Audio Codecs

Audio Codec Support

This section summarize audio codec support with and without (pass-through) transcoding selectable on the

.

Audio codec relay is supported in SIP-SIP, SIP-H.323 and H.323-H.323 interworking calls.

Audio Codecs Supported (Transcoding and Pass-Through)

Audio Codecs Supported (Pass-Through Only)

The

selects codecs on a call-by-call basis, and negotiates codec use with destination gateway during initial call setup. The  also renegotiates the media during a call. The  defines configuration parameters such as audio codec, packet size, and TOS to apply to individual call legs.

For direct media connections, bandwidth and policing requirements do not apply. Audio streams must have different IP port numbers, but may have the same or different remote IP addresses. Audio streams for each call leg can be allocated on the same or different IP interface.

Opus Codec Support

The 

, ,  and platforms support the Opus audio codec in accordance with RFC 6716 and draft-ietf-payload-rtp-opus-01 (see Supported Standards page). There are no licensing requirements for this codec.

Opus is an open, royalty-free, highly versatile audio codec consisting of a combination of SILK (LPC) and CELT (Constrained-Energy Lapped Transform) codecs. At any given point of time the LP layer, the MDCT (CELT) layer or both may be active. Opus is used for the following applications:

• VoIP and video conferencing
• Music/video streaming and storage
• Remote music jamming
• Wireless speakers/headphones/microphone
• Audio books
• Virtualization/sound servers

Supported Opus Features

The 

platforms include the following Opus functionality:

• Input sampling rates of 8k Hz (NB) and 16k Hz (WB) are supported on the IDP interface (encoder input and decoder output)
• Opus-to-Opus pass-through calls
  • Encoding output bandwidths of 8, 12, 16, 24 and 48 kHz
  • 6 kbps to 510 kbps bit rates
• Opus transcoded calls
  • Max bandwidth of 16 kHz
  • Max bit rate of 20 kbps
• Mono mode for both pass-through transcoded calls
• Stereo mode for Opus-to-Opus pass-through.
• Inband FEC mode
• Variable bit rate (VBR)
• DTX mode
• Single channel mode is supported
• Fax/Modem tone (FMTD) detection is not supported in Opus leg
• DTMF (Inband DTMF detection is not guaranteed)

Currently on

, Opus codec does not support DTX on the transmitter side (i.e. the encoder does not go into DTX mode during silence or background noise) nor FEC in neither transmitter or receiver side (i.e. the encoder does not encode FEC frames and the decoder does not attempt to recover any lost packet from FEC frames in future packets).

On 

, Opus DTX is fully supported on both transmitter and receiver sides; however, packets are generated approximately every 200 milliseconds during DTX period.

 

UXPAD Operational Modes

As with other currently-supported compression codecs, the

supports Opus in UXPAD only. The following UXPAD operational modes are supported by the :

• WB to WB transcoding using two UXPADs connected back to back over IDP-WB format. The wideband voice features should be preserved while performing the transcoding in two stages using PADs, the intermediate IDP should always be wideband.
• WB to NB transcoding (except for G.711-RTP) using two UXPAD connected back to back over IDP-NB format.
• WB to G.711-RTP transcoding using single UXPAD.
• Opus to TNAPAD to play out any tones or announcements.
• Opus-to-Opus transcoding using two UXPADs connected back-to-back over IDP-WB format. This is used only for Opus-to-Opus transcoding that may occur due to existing Packet Service Profile (PSP) flags:
  • Transcode always
  • Conditional Transcoding > Conditions in addition to “No common Codec”:
    
    • Different ptime/Silence Suppression,
    • Different DTMF relay.