Re: Outputting Multichannel sound from stereo sources
Re: Outputting Multichannel sound from stereo sources
- Subject: Re: Outputting Multichannel sound from stereo sources
- From: "Simon Thorpe" <email@hidden>
- Date: Fri, 13 Aug 2010 11:44:56 +0200
- Importance: Normal
Thanks Brian
You say that iTunes is not designed for surround. But it can quite happily
play back multichannel quicktime movie files with (I believe) no pratical
limit on the number of discrete channels. I've got a test file with 8
different discrete channels that I made with Logic Audio, and it comes
over fine when played with iTunes or QuickTime Player. The ability to play
discrete multichannel recordings has been there for years - it's just that
it never seems to be used.
You then say that using unencoded discrete audio channels is tough to set
up - "requiring an audio interface with multiple channels, and a
surround system which accepts discrete input channels." But this is no
longer a problem. Starting with the new iMacs released last autumn, all
the new Macs will output 8 channels of uncompressed 192 Khz 24 bit sound
via the minidisplay port - you just need to get a minidisplay to HDMI
adapter to be able to plug the Mac directly into a Home Theatre amplfier.
Indeed, the new Minimac actually comes complete with an HDMI connector
that can output unencoded multichannel audio.
At the other end, I think that virutally all modern 5.1 and 7.1 amplifiers
have a Direct Audio feature that just plays the multichannel signals
without having them messed around by surround sound technologies built
into the amplifier.
What I find incomprehensible is that despite all this, anyone who connects
a Mac to a 5.1 or 7.1 amplifier and hopes to be able to listen to
multichannel recordings is currently going to be very disappointed. As you
say, nobody is apparently interested in making multichannel recordings
available except on pressed optical media. And if you try an play anything
that you buy from the iTunes store or via Spotify or Deezer or whatever -
the sound will just come out of 2 of your 8 speakers, unless you decide to
allow the Dolby Pro Logic hardware on your amplifier to invent the signals
for the other ones....
Honestly, it seems pretty ridiculous to me. I can think of no good reasons
why (i) people don't distribute multichannel music in uncompressed
discrete formats, and (ii) there are no mac based ways of turning stereo
recordings into multichannel outputs using an Audio Unit that does the
same sort of thing as Dolby's Pro Logic hardware.
If someone can clarify why we have these self imposed restrictions, I
would love to hear from them
Simon
In other words,
On Thu, August 12, 2010 21:41, Brian Willoughby wrote:
> There are two ways to handle surround sound, and CoreAudio supports
> them both. iTunes, however, could only support one method, since it
> is not designed for surround.
>
> The bulk of the commercial surround media is encoded, such that the
> bandwidth typically allocated to a stereo bitstream represents 5.1 or
> 7.1, et cetera. But this content sounds like noise unless you have a
> pure digital link from the media source to a decoder. CoreAudio
> supports this with Hog Mode - a method to tell the audio device that
> the content is un-mixable. Once you play encoded surround media, you
> cannot support multiple apps playing sound at the same time, because
> there is not enough automated technology to decode all of the
> channels, mix in multiple applications' sound (each in potentially a
> different encoding), and re-encode. But after CoreAudio is
> instructed to allow only one application on an interface, it can pass
> through the digital data unaltered (unmixed), and your surround
> decoder can produce multiple channels of output. Since most consumer
> surround systems only handle encoded surround data inputs, this is
> your best bet. Also, iTunes can pass through this content if you set
> the volume to 100% and turn off all crossfading and volume
> normalization, but I don't think you can tell iTunes to ask for hog
> mode when playing.
>
> The more straightforward method for surround is to literally have
> discrete channels that are not encoded. This requires source
> material with multiple channels, an application that supports
> multiple channels, an audio interface with multiple channels, and a
> surround system which accepts discrete input channels. You're going
> to run into barriers at each step of the way. Nobody sells discrete
> multichannel media, so you'll have to create your own. iTunes does
> not support more than two channels, so you have to work in another
> application. Audio interfaces abound which support multichannel, but
> not built-in. Finally, discrete multichannel connections are
> somewhat rare on consumer surround systems, and even those with
> discrete 5.1 inputs may have certain limitations on volume control or
> bass management when using those inputs. Thankfully, CoreAudio and
> Audio MIDI Setup allow the user to describe their collection of
> speakers and the placement of the speakers in the room. Although few
> applications take advantage of this, it theoretically allows any
> number of source channels to be remixed on the fly for any number of
> output channels.
>
> On another note, I have licensed the DTS encoder, which allows me to
> create surround music CD media or even just DTS surround files like
> those that would be put on a DVD soundtrack. With this software, I
> must record or create the multiple channels for surround, mix them
> down with Logic, and then apply the encoding with carefully
> controlled settings. The resulting files can be played in hog mode
> through CoreAudio over an audio interface that has a digital output
> such as SPDIF (coaxial or optical) or AES3. This works fine for me,
> but it is vastly easier to just burn a disk and play that disk with
> my surround components instead of the computer.
>
> The industry outside of Apple does not appear to be ready for
> consumers to listen to surround sound except via commercially-
> produced and purchased media. There isn't even really a distribution
> format for files - only pressed optical media. iTunes, also, is not
> designed to allow this beyond very savvy users. The good news is
> that CoreAudio is well-prepared for surround. Your best bet is to
> learn a great deal and work with different software than iTunes.
>
> Brian Willoughby
> Sound Consulting
>
>
> On Aug 12, 2010, at 04:21, Simon Thorpe wrote:
>> I've got another question that has been puzzling me. I have a 7.1 home
>> theatre system and use iTunes to play music. To my knowledge, when
>> playing
>> a stereo file, iTunes will only use two channels. Of course, I can
>> use the
>> amplifier to add propriety surround sound to use all the channels. But
>> suppose that I want to use the Mac to control the sound layout.
>> What are
>> the options?
>>
>> Well, I know that I can use an application such as Audio Hijack Pro
>> to add
>> all sorts of Audio Units to do processing. However, bizarrely,
>> stereo to
>> multichannel surround sound doesn't seem to be at all standard.
>>
>> Looking through the list of available AU plugins at (for example)
>>
>> http://www.macmusic.org/software/cat.php/lang/en/id/7037/view/AU
>>
>> there seem to be few modules that are not either (a) very
>> expensive, or
>> (b) completely out of date.
>>
>> SRSLabs does do a module for adding effects to iTunes playback
>> (http://www.srslabs.com/store/store/comersus_viewItem.asp?
>> idProduct=18 )
>> but it only seems to process a stereo signal into another stereo
>> signal.
>>
>> Am I missing something? Is there some reason why iTunes couldn't
>> just have
>> a spatialized sound output? Is there a simple solution out there? Has
>> anyone produced a user configurable Audio Unit that could be used
>> to do
>> surround processing on iTunes playback?
>
>
__________________
Simon Thorpe
CerCo, Université Toulouse 3, CNRS
Faculté de Médecine de Rangueil
31062 Toulouse Cedex 9 - France
Tel 33 (0)5 62 17 28 03. Fax 33 (0)5 62 17 28 09
http://www.cerco.ups-tlse.fr
http://www.spikenet-technology.com
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