Taking the front speakers from the middle of the screen to a left and right stereo pair created an unfortunate problem. The dialogue that used to be clearly locked with the picture in the middle of the screen had been split in two with bits of dialogue coming from both sides. This proved extremely distracting to the audience, as only the people sitting in the very centre of the cinema could get some form of stereo imaging and have the dialogue in the middle of the picture, where it was supposed to come from. People sitting on either side of the cinema, got most of their sound from either left or right speaker, which was very distracting as the on-screen movement and sound were separated from each other. This led to the development of an additional, centre channel loudspeaker in the middle (as the name suggests) behind the screen to lock the dialogue to the centre of the image on screen.
This was a vast improvement to Dolby Surround and was called Dolby Pro Logic, which is a four-channel system, which consists of Left, Centre, Right and Surround with the centre and surround signals processed out of the left and right stereo signals. As the processing was done with basic decoders, the surround effect was effective, but quite subtle and was limited to four channels. The back two channels were mono (they played the same information) and the centre as well as the back speakers were limited in dynamic range and frequency response.
Digital technology and new surround sound formats.
As time progressed and audio entered the digital era, it quickly became evident that since sound was now being stored and transmitted digitally, sound could now be processed digitally as well, resulting in a huge step forward for surround sound and effects.
The main difference between the previous analogue surround formats and new digital surround formats was the fact that with analogue systems, the maximum number of recordable channels were two. Any desired additional channels of information (centre and surrounds) had to be imbedded within the existing left and right channels and be extracted with a basic decoder.