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  1. #1
    Registered User
    Join Date
    Nov 2007

    APS buffer with header 0?

    My observations with APS buffers:
    With (CNG -0/1 and VAD 0) -> All APS buffers always has header value 1 (Hopefully noise is also generated).

    With (CNG-0/1 and VAD 1) -> All types of header 2,1,0 are seen, with header 2 always precedes 1.

    The APS 2.4.1 doc says that header value 0 is unvoiced non-sid frame and contains body filled with zeros. But I observe a combination of both data and zeros (few data samples and zeros succeeds till the end).

    Can someone elaborate on Buffers with header value 0?
    - Can the buffer with header 0 contain samples as well as 0’s? Should we consider playing these buffers also if CNG is enabled (ideally we need not give any buffers if CNG is enabled)?

    -Even if VAD identifies noise, do we still get APS buffers with header 0 every 20ms?

  2. #2
    Registered User
    Join Date
    Sep 2006

    Re: APS buffer with header 0?


    Which codec are you using?

    My experience is mainly with the G.729 codec. Every silence [header 00] frame is always full of zeroes. When VAD is enabled, I get all 3 types of frames. Silence frames are always preceded by a SID update [02] frame. The decoder outputs comfort noise based on the last SID update frame until it receives a normal [01] frame or a new SID update.

    You have to keep constantly feeding the decoder new frames, even if CNG/VAD is active, otherwise it will underflow. So, if you want to play back comfort noise, send one [02] frame to the decoder followed by frames that are full of zeros.

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