30-07-2012, 11:03 AM
DOLBY SOUND SYSTEM
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INTRODUCTION:
Dolby sound system is an advanced form of digital audio coding that makes it possible to store and transmit high-quality digital sound far more efficiently than was previously possible. First used in 1992, it is the result of decades of experience by Dolby laboratories in developing signal processing systems that exploit the characteristics of human hearing. Going to the movies today is more exciting and involving than ever before, the history of cinema sound over the past two decades closely mirrors the history of Dolby film sound technologies.
Dolby digital is an industry-standard surround audio codec designed to deliver 5.1 channels of audio to many popular forms of entertainment including DVDs, broadcast, satellite TV programming, and even video games. An advanced encoding/decoding technology, Dolby digital reproduces multichannel audio to deliver a cinematic audio experience Dolby digital audio is available from laser discs, DVD-video discs, DVD-ROMs discs for computers, digital cable systems, direct broadcasts (DBS) systems, and digital broadcast TV (DTV).
DOLBY’S METHOD:
DR. Ray Dolby introduced a novel system for providing 10-15 dB improvement in recording and playback tapes. When the strength of signals falls below a pre-determined level (say, 40 dB over the noise novel), the circuit boost the strength before recording. All signals which are 40 dB or higher, pass through the Dolby system direct without any change. The lower level signals pass through the boosting stages which boost these signals by 10-15 dB. Boosting is done before recording.
Dolby – A System:
In this system boosting is done in 4 bands:
1. Below 80 Hz,
2. 80 Hz to 299 Hz,
3. 3000 Hz and above,
4. 9000 Hz and above.
Bands 3 & 4 overlap above 9000 Hz, so that the high frequency noise i.e, hissing sound is substantially reduced. Each band is processed separately by using low pass, band pass and high pass filters and limiters. The 16 Hz to 80 Hz signals goes to a low pass filter which causes improvement in signal to noise ratio with respect to hum and rumble. 80 Hz to 2999 Hz signal goes to band pass filter which deals with the mid-band noise. Most of the sound energy for music is concentrated in this band. The 3000 Hz and 9000 Hz high pass filter improve signal to noise ratio with respect to hiss and modulation noise. The output of the four separate units is added. All this is done in a side branch, and this branch is known as differential network.
Dolby – B System:
Dolby – B System is more simplified than Dolby-A. It is used for home use. In Dolby-B system, the encoded signal covers a signal frequency band from 500 Hz upwards. Signal below a specified threshold value in level are passed through a variable filter which increase the level by about 1 dB at 500 Hz and increases it progressively as the frequency increases until the increase becomes 10 dB at 10 KHz and then remains at 10 dB. Thus the low level signals in high frequency range, where hiss and modulation noises are more prominent, are boosted by 10 dB.
THE EVOLUTION OF DOLBY FILM SOUND:
Going to the movies today is more exciting and involving than ever before, thanks in large part to a continuing effort to improve film sound undertaken by Dolby Laboratories in the early 1970s. Indeed, the history of cinema sound over the past two decades closely mirrors the history of Dolby film sound technologies.
Optical Soundtracks:
The photographic, or “optical,” soundtrack was the first method of putting sound on film. Today it remains the standard, in both analog and digital forms. The classic analog optical soundtrack consists of an opaque area adjacent to the picture containing narrow, clear tracks that vary in width according to variation in the sound (Figure 1).As the film is played, a beam of light from an exciter lamp or LED in the projector’s sound head shines through the moving tracks.
Variations in the width of the clear tracks cause a varying amount of light to fall on a solar cell, which converts the light to a similarly varying electrical signal. That signal is amplified and ultimately converted to sound by loudspeakers in the auditorium. Economy, simplicity, and durability are among the advantages that have contributed to optical sound’s universal acceptance. The soundtrack is printed photographically on the film at the same time as the picture and can last just as long, which with care can be a long time indeed. And the optical sound head within the projector is itself economical and easily maintained.
Magnetic striping And Multichannel Sound:
In the early 1950s, as the film industry sought to woo viewers away from their Fascinating new television sets, a new method of putting sound on film was introduced. After the picture was printed, narrow stripes of iron oxide material (similar to the coating on magnetic recording tape) were applied to the release print. The sound was then recorded on the magnetic stripes in real time. In the cinema, magnetic prints would be played back on projectors equipped with magnetic heads similar to those on a tape recorder, mounted in a special sound head assembly called a “penthouse.” Magnetic sound was a significant step forward, and at its best provided much-improved fidelity over the conventional optical soundtrack. It also enabled the first multichannel sound reproduction, dubbed “stereophonic sound,” ever heard by the public. The voice of an actor appearing to the left, center, or right of the picture could be heard coming from speakers located at the left, center, or right of the new wide screens also being introduced at this time. Music took on a new dimension of realism, and special sound effects could emanate from the rear or sides of the cinema. The two main magnetic systems adopted were the four-track
35 mm Cinemascope system, introduced with The Robe, and the six-track 70 mm Todd-AO, first used for Oklahoma!