Re: In D/A conversion, is sample-and-hold necessary?

On Jul 23, 5:05 am, "m26k9" <maduranga.liyan...@xxxxxxxxx> wrote:
Hi,

I had to open another thread because this keeps bugging me.

I have a confusion in the process of DAC.

(Method-1)
Couple of books (Bernard Widro/ Proakis) states that DAC process consists
of sample-and-hold filter followed by a low-pass filter. The LPF smooths
the sharp edges of S/H filter.

(Method-2)
But, the 'Whittaker–Shannon interpolation formula'
(http://en.wikipedia.org/wiki/Whittaker%E2%80%93Shannon_interpolation_...)
states that signal can be reconstructed (DAC) by sending the digital signal
through a sinc filter. And I have read in other texts that DAC process is
basically sending the discrete samples through a interpolation (low-pass)
filter.

Thinking about it, at sampling instants, both method-1 and method-2 have
the same values. Disregarding the values between sampling instants (which
are not required to when ADC at the other end), is there a difference
between the methods?

Thank you.

Yes you can implement a DAC without a sample and hold. An example is
in the Yamaha nine channel synthesizer chip, YM2413. Each channel's
output is a stream of impulses. The 9 different outputs are combined
together by simple multiplexing. Thus 1 single sample period is
divided into 9 time slices. The actual pulses are narrower in time
than 1/9th of the sample period, so they are very much like impulses
even in this multiplexing scheme. Now this was done in this case to
simplify the hardware - i.e., no mixers. A drawback of this approach
is with the chip running on 5 volts, the pulses have a maximum
amplitude of 5 volts, so when the pulses go through the lowpass
filter, the amplitude ends up in the millivolt range. This was okay in
the application, since it is assumed the output after the lowpass
filtering will go right into a "microphone" level input of an
amplifier.

Apart from the Yamaha applications, I have seen the lack of a sample
and hold used a few times, for example Motorola use to do this their
CVSDM (Continuously Variable Slope Delta Modulation) chips.

I thought you should know that if there is more than one way to do
something, it probably has been implemented since each way often
offers unique advantages that can sometimes make it worth it.

IHTH,
Clay




.

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