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Thread: Dolby 5.1

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    Dolby 5.1

    Miracle Premium Twin HD - various VIX Apollo builds 030 to 39

    When viewing a Freeview (UK Terrestrial) HD program with Dolby 5.1 audio what audio standard is output from the Micraclebox Premium Twin HD on the HDMI and on the S/PDIF (TOSlink) optical sockets.

    I have an old (pre HDMI) Onkyo AV amplifier with only S/PDIF optical/coax digital inputs and can only get the Miraclebox to output audio if I set the AAC downmix to 'yes'. This doesn't give 5.1 over the S/PDIF (TOSlink) link which suggests that what is coming down the link is in a 2 channel PCM format.

    If I could connect an AV amplifier via HDMI I would I get 5.1. via 5 channel PCM?

    Or is the output format something else?

    Some background for my conclusion (which may be incorrect) can be found in posts #5 and #16 from:
    Code:
    http://forums.digitalspy.co.uk/showthread.php?t=1761159

    I also base my conclusions on the same AV amp being connected to 3 other sources of Dolby 5.1 which the AV amplifier correctly identifies as 5.1 because it is coming down the S/PDIF (TOSlink) links in a different format.
    Last edited by judge; 29-09-14 at 14:39. Reason: Please code external links.

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    According to the linked thread Freeview HD is encoded in AAC, not Dolby Digital 5.1, so yes you would have to use AAC downmix. For sure, S/PDIF hardware limitations mean that it only supports 2 channels of uncompressed audio, so no way of getting a six channel PCM stream there.

    Do you have any way of trying the equivalent channel on Freesat? On that platform there are occasional actual DD 5.1 programmes, which should be ok on S/PDIF as it supports compressed audio. You would have to ensure that DD downmix is off in that case.

    HDMI will support six channels of uncompressed PCM audio, but I don't know if the MB Twin is capable of outputting multichannel PCM properly. There are settings on the menu, but I've tried six channel FLAC high-res audio and the MB (and the Quad Plus) struggle to handle any audio data with a sampling rate over 48kHz. What seems to happen in that case is that it tries to downsample everything to 48kHz and output as two channel over HDMI.

    With a compressed AAC multichannel stream it MAY output this correctly as six channel PCM audio over HDMI, but I have no real way of testing this as I'm outside the UK Freeview transmission area.
    GB Quad Plus, Mut@nt HD51, AX HD61, 80cm dish and Supreme Dark motor. Sony STR-DN 1060, Sony UHP-H1 Bluray, Odroid N2+ (CoreElec), Monitor Audio Bronze 5.1 speakers

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    Quote Originally Posted by fat-tony View Post
    Do you have any way of trying the equivalent channel on Freesat? On that platform there are occasional actual DD 5.1 programmes, which should be ok on S/PDIF as it supports compressed audio. You would have to ensure that DD downmix is off in that case.
    I've only got (hybrid) terrestrial tuners in my Miraclebox but my Humax HD satellite box gives Dolby Digital 5.1 over S/PDIF.

    HDMI will support six channels of uncompressed PCM audio, but I don't know if the MB Twin is capable of outputting multichannel PCM properly. There are settings on the menu,
    The only menu settings on the VIX image seem to be to switch downmix on/off. I've found no other menu settings for audio with respect to HD terrestrial TV apart for finding the option of switching between the Audio Description (AD) channel and the real audio. I assume that the Miraclebox/VIX doesn't support mixing these two audio channels.

    With a compressed AAC multichannel stream it MAY output this correctly as six channel PCM audio over HDMI, but I have no real way of testing this as I'm outside the UK Freeview transmission area.
    Another Miraclebox user, on another forum, suggests that his AV box may be correctly decoding multi-chanell PCM over HDMI.

    As my AV box doesn't support HDMI for audio inputs I'm currently stuck with a 2 channel input from the Miraclebox. Sometime in the near future I'll probably upgrade my AV amp, hence the question about finding out what exactly the Miracle box does output, especially for UK Freeview HD terrestrial TV. It obviously doesn't transcode into Dolby 5.1 but so far I've found zero documentation about what it does.

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    I have a Humax Freesat box (now retired for the Quad Plus) which did DD 5.1 over S/PDIF or HDMI from sat. The Quad Plus also does this as does the MB. I haven't come across AAC audio on broadcast yet. The Irish Freeview system is only using MPEG-2 audio at the moment although the specs for any boxes used on the system require AAC support. I presume they will turn that on at some stage when they start experimenting with surround audio, but it's not available yet.

    I wasn't actually aware that Freeview HD was using AAC. I presumed it was DD (or AC3 to be more correct) like Freesat. In which case you wouldn't have an issue with S/PDIF!

    The MB may well decode DD or AAC to six channel PCM over HDMI, but I'd have to swap around the boxes to check but can't do so at the moment - sorry!
    GB Quad Plus, Mut@nt HD51, AX HD61, 80cm dish and Supreme Dark motor. Sony STR-DN 1060, Sony UHP-H1 Bluray, Odroid N2+ (CoreElec), Monitor Audio Bronze 5.1 speakers

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    Another observation

    There is a difference in volume level by a (hearing) factor of 2 when tuned into a HD channel between HD programs with surround sound and HD programs without. This is not confined to the SPDIF link as it can be also heard on the right/left analogue RCA outputs from the back of the box. This has the effect that at the end of a surround sound broadcast (and even through the credits) the continuity announcer is twice as loud as the preceding content.

    I wonder if the downmix for 5.1 AAC content is actually correct on a Miraclebox

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    I've noticed similar issues on surround content (AC-3) on satellite but it's not confined to the enigma boxes. I've also seen this on Humax boxes. Part of the issue is down to the fact that there is no volume control in operation on a DD 5.1 broadcast. You are stuck with whatever level the broadcaster sets. Try it - the volume control on the box has no function other than mute during DD (or presumably AAC) surround playback unless it's downmixing. In normal 2.0 MPEG audio the audio signal level is varied by the volume control setting on the box. You have to try and balance the normal 2.0 output by reducing the volume control to match the 5.1 output. You'll see this issue raised on various forums across all manner of boxes in the UK.
    GB Quad Plus, Mut@nt HD51, AX HD61, 80cm dish and Supreme Dark motor. Sony STR-DN 1060, Sony UHP-H1 Bluray, Odroid N2+ (CoreElec), Monitor Audio Bronze 5.1 speakers

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    Quote Originally Posted by adm View Post
    Another observation
    .....
    This has the effect that at the end of a surround sound broadcast (and even through the credits) the continuity announcer is twice as loud as the preceding content.
    Thanks. I've wondered why I sometimes need to use a volume setting of 17, and other times ~30 (that's the TV settings - I have HDMI CEC forwarding the volume control to the TV).
    Now I know it's just two settings I'll know just to switch between them and not expect some programs to be in between.

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    Do as Tony suggests: set the volume of the box a bit lower so the volume on MGP & AC3 sound is aprox the same.

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    Quote Originally Posted by fat-tony View Post
    You are stuck with whatever level the broadcaster sets. Try it - the volume control on the box has no function other than mute during DD (or presumably AAC) surround playback unless it's downmixing.
    But it is downmixing as most (all?) AV boxes cannot decode AAC.

    I can confirm that the miracle box volume control does work with a downmixed AAC signal both for live broadcast and playback. The dramatic change in volume is between a ACC downmixed non-sourround sound broadcast and a ACC downmixed surround sound broadcast. There is no way of balancing the output as suggested.

    Perhaps an upgrade for VIX, and/or other images, is to have an volume offset option for surround sound content?

    Sods law will however dictate that none of the UK broadcasters will adopt a common standard and those UK broadcaster noted for their technical incompetence in will break the 'rules' anyway. On commercial channels the broadcasters will use the limitations of some of the boxes to make advertisements a LOT louder than the program content (over and above the compression tricks they already used to do the same)!

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    Quote Originally Posted by adm View Post
    AV boxes cannot decode AAC.
    Except those that can

    There are indeed amps that are capable of decoding AAC. But because most AC3-capable amps can't, there is a dedicated downmix for AAC available on many boxes.

    And as has been suggested before: you can turn the volume of the box a bit lower, so all PCM sound will be less loud.

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    Quote Originally Posted by adm View Post
    But it is downmixing as most (all?) AV boxes cannot decode AAC.

    I can confirm that the miracle box volume control does work with a downmixed AAC signal both for live broadcast and playback. The dramatic change in volume is between a ACC downmixed non-sourround sound broadcast and a ACC downmixed surround sound broadcast. There is no way of balancing the output as suggested.

    Perhaps an upgrade for VIX, and/or other images, is to have an volume offset option for surround sound content?

    Sods law will however dictate that none of the UK broadcasters will adopt a common standard and those UK broadcaster noted for their technical incompetence in will break the 'rules' anyway. On commercial channels the broadcasters will use the limitations of some of the boxes to make advertisements a LOT louder than the program content (over and above the compression tricks they already used to do the same)!
    Ok - I see your issue. You're talking about the scenario where the AAC decoder in the STB is downmixing to PCM for either S/PDIF or HDMI (or even analogue audio through the phono leads) rather than passing the the raw AAC to the AV amp. My AV amp doesn't handle AAC even though it was pretty much state of the art about 5 years ago. Handles all kinds of Dolby/DTS hi-res surround you might get on a Blu-ray, but no AAC.

    In my situation I have the STB set to pass DD through without downmix and control the level with the AV amp. When the programme type changes to MP2 (2.0 audio) the volume control on the STB brings the volume down to a reasonable balance. If I had DD downmix set on, then I would have the situation you're describing because you have to have AAC downmix on. I just don't have any exposure to AAC audio.

    I agree with you that there's quite definitely a difference in audio levels between 5.1 and 2.0 on the BBC and ITV, quite apart from the ridiculous levels of compression used in the advertising breaks and other audio and video weirdness experienced on the budget channels where they transmit the wrong aspect ratios or play the main and narrative audio tracks at the same time.

    Maybe there's something that can be done within the AAC or DD decoder settings in the software which could provide a menu control to pull down the relative audio level on 2.0 before the downmix? I know that in xbmc there are various audio level "tweaks" that can be done outside of the normal volume control. That said, xbmc is a full-featured media centre, rather than a sat/terrestrial/cable PVR box, so I wouldn't hold out too much hope of getting that level of sophisticated control in enigma images.
    GB Quad Plus, Mut@nt HD51, AX HD61, 80cm dish and Supreme Dark motor. Sony STR-DN 1060, Sony UHP-H1 Bluray, Odroid N2+ (CoreElec), Monitor Audio Bronze 5.1 speakers

  12. #12
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    Quote Originally Posted by Rob van der Does View Post
    And as has been suggested before: you can turn the volume of the box a bit lower, so all PCM sound will be less loud.
    This may work with AV amp that decodes AAC but not with any connection reliant on the downmix and output as PCM. The MB volume control controls the downmix volume for both surround sound and non-surround sound broadcasts (HDMI, SPDIF and Analogue RCA) so there will always be a perceived 2:1 difference in loudness when the broadcaster changes, which usually occurs during the continuity announcements at the end of programs or for adverts.

    Set the MB volume to, say, half scale for a surround sound encoded broadcast and set the AV amp for an optimum sound level and it will change to around twice that perceived level when the broadcast program ends.

    Even if you take the AV amp out of the equation by feeding the HDMI directly into a non-surround sound capable TV and using the TV speakers or using the RCA analogue outputs to an 'ordinary' stereo amplifier there is a perceived 2:1 sound level difference.

    Sound balancing for UK HD terrestrial TV is probably impossible with the MB unless connected to a AAC capable AV amp that bypasses the MB downmix.
    Last edited by adm; 30-09-14 at 13:56.

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    Did you try the plugin 'automaticvolumeadjustment'?

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  14. #14
    adm's Avatar
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    Quote Originally Posted by Rob van der Does View Post
    Did you try the plugin 'automaticvolumeadjustment'?
    Not yet - I will give it a try. At the time of writing the server is unavailable (feeds are down for maintenance) - I will try later.

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    No: Ocram is down. Just rename the Ocram file in /etc/opkg for the moment and try again.

    Help asked via PM will be ignored.
    The forum is there for help and all will benefit from your questions.
    NO CARD SHARING TALK WILL BE TOLERATED, LAN OR WAN, IN OPEN FORUM OR PM !

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    I apologise for all my grammar, spelling and idiom errors.

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