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  1. Member
    Join Date
    Jul 2009
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    Australia
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    I'm ripping my Thunderbirds DVDs to MKV using HandBrake and found the following:

    Original DVD VOB file:
    Audio
    ID : 189 (0xBD)-128 (0x80)
    Format : AC-3
    Format/Info : Audio Coding 3
    Commercial name : Dolby Digital
    Muxing mode : DVD-Video
    Duration : 26 min 29 s
    Bit rate mode : Constant
    Bit rate : 448 kb/s
    Channel(s) : 6 channels
    Channel layout : L R C LFE Ls Rs
    Sampling rate : 48.0 kHz
    Frame rate : 31.250 FPS (1536 SPF)
    Compression mode : Lossy
    Stream size : 84.9 MiB (8%)
    Service kind : Complete Main
    It looks like AC3 Dolby Digital 6 channels.

    In HandBrake, when I open the file for ripping, the default audio option is:
    Track: Unknown (AC3) (5.1 ch) 448Kbps
    Codec: AAC (avrcodec)
    Bitrate 160
    Mixdown: Stereo.

    So I assume I would have to change this to:
    Track: Unknown (AC3) (5.1 ch) 448Kbps
    Codec: AC3 Passthru

    My amp has Passthru capabilities such as DTS, DTS-MA, etc.

    I am thinking of adding another track in case I'm playing this file on a device without Passthru capabilities, the track would be:
    Track: Unknown (AC3) (5.1 ch) 448Kbps
    Codec: AC3
    Bitrate 160
    Mixdown: Dolby Pro Logic II

    I've tested the above on my AV setup and to me it seems to sound clearer and more "broad" when playing the AC3 Passthru track.

    Am I doing this right?
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  2. Member
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    Originally Posted by JDMils View Post
    I've tested the above on my AV setup and to me it seems to sound clearer and more "broad" when playing the AC3 Passthru track.
    Which is to be expected since 160 kBit/s for stereo AC3 is a low bitrate. Generally, 192 kBits/s is about the lowest I would do with stereo AC3. Below 192 quality drops dramatically.

    Which brings the next question: Why AC3 for the stereo downmix? Why not AAC or even MP3? Both would be a better choice than AC3 if it isn't for some playback compatibility.
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  3. Member
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    No reason for choosing AC3, if you reckon AAC or MP3 is OK then i'll try that.

    PS: I reckon the bitrate is 160 as this was probably the best they could do back in 1966!
    Last edited by JDMils; 13th Jun 2021 at 22:34.
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  4. Member Ennio's Avatar
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    A higher bitrate and number of channels can contribute to "higher quality" perception. AC-3 5.1 contains 6 independent, full-range channels for left, right, center, surround left, - right and subwoofer.
    AFAIK a "2 channel Pro-logic downmix" will contain 4 channels. Three for left, right and center, and 1 (monoural) limited range channel for surround.

    When such a downmix is needed, I'd stick to the original 448 bitrate to save as much "quality" as possible. You are going from lossy source to again lossy encoding. By all means, do try & find out for yourself.
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  5. If your player can deal with it use AC3 Passthru That retains the original AC3 audio.

    Note that AC3 wasn't officially added to the MP4 spec until a few years ago. Older Apple devices can't play AC3.
    Last edited by jagabo; 14th Jun 2021 at 20:51.
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