Lecture 18CD, DVD StorageAudio CompressionDigital Music SynthesisFinal Exam Review

Instructor: David Kirkby (dkirkby@uci.edu)

                                                                                 

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MiscellaneousI will be traveling Fri-Wed. Office hours will beWed (Dec 11) 3:15-5:15pm during exam week.

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Compact DiscsCompact discs record digital signals as pits in a plastic surface. The features on a CD are much smaller than on a vinyl record:

http://www.mta.ca/~jehrman/demos.htm

CD pits (or bumps) are about 0.5m wide. Information on adisc is recorded along a spiral track starting from thecenter, with tracks spaced 1.6m apart. (Human hairs are40-300m thick).

                                     

    

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A CD player reads the pattern of pits on a disc using a laser. The laser is reflected from a shiny metal layer on one side of the disc.

Pits disrupt the reflected laser beam, but where there is no pit, the reflected laser light bounces back towards the laser where it can be monitored electronically.

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Digital Versatile Disc (DVD)DVD is similar to CD, but stores 26 times more data (up to 17 Gb instead of 650 Mb).

This increased storage capacity is made possible by 3 main innovations:

•Narrower tracks of pits (0.74 m instead of 1.6 m)

•A shorter-wavelength laser (green instead of infrared)

•Up to 4 layers instead of 1.DVD was originally targeted at video storage, but in 1999 a DVD-Audio format was announced with up to 6 channels sampled up to 192 kHz with up to 24 bits.

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Audio CompressionSo far, we have only talked about digital music storage.

Compression is the process of packing the “same information” into less space.

There are two types of compression, corresponding to different definitions of the “same information”:

•Lossless compression stores an exact replica of the original digital signal in less space.

•Lossy compression stores an almost equivalent replica of the original digital signal in less space.

Lossy compression is more powerful, but means some degradation of the original signal. MP3 is a lossy compression scheme.

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There are three general ways in which we can pack the “same information” into fewer bytes:

•Reduce the quality (sampling rate, number of amplitude levels) of the digital encoding.

•Exploit any redundancy (predictability) in the original signal.

•Exploit the limitations of human hearing by eliminating components of the signal that will not be perceived by most listeners.

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MPEG / Audio Layer III (MP3)The Motion Pictures Experts Group (MPEG) is a large consortium that has developed standards for video compression. One of these standards (Audio Layer III) is for compressing a video soundtrack.

MP3 is a lossy compression scheme that exploits redundancy as well as the limitations of human hearing. For example:

•Some sounds cannot be heard at all,•Some sounds appear louder than others,•A loud sound will mask quiet sounds.

We call this perceptual compression.

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An MP3 encoder first divides an input sample into small chunks, and then breaks each chunk into 32 frequency bands that are approximately a critical bandwidth in size:

                                 

         

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The final stage is to budget how many bytes to allocate for each frequency band, based on how much that band will contribute to your perception of the overall sound in that time chunk. For example:

http://www.howstuffworks.com/mp31.htm

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Compare these MP3 versions of our original test sample:

•Original 100%•128 kbits/sec 18%•64 kbits/sec 9%•32 kbits/sec 4.5%•16 kbits/sec 2.3%•8 kbits/sec 1.1%

One refinement to the algorithm is to allow more complex chunks to use more bits than simpler chunks, as long as as the average bit rate is kept on target. The second set of samples above use this Automatic Bit Rate (ABR) method, instead of the usual Constant Bit Rate (CBR) method.

Compare with thesereduced-quality 5%samples (p.23):

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Stereo vs Mono SignalsSo far, we have focused on a mono digital sample. Stereo samples require twice as much space when uncompressed, but MP3 can significantly reduce this overhead when the left and right channels are sufficiently similar to each other.

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Digital Music SynthesisComputer generated music is a vast subject.

We will briefly discuss techniques for creating virtual musical instruments (sysnthesis) on a computer. We will not have time to cover other aspects of this subject such as algorithmic score generation or human-computer performance interfaces.

The computer could be either a general-purpose PC or a special-purpose synthesizer:

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Synthesis MethodsHere is a basic classification of some popular synthesis methods:

FourierSynthesis

GranularSynthesis

AM/FMSynthesis

AdditiveSynthesis

SubtractiveSynthesis

LinearSynthesis

Non-linearSynthesis

http://www.sfu.ca/sonic-studio/handbook/Sound_Synthesis.html

PhysicalModeling

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Fourier SynthesisFourier synthesis involves building an instrument’s sound out of its pure-tone components. Fourier Analysis tells us that this is always possible and how to do it.In principle, this approach can generate arbitrarily good approximations to any sound.

In practice, the large amount of information required to specify an interesting sound is a limitation of this method.

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Granular SynthesisGranular synthesis also builds a sound up from its components. The components of granular synthesis are short (<50ms) “grains” of sound that can be overlaid to build a rich texture.

<50 ms

The inspiration for granular synthesis are sounds like these:

• carding wool

• guiro

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Granular synthesis usually uses small fragments of a recorded sound for its grains.

Here is an example ( ) of granular synthesis using this fog horn sound ( ). Here is another example ( ).

Granular synthesis is especially efficient at generating many environmental sounds.

It is also useful for changing the timing of a recorded sound without altering its frequency.

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Subtractive SynthesisSubtractive synthesis takes the opposite approach to additive synthesis: instead of starting from nothing and adding together simple components, subtractive synthesis starts from a complex sound and removes specific components.

The starting point for subtractive synthesis is usually some form of noise, which contains a broad range of frequencies:

•White noise•Compare with the sound of the Athabasca River

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The main tool for subtraction is a frequency filter:

Subtractive synthesis is particularly efficient at generating transients and breath sounds.

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Frequency-Modulation SynthesisFM synthesis is non-linear and so does not obey the principal of superposition: the frequencies coming out are not just the sum of the frequencies going in.

FM synthesis consists of adjusting the frequency of a carrier wave using the amplitude of a modulation wave:

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FM synthesis is a particularly efficient method for generating a complex timbre with a simple recipe.

The technique was discovered accidentally by John Chowning in the late 1960s, working at Stanford. By the early 1980’s, Chowning’s work was the basis for a new generation of electronic synthesizers (eg, Yamaha DX7) that revolutionized the industry.

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Physical Modeling SynthesisThe basic idea of this technique is to generate sound with a physically realistic acoustical simulation of an instrument (real or imagined).

This technique is not particularly efficient, but can generate extremely realistic sounds and gives the composer/performer control over many physically meaningful aspects of the sound production (eg, string tension, bowing speed and location) that influence the subtle nuances of a realistic sound.

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Here are some examples from the Yamaha Virtual Lead (VL) synthesizer:

•Oboe and bassoon•Shakuhachi•Trumpet•Saxophone

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Final Exam ReviewSee the handout, also linked to the course web site at:http://positron.ps.uci.edu/~dkirkby/music/html/final-review.pdf