Table of Contents
Advertisement
Filter
The output from the D/A converter is a discrete
waveform formed of samplied value pulses, and a low
pass filter is used to extract the low range component
only, which is the data component.
D/A converter output
waveform
u
sideband component wafeforms
¢
frequency
spectrum
|
—_—_—_
|
\
low range component and
Ub
level
fs/2
fs
2fs
——
frequency
r Output Waveforms and Frequency
ter output waveform is the same as
ipled waveform; the frequency com-
d of a sideband component
whose
e integer of the fundamental wave
( ymponent and sampled frequency fs.
There "is
'fold component right above the funda-
mental
wave: component
upper
limit, and if these
components go to the amp and speakers as is, modu-
lation distortion and tweeter damage may occur.
In order to eliminate the unnecessary components
over
20 kHz,
a filter
having
sharp
attenuation
re-
sponse is required, as the fold component frequency
lower limit is 24 kHz, relatively near.
Digital Filter
The
digital filter processes
the discrete
sampled
signal frequency component at the digital data stage,
so of course it is before the D/A converter.
sampled
sampled waveform
spectrum
|
ie
frequency
range determined at A/D side
@® fs
fs/2
fs
2fs
@ 2fs
fs/2
2fs
digital filter
actually digital data
— Ts/n —
delay
input
|
abit
a
4
oa
kh
multiplier
adder
| bk
Ts/n
The theory and structure is based on complicated
logic, and therefore is rather difficult to understand,
but for the D/A low pass filter, it can be thought of
as sampled
frequency.
multiplication.
This is illus-
trated in the above figure.
If the analog signal range controled
to less than
1/2 of the sampled frequency fs is sampled by fs, the
waveform
and spectrum
become
as shown at @) in
the diagram.
This corresponds to sampling the signal
up to 20 kHz on CD with 44.1 kHz.
If the same
range signal is sampled
by 2fs, that
waveform
and range should become as shown by
in the diagram.
The played back sample data cor-
respond to the sample values in @) , but if the sample
data
obtained
by sampling
this sample
data at 2fs
could be made at the middle of each sample, the out-
put waveforms
would
be as shown in @), and side-
band components having fs as center could be elimi-
nated.
Actually, true sample data can not be made, and
there is error, but this can be sufficiently minimized
by performing
many
different
operations.
In this
way, the sampled frequency is raised 3 and 4 times
equivalently,
shifting
the
high
range
components
higher,
and finally eliminating
them
with a simple
analog filter.
,
multiplication coefficient
Advertisement
Table of Contents
