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smooth streaming protocol and AAC extensions RRS feed

  • Question

  • Hi, 

    i would like to ask what is the correct sampling rate that should be reported in the manfiest's audio fragments when AAC+ encoding (HE-AAC v1 v2 etc.) is used. In general these files can be reproduced at different sampling rates depending on what extensions a player supports. Specifically, the base AAC-LC (higher) frequency or the AAC+ (lower) frequency should be used?

    -thank you

    ps. in all available versions for download i found information about AAC only

    Thursday, October 4, 2018 1:17 PM

All replies

  • Hi gepgep,

    Thank you for your question. One of our engineers will respond soon to assist.

    Thanks,


    Jeff McCashland | Microsoft Protocols Open Specifications Team

    Thursday, October 4, 2018 6:05 PM
    Moderator
  • Hi gepgep, I will be assisting you on this issue. I am currently researching the problem and will provide you with an update soon. Thank you for your patience.

    Regards,
    Sreekanth Nadendla
    Microsoft Windows Open specifications

    Friday, October 5, 2018 2:10 PM
    Moderator
  • Hi and thank you.. I have made an error in the question as the base LC sampling rate is usually the one with the lower sampling rate. However the question is valid; if you probe a file that contains an AAC+ stream you 'll get at least two sampling rates (see for example https://video.stackexchange.com/questions/17677/why-does-aac-have-two-sampling-rates).

    Being a i little more concrete with a smooth manifest audio entry example:

    <QualityLevel Index="0" Bitrate="64000" CodecPrivateData="130856E59D48B1" SamplingRate="24000" Channels="1" BitsPerSample="16" PacketSize="2" AudioTag="255" FourCC="AACP"/>

    so the player should use samling_rate = 24000 * 2 if it supports the AACP extensions or 

    <QualityLevel Index="0" Bitrate="64000" CodecPrivateData="130856E59D48B1" SamplingRate="48000" Channels="1" BitsPerSample="16" PacketSize="2" AudioTag="255" FourCC="AACP"/>

    which is the actual sampling rate of the stream (and perhaps divide by two as a fallback solution for older players)

    Monday, October 8, 2018 9:26 AM
  • Hello gepgep, are you saying the following ?

    when the actual sampling rate is 48000, which of the following definitions (See A, B below) is used in the manifest

    A) <QualityLevel Index="0" Bitrate="64000" CodecPrivateData="130856E59D48B1" SamplingRate="48000" Channels="1" BitsPerSample="16" PacketSize="2" AudioTag="255" FourCC="AACP"/>

    OR

    B) <QualityLevel Index="0" Bitrate="64000" CodecPrivateData="130856E59D48B1" SamplingRate="24000" Channels="1" BitsPerSample="16" PacketSize="2" AudioTag="255" FourCC="AACP"/>


    Regards,
    Sreekanth Nadendla
    Microsoft Windows Open specifications

    Monday, October 8, 2018 8:31 PM
    Moderator
  • yes 

    The problem is that both sampling rates are correct.. AAC+ audio encoding profile offers 48Kz for AAC+ playback AND fallbacks at 24Khz for AAC(LC) playback for older players that cant play aac+ audio. 

    For example, If you probe an mp4 file with AAC+ audio you get something like this:

    --------> Format profile : HE-AACv2 / HE-AAC / LC
    --------> Sampling rate : 44.1 KHz / 44.1 KHz / 22.05 KHz

    We need to know which of the two should be reported in the manifest.

    Tuesday, October 9, 2018 7:44 AM
  • Hello gepgep, The recommended FourCC is AACL for AAC-LC audio. Also the SamplingRate recommendation is either 44100 or 48000.

    Please note that such details are specific to codecs and they are not part of open specification documents such as  MS-SSTR.  

    Regards,
    Sreekanth Nadendla
    Microsoft Windows Open specifications


    Tuesday, October 9, 2018 8:30 PM
    Moderator