|For More Information:
Circle Surround is an improved surround sound decoder that offers 5.1 channel audio performance that nearly matches that of digital AC-3 decoders, but can work with any analog stereo source. Unlike the passive matrix surround decoders that are a standard feature in most AV amplifiers, Circle Surround can produce true stereo in the rear channels with 30dB separation. In addition, the surround channels are capable of full range playback. In some cases, DVD movies decoded by Circle Surround result in better audio quality than that of digital decoders. Besides TV and home theater AV applications, a Music Mode option is included that produces an even distribution of the audio to all channels, ideally suited to both 5.1 channel audiophile listening and 4.1 channel car stereo systems with Phantom Center channel. The best results are achieved with a Circle Surround Encoded source, however 5.1 channels of surround information can be derived from all passive matrix encoded material, as !
Background - Disadvantages of previous decoders:
Dolby Pro Logic(R) was specifically designed for movies that contain a matrix-encoded mix of Lt/Rt stereo. The "Left-total" and "Right-total" signal includes pure Left, pure Right, Left-plus-Right, and Left-minus-Right components. Left and Right decode to the L and R channels. The L+R contains audio taken from L and R that is common to both channels, and is decoded to the center channel. The L-R is a difference signal that is directed to the surround channels by the decoder. All of these components co-exist in a stereo pair. It was designed such that the original stereo signal remains unchanged when played using a stereo amplifier, without a surround decoder.
Audio that has been matrix encoded, as found on most Hi-Fi stereo VHS tapes, nearly all laser discs (LD), most VCD's from Asia, and a few CD's that bear the Dolby Surround logo, are ensured of having adequate surround. However, it is hard to find music CD's and other stereo sources that have surround information that decodes just as well.
The deficiency of most matrix decoders is evident when it comes to playing music CD's. It is normal for the majority of the audio in a movie to be decoded into the center channel because so much of the sound is dialog. Most of the action on the screen normally is placed in the center of the room. The Left, Right, and Surround, are incidental to the main action, except where it moves off-screen momentarily to the extreme side or rear. Therefore the realization that most of the audio is directed to the center channel goes unnoticed.
The Dolby Surround and Pro Logic systems were designed to operate with a single dominant channel at any point in time. With music, the center channel dominates and causes most of the soundstage to collapse into a mono signal coming from the center, with very little remaining in the Left and Right channels. This is because the surround decoding algorithm used by most stereo audio equipment demands that the Center channel be developed by not only steering the L+R signal to the Center speaker, but removing it from the Left and Right speakers with a high degree of attenuation. In a music CD, most of the instruments and vocals, while spread across the soundstage, are located in a central image, from half left to half right, that is decoded to the center channel. Little remains in the Left and Right speakers, resulting in a primarily mono output. To exacerbate the problem, the mono center image in the rear further tends to accentuate the monaural audio along an axis in the middle of! the speakers. To compensate, delay is added to the surround to de-correlate it from the front. When listening to music, delay only smears the audio between front and rear creating an artificial effect, certainly not what the recording artists had intended.
While digital encoding on DVD's has solved most of these problems, digitizing is not without some tradeoffs. The audio on a DVD is highly compressed, by a factor of 10 to 12. For five full bandwidth, uncompressed channels of surround audio at 44.1 kHz sample rate, nearly 5Mb/sec data rate is required, more than three times that on normal CD's. This would consume almost a quarter of the available data on a DVD, which needs about 20Mb/sec for acceptable video quality, which is also compressed. Therefore, all 5.1 channels of audio are combined and compressed into a single 384kb/sec or 448kb/sec serial data stream to make room for video. This audio compression, whether using AC-3, MPEG, or DTS, is not loss-less. The psychoacoustic algorithm employed has minimal degradation in most cases. However, when there is high amplitude audio on all channels simultaneously, not an unusual situation in both movies and music recordings, the compressed data must divide the bit pool evenly betwee! n all five channels, to 76.8kb/sec from 384kb/sec. When this happens, the audio quality is reduced much below that of 16-bit CD's, with audible artifacts. With five discrete channels of digital audio, the decoder has little discretion on the way it is decoded. Depending on the way it was mixed before encoding, the audio separation on a DVD may be too severe, with abrupt, unnatural transition across the room from channel to channel.
of Circle Surround
Another feature of Circle Surround is improved separation between channels from Dolby Surround encoded sources and other normal stereo signals. The stereo separation between the surround channels is about 30dB compared to zero in the mono surround of other adaptive matrix decoders. It is possible to encode an audio source with a Circle Surround encoder such that the separation increases significantly higher. Playing a noise sequencer test track that has been encoded with Circle Surround allows each speaker, including each surround channel, to output a single, separate audio track in succession, one at a time (Ls, L, C, R, Rs, Sw). Again, this is possible from a stereo, analog source. The only limitation is that Circle Surround is unable to decode five independent channels of audio simultaneously, which is possible with AC-3 decoders. It is rare to find this kind of recording except on special test tracks specifically designed to demonstrate this point.
frequencies are steered separately with longer, appropriate time constants.
It is not necessary that the lower band be steered to the same side. It
is possible in some situations that the high frequency may go to the Left
Surround, for example, while the low frequency surround is steered to
the opposite side in the Right Surround channel.
Further improvement is obtained when the lower frequency steering is limited to a band-pass mid-frequency filter, with a third band for the bass frequencies. Since the lowest frequency range contains little directional contribution to the audio, this band can be fixed, without steering, for a mono signal equally to both surround channels, as in conventional systems, with savings in processing complexity.
The benefit of multiband steering is a more realistic surround sound without the objectionable artifacts of conventional surround steering. The improvement is most noticeable with high speed transients in the higher frequencies, which is very common in musical recordings.
Variable Gain Steering Multiband steering solves only half of the deficiency of transient response in previous decoders with full bandwidth steering. Depending on the circuit design, the time constant chosen has a fast capture speed, but the release time may be many time constants longer, up to 20ms, before proper directional control is established. DSP implementations are more precise with symmetrical time constants, but at higher cost.
Circle Surround uses variable multipliers in the multiband steering circuit that provide variable gain to the VCA:s in the surround channels. The gain increases exponentially with dominant surround information. The result is smoothly panned audio between left and right surround or between front and rear. A test that will demonstrate the superiority of Circle Surround is the audio that steers hard left, for example, then instantaneously steers hard right.
Music Mode Unlike a decoder system that relies on steering to a dominant channel, Circle Surround is designed to work in Music Mode, which alters the steering in the front channels. In the Dolby-style system, only a slight imbalance, 1-3dB in the ratio of Left-Right and Front-Surround causes the audio to change from Center Channel to Left or Right. Center channel audio is not only steered to center, but removed from Left and Right, producing a wandering image.
The Left and Right Channels of the Circle Surround decoder in Music Mode remain unaltered. The Music Mode also incorporates a dynamic center channel, which decreased the center channel level by as much as 1 OdB in the absence of strong center channel information derived from the L+R inputs. The Center attenuation may be continuously reduced further only under hard steering situations. This prevents the collapse toward the center when the signal is panned across the front, producing a very stable stereo image, even with diffuse, non-correlated audio, as in a live recording. All the benefits of a center channel are preserved without destroying the normal stereo soundstage. The dynamic center operates equally well in 4-channel systems with an equally divided phantom center in the front speakers. This makes Circle Surround ideal for car stereos. While surround delay may be used in Movie Mode, or to achieve soundfield effects such as Hall or Stadium modes, in Music Mode, delay is not applied to the surround channels. Use of time delay only acts to smear the sounds between front and rear, detrimental to music listening. It was important in the Circle Surround design to avoid any artificial information not in the original recording, such as reverberation, phase correction, pitch changes, or harmonics added.
The applications where Circle Surround is ideally suited are home theater A/V amplifiers and car stereos. Used as an additional surround mode in a Dolby Surround receiver, audio from any stereo source, including DVD, is enhanced for both movies and music. In the car, all the passengers can hear good stereo surround with equal quality. Circle Surround maintains the characteristics of the original audio while providing a pleasant and effective multichannel surround experience.
Simplified 3-Band Circle Surround Decoder
5dot1.com is sponsored by Kelly Industries