Forum Post

EdgarRothermich (DingDingMusic.com) asked a question.

AC4-IMS, a 2-channel Binaural Audio signal without metadata without any Rendering during playback?
Is the AC4-IMS a Binaural Audio signal derived from the ADM BWF file using the BRM? So, technically a hard-coded 2-channel signal without any metadata. What happens if playing back a Atmos song on Tidal over speakers?

  • AdamP (Dolby Labs)

    Hi Edgar,

    AC4-IMS uses the Binaural Render Mode metadata in an Atmos master file during the encode to create a two channel bitstream. When the AC4-IMS bitstream is decoded in a mobile playback device it present binaural audio when listening on headphones. There is additional metadata in the bitstream that is used to provide immersive playback from mobile device speakers.

    When Tidal is being listened to on speakers via a device that has an HDMI output (connected to an AVR or Soundbar), DDPlusJOC is being used. DDPJOC uses Spatial Coding and is encoded as a 5.1 bitstream. It does not use the Binaural Render Mode metadata. A device with a DDPlusJOC decoder uses the bitstream metadata to reconstruct the Spatial Coded audio which is rendered to the listeners speaker layout.

    Hope this helps.

    Best,
    Adam
    Expand Post
    • Adam i simply dont understand anything..
      I have in hands now AC4 IMS stereo music file. The only one player in Windows can play it - Dolby Reference Player. So what is the difference between this file with "special" codec and WAV file with same binaural music??
      I believe AC4 IMS file is something multichannel which have inside everything with metadata but rendering as a STEREO for mobile devices. So i was pretty sure that i can extract initial 5.1 channels from it and reproduce on my sound system.

      EAC3-JOC - allow me to do this. But AC4-IMS doesnt have anything inside and its a finished stereo pre-rendered sound file like WAV?

      Also i wanna understand - how i can use TOP speakers in Windows? For example i have True HD file which i can listen in Dolby reference player - i can attach only 5.1 channels because they physically available in windows. But top speakers i cant assign for LTF RF LTR RTR in this player.
      Expand Post
      • AdamP (Dolby Labs)

        Decoded AC4-IMS is a two channel file that is indeed nearly identical to the binaural re-render. However, the coded contains additional metadata used by the encoder to help optimize playback for a given mobile device.

        You cannot decode AC4-IMS beyond two channels.

        EAC3-JOC is a 5.1 channel codec with metadata to recreate 16 dynamic elements which are renderer to speakers.

        TrueHD is not necessarily Atmos. If it is an Atmos file it will be a 7.1 core used with metadata to likewise recreate 16 dynamic elements which are renderer to speakers.

        If you ASIO device can handle 12 outputs you can set Dolby Reference Player to decode to 7.1.4 and activate the overhead speakers with Atmos content.

        Best,
        Adam
        Expand Post
      • My device - soundbar connected by HDMI to PC. Physically i have 5.1.4 config. However windows doesnt have any built in functional to call and control top speakers. We have what we have - simple 7.1 config. The only way to play top speakers - use passthru , and soundbar starts encoding these channels onboard. I cant say windows - play any of top speakers, they dont exist in operating system
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      • AdamP (Dolby Labs)

        You'd need to get the Dolby Access App from Microsoft to get Atmos over HDMI from your PC for playback. I still don't think you'd be able to use Dolby Reference Player for this purpose as it is already decoding the audio and is looking for an ASIO device.
      • I still dont understand purpose of Dolby Access app. My spatial sound in windows now OFF, your dolby access app said - i am not able to hear lovely dolby atmos, however if i launch any demo in your application - my soundbar show me Dolby atmos logo and all my 5.1.4 system sounds as meant to be. So what is the Dolby access app? its a surround DSP making sound a little bit interesting or what?
        Okay i can understand that this is for headphones and could add binaural effects. But for why you have also HOME THEATER function if it works flawless without it? Can you explain
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      • AdamP (Dolby Labs)

        Hi, I suggest having a look at this separate knowledge base: https://kb.dolby.com/access?catsearchtype=Dolby_Access. And using the contact us function on that page if you don't find the answers you are looking for re: Dolby Access. That would put you in contact with the experts. This forum is for content creation topics so this is going a bit off topic.
  • EdgarRothermich (DingDingMusic.com)

    So if I understand correctly, then the AC4-IMS works backwards. It carries a rendered 2-channel Binaural Audio signal, but the metadata that is carried in the signal makes it possible to create the Speaker Render signal if necessary.
    The second part of your answer I don't understand. You say that the DD+JOC signal is used when listening Tidal over devices. However, I though Tidal only carries the AC4-IMS signal, so where is the DD+JOC signal all of a sudden come from?
  • AdamP (Dolby Labs)

    Dolby Atmos content on Tidal is encoded to AC4-IMS for streaming to compatible mobile devices and encoded to DD+JOC for streaming to other devices.
  • EdgarRothermich (DingDingMusic.com)

    Wow, I never read anything about that. That means Tidal stores both codecs on their system and streams one or the other depending on if it detects whether the user is listening through headphones or connected to an HDMI connected streaming device.
    I read that Dolby Atmos technology is embedded in the Android OS that makes it easier for this kind to headphone detection,
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