Working in B+ Format

Thomas Chen

What is B+ Format ?


B+ format is the standard 4 channels of B format material known as W, X, Y, and Z plus 2 channels of L/R dry stereo recording.  This method of recording separates the dry/direct sound from the ambient/room sound in recording and reproduction.  Each serves a distinct purpose and is ideally suited to it.

Why B+ Format

In the development of B+ Format I have attempted to create a sound system philosophy that will provide the following attributes.

A Universal Medium-An Answer

B+ Format consists of B format ambient-recorded signal plus two channels of L/R direct recorded signal.  It is recorded on 6 channels and when reproduced it is decoded into however many channels you have at your disposal.

By using the two channels of L/R direct recorded signal presented only in the front we will have the directional sound clues from the direct signal as well as the sound of the instrument.  There is no vertical height information in this pair and it is added to the frontal speaker array.

The B format consists of 4 channels of sound, which are W, X, Y, and Z.  X, Y and Z are the differential signal which gives the directional information to decode the W omni signal.  B format does not provide accurate localization but rather presents the ambient information and spatial clues.  This includes vertical height information.  I have chosen to use two rings of speakers to reproduce the vertical height information.

By having the ambient and direct information stored separately you can now chose the balance of direct and ambient information at the time of reproduction.  This will allow for many different sized reproduction spaces.  The larger space will need less ambient information while the smaller spaces will need more.  In addition you as a listener can chose the amount of ambience that you prefer, very much like choosing your seat in a theater.

Time, Location and Spatial Representation

The direct sound best gives directional and instrumental timbre information.  The direct sound should arrive earlier than the ambient signal.  The direct sound doesn't have any spacious or enveloping qualities.

The ambient sound best gives spaciousness, spatial depth and envelopment to the reproduction of sound.  It is well known that B format presentation is not accurate in presenting location and many are trying to improve the reproduction with higher order of B format decoding.  First order of B format is probably satisfactory for presenting the ambient sound because it is not well localized.

Storage Requirements

B+ Format recordings require 6 channels of storage.  This is now obtainable with either SACD or DVD-A using MLP compression.  The advantage of using B+ format disks is that you are not limited to the speaker arrangement that the format will dictate but rather a decoder will determine your listening set up or you can set the decoder for your listening set up.

Technique of Recording B+ Format

Direct Recording

Stereo Mics

The recording of the direct channel is done by conventional means and can use stereo microphone techniques of X/Y, M/S or OCT and can be stored as L/R or M/S information.  The direct signal should contain little reverberant information and is recorded using direction microphones.

Spot Mics

You can use spot microphones or flanking microphones to help balance your recorded sound.  You will need to add ambience to the spot microphones and may need to add ambience to flanking microphones.  The ambience added should be in B format and added to the B format channel.

Multi Track Techniques

Multi-track techniques can be used to generate the direct signal and you will then need to add ambience in B format.  If you wish to locate images not in the frontal plane you can present images in B format in the rear locations but remember their localization will not be as good as the frontal image.

B-Format Recording

Room Recording

Room ambient recording is done with the Soundfield microphone generating a B format output, which is a combination of direct and ambient sound.  The location of the Soundfield microphone is typically at the location in the room where the direct and reverberant sounds are equal, i.e. the critical distance.  This gives accurate spatial information about the recorded location

B-Format ambience by convolution

Another method of obtaining a B format ambient sound is by convolution.  This is a digital technique where a room ambience is measured in B format and can then be impressed upon another signal.  This is also accurate.  You can chose whether you use any direct signal in the impulse response.

You can also obtain B format ambient information with multiple reverberation devices.  It will take at least 3 engines to generate B format reverberance, one for each axis.  I have developed a B-format mixer to help generate the B-format ambience.

Techniques for Monitoring B+ Format

Ideal decoding with 16 channels

Decoding of the B+ format is ideally presented as 16 channels, which is arranged as two rings of 8 speakers with 45 degrees of separation between them.  From stereo we know that 60 degrees is the maximum separation between speakers without a hole in the middle.  By using an upper and lower ring of 8 speakers I am able to present vertical/height information.  You will present the L/R stereo on the front speakers without height information.  You should be able to adjust the balance of dry versus ambient sound.  I have developed decoders for 16, 12, 10 channels of 3D surround or without height as 8, 6, or 5 channels.

Decoding in your present control room

You can use your present control room to work in B+ format.  The decoding will be done with 60-degree speaker spacing with two rings of 6 speakers.  The front up and down speakers will be combined and your present control room speakers will be used for the sum of the front up and down signals.  You can use your present console to obtain the L/R stereo mix and use a pair of auxiliary sends to sent to the B format ambience generator.

In conclusion, I believe B+ Format is a universal media and is an answer to the delivery of surround sound now and in the future.

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