The VideoLAN Forums

Hi all -

Here's my once-and-for-all 'answer' to all of those out there, still suffering with files that contain Dolby Digital Plus.

Below, I have documented with hopefully enough detail - to explain why each of you are having a few problems, and what can possibly be done to overcome some of these issues.


Firstly, I have used nothing but the very best, ( VLC of course! ), since OS X v10.0 "Cheetah".

I too, have been having some serious problems recently with any of my files that contain E-AC3 Audio, ( otherwise known as Dolby Digital Plus. )

I'm currently cruising along with a Mid-2011 MacBook Pro, ( the last of the Pre-Retina's ), running OS X Mountain Lion v10.8.5 - with VLC v3.0.71

For the delivery of all VLC media coming from my MacBook Pro across to my Amp, I'm using the Thunderbolt Port - located on the left-hand side of the laptop.
( For those of you that don't know, this port also contains everything needed to be realised as a complete mini-DisplayPort and thus is also able to be seen as HDMI with Audio ), thus terminating directly to my Amp via a genuine Apple Thunderbolt-to-HDMI adapter.

My Amp is a Harmon/Kardon AVR151 - which is capable of natively decoding the following digital audio formats ;

- Good Ol' Dolby Digital 5.1 ( AC3 ),
- Dolby Digital Plus ( E-AC3 ),
- Dolby Digital TrueHD 5.1 and 7.1
- Good Ol' DTS 5.1
- DTS-HD 5.1 and 7.1, ( both Master Audio and High Resolution Audio )

Care has also been taken - to ensure that all of the necessary settings have been attended to within the constrains of OS X Mountain Lion.

For example, I have ensured that my Amp has been correctly connected to the Mac via HDMI - and thus identified and then selected as the native sound device within 'Output', ( found inside the settings for 'Sound', contained within System Preferences. )

Additionally, ( and while ensuring that no Audio is currently playing out of any Application ), I have also made certain to select 'HDMI' - found specifically within the left-hand pane of 'Audio MIDI Setup' Utility - and have then deliberately chosen '48000.0Hz' and 'Encoded Digital Audio', underneath the respective 'Output' Tab for this device.
( This is to ensure that both Dolby Digital 5.1 and DTS 5.1 digital audio streams, are successfully able to be passed through unadulterated - without facing any form of downconversion by Apple's CoreAudio API. )

Finally, when playing any file from within VLC - make sure to confirm that inside the 'Audio' drop-down list, ( found up upon the Menu Bar, in-between the 'Playback' and 'Video' drop-down lists ), that you have selcted the 'Audio Device' as 'HDMI (Encoded Audio)'.

With these parameters covered - you're all set...

...that is, until you start having to face any number of files containing Dolby Digital Plus.


( See next post - )

( Continued - )


While using VLC, playback of any file which contains either standard Dolby Digital 5.1 or standard DTS 5.1 Digital Audio - results in the audio stream delivered straight through to the Amp 'in passthrough mode'.
( Meaning that the raw digital audio bitstream is fed straight from VLC, via the HDMI Cable, across to the Amp - with the Amp performing the decoding of these digital audio streams natively - as my Amp's display shows that it has both detected and is then decoding either a Dolby Digital 5.1 Digital Audio stream, or a DTS 5.1 Digital Audio stream. )

However, the instant any attempt is made to play a file which contains either a Dolby Digital Plus audio stream ( E-AC3 ), a Dolby Digital TrueHD 5.1 or 7.1 audio stream, or a DTS-HD Master Audio or High Resolution Audio 5.1 or 7.1 audio stream - it is no longer fed straight to the Amp in passthrough mode.

Instead, these files appear to be having their embedded audio streams now fed to the Amp strictly in PCM - indicating that each of these audio streams are first being down-converted, ( or stripped if you will ), out from within their native digital audio format, before being passed out to the HDMI cable that goes across to the Amp.

For all of my files that specifically contain either a Dolby Digital TrueHD or DTS-HD Master Audio / High Resolution Audio digital audio stream, the channel mapping at least appears to be correct and intact.
( Meaning that my Amp's Front Panel Display, correctly shows the number of PCM Channels that it's receiving from the HDMI cable, and that each Channel seems to be 'mapped' to the correct location in the room - for each of these now 'stripped out' PCM Audio streams. )

The real ugliness however, is when I attempt to play any file which contains Dolby Digital Plus ( E-AC3 ), as these files appear to suffer from a very messy downconversion into PCM Audio streams, before being sent out of the MacBook Pro and onto the HDMI cable.

It appears as though the Centre Channel, ( typically used for most spoken dialogue ), is mapped to the Left Rear Surround and that some of the other channels also appear to be incorrectly mapped.

Additionally, the overall level of audio also appears to be down, with an element of 'muffled-ness' or 'muddiness' also dampening the quality of the soundtrack.

For the purposes of evaluation, I have been using a small number of Dolby and DTS 'test reference files' for each of these tests and verification activities, found over at thedigitaltheatre d o t com - ( feel free to try them out for yourself. )


Upon hunting around really hard for an answer to all of these woes, I managed to find some clarity within a couple of obscure forums.

It seems that Apple has deliberately limited the functionality of its CoreAudio API to only permit the passthrough of digital audio for Good Ol' plain Dolby Digital 5.1 and Good Ol' plain DTS 5.1, ( the digital surround formats originally found on DVD. )

Anything beyond these digital audio formats is automatically down-converted by OS X, into individually separate PCM Audio streams - before sending them onwards and outbound to your external Amp.
( This explains why you can still enjoy 5.1 and 7.1 Audio from any file that you may have, which uses any of these higher order encoding standards - such as DTS-HD Master Audio for example - but you must remember that Apple's CoreAudio API always limits the down-converting of any of these high-order digital audio streams - to that of up to a maximum of eight individual PCM Streams of 24-bit resolution, sampled no higher than at 96kHz per PCM Channel... )

Say what? Let me explain -


For example ;

Located on your Hard Drive is 'File.mkv' or 'File.mp4'.

This File contains a Video Stream of H.264 1080p/60Hz Video, and an associated Audio Stream of DTS-HD Master Audio 5.1 -

- VLC will push the full-rate Video out over HDMI as expected ; 1080p at a 60Hz refresh rate.
- However, OS X will simply refuse to pass the associated DTS-HD Master Audio 5.1 digital audio bitstream down the same HDMI cable. Instead, it will internally down-convert, ( or 'extract' if you will ), each individual channel of audio contained within this high-order digital audio bitstream, ( even the .1 Sub-Woofer Channel ), and stream each of these channels out to your Amp as six separate channels of PCM Audio.

Your Amp simply maps each of these individual PCM streams 'as-is' out to each respective Speaker, with the Amp performing literally zero decoding of this audio.

What sucks about this, is that you are potentially being denied the real clarity, the true dynamic headroom and the outright frequency response that the original audio may have had - when it was encapsulated within its original high-order digital audio bitstream.

For example, Audio contained within a DTS-HD Master Audio 5.1 digital bitstream, ( when sent externally to an Amp for decoding ), can deliver each channel with 24-bit resolution, sampled at up 192kHz per audio stream.

This compares poorly to what Apple's CoreAudio API delivers, as once CoreAudio has extracted each of these high-order channels of Audio from the original DTS-HD Master Audio digital bitstream, it only provides each channel to the Amp as a PCM channel with 24-bit resolution, sampled at a maximum of only 96kHz.

This sample rate limitation of Apple's CoreAudio API only really applies to digital audio streams that are originally Dolby Digital TrueHD 5.1 or DTS-HD MasterAudio/HighResolution Audio 5.1 - as these two high-order digital audio bitstreams are capable of delivering each channel to the Amp for decoding at 24-bit resolution, sampled at 192kHz.


( See next post - )

( Continued - )


Dolby Digital TrueHD 7.1 and DTS-HD Master Audio/High Resolution 7.1 digital audio bitstreams however are fine, as the individual audio streams contained within these high-order digital audio formats - are in fact limited to 24-bit resolution, sampled at a maximum of only 96kHz.
( Apple's CoreAudio API, simply grabs the audio streams out from within each of these digital audio bitstreams - and streams them out over HDMI as individual PCM Audio streams at the exact same resolution and sample rate as the original high-order digital audio bitstream originally had - that being, 24-bit resolution, sampled at 96kHz. )


All of this only really starts to suck hard - when you have files that contain a Dolby Digital Plus, ( E-AC3 ), digital audio bitstream.

Because Dolby Digital Plus does not permit any kind of downconversion, ( with regards to tampering with the original sample rate of each Audio stream contained within it ), it becomes nothing but a road accident when Apple's CoreAudio API attempts to grab what it simply perceives as yet another audio stream, suitable for down-converting into a PCM Audio stream.

Dolby's Digital Plus format was developed inline with Dolby's potent ATMOS format, and is specifically designed to permit significantly more channels of audio to be delivered - when compared to legacy Dolby Digital 5.1 - but additionally permit these additional channels of audio to be delivered, within bandwidth-constrained environments. ( Such as Digital Free-to-Air Television and Cable TV for example... )

This should explain the perceived sudden explosion of these files, as online streaming providers reach to this new digital audio format - in order to provide their subscribers with a greater level of surround sound channel realisation and complexity.

Even if your Amp can natively decode this format when it is delivered to it via HDMI - you're just plain outta luck with OS X Mountain Lion, all thanks to Apple's CoreAudio API steadfastly refusing to simply permit 'passthrough' of any of these more advanced digital audio formats.

The second any Dolby Digital Plus audio stream faces any form of downconversion - with regards to its original sample rate - It. Is. Toast.

All that remains to be asked is that if anyone in here - specifically within the constraints of VLC's Dev Crew - possibly cares enough to introduce an advanced mode within VLC, capable of pre-emptively decoding the individual audio channels found within a Dolby Digital Plus digital audio stream, hopefully, directly down into eight PCM audio streams - just before Apple's CoreAudio API gets its hands on it...


Technical Summary of each digital audio format ;

Dolby Digital 5.1 -
Maximum of Five Channels of full-range audio, with the addition of a low frequency only, ( sub-woofer/bass ), channel - hence '5.1'
Channels are somewhat 'lossy' in quality when compared to the original master studio source, but when encoded at the maximum rate permissible, the quality is deemed to be almost indistinguishable.
All delivered within a digital audio stream, encoded at a bitrate no higher than 640kbits/sec.
- Apple's CoreAudio API permits 'passthrough' of this digital audio format, allowing your Amp to natively decode this digital audio stream.
- Can be delivered to your Amp via legacy S/PDIF Coaxial or Optical connections, or via HDMI as well.

DTS 5.1 -
Maximum of Five Channels of full-range audio, with the addition of a low frequency only, ( sub-woofer/bass ), channel - once again, hence the '5.1'
Channels are also somewhat 'lossy' in quality when compared to the original master studio source, but when encoded at the maximum rate permissible, the quality is deemed to be better than Dolby Digital 5.1 at its maximum rate.
All delivered within a digital audio stream, encoded at a bitrate no higher than 1536kbits/sec.
- Apple's CoreAudio API also permits 'passthrough' of this digital audio format, allowing your Amp to natively decode this digital audio stream.
- As with Dolby Digital 5.1, can also be delivered to your Amp via legacy S/PDIF Coaxial or Optical connections, or via HDMI as well.

Dolby Digital TrueHD 5.1 -
Limited to a maximum of Six Channels of Audio, including the same additional low frequency, sub-woofer/bass channel.
However, substantial improvement in Audio Quality - ( deemed as a high quality 'lossless' compressed digital audio format, thus closely representing the original source. )
Each Audio stream can be originally encoded with 24-bit resolution, sampled at up to 192kHz.
All delivered with a massive increase in required bandwidth, ( many Mbits/sec. )
- Apple's CoreAudio DOES NOT permit 'passthrough' of this audio format - instead stripping each audio stream out of its original high-order digital audio stream, and passing it out to your Amp as six separate PCM Audio streams.
- Each of these six PCM Audio streams is reduced from 192kHz sampling, down to 96kHz sampling, ( although maintaining 24-bit resolution ), hence resulting in somewhat of a loss in quality.
- At the down-converted 96kHz sample rate, can ONLY be delivered to your Amp via HDMI cable, due to the high bitrate when compared to Dolby Digital 5.1.


( See next post - )

( Continued - )

Dolby Digital TrueHD 7.1 -
Limited to a maximum of Eight Channels of Audio, including the same additional low frequency, sub-woofer/bass channel.
However, substantial improvement in Audio Quality - ( deemed as a high quality 'lossless' compressed digital audio format, thus closely representing the original source. )
Each Audio stream can be originally encoded with 24-bit resolution, but unlike Dolby Digital TrueHD 5.1 - is limited to a maximum sample rate of only 96kHz per channel of audio.
All delivered with a massive increase in required bandwidth, ( many Mbits/sec. )
- Apple's CoreAudio DOES NOT permit 'passthrough' of this audio format - instead stripping each audio stream out of its original high-order digital audio stream, and passing it out to your Amp as eight separate PCM Audio streams.
- This represents the maximum number of channels that Apple's CoreAudio API is capable of down-converting and simultaneously streaming, as independent but concurrent PCM Audio streams.
- Each of these eight PCM Audio streams is delivered with the same 96kHz sampling rate, as found within the original high-order digital audio format, ( also maintaining 24-bit resolution ), hence resulting in an in-perceivable difference in quality when compared to natively decoding this digital audio format within your Amp.
- At the 96kHz sample rate, these eight PCM Audio streams can ONLY be delivered to your Amp via HDMI cable, due to the high bitrate when compared to Dolby Digital 5.1 or DTS 5.1.

DTS-HD Master Audio 5.1 -
Limited to a maximum of Six Channels of Audio, including the same additional low frequency, sub-woofer/bass channel.
However, substantial improvement in Audio Quality - ( deemed as a high quality 'lossless' compressed digital audio format, thus closely representing the original source. )
Each Audio stream can be encoded with 24-bit resolution, sampled at up to 192kHz.
Considered to be superior to Dolby Digital TrueHD.
All delivered with a massive increase in required bandwidth, ( many Mbits/sec. )
- Apple's CoreAudio DOES NOT permit 'passthrough' of this audio format - instead stripping each audio stream out of its original high-order digital audio stream, and passing it out to your Amp as six separate PCM Audio streams.
- Each of these six PCM Audio streams is reduced from 192kHz sampling, down to 96kHz sampling, ( although maintaining 24-bit resolution ), hence resulting in somewhat of a loss in quality.
- At the down-converted 96kHz sample rate, can ONLY be delivered to your Amp via HDMI cable, due to the high bitrate when compared to DTS 5.1.

DTS-HD Master Audio 7.1 -
Limited to a maximum of Eight Channels of Audio, including the same additional low frequency, sub-woofer/bass channel.
However, substantial improvement in Audio Quality - ( deemed as a high quality 'lossless' compressed digital audio format, thus closely representing the original source. )
Each Audio stream can be originally encoded with 24-bit resolution, but unlike DTS-HD Master Audio 5.1 - is limited to a maximum sample rate of only 96kHz per channel of audio.
Considered to be superior to Dolby Digital TrueHD.
All delivered with a massive increase in required bandwidth, ( many Mbits/sec. )
- Apple's CoreAudio DOES NOT permit 'passthrough' of this audio format - instead stripping each audio stream out of its original high-order digital audio stream, and passing it out to your Amp as eight separate PCM Audio streams.
- This represents the maximum number of channels that Apple's CoreAudio API is capable of down-converting and streaming, as independent PCM Audio streams.
- Each of these eight PCM Audio streams is delivered with the same 96kHz sampling rate, as found within the original high-order digital audio format, ( also maintaining 24-bit resolution ), hence resulting in an in-perceivable difference in quality when compared to natively decoding this digital audio format within your Amp.
- At the 96kHz sample rate, these eight PCM Audio streams can ONLY be delivered to your Amp via HDMI cable, due to the high bitrate when compared to Dolby Digital 5.1 or DTS 5.1.

Dolby Digital Plus -
Represented as the next-in-line replacement for the original Dolby Digital 5.1 audio format.
Designed to be released as the 'broadcast compatible version' of the similarly related, but substantially more capable multi-channel Dolby ATMOS digital audio format.
Limited to a maximum of fifteen full-bandwidth audio channels, with an additional low frequency, sub-woofer/bass channel.
As realised within the realisation of Dolby Digital TrueHD, a significant improvement achieved with this format's improved compression - still not lossless, but very high quality when compared to Dolby Digital 5.1.
Maximum encode rate of just over 6Mbits/sec.
- Apple's CoreAudio COMPLETELY OBLITERATES the playback of this digital audio format - all due to CoreAudio's unnecessary downconversion of each of the embedded audio channel's original sample rate.
- Annoyingly, running Windows on your Mac, allows your laptop to deliver unadulterated Dolby Digital Plus digital audio streams - directly to your Amp over HDMI - demonstrating that as usual, there ain't nothin' wrong with your Hardware...


So, come one - come all,

Let rip and share your thoughts...

Thank you for this explanation. I have spent hours and hours trying to figure out why my mac couldn't passthrough TrueHD 7.1 and DTS-HD 7.1. This post was a real life saver as I was most likely going to spend another few hours of research. Thank you, much appreciated.

Thanks for this informative and well considered post. It's been driving me insane for years; so I take it a Windows machine running the latest VLC will passthrough the EAC3 signal so that my amp can decode it?

Hello !
Many thanks for the explanation !! As the others, I have searched on the internet for no answers... I've even tried a lot of different cables to see if there was a difference, but no.
It was when I tried my amp with my very old and broke surface pro that I understood the problem was from MacOs.

Your post is now two years old, my question is : did you finally found an answer ?? Did VLC did ? I hope so...

Thanks again for your research !