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PCM2902 and ac3 stream

Heinrich Geiger
Prodigy 50 points

Other Parts Discussed in Thread: PCM2902

Hello together,

I've a question regarding PCM2902, I didn't found something about this in the documentation.

So, I would like to play AC3 stream (DVD, Dolby Digital or DTS) in "PassThrough" mode.  So, that means the should not do any modifications on stream, jest route them directly to SPDIF-output.

It is possible with this chip?

Thanks & kind regards,

Heinrich

  • 0

    I have the same question.  Can the PCM2902 pass through DTS / AC3 unimpeded over SPDIF?

    http://www.ti.com/lit/ds/symlink/pcm2902.pdf

    Patrick G - Any chance you can put up a definitive answer.  There are a few USB sound card manufacturers (one starting with B) that leverage the PCM2902E chip and refuse to definitively state whether an AC3/DTS signal can pass through.

  • 0 in reply to Matthew Shannon
    TI__Guru**** 189401 points

    Hi, Guys,

    Welcome to e2e, and thanks for your interest in our products.

    I see no reason why it wouldn't be able to pass DTS/AC3, but I have never personally tried it.

    -d2

  • 0 in reply to Don Dapkus

    Hi Don

    I'm leveraging a UCA202 Behringer card. 

    The spec for the pcm2902 chip states that ...

    The following data formats are accepted by the S/PDIF input and output. All other data formats are unable to use
    S/PDIF.
    · 48-kHz 16-bit stereo
    · 44.1-kHz 16-bit stereo
    · 32-kHz 16-bit stereo
    Mismatch between input data format and host command may cause unexpected results except in the following
    conditions.
    · Record monaural format from stereo data input at the same data rate
    · Record 8-bit format from 16-bit data input at the same data rate
    A combination between the above conditions is not accepted.
    For the playback, all possible data rate source is converted to 16-bit stereo format at the same source data rate.

    Using the test media at

    http://www.diatonis.com/surround_sound_music.html

    I attempt to play the AC3 5.1 and DTS 5.1 media which are at 44.1 and 48khz using passthrough.

    My amplifier shows PCM 44.1 or PCM 48, but makes a horrible noise.

    If I change the audio player to convert the AC3 / DTS media to stereo and reattempt to play the files, they play fine (and once again show PCM 44.1 or PCM 48 etc), but are missing the surround channels.

    Would this indicate that there is some type of processing that is occurring either in the pcm2902 chip (or somewhere in the sound card) on the compressed input single, that does sufficient modifications to render the output spdif stream in such a state that it can not be decompressed/decoded by the amplifier?

    I can play the same media back using an Apple TV over optical to the same amplifier perfectly.

    This post seems to indicate that it is possible to get the UCA202 to output AC3 / DTS using the SPDIF-as-PCM trick using AC3Filter with stereo channels reversed ...

    http://www.ac3filter.net/forum/index.php?topic=10.msg351#msg351

    SPDIF-as-PCM trick

    "Sound card tricking may solve problems number 2 and 3. Because of PCM nature of SPDIF transmission we can trick the sound card and say that we want to transmit PCM data, but send compressed stream instead. This is controlled by “Output SPDIF as PCM” option.

  • 0 in reply to Matthew Shannon
    TI__Guru**** 189401 points

    Hi, Matthew,

    Sorry, but I am unfamiliar with the UCA202 Behringer card, so I can't really comment on what it may or may not be doing.

    Suggest you contact them directly with your questions.

    -d2