Subject: RE: [linux-audio-dev] surround encoding
From: Richard W.E. Furse (richard_AT_muse.demon.co.uk)
Date: Thu Oct 18 2001 - 23:38:50 EEST
> -----Original Message-----
> From: owner-linux-audio-dev_AT_music.columbia.edu
> [mailto:owner-linux-audio-dev_AT_music.columbia.edu]On Behalf Of Nick
> Bailey
> Sent: 18 October 2001 13:18
> To: linux-audio-dev_AT_music.columbia.edu
> Subject: Re: [linux-audio-dev] surround encoding
>
>
> Steve Harris wrote:
>
> >
> > Thanks, I'm very interested in ambisonics, but was put off by
> the price of
> > the equipment, looks like decoding is covered then. Do you know if its
> > posible to build a soundfield type mic using cheap elements, or does it
> > require really good ones?
> >
> > - Steve
>
> I believe so: you need an omni and two figure-of-eights for a
> minimal setup if
> I remember. The 8s are places front-back and left-right in the
> horizontal plane,
> and coincident with the omni. Of course, depends what you mean
> by cheap (most
> cheap mics are Cardiod, aren't they.)
Yep - I believe many of the original recordings were done this way. You need
another vertical figure-of-eight to capture the height information (Z
channel). Re PZM: this will probably allow you to construct a good
figure-of-eight and omni response, but there's nowhere to put the other
mikes. The figure-of-eight Blumlein arrangement mentioned is again probably
adequate for 2D although you'd need a little maths to turn it into B-Format.
The problem with all these approaches is getting the mike capsules
coincident enough to avoid phase problems. The ST250, Mk5 (and the other
one) get around this by use of tetrahedral capsules and a bit of matrix
maths. This also reduces the S/N ratio (essentially by averaging) - the
ST250 I own is wonderful.
The best cheap option for construction of soundfields in the studio (rather
than in the field) is probably to record in mono and spatialise into an
Ambisonic soundfield using the encoders such as are included in the CMT set
(or VSpace if you want a proper simulator).
--Richard
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