1 Storing Digital Audio. 2 Storage There are many different types of storage medium and encoding methods for the storage of digital audio CD DVD.

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Storing Digital Audio

Storing Digital Audio

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StorageStorage

There are many different types of storage medium and encoding methods for the storage of digital audioCD DVDDATMiniDisc

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Optical StorageOptical Storage

Optical storage can provide about 100 times the storage capacity for the same size as magnetic media

Greater life expectancy than magnetic media

Less susceptible to damageImpervious to magnetic fieldsNo danger of head crashes

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Audio Compact DiscAudio Compact Disc

Audio signal is initially represented as 16-bit PCM data

Requires CIRC (Cross Interleave Reed Solomon Code) error correction encoding

EFM ModulationSubcode and synchronization words

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All data on a CD is formatted by framesProvides a means of distinguishing

between:Audio dataSynchronization word ParitySubcode

Compact Disc FramesCompact Disc Frames

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Compact Disc FramesCompact Disc Frames

0.163mm

1 Frame

Sync (24 bits)

Sub code

Data (96 bits) Parity (32 bits)

Data (96 bits) Parity (32 bits)

8 bits P,Q,R,S,T,U,V,W

Compact Disc

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FramesFrames

Twelve 16-bit PCM audio sampling periods (alternating L and R) are grouped together – 192 bits

These are divided to provide 24 8-bit audio symbols

To scatter possible errors symbols from different groups are interleaved into a frame

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Error correctionError correction

Two 32-bit parity symbols are generated per frame – one in the middle and one at the end

Interleaving and parity bits constitute error correction encoding based on the Cross-Interleave Reed-Solomon code (CIRC)

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Subcode Subcode

One subcode symbol (8-bits P,Q,R,S,T,U,V,W) is added per frame.P subcode – music track separator flag

• One at the start of each selection

• Zero during music and lead in track

Q subcode – control information such as track number and time

R to W – advanced features such as storing text and images

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Eight to Fourteen Modulation

Eight to Fourteen Modulation

Converts the 8-bit sequences to 14-bit sequencesAvoids small pits by minimizing the number of

0-1 and 1-0 transitionsError correction is helped because more

unique patterns can be selected than if 8-bit words were directly recorded

Applied to all of frame except synchronisation word

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Eight to Fourteen Modulation

Eight to Fourteen Modulation

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SynchronisationSynchronisation

A 24-bit synchronisation word is added at the start of each frame and has two main functions

1. Indicates the start of each frame

2. Sync word frequency is used to control the player’s motor speed

The complete final frame now comprises of 588-bits

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Complete FrameComplete Frame

A complete frame consists of:One 24-bit synchronisation word14 bits of of subcode24 words of 14 bit audio dataEight words of 14 bit parity data102 merging bits

588 channel bits per frameEncodes 192 bits of audio data

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CD-ROMCD-ROM

Extends the CD audio format to the broader application of information storage in general

Not tied to a specific application (like CD audio)

Ninety-eight CD frames are summed to form a data block 2352 bytes in length192 bits per frame = 24 bytes per frame24 x 98 = 2352 bytes

Each disc has a maximum of 330,000 blocks330000 x 2352 bytes = 768 MBytes of data ?????

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CD-ROM Data BlockCD-ROM Data Block

First 12 bytes of the block form a synchronisation pattern

Next four bytes are the header field:Three bytes – address field

• Minutes

• Seconds

• Block

Mode byte

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CD-ROM Modes 1 and 2CD-ROM Modes 1 and 2

Mode 1Most commonly used mode for data storage2048 data bytes (Not 2352)280 bytes used for extended error detection

and correction (EDC/ECC)Mode 2

Full 2336 bytes used for data storage relies on CIRC CD audio error correction only

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CD-ROM Modes 1 and 2CD-ROM Modes 1 and 2

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DATDAT

Digital Audio TapeUses helical scan recodingLike CDs available in audio or

digital data storage (DDS) format

Audio DATs normally store approx 2hrs of audio

32KHz, 44.1KHz, 48KHz and sometimes 96KHz sampling supported

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MiniDiscMiniDisc

First recordable, erasable optical-disc audio format for consumer applications.

64mm diameter optical disc permanently housed in a plastic case

Random data accessLarge memory buffer

provides anti-shock

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MiniDiscMiniDisc

Two types1. Pre-recorded – similar to CDs2. Recordable – uses magneto-optical

technology• Both a laser and a magnetic head are used to

change the disc recording area• Data is read by detection of the polarization of

light reflected from the MO surface rather than by light intensity as with CDs

MiniDisc players can read both types of media

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