Audio Engineering

446 Chapter 15

are referred to as words. Thus a three byte word will comprise 24 bits (though word is by
now being used mainly to mean two bytes and DWord to mean four bytes). Blocks are the
next layer of structure perhaps comprising 512 bytes (a common size for computer hard
discs). Where the arrangement of a number of bytes fi ts a regular structure the term frame
is used. We will meet other terms that describe elements of structure in due course.

Conventionally we think of bits, and their associated patterns and structures, as being
represented by one of two voltage levels. This is not mandatory and there are other ways
of representing the on/off nature of the binary signal. You should not forget alternatives
such as the use of mechanical or solid state switches, presence or absence of a light,

(a)

Clock

QA

QB

QC

Reset

Clock

Reset

S

A

R

OQ

Q

S

B

R

DQ

Q

S

C

R

DQ

Q

Figure 15.5(a) : The simplest counter is made up of a chain of edge-triggered D-type FFs.

For a long counter, it can take a sizeable part of a clock cycle for all of the counter FFs to

change state in turn. This ripple through can make decoding the state of the counter diffi cult

and can lead to transitory glitches in the decoder output, indicated in the diagram as points

where the changing edges do not exactly line up. Synchronous counters in which the clock

pulse is applied to all of the counting FFs at the same time are used to reduce the overall

propagation delay to that of a single stage.