Audio Engineering

Fundamentals and Instruments 885

then clocked through a shift register into a 16-bit digital-to-analogue converter (DAC).
If data chosen are those for a 16-bit encoded sinusoidal waveform, the typical intrinsic
purity of the output signal will be of the order of 0.0007%, improved by the use of low-
pass, sample-and-hold fi ltering.

Moreover, if digital fi ltering is used, prior to the DAC, this can be made to track the
frequency of the output signal. As in the previous design ( Figure 30.9 ) the output signal
frequency is related to, and controlled by, the clock frequency.

30.3 Alternative Waveform Types .................................................................................

A range of waveforms, including square- and rectangular-wave shapes, as well as
triangular and “ ramp ” type outputs, are typically provided by a “ function generator. ”
The outputs from this kind of instrument will usually also include a sinusoidal waveform
output having a wide frequency range but only a modest degree of linearity.

An IC that allows the provision of all these output waveform types is the ICL ‘ 8038 ’ and
its homologues, for which the recommended circuit layout is shown in Figure 30.11. The
output from this is free from amplitude “ bounce ” on frequency switching or adjustment
and can be set to give a 1-kHz distortion fi gure of about 0.5% by adjustment of the
twin-gang potentiometers RV 4 and RV 5. As shown, the frequency coverage, by a single
control (RV 2 ), is from 20 Hz to 20 kHz.

Since a square-wave signal contains a very wide range of odd-order harmonics of its
fundamental frequency, a good quality signal of this kind, with fast leading edge (rise)
and trailing edge (fall) times, and negligible overshoot or “ ripple, ” allows the audio
systems engineer to make a rapid assessment both of the load stability of an amplifi er and
of the frequency response of a complete audio system.

ROM Shift register

Clock

DAC

Output waveform

LPF

Figure 30.10 : ROM-based waveform synthesis.