Dolby Pro Logic

**Dolby Pro Logic**, sometimes referred to as **Dolby Surround Pro Logic**, is a sound processing system developed by Dolby Laboratories and was introduced in 1987 as an upgrade for the original Dolby Surround.

Improving on the original decoding technology of Dolby Surround, this was done as a means of adapting for the rapidly evolving home theater market of the mid-1980s. Some of Dolby Surround's limitations were resolved in this new iteration, such as the enabling of decoding the full 4.0 Dolby Stereo soundtrack by including a proper center channel rather than the blended left/right audio channel Dolby Surround employed for its center channel implementation. Another improvement of Dolby Pro Logic includes the decoding process, which is considerably more involved as it now makes use of an *active* decoder rather than Dolby Surround's *passive* decoding.

This system was supplanted by the **Dolby Pro Logic II** family in 2000, which incorporated a fifth channel for more directional effects in the rear channels.

## The decoding process

To solve numerous issues relating to passive decoding relating to audio crosstalk (surround audio effects in left/right channels, etc.) along with gaining an extra degree of precision with audio soundtracks, Dolby Pro Logic makes use of a more involved encoding process known as active decoding, making use of Steering Logic to do so via an enhancement circuit combined with another passive decoder. Directional enhancement is much improved here.

To eliminate dialogue leakage, a number of concepts were explored, such as Gain Riding (rejected due to high incidences of decoding issues) and Noise Cancellation (worked much better due to its destructive interference).

In the end, the main system used was the form of *dominant sounds* - when the signal first enters the decoder, dominant sounds (determined by the logarithm of each possible signal and subtracting one from the other) are placed within the main point of the origin of a 360-degree circle, while non-dominant sounds are pushed to the other speakers. Only one dominant sound can be used at a time. The direction and angle of the dominant sound are then determined from an X/Y coordinate plane (where "X" is the left/right channel and "Y" is the center/surround channel) by taking the magnitudes of the signals from each axis and then converting them to rectangular and then polar coordinates along the sound space. Dominance in the channel is then represented as a vector with the angle and magnitude of the sound being represented in the dominant sound's characteristics.

This dominant sound determination (all part of a real-time monitoring of the soundtrack) marks the core tenet of the Pro Logic decoder, the main difference between this system and the older L-R differential system Dolby Surround used. At first, the Lt and Rt inputs go through an input balance control, and then a noise sequencer. Then, both signals go through the adaptive matrix, where a series of bandpass filters, amplifiers, conversions to waves, threshold switches, dual-time constances, polarity switchers, and calculations are all determined to find what the dominant and non-dominant sounds in the Pro Logic soundtrack are. Once all of the channels are defined in the adaptive matrix after combining the networks of sound, it then goes through a very similar process to Dolby Surround's decoding process, where the center and surround channels are defined, and then finally output to a speaker system. Steering Logic is used to drive the amplification of each channel by dynamically changing the audio volume with careful and constant signal energy.

While a theoretical separation map of 37 dB can be seen of an infinite distance of space, in reality typical seperation maps usually averaged around 30db of space in the ordinal directions, with a 30 dB distance between the speakers. Regardless, it marked a big leap forward from the original Dolby Surround in terms of audio distance and clarity with the usage of passive decoding.

## Trivia

* Pro Logic decoding was not normally mentioned in packaging of various home media releases or games (the Dolby Surround label was often used instead), but was fully backwards compatible with the older system - allowing for more clear decoding of older releases.
* Due to the success of Pro Logic, Dolby Surround was retroactively added to the Dolby Pro Logic family.