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The goal of this article is to provide
step by step instructions
for converting Ambisonic Stereo UHJ interleaved audio files to
horizontal first order B-Format three channels audio stream and files. If
you're not familiar with Ambisonic surround sound technology, we
suggest you read this article and this article or the Wikipedia entry first. You must also be familiar with Steinberg Nuendo.
DISCLAIMER: we are not satisfied with the resulting audio output of the process suggested. End results are really, really, really, not guaranteed.
The software used for this demonstration is Mac OS X 10.4.11/10.5.8/10.6.2, Steinberg
Nuendo 3.2.1/4.3.0, Angelo
Farina UHJ to B-Format impulse
responses, Dave Malham & Ambrose Field Ambisonic
VST plug-ins and York University Ambisonic
VST plug-ins and a Stereo Convolver VST plug-in of your choice. Apart from OS X,
all the software is compatible with Windows OS
The information on this page is provided as is,
without any guarantee regarding performance or end result.
People familiar with Ambisonic know about
Stereo UHJ: it's a way of encoding first order B-Format
into two channels for compatible playback on traditional stereo
equipment and for decoding in surround sound with the appropriate
Ambisonic decoder. Meridian is a manufacturer of such hardware
decoders.
Ambisonic Stereo UHJ compact discs are available
mainly from Nimbus Records and a few other companies.
If you don't want to construct the Stereo UHJ to
B-Format Nuendo project, here's a Nuendo template.
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This is the signal processing
path for the Stereo UHJ to B-Format conversion that will
be followed in Nuendo.
Signal processing path and
image by Angelo Farina. |
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2 |
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In Nuendo, create a New Project. In the Outputs pane of the VST Connections window, create a Stereo
output bus and LRS output bus.
You can rename the output busses Stereo UHJ and B-Format. |
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In the Project window, add a Stereo Audio
track and three Stereo Group Channel
tracks.
You can rename the Stereo Audio track Stereo UHJ and the stereo Group Channels UHJ to W, UHJ to X and UHJ to Y. |
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4 |
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In the Mix
window, assign the Stereo UHJ Audio
Track to the Stereo UHJ output bus.
Assign the UHJ to W Group
Channel to the Left channel of the B-Format output bus.
Assign the UHJ to X Group
Channel to the Right channel of the B-Format output bus.
Assign the UHJ to Y Group
Channel to the Center Surround channel of the B-Format output
bus.
Pan the three Group Channels to the
center.
Insert an instance of Nuendo
TestGenerator and a Stereo Convolver in each of the Group
Channels, but leave them off.
Optionally insert an instance of York's B-Proc and B-Mic in the B-Format output bus: this will give
you the opportunity to manipulate the B-Format and to compare
the undecoded Stereo UHJ to a straight stereo decoding of the
B-Format. |
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5 |
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In the Mix
window, turn on three Sends in the Stereo UHJ Audio Track, one to each
of the Group Channels. The Sends are post fader
and at 0 dB level. |
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6 |
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You can verify that
the UHJ to W Group Channel is
going to the W channel (Left of LRS) with the
TestGenerator. |
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You can verify that
the UHJ to X Group Channel is
going to the X channel (Right of LRS) with the
TestGenerator. |
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You can verify that
the UHJ to Y Group Channel is
going to the Y channel (Center Surround of LRS) with the
TestGenerator. |
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Activate the Stereo Convolver plug-ins in the
three Group
Channels. Load the appropriate impulse response
in each (uhj_w_**.wav, uhj_x_**.wav, uhj_y_**.wav) with the sample rate corresponding
to the Nuendo project.
Download the Stereo UHJ to B-Format impulses responses. |
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To test the
conversion process, use the 200 Hz, 20000 Hz and pink
noise files, encoded in UHJ, available here.
The files are two channel interleaved 16 bit
FLAC file at 44.1 kHz sample rate. |
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The first minute of
the UHJ encoded test files contains W only signal. The
conversion should look this.
W = 0.5*(0.982*L + 0.982*R + j*0.164*L -
j*0.164*R)
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The second minute of
the UHJ encoded test files contains X only signal. The
conversion should look this.
X = 0.5*(0.419*L + 0.419*R - j*0.828*L +
j*0.828*R) |
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The third minute of
the UHJ encoded test files contains Y only signal. The
conversion should look this.
Y = 0.5*(0.763*L - 0.763*R + j*0.385*L +
j*0.385*R) |
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Replace the test
files in the Project window with the
Stereo UHJ sound files to convert to
B-Format. |
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At this point, you
can do the following:
Activate the B-Proc in the B-Format output bus to manipulate the sound
field.
Compare the Stereo UHJ output bus to a stereo decode with the B-Mic in the B-Format output
bus (don't forget to mute one output or the other). |
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© Daniel Courville, 2006-2007 |
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