• Standard Surround Configuration

• Surround Channels
• Front Channels
• Implementing a Surround Format for Music
  • More Information On RSP Circle Surround

Introduction

"Circle Surround" is RSP Technologies' patented surround sound processing system. It is a powerful multi-dimensional sound format designed for exclusive-audio as well as video-sound applications, and is intended to be used with four or five speaker sound systems. Circle Surround creates a listening environment which places the listener "inside" music performances, and dramatically improves both hi-fi audio and conventional surround-encoded video material. Unlike other popular surround systems, Circle Surround provides composite stereo rear channels to greatly improve separation and image positioning - adding a heightened sense of realism to both audio and A/V productions.

Utilisation of the complete Circle Surround system (both encode and decode) allows audio engineers to place any voice, instrument or sound effect at any predetermined location within the 360 degree radius surrounding the listener. This white paper provides a brief background of surround sound/ matrix systems, including discussion of some of the more common surround systems, followed by an explanation of Circle Surround - both how it works as well as its advantages.

Background

Methods of providing multi-dimensional sound for both film and music applications have been in existence since at least as early as the 1960s. One of the original matrix systems was developed by David Hafler, founder of Dynaco. Hafler's system was strictly a passive decoder which decoded a standard stereo recording into four channels. In the late 1960s, Peter Scheiber filed US. Patent No. 3,632,886, in which he disclosed an encode/decode matrix system which was the basis of one of the major competing formats for quadraphonic sound in the early 1970s. Of the many attempts that have been made to introduce a multidimensional sound system, some of the most notable are the several rival quadraphonic systems introduced in the early 1970s.

In hindsight, it is easy to see why the quadraphonic era was short-lived. All of the systems introduced had considerable technical problems, and were incompatible with one another to some degree. None of the systems had a good variety of software available and, in addition, the public had to be persuaded to buy extra speakers and power amplifiers. They did not work particularly well with non-encoded material, as they suffered from adverse image wandering effects due to the broad- band gain riding implemented.

Even so, the aforementioned Scheiber patent, as well as his subsequent patent numbers 3,746,792 and 3,959,590, are the patents cited by Dolby Laboratories for the "Dolby Surround" system. In light of this fact, it becomes evident that the original concepts of what has come to be known as "Dolby Surround", as well as all other matrix systems, were originally founded by Peter Scheiber in his efforts to develop a system for quadraphonic audio use with phonograph records in the late 1960s and early 1970s.

As mentioned, the original passive decoding system accepted a standard stereo signal and, from it, produced four signals which could each be fed to a speaker located at specific position around the listener.

The systems disclosed in the Peter Scheiber patents are known as 4-2-4 matrixes - wherein four discrete signals are encoded into a two channel stereo signal. The encoded signal can then be played back through a decoder to extract the original four signals from the encoded two-channel stereo signal - allowing each signal to be fed to its intended speaker location.

Although passive matrix systems are capable of providing infinite separation between the left and right channels as well as the front and rear channels, a primary disadvantage lies in that such systems are only capable of approximately 3dB of separation between adjacent channels (i.e. left/center, center/right, right/surround and surround/left). Due to this drawback it was desirable to develop a steered system, incorporating gain control and steering logic, to enhance the perceived separation between channels. The inventions disclosed in the US. Patents issued to Peter Scheiber also incorporate early implementations of such technology, which is further discussed later in this paper.

Cinematic Considerations

Following the demise of quadraphonic sound, companies such as Dolby Laboratories adapted the matrix scheme to cinematic applications in an attempt to provide additional realism to feature films. However, the nature of the cinematic listening environment requires that different criteria be met than would be needed for 4- channel matrixes music applications.

In cinematic applications, it is desirable that character dialogue be localised to any characters shown on the motion picture screen, therefore a centre channel was incorporated to exclusively provide dialogue which emanates from a speaker located behind the screen. The need for a hard centre channel with its own respective speaker (rather than utilising a phantom centre within a four speaker system) results from the need to stabilise the centre image for listening positions which are off to either side of the theatre.

The left and right front channels provide stereo information and sound effects, and the surround channel is used primarily for ambience and effects.

Standard Surround Configuration

The standard Dolby ProLogic system in use for home theatre applications utilises five speakers - configured as left front centre, right front and left and right surround. (However, the left and right surround speakers an both fed from the same mono surround channel.) The steering logic of the system was designed specifically for cinematic applications, where the main criteria was keeping dialogue focused at the picture screen (i.e., in the centre channel) and out of the left right and surround channels.

Typical Surround Operation

Surround Channel

The rear channel of the ProLogic decoding system is derived by subtracting L-R and feeding this mono difference signal to each of the rear surround speakers. A limited 100Hz-7kHz bandwidth and a simplified implementation of Dolby B noise reduction is applied to reduce any perception of sibilance splattering in the surround channel. This is primarily due to the inherent inefficiencies of the medium used for cinematic reproduction (i.e. optical soundtrack on 35mm film). Even today, high fidelity VHS tape is capable of better balance and frequency response than can be provided from an optical soundtrack on motion picture film.

In addition, the limited bandwidth is applied to the rear channels due to the fact that the surround speakers often used in the typical home theatre system are very small, and not capable of reproducing any bass information below 100Hz. Delay is also added to the rear channels to contribute to the Haas effect (also commonly referred to as the "precedence" effect). This slight time delay applied to the rear channels ensures that any leakage of dialogue that may unintentionally emanate from the rear channels will still be perceived by the listener to originate from the front channels. The actual delay time applied for this purpose is based on the distance between the front and rear speaker locations, and is typically calculated at 1 millisecond per foot.

Front Channels

The centre channel of the ProLogic decoder is derived by adding L+R information (i.e. all elements of the left and right channels are fed to the centre channel). The left channel is the pure left signal from the left stereo input, while the right channel provides the pure right signal fed from the right stereo input. However, the operation of the ProLogic decoder is such that, at any given point in time, one channel is considered to be the "dominant" channel. To accomplish this, the system monitors the level between front and back, and also between left and right When a defined threshold has been exceeded anti-phase information is added to both the left and right channels from their respective opposite channel to cancel out any centre channel (L+R) information that is present in the front left and right channels. This acts to increase the separation between the Ieft centre and right channels.

While this technique succeeds in removing the centre channel information from the front left and right channels, it also removes the bass components. Therefore, a 6dB/octave low pass network is employed which adds low band information back into the left and right front channels.

Conversely, the centre channel is also configured such that hard left or high right detected input will result in an attenuation of the centre channel.

Implementing a Surround Format for Music

With the added dimension and notable improvements that the surround environment provides the film-viewing community, it would seem only natural to apply such principles to music applications to greatly enhance the experience of listening to music.

However, a number of significant drawbacks become apparent when attempting to utilise a Dolby-style surround decoding system for exclusive music applications. One such drawback is that the rear channel of the Dolby-style surround decoders is mono and as a result, the rear audio lacks any of the directional realism of a common stereo recording. Automotive sound systems incorporating four-speaker stereo have provided stereo operation in the rear channels for many years. Therefore, attempting to implement a system providing mono rear channel operation, such as Dolby, would be less than desirable. Add to this that the rear channel of such systems also consists of primarily ambient information - thereby not providing the required bass response through the rear speakers. Based on the fact that automotive sound systems derive the bulk of the system's bass through the rear speakers, any attempt to adapt an audio surround system to automotive applications requires that the bass be amplified through the rear channels. Thus, it therefore becomes obvious that such a system could not be applied to automotive applications.

In addition to the shortcomings caused by the operation of the rear channel, additional drawbacks are present due to the operating nature of the front channels. A pronounced monophonic emphasis is produced across the front three channels when music is played exclusively through a common decoder designed for cinematic applications. Though this effect is not apparent when monitoring exclusive dialogue (as would be found in a feature film), it is unacceptable for the stereo imaging required for music related applications.

The operation of the Dolby-style adaptive matrix dictates that the system produce a slight cancellation of signals in the left and right channels when input signals are not steered hard left or hard right. This condition is always present unless a hard left or hard right input is detected. Therefore, the system is most often steered between a centre-steered signal and the pure left and right that is input to the system - thus clouding the stereo imaging of the front three channels and producing a decidedly more mono sounding front soundfield.

In addition, the detection of strong centre channel input results in the left and right channels suddenly converting to difference signals - thereby producing undesirable image wandering effects across the front three channels, as well as a mono left and right signal. This audible side effect is very objectionable when listening to high fidelity music.

These numerous drawbacks clearly illuminate the fact that a different type of surround system is required for music applications than has been commonly known and used for cinematic applications.

More Information On RSP Circle Surround

• Rear Channel Operation - Music Mode
• Front Channel Operation - Music Mode
• Circle Surround - Video Mode
• Circle Surround Encoding Notes
• In Conclusion

The Advantages of Circle Surround

Other surround system developers have come up with different designs to essentially duplicate the end result of the Dolby system, striving for better performance, better speed and channel separation. Circle Surround however, was developed under a different approach than any of these other systems. It was developed to provide a multidimensional surround sound system specifically for high fidelity audio applications. It was critical that the Circle Surround system operate effectively with both encoded and non-encoded material. The development of Circle Surround's "Cinem" mode for use with video applications followed only after the system was perfected for use in studio-related applications.

All of the inherent disadvantages of implementing a Dolby-style surround system for exclusively audio-based applications were addressed in the development of the Circle Surround Music mode.

Rear Channel Operation - Music Mode

Unlike Dolby decoding systems, the rear channels of the Circle Surround system provide full bandwidth, stereo operation. The 100Hz-7kHz bandwidth limitation is not applied to the rear channels, as Circle Surround is most commonly used in music and home theatre environments - where the inefficient optical soundtrack transmission medium is not used. The time delay applied to the surround channels of typical surround systems can also act to "smear" sounds between the front and rear channels. It is for this reason that a time delay is not applied to the rear channels in the Music mode of Circle Surround. The only condition in which it would be desirable to apply a time delay to the rear channels in a music application would be when a music system is installed in a very large venue, where the distance between front to rear speaker locations would require a time delay to compensate for the time required for the sound to arrive to the listener from the front vs. the rear.

When developing Circle Surround, it was critical that the system provide rear channel directional steering without the necessity of adding any artificial information (such as delays, reverb, phase correction or harmonics regeneration) that was not already present in the original source material. Most manufacturers of home theatre equipment have recognised the need for a music reproduction mode. Generally, artificial reverberation and/or delays are applied to the rear channels to simulate the effects of rear reflections in a performance hal.l At best, such systems only add an artificial element to the music which is simply not present in the recording - and not intended by the music's producers.

Other systems have attempted to provide directional rear channel steering capabilities, but have done so using broadband steering designs. Under many conditions, broadband steering is objectionable due to the unnatural pumping effects inherently produced from such steering schemes. To help compensate for this, these systems typically limit steering to something on the order of 10- I5dB.

Other methods have also been applied to enhance the performance of the rear channels of surround systems. Lucasfilm's THX system, for example, applies "decorrelation" technique to the single monophonic surround channel. This is accomplished by splitting the mono surround channel into two signals and applying pitch shifting techniques to at least one of the channel channels. Methods such as this produce unacceptable results for music applications, as they merely detune information in the surround channels and still do not place instruments in the left or right rear channel based on their location in the original panoramic soundfield.

Therefore, it was critical as well that the system provide rear directional steering without encountering the objectionable pumping effects perceived with a single band system. This is accomplished by initially deriving a composite rear signal by subtracting L-R. This L-R signal is divided into at least two bands - mid and high. The crossover for the rear channels of a two band implementation of Circle Surround is a Linkwitz-Riley design with 24dB/ octave response and a crossover frequency of 2kHz. This ensures both good separation between the bands as well as correct phase response at the crossover point This also allows the portion of the audio spectrum which contains most of the high frequency transient and directional information to be processed with proper speed and accuracy. The audible side effect of pumping is greatly diminished as a result of steering the highs separate from the mids and lows. With an instantaneous left band signal in the input the high band portion of Circle Surround will steer the rear high band to the left in approximately 5OOus.

It has been documented that the human ear acts as an integrator to signals in the first millisecond, therefore was imperative that the system respond faster than the ear to transients in order to provide the proper definition and transparency. In complex music, the mids do not necessarily follow the high frequency transients - this means that the mids will steer based on the mid band directional bias in the input audio. When a broadband system steers complex audio to the left or right, the opposite channel will produce an absence of audio across the entire spectrum - creating "gated" effect and increasing the perception of pumping. The multiband scheme of Circle Surround eliminates this problem, as a high frequency left or right bias in the input audio will not necessarily provide a mid band bias in the same direction. This means that the mids will properly track the mid band bias and provide a correct rear sound field without the objectionable pumping or "gated" effect.

The typical method of producing the control signals to steer the surround matrix uses a single capacitor charged positive for one directional dominance (such as left) and will alternately charge the capacitor negative for the opposite directional dominance (i.e. right). The Dolby matrix operates this way, using two RC networks charging both positive and negative - with one network providing a fast time constant, and the other providing a slow (or long) time constant to help avoid the side effects of pumping. Threshold detectors determine when there is a dominance signal present and will switch in the fast time constant so as to improve steering speed This signal is fed to a polarity splitter which will produce a left output when the voltage is positive on the capacitor, and will produce a right output when the voltage is negative on the capacitor. Thus, the polarity splitter basically functions as a half-wave rectifier to produce the left and right control voltage from the single time constant.

Another high end decoder design which utilises broadband rear steering implements what is referred to as the "Servo Logic" system. In this design, the broadband left/rear steering also uses a single capacitor charged positive for one directional dominance and negative for the opposite directional dominance. The design switches an analogue switch on and off to short out the resistor in the RC time constant circuit. The analogue switch is controlled by a PWM (Pulse Width Modulation) circuit which is modulated based on the presence of a directional dominance signal. (The idea is to increase the charging time constant so as to provide a faster response time, which is claimed to be 3.5 milliseconds.)

There is an inherent flaw in these designs which greatly reduce their ability to steer fast enough to satisfy the requirements for music. If, for example, a left dominance signal is detected - causing the timing capacitor to charge positive corresponding to a 20dB increase in the left input - and this was instantaneously followed by a right dominance signal, the design flaw becomes apparent. At this instant, the system not only has to charge the capacitor negative corresponding to the right dominance signal, but it also has to overcome the positive charge associated with the original left dominance signal.

Thus, it becomes obvious that the actual time period may be several time constants (upwards of 20 milliseconds) before the system can actually provide the proper directional control for the matrix. By this time, the initial transient and directional information is either incorrectly decoded or has been smeared across the channels. This condition is not nearly as common in cinematic productions as it is in music applications. Because Circle Surround was designed for music applications first and cinema second, it was an initial design requirement to eliminate this problem. This condition is resolved by individually detecting left and right dominance in multiple bands and producing separate time constants for the left and right dominance signals.

In a simplified version of the Circle Surround Time Constant Generator, where the output of the difference amplifier is split and the lower path is inverted, the polarity is divided at the output of the difference amplifier- as opposed to the method when the polarity is divided after the time constant has ban generated. Thus, the capacitor in the upper (non-inverted) path will charge only when a left dominance signal is detected at the input Conversely, the capacitor located in the lower (inverted) path will charge only when a right dominance signal is detected at the input. Utilising this method, the left time constant is generated without the right directional dominance having any impact on the attack time of the left. Conversely, the right channel control is generated without the left directional dominance affecting its attack time.

Unlike the Dolby design, where the capacitor is charged through a resistor - slowing the attack time based on the value of the resistor, the Circle Surround method employs a peak detector directly driving the capacitor. Utilising this method, there is no resistance in series with the capacitor to slow the attack time, and the charging current is limited only by the current that the driving amplifiers can source. This means that the left and right steering can be accomplished in the required time period less than 1 millisecond to provide the accuracy and detail required for music applications.

It should also be noted that, by providing a system which processes limited portions of the audio spectrum separately, intermodulation distortion that would be associated with steering a broadband system is also avoided. The absence of predominant signal energy in the left or right input for a specific band will result in the rear channel remaining mono in that band.

Virtually all other surround processors generate steering control signals by monitoring the levels between left/right and between front/rear. These control signals are then used to control the entire matrix for all four channels (left, right, front and rear)- even in a split surround system utilising broadband staring. However, the Circle Surround system incorporates a circuit dedicated to generating the control voltages for the rear steering independent of the front channels for improved performance. The method of deriving directional information utilised by the Circle Surround system also allows for localising simultaneous images in the rear speakers, such that predominant right mid band information will cause the mid band to steer to the right while the high band can be simultaneously steered to the left. The advantages derived due to the enhanced operation of the multiband scheme provided by Circle Surround provides the perception that there are two virtually discrete channels in the rear.

The rear steering vectors are different depending on the current operating mode selected for the system. In the Music mode, for example, a signal panned hard left will emanate from both left front and left rear speakers, with attenuation of the signal in that specific band into the opposite rear channel. Additionally, the left rear channel will provide a 3dB increase in output level when the signal is panned hard left.

This increase in output level allows a producer encoding a program the ability to position a sound so that it is perceived to emanate from the left rear when material is panned hard left. The signal can be panned back towards centre to reduce the gain in the left rear channel such that the level in the left rear is equal to that in the left front - thus producing "side" image. Panning the signal slightly further toward centre will reduce the left rear channel level to provide a signal that emanates predominantly from the left front speaker location, while panning the signal "dead centre" will result in the signal emanating equally from the left front centre and right front outputs. These characteristics are equally true when panning a signal from a hard right position to centre, and work to increase the producers capability to localise specific sounds wherever they are desired to be placed in the 360 degree listening radius when encoding material intended for surround playback.

Image positioning can tend to collapse to a small degree when placing simultaneous signals in the encode process. However, much of it is retained due to the multiband steering scheme in the rear channels - unlike a typical surround system which requires a single dominant channel at any given point in time.

As previously described, the rear channels in the Circle Surround Decoder are derived by subtracting L-R. This signal is fed to the left output as a L-R signal. However, the right channel is phase inverted so that it then becomes an R-L signal. This is done to preserve the phase relationship between the rear channels and the front channels, and ensures the possibility for side-imaging This creates an out-of-phase condition in the rear channels, which will tend to reduce the possibilities for "dead centre" rear imaging. While this condition does exist it can be nullified with the insertion of allpass networks in the left front, right front and surround channels to produce an output that is never 180" out of phase with any other output. In many systems, this is omitted and is not a specific requirement as listening tests have proven that this is not a critical aspect to the proper localisation of the surround soundfield.

All Circle Surround systems currently available provide a three band system in the rear, with at least two bands (mid and high) steered. Future systems may provide greater rear channel resolution (i.e., a larger number of bands that are steered in the rear channels) to further enhance the performance of Circle Surround.

Unlike the Dolby system, which allows for the use of limited bandwidth speakers in the rear, it is recommended that speakers of the same type and power rating be used for all channels, with equal power applied to all channels. It is also recommended that all speakers be located at an equal distance from the listener.

This method for deriving rear channel steering is part of the basis of US. Patents #5,3 19,713 and #5,333,20 1.

Front Channel Operation - Music Mode

Due to the de-emphasis of the stereo image and the image wandering effects produced by the steering scheme of the common decoders, it was imperative that the left and right channels of the Circle Surround system remain unaltered. Like the other systems, the centre channel signal consists of L+R information. However, Circle Surround incorporates a dynamic centre channel - where strong, predominant centre channel information results in the centre channel level increasing to unity gain. If a strong centre signal is not detected, the centre channel level is reduced by as much as 10dB to avoid collapsing the stereo imaging of the left and right front channels. Input signals panned hard to the left or right will cause the centre channel to steer down completely to eliminate any collapse toward centre of signals panned hard left or right. When used in a four speaker configuration without a dedicated centre channel, centre channel information is divided equally between the left and right front channels. However, the centre channel still operates dynamically in such a configuration. (Automotive applications would require a configuration such as this.)

A signal panned hard to surround will result in the attenuation of the left and right front channels to provide a dominant signal in the surround channels. This provides the producer additional potential directional impact (for effect only) far signals panned hard to the surround position in the absence of any other audio. Antiphase information in the left and right channels does not appear in the centre channel therefore centre channel steering is not required. When used with non-encoded material, this condition can never exist - therefore the decoder will not provide any dedicated directional surround information as a stand-alone signal. No objectionable impact will be apparent due to these steering characteristics, as this steering condition will only occur under a hard surround pan - which can only be achieved when intentionally encoding material to take advantage of this particular feature.

The incorporation of the dynamic centre channel, coupled with the pure, unaltered left and right channels, results in a very stable front sound stage where the stereo imaging is not adversely affected - even in the presence of diffuse, non-correlated audio. Thus , all the benefits of having a centre speaker are gained without destroying the normal stereo image.

Circle Surround - Video Mode

Although Circle Surround was initially developed as a surround system for music applications, it also provides a Video mode for Cinematic use. The Circle Surround Video mode also provides additional improvements over the standard surround systems.

An 18dB per octave low pass network is applied to the front channels to maintain a stable low band when steering is taking place. Though other systems, such as Dolby, typically utilise a 6dB per octave low pass network for this purpose, an 18dB per octave network is implemented to attenuate mid band dialogue information to a greater level in the left and right front channels.

As previously mentioned, the Dolby system has been designed to provide a single dominant channel at any point in time. As a result, the front soundfield tends to collapse towards centre instead of maintaining a wide stereo front image. Coupled with the fact that the rear channels are mono, this produces a very one-dimensional soundfield which goes almost directly from front to rear. As previously stated, this is due the fact that the Dolby matrix produces a slight cancellation of the signals in the left and right channels when it is not steered hard left or hard right.

Therefore even in cinema applications where program material contains stereo background music information, the system will collapse toward centre and produce a notably narrower soundfield than would be derived with a normal stereo signal. Circle Surround has been designed to avoid this drawback and provide full high fidelity left and right stereo information under conditions where a dominant centre signal, or a dominant left or right signal, is not present.

The centre channel operates dynamically, and is described in the Music mode, so as to avoid collapsing any stereo imaging that may be present toward the centre channel. The centre channel level rises to unity gain only under hard centre conditions, and attenuates under conditions involving stereo music in the background of a cinematic production. This works to maintain a wide left/right soundfield in the front channels.

The Video mode provides selectable rear mode operation. In the Video mode, the system can provide the wide surround soundfield as described for the Music mode or, if desired, it can provide response characteristics typical of a Dolby-style surround decoder - where material panned hard left or right will result in the attenuation of the surround soundfield.

Other surround encoded material (including Dolby Surround encoded material) can be decoded with great compatibility and increased performance results using a Circle Surround decoding system. The Circle Surround decoder does not apply any bandwidth limitations to the rear channels, or the modified Dolby B noise reduction. The system is also compatible with any of the 2-channel enhancement formats, such as Q-Sound or the Roland RSS system.

Circle Surround Encoding Notes

The Circle Surround Encoder only requires Left Right Centre and Surround inputs. This is due to the fact that hard left or right pans can be used in music productions to place sounds behind the listener, and less drastic panning can be used to provide side sound images.

Unlike the Dolby encode format, the limited bandwidth restriction applied to the surround channels and the noise reduction processing are not required, and are therefore not included in the Circle Surround encode format.

It should be noted that Circle Surround encoded material can be decoded by any of the matrix surround decoding systems available. However, the performance will be reduced due the limitations imposed by the particular decoding system used.

In Conclusion

Circle Surround is the pinnacle of evolution in surround matrix technology, offering tremendous benefits to the high-end audio community. In addition, Circle Surround also furthers the performance previously available from surround matrix decoders for video applications.

The Circle Surround system was co-developed by Derek Bowers, who also devised the original concepts of the system, and worked through the development of the system to its current embodiment.

The Circle Surround system is currently covered by US. Patents #5,319,713 and #5,333,201, with other patents pending and foreign patents pending.

This text includes some material taken from the Rocktron Corporation/RSP Technologies white paper, "The Circle Surround Audio Surround System" by James K. Waller, Jr.