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La Scena Musicale - Vol. 8, No. 2

Audio in a DVD-Video World

by Geoff Martin / October 2, 2002

Version française...

Music is an integral part of a movie experience, creating moods, underlining key scenes and punctuating action sequences. But on DVD all is not as it seems. A DVD movie has one video as well as six channels of audio (five main channels and a low frequency effects signal). Most people don't realize that it's impossible to fit an entire movie along with all these channels of audio on a disc. To make it all fit, the DVD relies on your inability to hear everything.

The threshold of hearing is the level when a sound becomes audible; it is different depending on the frequency. For instance, you are most sensitive to tones in the range of 3000 Hz to 5000 Hz (this translates to an F#7 up to D#8 in a system where Middle C is C4 and the semitone below it is B3). Furthermore, this threshold is dynamic--it changes according to the sounds hitting your eardrums at any given time. If a loud tone is played simultaneously with a quiet (but normally audible) one at a close frequency, you won't be able to hear the quieter tone. This effect of louder sounds "drowning out" quieter ones, called psychoacoustic masking, has been studied for decades, and is used to make music fit on a DVD or in an internet connection.

Before the sound is recorded on a DVD, it goes through a digital signal processor (DSP) that has been programmed to predict what you will and will not be able to hear. If the DSP decides that there is a component in the audio signal that you probably won't be able to hear--either because it's too quiet or because it's being masked by another sound--then it removes that component from the audio signal. This component never makes it onto the disc. Sometimes it is a sound like the end of a decay of a cymbal, other times it's an overtone from a violin getting masked by an oboe harmonic. In short, a very large part of the original recording never makes it onto the DVD. That's the bad news. The good news is that, unless you learn how to hear the extra garbage that this encoding creates (called artifacts), in most cases, you won't notice. (Should you want to learn, listen for "gurgling" noises in the high frequencies or a little "ffft" noise right before the attack of a percussion instrument. In the case of pitched percussion, that noise will have the same pitch)

There are many types of this "intelligent" coding that are sold under various trade names--Dolby Digital (also known as AC-3) and DTS for audio with picture, MP3 and RealAudio for Internet connections, ATRAC for MiniDiscs, other types for cell phones and so on. Each has a slightly different set of parameters, but they all do basically the same thing: they omit parts of the sound to make the signal fit in a space where it normally couldn't. With all of these systems, you are not getting as high an audio quality as you would with a good old-fashioned CD. You get more channels, but at the expense of a lower audio quality.

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