Dolby Pro Logic

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Dolby Pro Logic is a analog home theater - multichannel audio company Dolby . Dolby Pro Logic decodes surround sound information from a stereo source .

history

Dolby Pro Logic is based on Dolby Stereo, developed in 1976 . Dolby Pro Logic itself was developed in 1986 . From today's point of view, Pro Logic has a comparatively lower degree of selectivity; only a small bandwidth was available between the individual channels and for the return channels.

In 2000 , a further development was presented with Dolby Pro Logic II . It expanded Pro Logic with an additional return channel and a subwoofer channel , a so-called 5.1 system. Pro Logic II is still used today, e.g. B. when decoding a 5.1 sound from a stereo source. Examples of this are analog television and CDs, but also game consoles such as. B. Wii from Nintendo . For cost reasons, separate channels were not used here.

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

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

functionality

Coding

Dolby prologic encoder

The four signals left, right, center and surround sound (L, R, C, S) are combined into two program channels, which are called Left total (Lt) and Right total (Rt).

The two input signals on the left and right are routed directly to the program channels on the left and right without any change and the center channel is evenly added to the two channels with a level reduction of 3 dB .

The signal of the surround sound channel first passes through a band pass from 100 to 7000 Hz and a Dolby B compressor modified to −6 dB in order to minimize crosstalk of the high frequencies (hissing effects) from the front channels into the rear channel during decoding. Just like the center channel, the signal is then lowered by 3 dB and, shifted in phase by -90 ° , is added to the left program channel. The right program channel receives the same signal, but shifted by + 90 °. This results in an anti-phase signal with a phase shift of 180 ° for the surround sound channel in the two program channels.

Decoding

Passive-dolby-prologic-decoder

The simplest decoder for recovering the surround sound signal consists of a subtracter . Since the center signal is in phase on both stereo channels, it simply falls out when you subtract the stereo channels from one another. The channels are therefore very well decoupled from one another.

It becomes problematic with asymmetrical sound events on the stereo channels, since these again result in a difference signal at the subtracter.

In order to reduce these effects, there are complex control circuits that constantly adjust level and phase in order to achieve the highest possible decoupling of the channels. These systems were implemented in Pro Logic II .

literature

  • Roland Enders: The home recording manual. 3rd edition, Carstensen Verlag, Munich, 2003, ISBN 3-910098-25-8
  • Thomas Görne: Sound engineering. 1st edition, Carl Hanser Verlag, Leipzig, 2006, ISBN 3-446-40198-9