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

[Gregory Maxwell]

I've been doing a fair amount of work with Vorbis support for Ambisonics,
which seems to be going along nicely. It seems that there is signifant
interest in coding 5.1 material with Vorbis esp as tarkin becomes more
complete, so I've decided to take a break from pure ambisonic work to look
into this.

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.

To accomplish that I produced a model of an idealized 5.1 speaker
arrangement (with pointsource massless speakers) and a simulated ambisonic
microphone at the listening location. I then measured the ambisonic
impulse response from each speaker and produced convolution filters for
the audio.

The nice thing about this is that you can take the ambisonic decoded
output from vorbis and convolve it with inverse of these filters and get
pretty much the orignal input back or you could also convolve it to match
your own speaker layout and get better results than you would with the
orignal 5.1 material.

My only problem right now is with the LFE (subwoofer channel). I had been
intending to just mix it into the W signal, which I think matches the
intent of the channel. Obviously this really couldn't be recovered on
decode.

My alternative is to use a seperate LFE channel in the vorbis stream,
which is just ugly.

So, what I'm looking for is some guidance about how the LFE is normally
and best used in 5.1 mastering. If I had an infinite supply of perfectly
flat really loud full spectrum speakers and I was setting up an ideal 5.1
setup, would I mix my LFE into my five speakers, or would I grap another
one of those perfect speakers and lay it in the corner with the LFE feed
going into it?

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