Re: Outputting Multichannel sound from stereo sources
Re: Outputting Multichannel sound from stereo sources
- Subject: Re: Outputting Multichannel sound from stereo sources
- From: Brian Willoughby <email@hidden>
- Date: Thu, 12 Aug 2010 12:41:46 -0700
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?
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