Let’s Talk Surround
A Bit of History
With the movie industry continually trying to entice more people into the theaters, you would expect that surround sound would be another weapon in their arsenal and indeed it was. Surround sound in the movies started back in 1941 with the movie Fantasia put out by Walt Disney. Only two movie houses put in the equipment which cost in excess of $80,000, so it was some time before surround sound at the movies became mainstream. This early system required synchronizing two projectors; one for the film and the other for the sound. It was a four channel audio system.
Later a magnetic media based surround system became popular for a while which could deliver up to 6 channels of audio. The problem with these early systems is they needed to be synchronized with the film projector and they were very troublesome to operate. They also applied magnetic strips to the film, but the cost of these films were about 10 times that of optical tracks. Additionally the optical tracks allowed for universality as the films could be played all over the world.
The Big Breakthrough
Then in 1966 Dolby Labs came out with a breakthrough in sound processing, the Dolby A-type noise reduction system. This would finally allow the optical audio on films to carry a stereo sound track. This was huge as it allowed movies to be shown on a single machine with a two channel (stereo) soundtrack. Soon thereafter the Dolby stereo system became a major breakthrough in movies with surround soundtracks. Using a process called Matrixing, Dolby encodes the stereo soundtracks with four channels of information. At first the Dolby Surround was three channels and did not have the center channel. Later the Dolby Pro-Logic include a fourth channel as shown in the following figure.
As shown there are four discrete channels:
5. Optional LFE channel
The left and right channels contain all of the four channels and are decoded by a Pro Logic decoder. When there are sounds which are of equal volume and phase in the two channels the decoder will divert those sounds to the center channel. Primarily this will be voices that are centered in the picture. To create the surround channel the surround sounds are recorded at equal levels, but 180 degrees out of phase with each other, thus allowing the decoder to divert the sounds recorded in such a manner to the surround channel. Also the decoder had to send signals to the appropriate channels at the appropriate levels for when the sound origin was in between the left-center-right. The positional accuracy depended a lot on the quality of the mix on the film as well as the quality of the playback decoder, and while was very good in the theaters, was not so good in some of the early Pro-Logic home systems and the front positioning would actually be better without the center channel on many films. Other issues that limited Pro-Logic are numerous, but the overall effect was a leap and a jump over stereo and could be easily recorded on VHS prerecorded movies.
One such problem that the four channel matrixed system had in addition to the positioning at the front of the movie was the fact that only a single surround channel could be encoded. Therefore it was best used for non-positional sounds like the noise of a crowd, thunder, ocean noise, etc. If, for example, the director wanted to have the sound of a train traverse from one side to the other at the rear of the theater, the Pro-Logic system could not accomplish that task. Another problem is that many home processors will pop and hiss due to a weird combination of frequencies that could confuse the decoder. This would result in a surround pop when an actor said a word that started with a hard P sound.
The LFE channel was simply a crossover of the low frequencies that were in phase and sent to a separate subwoofer amplifier and speaker.
The Digital Era Arrives.
With the advancements made in the digital technologies, the first Dolby digital format (AC-1) arrived in 1984 followed by the AC-2 improvement. Dolby Digital (AC3) arrived in the cinema in 1992 which featured five channels plus a LFE channel:
4. Left Surround
5. Right surround
While Dolby called this AC-3, it is commonly called 5.1 also. The improvement over Pro-Logic is more than just the additional channel. Now with each channel being discrete, the limitations of matrixing are eliminated. Now trains could indeed travel across the rear of the theater, or at least side to side. The surround channels, left and right, are again well suited for sounds that are somewhat non-positional sounds like the noise of a crowd, thunder, ocean noise, etc, but with the two discrete surround channels much better effects can be achieved, such as a bird chirping only in the left surround channel. It is still not intended for rear imaging, which is why they are called surround channels rather than rear channels. In the future the 7.1 systems may add the three rear channels which would be required for rear sound positioning. Still, even with just surround being discrete right and left as opposed to the Pro-Logic, the crowd noises at a sporting event can sound much more realistic with separate left and right surround channels.
6.1 and 7.1 Systems.
The 6.1 system is a product of Digital Theater Systems, Inc. (DTS) who develops surround systems the same as Dolby. With a DTS 6.1 ES (Extended Surround) there is a center rear surround channel added. It can be matrixed, which is decoded from the left and right surround channels or a separate discrete channel which is encoded on the DVD. Even when there is a discrete rear surround channel the same information is matrixed in the left and right surround channels for compatibility with 5.1 systems. Equipment that have a 6.1 decoder will subtract the rear surround information from the left and right surround channels to restore them to discrete channels. The 6.1 systems you can currently buy will develop the center surround channel in much the same way as the Pro-Logic systems developed the front center channel when the discrete rear surround channel is not present. Since the source is digital, the decoder can be much more precise. Since there is naturally occurring matrixed information in Dolby 5.1 encoded DVDs, quite often the decoder will supply information to the rear surround channel, thus improving even 5.1 encoded discs.
Currently there are not any sources for 7.1 systems, although it is expected to be the next generation of theater sound. The 7.1 systems will have left and right surround channels as well as left and right rear channels and the rear center channel. Again the systems available today will just develop these additional channels from the 5.1 channels of the source.