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  DAT  MiniDisc

3 Optical 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 damage  Impervious to magnetic fields  No danger of head crashes

4 Audio Compact Disc  Audio signal is initially represented as 16-bit PCM data  Requires CIRC (Cross Interleave Reed Solomon Code) error correction encoding  EFM Modulation  Subcode and synchronization words

5  All data on a CD is formatted by frames  Provides a means of distinguishing between:  Audio data  Synchronization word  Parity  Subcode Compact Disc Frames

mm 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

7 Frames  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

8 Error 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)

9 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

10 Eight to Fourteen Modulation  Converts the 8-bit sequences to 14-bit sequences  Avoids small pits by minimizing the number of 0-1 and 1-0 transitions  Error 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

11 Eight to Fourteen Modulation

12 Synchronisation  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

13 Complete Frame  A complete frame consists of:  One 24-bit synchronisation word  14 bits of of subcode  24 words of 14 bit audio data  Eight words of 14 bit parity data  102 merging bits  588 channel bits per frame  Encodes 192 bits of audio data

14 CD-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 length  192 bits per frame = 24 bytes per frame  24 x 98 = 2352 bytes  Each disc has a maximum of 330,000 blocks  x 2352 bytes = 768 MBytes of data ?????

15 CD-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

16 CD-ROM Modes 1 and 2  Mode 1  Most commonly used mode for data storage  2048 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

17 CD-ROM Modes 1 and 2

18 DAT  Digital Audio Tape  Uses helical scan recoding  Like 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

19 MiniDisc  First recordable, erasable optical-disc audio format for consumer applications.  64mm diameter optical disc permanently housed in a plastic case  Random data access  Large memory buffer provides anti-shock

20 MiniDisc  Two types 1. Pre-recorded – similar to CDs 2. 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

21 Fin