Dolby Pro Logic

Dolby Pro Logic is an analog home theater multichannel sound system of Dolby. Dolby Pro Logic decodes surround-sound information from a stereo source.

History

Dolby Pro Logic is based on the 1976 developed Dolby Stereo. Dolby Pro Logic itself was developed in 1986. Pro Logic has currently possesses a comparatively lower selectivity between the individual channels and for the rear channels was only a small bandwidth.

In 2000 was presented with Dolby Pro Logic II is a further development. It extended Pro Logic by an additional rear channel and a subwoofer channel, called a 5.1 system. Pro Logic II is still used, for example, when decoding a 5.1 sound from a stereo source. Examples include analog TV and CDs, but also game consoles such as Nintendo Wii. Here was omitted for cost reasons on separate channels.

A further development, Dolby Pro Logic IIx, a 6.1 or 7.1 system supply, while the latest version, Dolby Pro Logic IIz can power a 9.1 system with sound information.

Pro Logic II, in turn, was partially replaced by digital multi -channel sound systems such as Dolby Digital and DTS, which have separate channels.

Operation

Coding

The four signals left, right, center, surround (L, R, C, S) are grouped into two program channels as links total ( Lt) and right total ( Rt) are referred to.

The two input signals left and right are left and right through without change directly to the program channels and the center channel adds a level reduction of 3 dB evenly across the two channels.

The signal of the surround channel first passes through a bandpass from 100 to 7000 Hz and a modified to -6 dB Dolby B compressor to minimize crosstalk on the treble frequencies ( hiss - effects) from the front channels to the rear channel when decoding. As well as the center channel, then the signal is lowered by 3 dB and, shifted by -90 degrees in phase, the mixed left channel program. The right program channel receives the same signal, but shifted by 90 °. This results for the surround channel in the two program channels an anti-phase signal with a phase shift of 180 °.

Decoding

The simple decoder for recovering the surround signal is composed of a subtractor. Since the center signal is in phase on both stereo channels, it falls when subtracting the stereo channels each other out easily. The channels are thus decoupled from each other very well.

The problem arises because these arise again at the subtractor, a difference signal in asymmetric sound events on stereo channels.

To reduce these effects, it is usually complicated circuits which level and phase match all the time to achieve a very high isolation between channels. These systems were implemented in Pro Logic II.