media section
  3. SETUP (video)
  4. SETUP (audio playback)
  5. SETUP (audio backchannel)
  6. PLAY
  7. Client starts sending audio to server
  8. TEARDOWN to terminate
tip

The client must not send audio before receiving a successful PLAY response.

SDP Direction Attributes​

To describe media direction, ITVDesk uses standard SDP attributes:

  • a=recvonly
    Media sent from server to client (video, normal audio)

  • a=sendonly
    Media sent from client to server (audio backchannel)

Example: SDP with Audio Backchannel (Multiple Codecs)​

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 97 98 99 100
a=control:rtsp://192.168.0.1/audioback
a=rtpmap:0 PCMU/8000
a=rtpmap:97 G726-16/8000
a=rtpmap:98 G726-24/8000
a=rtpmap:99 G726-32/8000
a=rtpmap:100 G726-40/8000
a=sendonly

The client selects one codec supported by both sides.

RTSP SETUP Example (Backchannel)​

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 response:

RTSP/1.0 200 OK
CSeq: 4
Session: 123124;timeout=60
Transport: RTP/AVP;unicast;client_port=6296-6297;server_port=2346-2347

Testing Audio Backchannel​

AudiocallBack channel ITVDesk AudiocallBack channel ITVDesk

ITVDesk VMS
https://itvdesk.eu/en/products/itvdesk-vms-video-management-system/download-itvstreamer

Test Procedure​

  1. Select microphone as audio capture device
  2. Open a normal RTSP stream from ITVDesk
  3. Click the microphone icon in the player
  4. When the icon changes state, the backchannel is active
  5. Speak into the microphone
  6. ITVDesk receives and plays the audio

Client Requirements​

  • RTSP client must support:
    • Require header
    • SDP direction attributes
    • RTP audio transmission
  • Microphone capture support
  • RTP over UDP or TCP (recommended for reliability)

Notes & Limitations​

  • One audio backchannel per RTSP session
  • Audio is not recorded by default unless explicitly enabled
tip

RTSP over TCP is recommended in NAT/firewall environments.

  • Backchannel is independent of video encoding

Summary​

The Audio Backchannel feature in ITVDesk provides:

  • Standards-compliant ONVIF implementation
  • Full RTSP Require-tag support
  • Multiple codec negotiation
  • Reliable two-way audio communication
  • Compatibility with professional VMS and RTSP clients

This makes ITVDesk suitable for intercom, PA systems, operator feedback, and advanced surveillance deployments.

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