Audio Engineering

Digital Audio Recording Basics 551

Figure 17.19 shows the principle. The instantaneous data rate of the disc drive is far
in excess of the sampling rate at the convertor, and so a large time-compression factor
can be used. The disc drive can read a block of data from disc and place it in the TBC
in a fraction of the real time it represents in the audio waveform. As the TBC steadily
advances through the memory, the disc drive has time to move the heads to another track
before the memory runs out of data. When there is suffi cient space in the memory for
another block, the drive is commanded to read and fi lls up the space. Although the data
transfer at the medium is highly discontinuous, the buffer memory provides an unbroken
stream of samples to the DAC and so continuous audio is obtained.

Recording is performed using the memory to assemble samples until the contents of one
disc block are available. These are then transferred to disc at high data rate. The drive can
then reposition the head before the next block is available in memory.

An advantage of hard discs is that access to the audio is much quicker than with tape, as
all of the data are available within the time taken to move the head. This speeds up editing
considerably.

After a disc has been in use for some time, the free blocks will be scattered all over the
disc surface. The random access ability of the disc drive means that a continuous audio
recording can be made on physically discontinuous blocks. Each block has a physical
address, known as the block address, which the drive controller can convert into cylinder
and head selection codes to locate a given physical place on the medium. The size of
each block on the disc is arranged to hold the number of samples that arrive during a

Silo

contents

Disk transfers samples to silo

Interblock

gap on

disk

Disk drive seeks,

silo empties

Figure 17.19 : During an audio replay sequence, the silo is constantly emptied to provide

samples and is refi lled in blocks by the drive.