Audio Engineering

Digital Audio Fundamentals 423

14.6 Time Compression and Expansion ........................................................................

Data fi les such as computer programs are simply lists of instructions and have no natural
time axis. In contrast, audio and video data are sampled at a fi xed rate and need to be
presented to the viewer at the same rate. In audiovisual systems the audio also needs to
be synchronized to the video. Continuous bit streams at a fi xed bit rate are diffi cult for
generic data recording and transmission systems to handle. Such systems mostly work on
blocks of data that can be addressed and/or routed individually. The bit rate may be fi xed
at the design stage at a value that may be too low or too high for the audio or video data
to be handled.

The solution is to use time compression or expansion. Figure 14.10 shows a RAM that
is addressed by binary counters that periodically overfl ow to zero and start counting
again, giving the RAM a ring structure. If write and read addresses increment at the same
speed, the RAM becomes a fi xed data delay as the addresses retain a fi xed relationship.
However, if the read address clock runs at a higher frequency but in bursts, output data
are assembled into blocks with spaces in between. Data are now time compressed.
Instead of being an unbroken stream, which is diffi cult to handle, data are in blocks
with convenient pauses in between them. Numerous processes can take place in these
pauses. A hard disk might move its heads to another track. In all types of recording and

Write address

counter

Input

clock

Data in Data out Output

clock

Read address

counter

Ring

memory

(RAM)

Figure 14.10 : If the memory address is arranged to come from a counter that overfl ows,

the memory can be made to appear circular. The write address then rotates endlessly,

overwriting previous data once per revolution. The read address can follow the write address

by a variable distance (not exceeding one revolution) and so a variable delay takes place

between reading and writing.