AFTEN to do 7.1 (or 6.1)

[rondari]

Hi all,

On www.ambisonia.com I've converting ambisonic recordings to DTS. Now I see that perhaps there is more playback support for AC3.

What I do is decode the Ambisonic recordings to 4 (square) speaker feeds (which works well for ambisonics), and I encode them to DTS (with a silent central speaker).

If I were to choose AC3 instead, and could deliver 6 channels of audio (which would be 7.1 with a silent central speaker)... then this would fascilitate people listening to ambisonics with a hexagonal speaker setup (which is a much improved listening experience for ambisonics).

My questions are :
1) can aften encode 6 speakers (known as 7.1)?
2) what would happen if someone only has a 5.1 setup? are the 2 rear channels of a 7.1 mix ignored? or do they get mixed in to the 2 side speakers somehow by the amp?
3) when DVD's offer 7.1 surround, does that mean they aldo have a separate 5.1 track... or is the 5.1 extracted from the 7.1 ?

Etienne

[DarkAvenger]

AFAIK, AC3 (not the plus format) doesn't know anything beyond 5.1. Dolby Digital EX has sort of 6.1, but the rear center is not discrete, but matrixed into the rear channels. The 7.1 DD receivers don't deliver real 7.1 for DD EX. The two rear speakers are playing the same channel, the center rear channel.

But you could offer 4.0 AC3 files, which would basically be the same as the DTS ones.

[honai]

The 7.1 DD receivers don't deliver real 7.1 for DD EX. The two rear speakers are playing the same channel, the center rear channel.

Actually, most 7.1 receivers come with Dolby Digital Pro Logic IIx, which converts 5.1 to 7.1. It's surprisingly good.

[DarkAvenger]

Nevertheless I am talking about (semi)discrete channels, not mumbo jumbo generated from more or less thin air... (Edit) But of course, you may be right, that those DPL IIx receivers play *different* content on those rear channels. Though I wonder how to master them...

[rondari]

...
But you could offer 4.0 AC3 files, which would basically be the same as the DTS ones.

Not sure what you mean by 4.0 AC3 files ... do you mean an AC3 file with 4 independent channels in it?

There's a good chance I will do that.

but I'm also investigating into how I can offer more than 4 channels (or more than 5.1) that can be played on a home consumer system that can do 6.1 or 7.1. A 6.1 system would be able to play back a 6 spekaer hexagonal arrangement ... which would give very good performance in terms of localisation of sounds.

[SeeMoreDigital]

Many PC sound cards now offer 7.1 (8Ch) analogue outputs. And many mid-range surround sound amplifiers offer 8Ch analogue inputs.

Instead of encoding to multi-channel AC3 (or DTS). Have you considered 8Ch WMA Pro or (my personal favourite) 8Ch AAC?


Cheers

[rondari]

Instead of encoding to multi-channel AC3 (or DTS). Have you considered 8Ch WMA Pro or (my personal favourite) 8Ch AAC?


I havn't really begun to look at AAC yet ... I have a couple of major issues when it comes to choice of formats:

1) how easily can I automate the conversion from ambisonic to AAC. The server is on Linux so any codec that has an encoder available on Linux is a huge benefit ... is there one available for AAC?

2) how easily can consumers play back that format? (does VLC support AAC playback?) ... what hardware / software combos are available.

BTW ... why is AAC your personal favourite?

[SeeMoreDigital]

1) how easily can I automate the conversion from ambisonic to AAC. The server is on Linux so any codec that has an encoder available on Linux is a huge benefit ... is there one available for AAC?Sorry, but I'm unable to help you with this...

2) how easily can consumers play back that format? (does VLC support AAC playback?) ... what hardware / software combos are available.Playback is very simple. VLC supports it as do other self contained media players such as Nero's ShowTime player and Apples QuickTime 7. And a whole host of direct-show based media players... Plus, if you have QT7 installed, it's possible to playback multi-channel AAC streams via a web page.

BTW ... why is AAC your personal favourite?Because it's very flexible

[nurbs]

1) how easily can I automate the conversion from ambisonic to AAC. The server is on Linux so any codec that has an encoder available on Linux is a huge benefit ... is there one available for AAC?

Nero offers a free command line encoder for linux. The linux version is a bit slow though. You can run the windows version of the encoder under wine for better speed. As an open source alternative there is faac, but quality is probably worse than nero.
I don't know about ambisonic, but the encoder can handle wav and stdin.

edit:
Sorry, I overlooked that you want to encode 7.1. I don't know if any of the encoders I mentioned does that.

[rondari]

... Plus, if you have QT7 installed, it's possible to playback multi-channel AAC streams via a web page.

I can not state how significant that is.

The 'user experience' that really brought video to the world was when you could play the videos directly from the page.

If it was possible to play ambisonic files (or other surround files) directly from a web page, that would be a HUGE boost to surround.

I need to do this.

Ok ... I guess I need to research about QuickTime plugins ...

[SeeMoreDigital]

Ok ... I guess I need to research about QuickTime plugins ...If you have QT7 installed and your soundcard correctly configured, try this sample: -

http://homepage.ntlworld.com/seemoredigital/Audio_Streaming_Tests.html


Cheers

[rondari]

If you have QT7 installed and your soundcard correctly configured, try this sample: -

http://homepage.ntlworld.com/seemoredigital/Audio_Streaming_Tests.html



I'm installing QT7 now ... but I have some questions for you...

The above web page states that the file is a 6 channel file ... is there anything included inside the file that tells the QuickTime player how to play it? ... for example, are the 6 channels just 5.1? ... how is this communicated?

With Ambisonics, the location of the sounds is described in the digital signal, meaning that it can be played back on _any_ speaker array. THis just means that QT would have to know that the 4 channel file is Ambisonic, and the QT ambisonic plugin player would have to allow the listener to configure the player for whatever speaker array that have (5.1, 6 (hex), 8 (cube), 12 (icosohedron etc.)).

AAC can apparently do up to 48 channels ... so I assume it also defines a way to define what the channels are.

[SeeMoreDigital]

The above web page states that the file is a 6 channel file ... is there anything included inside the file that tells the QuickTime player how to play it? ... for example, are the 6 channels just 5.1? ... how is this communicated?The creation and playback of AAC audio files is carried out in accordance to Part-3 of the ISO/IEC 14496 standard. So yes, each channel has a designated position within the AAC stream. All if which is then muxed into the .MP4 or M4A container, in accordance to Part-14 of the ISO/IEC 14496 standard.

With Ambisonics, the location of the sounds is described in the digital signal, meaning that it can be played back on _any_ speaker array. THis just means that QT would have to know that the 4 channel file is Ambisonic, and the QT ambisonic plugin player would have to allow the listener to configure the player for whatever speaker array that have (5.1, 6 (hex), 8 (cube), 12 (icosohedron etc.)).

AAC can apparently do up to 48 channels ... so I assume it also defines a way to define what the channels are.The usual approach to creating multi-channel AAC files is to start with either a multi-channel .WAV or .AC3 file which you then convert to a multi-channel AAC file, using audio encoding applications such as BeSweet (BeLight is my personal favourite).

That said, the full version of QuickTime7 (ie: QuickTime7 Pro) also allows you encode from one audio format to another - Although it does not allow you to input .AC3 or .DTS streams.

Are you able to upload an (multi-channel) AC3 version of one of your Ambisonic sources?


Cheers

[SeeMoreDigital]

Just in-case you are still interested. Here's a 6Ch AAC-LC 'Speaker Mapping' sample (http://82.10.220.174/Uploaded_Files/Doom9_Forum_files/6Ch_AAC-LC_Speaker_Tests.zip) for you to try.

There are actually 3No samples of the same source muxed in different containers. 1No 6Ch AAC-LC in .AAC, 1No 6Ch AAC-LC in .M4A and 1No 6Ch AAC-LC in .MOV.


Cheers

[rondari]

Just in-case you are still interested. Here's a 6Ch AAC-LC 'Speaker Mapping' sample (http://82.10.220.174/Uploaded_Files/Doom9_Forum_files/6Ch_AAC-LC_Speaker_Tests.zip) for you to try.



When QuickTime plays the AAC and the MOV, and I am listening in headphones, I hear the centre and surround positions. I'm wondering how Quicktime is pulling those other channels into the stereo mix.

I'm going to give AAC a try ... I'll post the resultant URL to this forum for people to try.

I just googled a bit and discovered that there are AV receivers that will accept a digital AAC stream. This is important because it offers a mechanism to achieve quality Digital to Analog, all whilst streaming directly from a webpage.

Will let you know when I get something up.