Number of audio channels, & sound quality

Discussion in 'OffTopic' started by Dr Forinor, January 10, 2016.

  1. Dr Forinor

    Dr Forinor Portal Pro

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    This is a 2 part question, the first of which I think I know the answer but if possible would like someone more knowledgeable to explain it so I understand better, and the second I have no clue.

    1 - Why does a video with only 2 channels of sound (lets say AAC at 90kbps) sound much better (louder?) through my TV speakers at a given volume, than a 5.1 channel sound (lets say AC3 at 448kbps)?



    2 - Why does "AAC 48khz 5.1 320kbps" sound better than "AC3 48khz 5.1 640kbps"? I thought AC3 was higher quality than AAC?
     
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  3. mm1352000
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    Hello

    If you really do mean louder when you say "better", the response I gave to a similar question yesterday may be relevant:
    https://forum.team-mediaportal.com/...ge-freezing-please.133208/page-2#post-1170405

    Otherwise I think your question is unclear, and requires more context including a precise definition of what you mean by "better".

    Again, the answer entirely depends on the context (audio setup, media source etc.) and what you mean by "better".

    As far as I'm aware, neither of the encoding schemes you mentioned are so technically superior (eg. capable of reproducing a wider frequency and/or dynamic range, more efficient compression etc.) that one would always sound noticeably "better" - whatever that means - than the other. Therefore I'd be surprised if the difference you are noting related to technical aspects of the formats.

    Rather, I think "better" would be a subjective judgement (ie. personal preference) of the match between your particular audio setup and the quality/intent of the mixing and mastering (encoding). For example, as mentioned in the linked comment above, AC3/DD for films seems to often be encoded with wider dynamic range in order to convey realism. The audio doesn't have to be encoded that way, but it often is because when whispers are almost inaudible and explosions blow you out of your seat then you're drawn into the story. Anyhow, such a mix/encode is perfect if you have a surround sound system, a home theatre room with little or no background noise, and don't have to worry about noise complaints. On the other hand, if you've got a stereo system, city/family noises all around, and a neighbour through the wall who is sensitive to noise, a 2 channel AAC track encoded with less of the low end (bass) effects and less dynamic range might be "better".

    What I'm trying to say is that audio formats are designed to be suitable for use in a certain range of applications, and audio tracks are mixed and encoded for specific application(s). If [for example] you're listening to a format/track that's intended for playback in a surround sound context (as AC3 often is) with cheap and tinny stereo TV speakers, it's no surprise that you'd be disappointed. Likewise if you're listening to a format/track that's optimised for stereo speakers and relatively low quality bit-rate (as TV AAC often is) then you may be disappointed when you listen to it with a quality sound system. However if you select a track that matches the context of your audio setup, you're less likely to be disappointed.

    An example of technical superiority - for a particular application/context - might be that AAC supports far lower sample and bit rates than AC3. Therefore if given a choice between AAC and AC3 for encoding dialogue that only needs to be intelligible (eg. podcast, phone audio), AAC would be the more appropriate choice.

    Does this make sense?

    Again, "higher quality" is so ambiguous as to be meaningless.

    I think all that I can say in response to this question/comment is that I generally prefer AC3/DD over MPEG, AAC or HE-AAC. I prefer it because when I encounter AAC (and MPEG) it's almost always in the form of a relatively low bit-rate stereo (2 channel) TV channel mix encoded for intelligibility even on the "lowest common denominator" built in TV speakers. Compared to a parallel 5.1 AC3 track, this means that the high frequencies sound tinny and over-emphasized while bass and/or low frequency effect content is almost or completely missing. My PC sound chip is just a motherboard-based Realtek jobbie but I have monitor-quality stereo speakers, so I can absolutely hear the difference. To my ear the AC3 is "better", hands down.
     
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  4. Dr Forinor

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    I noticed that 2 channel audio always sounded louder, particularly in dialogue situations. And yes in action sequences the sound was pretty empty.

    When I was comparing the AAC to the AC3, it was at the same point in the same film, but it again it was a dialogue scene where the AAC came across as louder than the AC3 at the same volume. I didn't try the "test" in an action scene.

    I didn't realise that you would have would have enable the dynamic compression setting. For some reason I just thought the sound would be processed automatically into the 2 channels, quite naively obviously.

    Your explanation has certainly enlightened me further, thank you :)
     
  5. mm1352000
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    To be clear, dynamic range compression makes the quietest sounds louder (and perhaps also makes the loudest sounds quieter). It does not "process" into 2 channels. Down-mixing is the process that "converts" the 5.1 channels into 2, and no that doesn't necessarily happen automatically. For example, when using the LAV codecs I think it has to be enabled and configured manually.

    No problem, happy to help. :)
     
  6. CyberSimian
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    I also encountered this when watching DVDs, and also when watching a very small number of TV programmes. It is the problem explained by mm -- the DVD or TV programme has a five-channel soundtrack, but I was watching with two-channel stereo and I had not enabled "mix down" in the LAV settings. I enabled mix down using the default settings (0.71, 0.71, 0.0), but I found that that was still not enough, so I increased the central bias and reduced the "surround" level (0.80, 0.60, 0.0).

    Although DVB-T2 has the capability to carry multi-channel sound, I think that at present very few programmes are transmitted this way. The only two that I have noticed are "Remember Me" (a 3-part contemporary ghost story starring Michael Palin), and "Wizards and Aliens" (a SF serial for juveniles, but enjoyed by older juveniles too ;) ). I suspect that the BBC use a multi-channel soundtrack only for "prestige" productions, where significant foreign sales are anticipated. However, big-name films may be broadcast with multi-channel soundtracks (but now that I have enabled mix-down, I am less likely to notice this :D ).

    -- from CyberSimian in the UK
     
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  7. MikeBouy

    MikeBouy Banned Users

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    Detailed Explanation. Very helpful Indeed
     
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