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Activity Forums Compression Techniques Compressor 5.1 AC3 Encoding Strangeness

  • Compressor 5.1 AC3 Encoding Strangeness

  • Matthew Keane

    November 21, 2005 at 7:19 pm


    OK, this is a bit long, but I’m wondering whether somebody can cast some light on this…

    I have 6 WAV files supplied by the sound studio to make the 5.1 soundtrack for the DVD I’m working on. I create a batch in Compressor and assign the tracks to the correct channels (I checked 3 times!) and, 45 minutes later on a 2×2.5 G5, I have my ac3 file.

    I build the DVD in DVDSP with the ac3 audio file and create a disk image to test it with the Apple DVD player. The sound is hooked up via the optical out to a Sony amp which shows the signal type and channels. It shows up OK as 3/2.1, but after listening for a while, I realise there is nothing coming out of the centre channel.

    So, I check the source files. The centre channel WAV is silent for the first couple of minutes before the narration kicks in, but in the ac3 file, it never kicks in. Is Compressor getting bored waiting for something to happen, and then forgetting to encode the channel?

    So, to check things out further, I use MPEG StreamClip to listen to the different channels in the ac3 file. The centre channel is indeed empty, and so are the rear L/R channels!

    Right, so maybe I screwed up the Compressor batch. I fire up Soundtrack Pro, drop in the tracks, check them, and export to ac3 via Compressor, making sure the channel attribution is right. Exactly the same results.

    OK, last ditch attempt. I take the sound files home and use my old copy of A.Pack to compress them. Same settings as at work. 45 minutes later – on a 2×1 G4 – I have an ac3 file. I check it out in MPEG StreamClip and – all the tracks are where they should be – hurrah!

    I haven’t had time to rebuild the DVD with the new ac3 file and check it out with the Sony amp, but why on earth is this job getting screwed up with Compressor, when A.Pack does it fine – and in the same amount of time on a much older machine?

    Anyway, if anybody has any ideas about where I’m going wrong with compressor, please let me know.


    PS. Compressor settings:
    Target system: DVD Video
    Audio Coding Mode; 3/2
    Low Frequency Effects: Enabled
    Data rate: 448
    Dialog Normalization: -31
    Bit Stream Mode: complete main
    Centre Downmix: -3
    Surround Downmix: -3
    Compression: None
    RF Overmodulation: Off
    Digital De-emphasis: Off
    Low-pass filter: On
    DC filter: On
    Low-pass filter: On
    90 degree phase shift: On
    3db attenuation: On
    – – –

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