How to: Prioritize Media Codecs Used by Speech Server

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Speech Server uses a prioritized list of codecs to negotiate the digital media format with SIP peers. To establish call audio connectivity, the SIP peers must use one of the following codecs supported by Speech Server:

  • RTAudio16KHz - Microsoft Real-Time audio codec that runs at 16 KHz and is used for wide bandwidth applications of Microsoft Unified Communications platforms.
  • RTAudio8KHz - Microsoft Real-Time audio codec that runs at 8 KHz and is used for narrow bandwidth applications of Microsoft Unified Communications platforms.
  • G723.1 (6.3) - Typically used for Voice over IP (VoIP) telephony. There are two bit rates for this codec. Speech Server only supports the 6.3 kbps rate.
  • G711u (mu-law) - Typically used for telephony in North America and Japan.
  • G711a (a-law) - Typically used for telephony in Europe and other parts of the world.

The listed order of the codecs determines the priority in which the codecs are used to negotiate the media format with SIP peers. If the SIP peer does not use a listed codec, the next one in the list is used until it is established that both endpoints (Speech Server and the SIP peer) are using the same codec. You can change the order based on the codec preference of the SIP peers communicating with Speech Server.

Note

If either of the RTAudio codecs (16 KHz or 8 KHz) is determined to be the negotiated media format, Speech Server only receives audio in that codec. To send audio, Speech Server uses the next mutually supported non-RTAudio codec in the list.

Prioritizing Media Codecs

To prioritize media codecs used by Speech Server

  1. Open the Speech Server Administrator console.

    For more information, see How to: Start the Speech Server Administrator Console.

  2. In the console tree, expand the applicable group, and then click Servers.

  3. In the details pane, double-click the applicable server.

  4. Click the Advanced tab.

  5. Select a codec, use Move Up and Move Down to change its priority position in the list, and then click Apply.

  6. To disable the use of a codec that Speech Server uses to negotiate with SIP peers, clear the check box next to the codec.

  7. Click OK.

    Note

    You can also prioritize the media codecs using a Windows Management Instrumentation (WMI) script. For more information, see the WireCodecList property in MSS Class.