Representation of Audio Signals 481imperfections of the switch mean that there can be signifi cant energy leaking from the
switching waveform as the switch is operated and there is the problem of cross talk from
the audio signal across the switch when it is opened. The buffer amplifi er itself must be
capable of responding to a step input and settling to the required accuracy within a small
fraction of the overall sample period. The constancy or jitter of the sampling pulse must
be kept within very tight tolerances and the switch itself must open and close in exactly
the same way each time it is operated. Finally, the choice of capacitor material is itself
important because certain materials exhibit signifi cant dielectric absorption.
The overall requirement for accuracy depends greatly on the acceptable signal-to-noise
ratio (SNR) for the process, which is much controlled by the resolution and accuracy of
the quantizer or converter. For audio purposes we may assume that suitable values for Fs
will be in the 44- to 48-kHz range. The jitter or aperture uncertainty will need to be in the
region of 120 pse, acquisition and settling time need to be around 1 μ s, and the capacitor
discharge rate around 1 V/s for a signal that will be quantized to 16 bits if the error due to
that cause is not to exceed / 0.5 LSB. The jitter performance is complex to visualize
completely because of the varying amplitude and frequency component of the jitter itself.
V (^1) C V 0
tac
T —^1
Fs
Figure 15.22 : An elementary sample and hold circuit using a fast low distortion
semiconductor switch that is closed for a short time to allow a small-valued storage capacitor
to charge up to the input voltage. The buffer amplifi er presents the output to the quantizer.