[vorbis-dev] Calling for 5.1 Mastering experience! (vorbis am bisonics and 5.1)

[Myles Buckley]

On 20030610: Gregory Maxwell  wrote:
 (in reply to Ralph Giles)

>> I assume you're aware of the technical documentation on dolby's site?
>> (http://www.dolby.com/pro/) In particular the surround mixing guide has
>> a lot of detailed guidelines. I don't have any practical experience
>> with it though, so I can't vouch for it.

>Yes I am. I've done a lot of research to determine the 'correct' speaker
>placement for my ambisonic conversion.

What resources did you use for 'correct' placement?  I *STILL* haven't 
set up my full rig at my new house - and i've been here for eighteen months.

>> In particular it says that the LFE track is for option reinforcement,
>> so while the idea is to encode it separately and mix it into the
>> subwoofer channel, mixing it into the other 5 is acceptable, as is
>> ignoring it. Don't know how safe that is in practice.

>I just can't figure out what the orignal logic is for 5.1 to even HAVE a
>seperate subwoofer channel.  But a lot of the decisions behind the dolby
>5.1 stuff are not mathmatically sound or even intutivly sane.

IIRC it was to use as few bits as possible, w/o distorting the cheapa$$ 
speakers most people throw in the corners behind the couch with bass signal.
(Mustn't take space away from the anti-piracy bits!).

Another guess is, with a choice of only two from this list: 
A|cheap encoding| B|cheap decoding| and C|excellent sound|
Dolby labs picked A and B.  the chips to encode and decode dolby are 
cheap (and sound cheap.)  5.1 is also easy to cookie-cutter a design.

Joe sixpack does not want to 'calculate' where the speaker belongs or 
'program' the delay and phase for its placement.  He just wants a pretty 
picture of what the room will look like, and NOT have his wife trip over 
wires.

>> Encoding it separately breaks the symmetry of the ambisonic encoding,
>> but is probably closer to the intent of the original mix. It should
>> compress quite well with it's own codebook given the lowpass, but I
>> think it also tends to share a lot of entropy with the W channel.

>I'd like to think that users of surround Vorbis (including people who are
>listening to content transcoded from 5.1 sources) will *not* be listening
>on speakers configured per the dolby 5.1 specs but rather listening on an
>(likely irregular) Ambisonic array sized according to their needs and
>resources. Because of this, I'm slightly more concerned with compatiblity
>with such setups than absoultly perfect mirror of the mix intent.

Bingo - the purist will use an Ambisonic array - but there are many still 
suffering with 5.1

>However, if it's common practice to do evil things with the LFE (say put
>the same LF content thats on the main channels but 180deg out of phase)
>then I will have to seriously consider using a seperate channel.

My kids Disney DVD titles have terrible LFE phase - for instance: Beauty 
and the Beast has the bass in the front channels 180 degrees out of phase
with the 
back channels.  (and all are +/-90 degrees out from the LFE channel to
boot.)

>> > I've decided that the best (from a pure elegance and patent avoidance)
>> > way
>> > to handle this is to basically decode the 5.1 input into a WXY or WXYUV
>> > ambisonic signal, which can then be handled by the Vorbis Ambisonic
>> > support.
>>
>> So U and V are the planar quadupole (m=+/-2) moments? I'm glad to hear
>> you can get by with the same number of channels.
>
>They happen to fit nicely with the proscribed speaker placements for 5.1.

>I haven't done testing to determine how well I'm really representing 5.1
>yet as doing so will require me setting up a proper 5.1 speaker system
>which seems horribly boring compared to my 14 channel ambisonic rig.

imple'nuff, Plug up you ears and bring your test tracks to your local 
big-box-retail store with a 'demo movie/music room'  Thats how I test 
my encodes for other peoples setups.... and the FutureShop boys just 
might like your "position impossible" soundscapes.

-Myles
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