225WAD (07-2009)

225WAD - 2nd Order Ambisonics Decoder

Needed Software :

VST's :

- DCKiller (not included in package)
- RMS Buddy (not included in package)
- Panorama (included)
- Wig Ambi Decoder 2nd order (included)
- MSAdjustGUI (included)
Some of these are not included, because they are available in other methods.

You can download the package here : 225WAD

Layout :

Settings :

Because this is second order Ambisonics and we want to convert to a maximum of
5 speakers, only quad 20 and the two ITU 20 sets are important here. Because
the center of an ITU conversion is very low in volume, you can add the center
directly from the stereo input with MSAdjustGUI. Do not change this setting !
You can also use it to make a quad conversion with a center.
Let's walk through the settings :

- Azimuth : these settings are responsible for the front to rear balance.
The value of +75/-75 gives an almost equal volume in the fronts and rears with
the default Decoder preset. Changing this value gives more or less sound to the
rears and thus replaces the postgains here. Try to achieve a final result of
center +0.5 Db, LFE -4 Db RMS and rears -1 Db compared with the fronts.
When you change to the quad option, you should also adjust the Azimuth Setting.
You will notice clipping in the fronts and a big difference between fronts and
rears with a value of '75'. Increasing this value to for instance 85 (+85/-85)
will get a better balance with the quad preset.
The quad setting gives a better result normally and should be your preferered
method. Notice that ITU gives a very low center and the MSAdjustGui takes care
of making it louder.

- Additional Bass Boost
This works different from the bassboosts in other layouts like DPLamb or QSxx,
where the bass is extracted from the stereo source. Here the bass is derived
from the 'weight' or Pressure value from the Ambisonics soundfield . However as
usual we advice not to use any bassgain at all. Let the player or receiver take
care of the bass !

- Outgain
Run the process once and you get the values in the RMSBuddies. If you want to
increase or decrease the volume, you must use this slider. As separate volume
changes for each separate channel would destroy the soundfield, the gain is for
all channels. When you add gain, make sure you don't get clipping by adding no
more than the value in the RMSBuddy with the highest value for the Peak level.

- Disable Distance Compensation
This setting will compensate a bit for the distance between the listening space
and the Center. It doesn't make a big difference though.

-Decoder Low and High pattern
This setting emulates different microphones. For both ITU sets, the setting
should always be 0.5 (which gives a value of '1.00').
With a decoder setting for quad, changing this setting has no effect. It
influences the surround sound and the placements of the instruments.
The pattern parameters control the polar patterns fed to the speaker feeds,
with a value of 0.0 representing omni-directional, 1.00 representing cardioid
and 2.00 representing a figure of 8 response

-Decoder TransitionF
As mentioned before you must take extra care of this setting !
Humans rely on several cues to localize sound sources. While some suit some
sound sources better, it is the combination of these cues that allows us humans
to determine the origin of sounds. The first cue works for sounds with
frequencies up to 700Hz and is known as Interaural Time Difference (ITD). This
occurs when sounds arrive at each ear at different times, which results in phase
differences. For this reason, this cue is also known as Interaural Phase
Difference (IPD). For sounds with frequencies greater than 700Hz, the Interaural
Level Difference (ITD) cue is used. This is a result of sounds arriving at one
ear before arriving at the other. The extra distance required for the sound to
reach the second ear results in a drop of amplitude in the perceived sound.
In an Ambisonic decoder, the spherical harmonic direction signals, W, X, Y and Z,
are passed through a set of shelf filters which have different gains at low and
high frequencies designed to match different ways the ear/brain localises sounds.
The different localisation mechanisms operate below and above the Transition
Frequency, about 500 - 700 Hz. In summary you can use any setting between 500-700
as 'ideal' setting, but some music benefits from higher settings. It's just a
matter of experimenting, but on 90% of the sources a value of 551 Hz gives the
best results.
NB. When you change in the DSP from 44.1 Khz to another, the value for the
TransitionF must be changed too !! Make sure it's about 550 Hz.

- Decoder use spatial EQ
Another setting that influences the sound and gives more or less spatial effect
to the low or high frequencies..
We use 0.0 as default. Then the spatial EQ setting is off. To put it on set it
to 1.0. And remember this setting is for ITU (=5.1) only !

- Some extra remarks :

- Don't be surprised when you use the 5.1 ITU set and are confronted with a very
low Center. (About 20 Db lower than the Left and Right front). This is normal as
in the Ambisonics layout for 5.1 the center is only there as support for the Left
and Right speaker. To compensate for this weak center we added MSAdjustGui.
- Do not use the upsampling/dowsampling technique ! Downsampling the mono wavs
from 88.2 Khz or 96 Khz to 44.1 Khz can have a negative result on the Ambisonics
soundfield. And because this method is "non-destructive" there is no need to up-
and downsample. However : sometimes your source can use a quality boost and then
you need to add some preprocessing on the stereo like BBE Sonic Maximizer. If we
put it after the whole process, it would destroy the Ambisonics soundfield. On
most older CD's a mild setting (as the default one) for BBE will do fine !
Be sure the setting of BBE will not lead to clipping !
- Note about clipping : this should always be avoided at any step in the process.
In other words : when you have a cllipping sound before the postgains, you cannot
make the clipping go away by adjusting it with the postgains. Always when you have
clipping, you should start with the pregain !


Special thanks to Bruce Wiggings for his excellent WAD 2nd order processor.


Layout and Guide by The Dynamic Duo
The Dynamic Duo : Kempfand & Eye of Horus
It's not allowed to re-publish this method !