Ambisonic Studio Ambisonic
   
Comparative Surround Recording  
 
 

H2 - ST350
                        - Native B

 

In September and October 2007 was held a surround sound production workshop at OBORO in Montreal. On October 14, we recorded an organ recital by Robert Sigmund at the Grand Séminaire de Montréal chapel using three surround microphone pickups: a Soundfield ST350, a native B-format assembly with AKG Blue Line microphones and a Zoom H2 portable digital recorder. Here is an account of the recording session with pictures and audio samples.

The OBORO workshop was aimed at giving participants a historical and theoretical overview of Ambisonic surround sound and a demonstration of the hardware and software production tools that can be used for Ambisonic. Without redoing the comparative surround recordings that took place in Austria and Germany in 2001 or redoing the comparative study between tetrahedral array-generated B-Format and native B-Format, we thought that the introduction of the Zoom H2 portable surround recorder in September 2007 warranted some new comparisons. This time around though, the "best surround microphone system for 5.1" criteria would not necessarily be the main interrogation for the comparison, but rather if the price point of each system is subjectively reflected in the timbre quality and the directional encoding.

The Zoom H2 is $200 (including the recorder), the AKG Blue Line native B-Format assembly is around $2000 and the SoundField ST350 is around $8000. It's important to remember that in the case of the native B-Format and the SoundField, a recording system must be added.

The Zoom H2 has four cardioid capsules arranged as a 90° front pair and 120° back pair (240° from center front), an arrangement reminiscent of the quad days of the seventies. A native B-Format assembly has one omnidirectional and two figure-8 (bidirectional) microphones to directly generate the Ambisonic B-Format. A SoundField microphone has four cardioid capsules mounted as a tetrahedron producing an audio stream called A-Format: the SoundField control unit must be used to convert the stream to B-Format.

Although the Zoom H2 can be used as an USB audio interface, it's in the first place a standalone recorder and it was used as such: since the H2 was the lightest system, it was put on top of the microphone stand. The ST350 was under the H2 and about 15 cm in front of it. The 3 meter cable that comes with the ST350 meant that we had to tape its control unit to the mic stand and then run four line level cables to its recording system, a Sound Devices 744T. The native B-Format assembly was under the ST350 and about 10 cm in front of the H2. The three mic level cables out of this assembly were run to its recording system comprised of two Metric Halo ULN-2 audio interfaces connected to an Apple iBook G4 1.33 GHz with OS X 10.4.10 and Steinberg Nuendo 3.2.1 DAW. The three recording systems were set to 24 bit and 48 kHz.



Comparative Surround Recording downloads

Three excerpts from the Livre d'Orgue de Montréal: Voix humaine, Trio and Dialogue.
Robert Sigmund, organ. Featuring Hugues St-Gelais, tenor.

Apart from the spatial processing necessary in some cases (B-Format decoded to Quad, Quad transcoded to B-Format and B-Format decoded to stereo), no other processing was done (EQ, compression, etc). The B-format RMS levels for the AKG assembly and the Zoom H2 were matched to the ST350 B-Format RMS levels based on an organ sustained chord and a RMS response time of 5 seconds. The output levels were match by ear and should be within 2 dB of each other. The B2G plug-in was used to decode the B-Format to Quad, the Quad2B was used to transcode the Zoom H2 quad surround to B-Format and the B2Stereo plug-in was used to decode the B-Format to stereo.

FLAC interleaved audio files, 24 bit, 48 kHz, zipped.

Quad contains:
- AKG Blue Line native B-Format decoded to quad surround (Zoom H2-like L, R, Ls, Rs, 90°/120°)
- SoundField ST350 B-Format decoded to quad surround (Zoom H2-like L, R, Ls, Rs, 90°/120°)
- Zoom H2 quad surround (L, R, Ls, Rs, 90°/120°, original four channels)

Ambisonic B-Format contains:
- AKG Blue Line native B-Format (WXY, original three channels)
- SoundField ST350 B-Format (WXYZ, original four channels)
- Zoom H2 quad surround transcoded to 1st order horizontal B-Format (WXY, three channels)

Stereo contains:
- AKG Blue Line native B-Format decoded to stereo (110° width/0.75 hypercardioid)
- SoundField ST350 B-Format decoded to stereo (110° width/0.75 hypercardioid)
- Zoom H2 quad surround converted to stereo through 1st order B-Format transcoding (110° width/0.75 hypercardioid)

On OS X, the latest versions of TwistedWave and Wave Editor can open, play and process multichannel FLAC files.


OS X audio utility downloads

X Lossless Decoder (XLD): utility for converting files between the following formats: Wave, AIFF, raw PCM, Wave64 (.w64), MPEG-4 AAC (.m4a), Apple Lossless (.m4a), FLAC (.flac/.ogg), MP3, Ogg Vorbis, WavPack (.wv). Can also read Monkey's Audio (.ape) and True Audio (.tta). Files can be multichannel if supported by file format.

Scott Wilson's De-Interleaver: utility for interleaving and de-interleaving audio files. Direct download.



Highslide JSOBORO's surround sound control room   The workshop at OBORO's surround sound control room.
Dynaudio Air 20 monitors are positioned in typical 5.1 fashion with left/right at 30° and Ls/Rs at 110°.
 
Highslide JSOBORO's surround sound control room screen   Highslide JSOBORO's surround sound control room screen showing Plogue Bidule   Left: the front screen with the center channel monitor behind it. On the screen is Wave Editor with an Ambisonic file converted to 5.1. As far as sound editors on OS X go, we also did demo the new comer, TwistedWave.

Right: the front screen again, but with the modular environment of Plogue Bidule.

 
Highslide JSExplaining 2nd order virtual microphones   Explaining 2nd order virtual microphones in the 2B2G decoder plug-in.
On screen is Nuendo with the 2B2G plug-in.
 
Highslide JSThree surround microphone systems on one microphone stand   Highslide JSThe recording gear includes two Metric Halo ULN-2   Left: In OBORO's Studio 1, checking the feasibility of mounting the three surround systems on the same microphone stand. In the background, Klein + Hummel O 300 monitors that were used in an Ambisonic birectangle rig for 3D sound reproduction.

Right: Testing the recording gear: on the table are the Metric Halo ULN-2s that will be used with the AKG Blue Line microphones. There's also the MOTU 828 audio interface that was used in playback for the Ambisonic birectangle 3D rig.

Klein + Hummel O 300 monitors courtesy of Sennheiser Canada
Monitor ceiling brackets and audio interface courtesy of UQAM Media School

 
Highslide JSTesting the recording gear: getting around the menus on the Sound Devices 744T   Testing the recording gear: getting around the menus on the Sound Devices 744T.
The 744T will be used for the SoundField ST350. We chose to disengage the 744T input level knobs and to set it at a fixed input sensitivity: that way, only the ST350 control unit is used for level setting and the four line level input on the 744T are ganged.

Sound Devices 744T recorder courtesy of Sonotechnique

 
Highslide JSThe Grand Séminaire de Montréal chapel before the recital   Highslide JSThe three surround pickups in the chapel   Left: a wide shot of the Grand Séminaire de Montréal chapel before the recital. The black bulge on the mic stand is the polypropylene foam-wrapped and gaffer-taped ST350 control unit.

Right: The three surround pickups in the chapel. Notice that the Zoom H2 is already running. Compared to the other two systems, we have at least fifteen minutes more of pre-recital recording on the H2. This is the kind of detail that must be taken into account when choosing the SD card that will go in the H2: we used a SanDisk SDHC 4GB card.

 
Highslide JSThe Guilbault-Therien French-Baroque style organ   Highslide JSThe three surround pickups in the chapel   Left: the Guilbault-Therien French-Baroque style organ of the Grand Séminaire de Montréal chapel with the microphones in front.

Right: another shot of the three surround pickups in the chapel.

 
Highslide JSThe control room in the chapel sacristy   Highslide JSAttentively listening to the recording in the control room   Left: the control room in the chapel sacristy.

Right: attentively listening to the recording in the control room. Although the Sound Devices 744T offers a "B-Format decoded to stereo" monitoring mode, there are no control for that mode. There was no way to modify the decoding and, monitoring on headphones, we did find the result to be quite narrow, to the point of almost being mono.

 
Highslide JSAttentively listening to, and looking at, the recording in the control room   By comparison, the stereo monitoring in Nuendo was done with the B2Stereo plug-in that offered much more monitoring options which proved to be more useful in checking technical aspects of the recording and more enjoyable for simply listening to the performance.
 
Highslide JSWrap-up after the recital   Wrap-up time after the recital.

Warm thanks to Robert Sigmund and Yves G. Préfontaine for letting us record the recital.

Photos by the workshop's participants.

 
Ambisonic Studio

© Daniel Courville, 2007-2016