|
Dolby Pro Logic is a surround sound processing technology developed by Dolby Laboratories, designed to decode soundtracks encoded with Dolby Surround. Dolby Stereo was originally developed by Dolby in 1976 for analog cinema sound systems. The format was adapted for home use in 1982 as Dolby Surround when HiFi capable consumer VCRs were introduced; it was then replaced by the newer and improved Pro-Logic system in 1987. However, the term "Dolby Surround" is still used to describe the encoding technology or matrix-encoded soundtrack, whereas Pro Logic refers to the decoding technology and processor. It is the domestic equivalent of the theatrical Dolby Stereo technology used in movie cinemas in the 1970s and '80s. The two technologies are mostly identical but a change in marketing was needed so as not to confuse cinema stereo which is at least four channels of audio with home stereo which is only two. Thus Dolby Pro Logic is the consumer version of theatrical Dolby Stereo. Dolby Surround/Pro Logic is based on basic matrix technology. When a Dolby Surround soundtrack is created, four channels of sound are matrix-encoded into an ordinary stereo (two channel) sound track. The centre channel is encoded by placing it equally in the left and right channels; the rear channel is encoded using phase shift techniques, typically an out of phase stereo mixdown. A Pro Logic decoder/processor "unfolds" the sound into the original 4.0 surround—left and right, center, and a single limited frequency-range (7 kHz low-pass filtered〔(【引用サイトリンク】title=Dolby Surround Pro Logic II Decoder Principles of Operation )〕) mono rear channel—while systems lacking the decoder play back the audio as standard stereo. A Pro-Logic decoder also uses 'Steering Logic', which drives amplifiers, to raise or lower the output volume of each channel based on the current dominant sound direction. For example, while a mono signal is played, the strong correlation to the center channel triggers the output volume of the left, right and surround channels to be lowered. This increases the channel separation achievable, to around 30 decibels between channels. By careful tuning of the response of the amplifiers, the total amount of signal energy remains constant and is unaffected by the operation of the channel steering. Additionally the response time of the system to changes in sound direction is important as too fast a response results in a twitchy feel, while too slow a response leaves sounds coming from an inappropriate direction.〔(【引用サイトリンク】title=Dolby Surround Pro Logic Decoder Principles of Operations )〕 In addition to 5db of noise processing, the surround channel is slightly delayed, so that any front channel sounds that leak into the surround channel arrive at the listener after the front channels, providing an illusion of greater separation. Although Dolby Surround was introduced as an analog format, all Dolby Digital decoders incorporate a digitally implemented Dolby Surround Pro Logic decoder for digital stereo signals that carry matrix-encoded Dolby Surround. One of the first was the MSP400 surround sound receiver and amplifier by RCA for their high-end Dimensia brand. It was released in 1987 for the Digital Command Component System. ==Dolby Pro Logic II== In 2000, Dolby introduced ''Dolby Pro Logic II'' (DPL II), an improved implementation of Dolby Pro Logic created by Jim Fosgate. DPL II processes any high quality stereo signal source into five separate full frequency channels (right front, center, left front, right rear and left rear). Dolby Pro Logic II also decodes 5 channels from stereo signals encoded in traditional four-channel Dolby Surround. DPL II implements greatly enhanced steering compared to DPL, and as a result, offers an exceptionally stable sound field that simulates 5 channel surround sound. Because of the limited nature of the original DPL, many consumer electronics manufacturers introduced their own processing circuitry, such as the "Jazz", "Hall", and "Stadium" modes found on most common home audio receivers. DPL II forgoes this type of processing and replaces it with simple servo (negative feedback) circuits used to derive five channels. The extra channel content is extracted using the difference between the spatial audio content between two individual channels of stereo tracks or Dolby Digital encoded 5.1 channel tracks and outputs it appropriately. In addition to five full range playback channels, Pro Logic II introduced a Music mode which includes optimized channel delays, and adds user controls to—for example—adjust apparent front sound stage width. Pro Logic II system also features a mode designed specifically for video gaming, and was frequently used in game titles for Sony's PlayStation 2, Nintendo's GameCube and Wii as an alternative to digital surround formats such as Dolby Digital, or DTS. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dolby Pro Logic」の詳細全文を読む スポンサード リンク
|