DBLX Consulting
Architectural Acoustics and Lighting

Multichannel Audio

Broadcasting and Recording Systems
The sound performance is recorded or transmitted from spaces called performance spaces such as studio, auditorium, and concert hall. Transmitted or stored sound programs are reproduced by the loudspeakers in spaces called listening spaces such as listening rooms, cinemas and cars. The sound fields in the performance spaces are not exactly the same as the sound fields in the listening spaces regardlees of the condition of the broadcasting/recording channels.
There are two concepts for the requirements of the transmission/recording systems. First one is to present the sound field in the listening space as close as possible to the field in the performance space. The other concept is to produce sound field different from the original one, however it gives the listeners a favorable impression that may generate the perception of the original sound field in the performance space.

Electro-Acoustics Systems in Broadcasting

Surround Mono1-1may be>=1
Incomplete Stereo2Y1Y2
Regular Stereo2-2-2
Enhanced Stereo4Y2Y4
Regular Quadraphonic4-4-4

The audio signals in both concepts may be subject to a significant modification by a special signal processing (digital or analog) as shown in the drawing. In practice, both concepts are incorporated into the electro-acoustics systems to produce a favorable sound in the listening spaces. This usually requires optimizing the microphone and loudspeaker locations and characteristics.

Stereo Systems
Mono system does not provide many characteristics of the performance space to the listeners. These characteristics include perception of the volume (spatial perception), localization of different sound sources, the reverberation process and many others.

Stereo Systems

Stereo system illustrated in the figure provide or improve the special perception; localization of the virtual sound sources (located between the two loudspeakers); perceptions of the sound filed of the performance space, and frequency characteristics of the reproduced sound sources.

Stereo-Sound Listening Area
All the characteristics that are described in the stereo reproducing sound will depend largely on the position of the listeners with respect to the loudspeakers. The following illustration shows the optimum area for stereo listening. Area 1 represent the listening locations where preferences of the stereo system compared to the mono system is more then 85%. In this area the listener can locate all of the virtual sources of the sound stage along the line between the loudspeakers.

Stereo Zone

When the listener move farther off the line of symmetry the virtual sound sources move toward the closest loudspeaker as well and the width of the reproduced sound stage reduced. Consequently, the preferences of the stereo system drop. The listening area where the preference is 60-85% represent the location where the listener can separate fewer virtual sound sources (at least 3 group) on the loudspeakers line (Base Line).
The stereo-effect area can be increased in many ways. One is to minimize the shifting of the location of the reproduced sound stage when the listener moves off the symmetry line. This can be done in different ways, one of which is to aim the loudspeakers as following:

Larger new stereo zone

Multichannel Stereo Systems
Multichannel systems refer to the system that incorporate two or more transmission channels or recording process and more then two loudspeakers. These systems are widely used in the cinemas and the home theater. The major issue for these systems is the compatibility between these systems itself for different applications as well as the compatibility between these systems and the regular stereo or mono system. This is the reason why these systems started to be standardized just in the last few years.
The main configuration of the system for home theater is the 5-1 or 3-2 system illustrated in the following drawing. This configuration may be applied with different transmission system or recording process and it is independent of the coding formats.

5-1 layout

Practically, room interiors may not be optimized for the loudspeaker layout and movement of some loudspeaker is needed. In this case, delay of some loudspeaker signals will be needed.

5-1 practical layout

The central front loudspeaker is usually supplied by the sound of vocalist, soloist, or dialogue. The front right and left loudspeakers are for stereo reproduction. The ambiance/surround sound is supplied to the surround loudspeakers. A subwoofer is used for low frequency signals that may be processed from independent channel in the transmitted or recorded system. There are few formats used in the industry for the coding that is applied in preproduction and postproduction of the channel signals (Dolby Digital and others).
Many other different electronic and acoustical details should be implemented to achieve the desired listening experience with this system. These details include: low frequency response of all of five loudspeakers is 80 Hz (with uniform polar response in defined angles), sound pressure level at the listening position from each loudspeaker is 82 dBC measured with
pink noise (signal of wide frequency spectrum), subwoofer signal filtering has cut off frequency less than 150 Hz, and smooth low frequency output from the subwoofer. This last point may require real time measurements and optimizing the shape and the finishing of the listening room as well as the location of the subwoofer.