Re: Outputting multichannel sound through the optical link??
Re: Outputting multichannel sound through the optical link??
- Subject: Re: Outputting multichannel sound through the optical link??
- From: William Stewart <email@hidden>
- Date: Fri, 5 Feb 2010 14:36:33 -0800
ok... so a couple of salient points I'd like to bring out here.
SPDIF's shortcomings have been described in some detail, but a point
that might be missed is that AC-3 or DTS encoding of the audio is not
the solution you really want. These encodings are lossy encodings, and
will degrade the quality of the audio signal. The encodings also add
latency, and while there are different versions of the AC-3 encoder to
minimise this for game play (the XBox supports this), it is still not
ideal.
So, the whole area of consumer multichannel audio has been a
problematic one for some years now. The development of HDMI and
DisplayPort as a means to easily transport both video and audio
(multichannel) is the way in which this problem has been addressed by
the CE industry.
With the release of the new iMacs from Apple, we now have a solution
for you.
The DisplayPort connector on these iMacs is capable (through an
adaption of the display port plug to HDMI) of carrying up to 24bit,
96kHz, 8 channels of linear pcm audio.
Griffin technologies provide an adaptor to adapt the display port
connection to HDMI, and once you plug this in, and then hook up a
receiver on the other end of the HDMI cable.
http://www.griffintechnology.com/products/video-display-converter
Now, HDMI is driven by an EDID (a descriptor that describes the
capabilities of a given device). So, what you will see in AMS (Audio
MIDI Setup) ***once you plug a receiver in*** is the capabilities of
that receiver. Some of the older HDMI receivers are only able to do 2
channels of linear pcm audio over HDMI, so that is all you will see.
If your receiver can do mutli-channel linear pcm over HDMI, then AMS
will show that to you.
Once this is there, this then behaves like any audio device you have
plugged in to your Mac. That is, any application (iTunes, etc), any
game, can output audio to that receiver up to the number of channels
it supports.
So, I think this is the one cable solution you are looking for.
Bill
On Feb 5, 2010, at 1:01 PM, Ryan Walklin wrote:
On 6/02/2010, at 4:57 AM, Simon Thorpe wrote:
From my understanding, in order to play a multichannel audio file,
I would have to use software like Compressor to generate a Dolby
Digiial or DTS or AC3 formatted file. This seems excessively complex.
Is this really necessary? Is there no way just to send the
multichannel audio directly out through the optical link? Are there
no home theatre amplifiers that could understand a non-proprietry
dumb multichannel stream?
As has been said, this is impossible given the SPDIF bandwidth, as
it's only able to support 2 channels at a time.
I've worked round the issue in Plex (www.plexapp.com) by using the
free AC3 encoder Aften (aften.sourceforge,net) to encode
multichannel PCM (e.g. from AAC/FLAC) to AC3 on the fly, and then
output it to SPDIF as Jeff describes, You also need to encapsulate
it as per IEC 60958 with a SPDIF header. This allows playback of
video with 5.1 AAC etc.
Something like Quicktime is a little harder, as it uses the system
sound output by default and won't allow access to the raw audio data
when playing back (Using QTKit anyway). So I used the
AudioReflectorDriver to create a 6 channel virtual device that
redirects sound to a daemon which does the encoding, then plays it
over the actual SPDIF device. This allows realtime 5.1 output of any
audio playing on the system, and even allows mixing. There's a
little latency involved in converting the audio, given the PCM
stream has to round trip through kernel space, but minimising the I/
O buffers drives it down to ~40ms, which I can't detect watching
video.
The source for Plex is freely available at github.com, and has a GPL
licence. I haven't released the code for my system-wide AC3 driver,
but it shouldn't be too bad as most of the components are open source.
One problem with this solution is that Dolby Digital (DD) is
patented and I'm unsure of the legality of Aften in the US. I'm in
NZ so it's not a problem.
Regards,
Ryan
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Coreaudio-api mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Coreaudio-api mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden