Example of B-format recording

This document provides information of an example of B-format recording, and of delivery of it on the internet, either as multichannel B-format (in .AMB format) and as plain, old stereo downixes.

The recording was done on 20 April 2007 by Adriano Farina, at La Casa della Musica, Parma, ITALY.

It was a concert of a small instrumental group, named Ensemble Arte Musica, directed by Francesco Cera. The soprano was Loredana Bigi. This soprano has a wonderful voice, and she also performed (a few years ago) some anechoic recordings, which are available here.

This is a picture taken during the concert:

The position of the Soundfield ST-250 microphone is evidenced in the above picture (a cyan ellipse).

The other equipment employed is:

• Metric Halo Mobile I/O firewire sound card
• Apple MacbookPro 17 computer

5 music pieces were played, and of each of them a short sound sample is made available for download. The list of performers and the details of each of the 5 music pieces (plus a "bis") are reported here.

The recordings, performed directly with the Metric Halo control panel, consist of 4 separate mono WAV files. These are packed together into a single 4-channels wave-ex file, and converted to .AMB format, employing the tools and the methods described on Ambisonia.com.

The resulting .amb files can downloaded from here.

However, these .amb files are for people who can manage to play them through an Ambisonics decoder, and a proper multi-loudspeaker rig. More info on the software and hardware required can be found, again on Ambisonia.com.

For making the recordings playable on everyone's system, however, it is advisable to provide also a stereo downmix, and this is what is described here in great detail. Basically, we have two possible approaches for creating a 2.channels downmix of a 4-channels B-format recording:

• Employing a complex UHJ matrix for encoding three of the 4 channels (namely, W, X and Y) into a stereo-compatible, "surround-encoded" stream (a proper UHJ decoder can later retrieve, at least partially, the surround information)
• Creating a "pure" coincident stereo recordings, by syhthesizing a couple of microphones (for example, two angled supercardioids, or two crossed figure-of-8).

I created three different stereo dowmixes of the "BIS" recording:

1. AF_BIS_uhj_OK.mp3 - This is UHJ, created employing the method described here.
2. AF_BIS_stereo.mp3 - This is a pair of supercardioids (directivity factor=1.3) angled at +/- 51 degrees, and with an elevation of +21 degrees
3. AF_BIS_blumlein.wav - This is Blumlein, a pair of figure-of-8 (directivity factor=2.0) angled at +/-45 degrees, and with an elevation of +35 degrees

The following images show AudioMulch, the program employed for processing the above three files, making use of suitable plugins (SIR for doing the convolution required for UHJ encoding, Visual Virtual Microphone for synthesizing a pair of microphones with proper directivity and aiming).