1.2.5 D/A Conversion
Most commercial DACs are zero-order-hold, which means they convert the binary
input to the analog level and then simply hold that value for T seconds until the next
sampling instant. Therefore the DAC produces a staircase shape analog waveform y
which is shown as a solid line in Figure 1.5. The reconstruction (anti-imaging and
smoothing) filter shown in Figure 1.1 smoothes the staircase-like output signal gener-
ated by the DAC. This analog lowpass filter may be the same as the anti-aliasing filter
with cut-off frequency f
=2, which has the effect of rounding off the corners of the
staircase signal and making it smoother, which is shown as a dotted line in Figure 1.5.
High quality DSP applications, such as professional digital audio, require the use of
reconstruction filters with very stringent specifications.
From the frequency-domain viewpoint (will be presented in Chapter 4), the output of
the DAC contains unwanted high frequency or image components centered at multiples
of the sampling frequency. Depending on the application, these high-frequency compon-
ents may cause undesired side effects. Take an audio CD player for example. Although
the image frequencies may not be audible, they could overload the amplifier and cause
inter-modulation with the desired baseband frequency components. The result is an
unacceptable degradation in audio signal quality.
The ideal reconstruction filter has a flat magnitude response and linear phase in the
passband extending from the DC to its cut-off frequency and infinite attenuation in
the stopband. The roll-off requirements of the reconstruction filter are similar to those
of the anti-aliasing filter. In practice, switched capacitor filters are preferred because of
their programmable cut-off frequency and physical compactness.
1.2.6 Input/Output Devices
There are two basic ways of connecting A/D and D/A converters to DSP devices: serial
and parallel. A parallel converter receives or transmits all the B bits in one pass, while
the serial converters receive or transmit B bits in a serial data stream. Converters with
parallel input and output ports must be attached to the DSP's address and data buses,
Figure 1.5 Staircase waveform generated by a DAC
INPUT AND OUTPUT CHANNELS