ACN8 was accidentally put on ACN7. However, rather than adding the missing
channel of silence, set up the coefficient map to only use the ambisonic
channels that can contribute to output.
Now front-center sounds are attenuated by roughly -4.5dB instead of -3dB. This
will help keep rear-panned sounds from attenuating too much while not making
front-panned also attenuate too much.
Similar to the previous, but includes the top and bottom HRTF responses. The
higher-order decode (for the "basic" HRTF output) also now uses 2H1P mixed-
order instead of 2H1V, which seems more stable.
This improves a stereo (front-left + front-right) sound "image" by generating a
front-center channel signal. Done correctly, it helps reduce the comb effects
and phase errors associated with using only two speakers to simulate center
sounds.
Note that it shouldn't be used if the front-center channel is already included
in the positional audio mix (the dialog effect is okay). In general, it may
actually be better to exclude the front-center channel from the positional
audio mix and use this to generate front-center output.
Not all speaker kits have a front-center speaker capable of outputing full-
range content. It's best to err on the side of caution and not include front-
center for normal positional sound by default, leaving it instead for the
dedicated dialog effect.
NFC filters currently only work when rendering to ambisonic buffers, which
includes HQ rendering and ambisonic output. There are two new config options:
'decoder/nfc' (default on) enables or disables use of NFC filters globally, and
'decoder/nfc-ref-delay' (default 0) specifies the reference delay parameter for
NFC-HOA rendering with ambisonic output (a value of 0 disables NFC).
Currently, NFC filters rely on having an appropriate value set for
AL_METERS_PER_UNIT to get the correct scaling. HQ rendering uses the averaged
speaker distances as a control/reference, and currently doesn't correct for
individual speaker distances (if the speakers are all equidistant, this is
fine, otherwise per-speaker correction should be done as well).
This change allows pair-wise panning to mostly go through the normal ambisonic
panning methods, with one special-case. First, a term is added to the stereo
decoder matrix's X coefficient so that a centered sound is reduced by -3dB on
each output channel. Panning in front creates a similar gain response to the
typical
L = sqrt(1-pan)
R = sqrt(pan)
for pan = [0,1]. Panning behind the listener can reduce (up to) an additional
-10dB, creating a audible difference between front and back sounds as if
simulating head obstruction.
Secondly, as a special-case, the source positions are warped when calculating
the ambisonic coefficients so that full left panning is reached at -30 degrees
and full right at +30 degrees. This is to retain the expected 60-degree stereo
width. This warping does not apply to B-Format buffer input, although it
otherwise has the same gain responses.
Both 5.1 Side and Rear configurations use 'surround51' to look up the
appropriate decoder file. The decoder loader already handles mapping between
rear and side channels, so there's no need for separate options.
This now takes advantage of the differences seen in generated decoder matrices
for first-order compared to second- and third-order, such that with the
appropriate frequency-dependent scaling applied to first-order content, the
result is identical with a higher-order decoder matrix compared to a first-
order matrix for the same layout.
