Matrixed Surround Sound Systems for Multichannel Formats Surround sound needs to use encoding systems that will allow storing the multichannel information into systems with fewer channels. The problem is that some of the information can get lost in the process. Hence careful dematrixing is required. Ideally one would like to store all 6 channels of a 5.1 mix discretely, but because data storage space and compatibility with other systems it may be necessary to use stereo media for delivery and reproduction. Dolby Stereo, Dolby Surround and ProLogic Dolby Labs developed several encoding system mainly used in cinema, one of which is know as Dolby stereo. This system can encode four channels of audio into two optically recorded audio tracks on the 35mm film format. The four channels of audio correspond to the 3-1 stereo configuration (LCRS surround)1. This configuration uses a mono center channel and a mono surround channel. Dolby Surround was created as an extension on Dolby Stereo for consumer environments. The decoding process was made the same so the sound could be transferred to television in a similar way than in the movie theater. The backside of Dolby Surround was that it did not use the Dolby-A noise reduction encoder/decoder2. Nonetheless the Dolby Stereo optical sound track for cinema did use it. The matrix of Dolby Stereo encodes the mono surround channel so that is added out of phase by 90˚ to the left and right channels. The center channel is also added to the left and right but in phase. The resulting sum is called Lt/Rt or Left Total / Right Total. During the decoding process it is simply needed to sum the Lt/Rt signals out of phase to extract the stereo difference of the signal and separate the mono surround signal. The center channel can be extracted by summing the Lt/Rt in phase. In Dolby Surround, in addition to the sum and differences encoding, the surround channel is delayed and band limited (100Hz-7kHz). A low pass filter and a delay are used at the output of the subtracted stereo signal to prevent crosstalk between channels since this could result in frontal signal coming out of the surround channel, specially at high frequencies. The delay is usually 20ms to 30ms long in consumer type systems and its use is 1 “3-1 Stereo” was developed for cinema reproduction by adding an “effects” surround channel to the three front channels routed to one or more speakers located behind and to the sides of the viewer. 2 Dolby-A was first developed for mulititrack recording in order to improve the signal to noise ratio. Its success made it one of the most indispensable units even in cinema. Nonetheless Dolby Surround did not use it some recent cinema formats have sue Dolby SR-type noise reduction system.

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based on the precedence effect to cause the listener to localize the first arriving sound as being the source, which will be now emanating from the front speakers. The surround signal also uses an NR type noise reduction to reduce distortion and decoding errors. A defect of Dolby Surround is the little separation existing between adjacent channels. For example, when a signal is fully panned to the left, it will also appear in the center channel but only 3dB down. Dolby ProLogic System overcomes this issue by sensing the location of a dominant signal (i.e. a signal with a higher level) and selectively attenuating the channels that do not contain the dominant signal. This system is very useful to give priority to dialogs for instance; i.e. if a dialogue signal is preponderant in the center channel, the Left, right and surround channels will be attenuated. This system seems to be adequate for film but maybe not be very useful in music application as the image and localization of the instruments would be constantly moving. Circle Surround This encoding formula was developed by RSP technologies as a matrix surround system with stereo surround capabilities. This would make it more appropriate for music since it allows phantom images from the rear speakers. The sum and difference approach is the same than in the Dolby systems but Circle Surround avoids the band limiting and the noise reduction decoding steps for the surround channels. These are steered separately according to a split band technique that would allow for a separation of high and low frequency components independently from each other. The rear channels are decoded using the difference L-R for the left surround and R-L for right surround. Essentially the Lt/Rt process is kept but with a 5-2 encoding instead of a 42 as in Dolby with the ability to then decode those five signals as independent sound sources which can be fed to any predetermined speaker location. SRS, (previously RSP technologies), claims that Circle Surround works as well as the digital AC-decoders but with an analog stereo 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. Circle Surround provides several “modes” that would fit better any given application (film, music, home theater reproduction…). These modes will not be discussed in this paper, nonetheless according to Francis Rumsey, the music mode, which seems to be the most popular, introduces a rise of 3dB gain in the rear channels for any front signal panned full right or left, which creates a rather confusing sensation. No denial of this statement was found but yet SPS claims that in Music Mode there is no delay applied to the surround

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channels since this acts to smear the sounds between front and rear, detrimental to music listening. SPS also states that the left and right channels remain unaltered in Music Mode as this incorporates a dynamic center channel, which decreased the center channel level by as much as 10 dB in the absence of strong center channel information derived from the L+R inputs. 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. Lexicon Logic 7 and Dolby EX Logic 7 appears as alternative to the Dolby Surround decoding system and makes part of a series of steering decoding processes that appropriately distribute the sound energy between a given number of loudspeakers depending on the gain and phase relations of the source material. Logic 7, as its name suggests it, distribute the sound among seven loudspeakers, two of which would be placed to the sides of the listener enabling a “more enveloping image and the possibility of placing rear speakers further to the back”3. These side channels would carry steered information that attempts to ensure that effects panned from front to rear speakers pass through the sides on the way, creating an enveloping spatial effect. Logic 7 is able to decode information encoded with a 3-2, 4-2 or 5-2 matrix. Dolby EX is the result of Dolby and THX joined efforts to create a surround system with an extra channel placed behind the listener. This system is also known as Dolby Digital Surround EX and it uses matrix-style center channel encoding and decoding between the left and right surround channels of a 5.1 mix. This way the center rear channel is driven separately from the L/R surrounds but its information its encoded/decoded in this surround channels using a similar system than at the one used for Dolby Stereo.

Digital Surround Formats New digital formats allow the storage and delivery of multiple channels discretely, which makes them more advantageous compared to the two-channel storage/delivery of the previous systems. However, distributing five channels of audio in full resolution PCM format in a discrete manner, occupies a too large storage space and transmission bandwidth. Therefore digital surrounds formats have come up with encoding algorithms that encode 3

Spatial Audio, Francis Rumsey; Focal Press

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the original information with a lower bit rate and still manage to get very high sound quality. Dolby Digital The advantage of this system is the possibility of delivering 5.1 mixes to either cinema or home environments bypassing the need of an analogue matrix encoder/decoder. The system relies on a low-bit-rate encoding /decoding process that enables multiple channels to be transmitted with no need of separation, summation, steering or differentiation like ine the previous matrix systems like Dolby Stereo. Dolby Digital compression formula is also known as AC-3, which stands for audio coding generation 3. It is widely used in movies as an optically stored digital sound track on the 35mm film4 or for music distribution on DVD Videos. AC-3 relies on a method known as perceptual coding that uses human auditory masking in that it divides the audio spectrum of each channel into narrow frequency bands of different sizes optimized with respect to the frequency selectivity of human hearing. This process reduces the quantization noise that results from the coding process. The data of the 5.1 mix is transformed into the frequency domain and then requantised to a lower wordlenght therefore creating additional quantization noise. By reducing or eliminating coding noise wherever there are no audio signals to mask it, apparently the sound quality of the original signal can be subjectively preserved. Thus AC-3 seems to be a form of very selective noise reduction. In the coding process, channels that require higher data rates than others can trade their bit rate requirements as long as the total bit rate is not larger than the constant bit rate. This result in a single bitstream with all the information of the 5.1 mix ay a regular data rate (320-384Kbits/sec). It is the general opinion that the differences between the original source material and the encoded/decoded are rather small and even become minimal as the bit rate increases. Dolby Digital can operate at sampling rates of 32, 44.1 or 48kHz. Given its limited bandwidth, the LFE is sampled at 240Hz. Besides being surround sound capable, there is a so called Downmix control that allows the decoding of a two track version of the 5.1 mix.

4 The data is stored between the sprocket holes of on the film, so the analogue optical sound can also be retained in its regular position together with the picture. This is done for compatibility purposes.

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DTS Digital Theater Systems, just like Dolby Digital, is a digital encoding system that provides delivering for consumer and/or professional technologies. Surround sound 5.1 mixes can be encoded with DTS format also use a low-bit-rate encoding technique similar to that of Dolby. It appears that among the differences between the two systems, the wider range of bit rates (32kbit/s to 4Mbit/s) provided by DTS with up to eight channels of information sampled at 192kHz would mark probable superiority5. Several options are also included in this format, as is the downmixing and control option also found in Dolby Digital. When pressed onto film format, DTS is encoded on a different area of the film than the analogue matrixed audio, Dolby Digital or SDDS, so multiple formats can coexist within the same film tape. Hence, when one sees on a movie theater the DTS, Dolby Digital and/or Dolby Surround are announced before the film, this means that the theater is able to reproduce any or all of this formats depending on the encoded systems used on the tape. DTS has the advantage of being capable to encode surround mixes on cd formats. With a cd/dvd player DTS decoding capable, which are becoming more and more popular in consumer multireaders (i.e. pioneer), surround recordings can be delivered with advantageous economical costs to consumers. DTS dvd’s are also an option and because the maximum data rate is typically higher than that of Dolby Digital, a reduced amount of low-bit-rate coding artifacts can be achieved, resulting in an apparently better sound quality. Smartcode Pro6 Smartcode Pro is a Protools compatible plugin that offers a very comfortable solution for encoding to DTS or Dolby Digital formats. All discrete channels of surround 5.1 mixes can be encoded to AC-3 or DTS files just by selecting the appropriate audio files and dragging them to the software plugin. The encoded file can then be put on a cd/dvd burning software for reproduction. Smartcode code pro seems to be the best option fro Protools users.

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It is not the intent of this paper to resolve for any particular format but to state what is found as a common agreement on the different lectures. 6 Smartcode pro is made by universal audio: http://www.uaudio.com/products/software/smartcodepro/. This paragraph is ment to give the reader an option to experiment with the results given by the previously discussed digital encoding formats.

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SDDS This format is yet another option to digital encoding systems of multichannel sources. SDDS stands for Sony Dynamic Digital Sound. It uses a proprietary reduction system called ATRAC and employs 7.1 channels instead of the 5.1 surround standard. 7.1 refers to different configuration than that of the one proposed by ITU. Here two extra channels are added to the front array of speakers resulting in a L, CL, C, CR, R plus SL, SR configuration. Hence, SDDS is only use in cinema applications as it can only be found on film prints. Some large ciname auditoriums use the 7.1 configuration to enhance the front imaging accuracy since Left, center and right signals would otherwise be too much apart.

THX THX is not an encoding system, nonetheless it is meant to enhance surround sound reproduction. Its name comes from its developer, Tomlinson Holman Experiment. Even though it was not design to do any encoding, it was developed to be a complement of Dolby Stereo in the improvement of sound quality in movie theaters. Apparently, THX works with the acoustics of the theater and the design of loudspeakers to reduce the noise levels of the cinema and to optimize the acoustic characteristics of it. THX licenses a proprietary form of loudspeakers and crossovers systems to the theaters. It seems that the system has to be installed and periodically tested by the company in order for them to give a license. THX also provides systems for home theaters and capable to work with Dolby Surround, Dolby Digital and DTS. On the Dolby Surround, the mono surround signal is decorrelated from signals sent to both surrounds channels and creating a higher amount of diffusion. Also the system has a different equalization than that of the cinema to compensate for the extra high frequency content used for large cinema theaters. The specifications of this system include amplifiers performance and loudspeakers directivity and positioning. THX also recommend a different speaker set up than regular 5.1 systems where 5 matched speakers plus a subwoofer are common. THX proposes two dipole speakers as the surrounds arranged so that the listener hears mainle reflected sound creating a large enveloppement sensation. THX claims that THX LCR Speaker and THX Dipole Surround

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designs psychoacoustically match the best surround sound performance of a movie theatre or mixing stage7.The subwoofer is also provided by the company.

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http://www.thx.com/mod/techLib/homeTheatreFAQ.html#iveHeard

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