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3.1 Anti-Aliasing Filters
Different
PCB revisions utilize 3 different
types
anti-aliasing
eliminate Nyquist errors. The 1st
is a four pole active filter, and will
be seen on the oldest PCB
revisions
(A
through
problems
occur
when
these older board revisions with
digital synth's, and some "hot"
guitar
pickups,
it
necessary to upgrade this filter to
a type 2. The type 2 filter is a six
pole version of the active filter,
and includes a pre-emphasis
curve. It will only be seen on
older board rev.s that have been
upgraded. The 3rd type of filter is
the LC block filter. This single
component filter is seen on
revision F, and later, boards. It
eliminates the need for active
circuitry, as well as facilitating
easier servicing.
3.2 Successive
Approximation
Successive approximation
is a heuristic approach to the
process
of
analog
conversion. The idea is to divide
the
process
into
manageable
sections.
significant binary weight (starting
with the Most Significant Bit) is
taken in turn, thus requiring only
16 comparisons to achieve a final
value.
The process begins with
the
input
"sample
circuit". 1/3 of the 4053 (U9) is
turned on, allowing the input
sample capacitor (C38) to charge
[or discharge] to the level of the
current input signal. When the
switch is turned off, the capacitor
will hold that level indefinitely
[barring internal leakage ]. At this point, the SAR (Successive Approximation Register-part of the
DASP 24 ASIC) will take over. Starting with the MSB, the SAR will set the bit, and compare the
output of the DAC, to the level of the input sample capacitor (via comparator U8). The results of the
Alesis QuadraVerb Service Manual------
QuadraVerb
filters,
to
E).
If
using
will
be
to
digital
short,
Each
and
hold
Diagram 2
Diagram 3
3
08/13/04
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