adav804 Analog Devices, Inc., adav804 Datasheet - Page 16

• Manufacturer Part Number: adav804
• Description: Audio Codec For Recordable Dvd
• Manufacturer: Analog Devices, Inc.

ADAV804
SRC FUNCTIONAL OVERVIEW
THEORY OF OPERATION
Asynchronous sample rate conversion is converting data from
at the same or different sample rate. The simplest approach to
an asynchronous sample rate conversion is the use of a zero-
order hold between the two samplers shown in Figure 9 In an
asynchronous system, T2 is never equal to T1 nor is the ratio
between T2 and T1 rational. As a result, samples at fS_OUT will
be repeated or dropped producing an error in the re-sampling
process. The frequency domain shows the wide side lobes that
result from this error when the sampling of fS_OUT is
convolved with the attenuated images from the sin(x)/x nature
of the zero-order hold. The images at fS_IN, dc signal images, of
the zero-order holdare infinitely attenuated. Since the ratio of
T2 to T1 is an irrational number, the error resulting from the re-
sampling at fS_OUT can never be eliminated. However, the
error can be significantly reduced through interpolation of the
input data at fS_IN. The sample rate converter in the ADAV804/
is conceptually interpolated by a factor of 2
CONCEPTUAL HIGH INTERPOLATION MODEL
Interpolation of the input data by a factor of 2
(2
both the time domain and the frequency domain of
interpolation by a factor of 2
2
samples between each f
interpolated signal with a digital low-pass filter to suppress the
images. In the time domain, it can be seen that f
closest f
the nearest f
significantly reduces the re-sampling error.
Figure 9. Zero Order Hold Being Used by fS OUT to Resample Data from fS_IN
20
20
would involve the steps of zero-stuffing (2
FREQUENCY RESPONSE OF fS_OUT CONVOLVED WITH ZERO-ORDER
HOLD SPECTRUM
−1) samples between each f
IN
S_IN
f S_IN =1/T1
× 2
S_IN
SPECTRUM OF ZERO-ORDER HOLD OUTPUT
20
sample in the case of no interpolation. This
sample from the zero-order hold as opposed to
SPECTRUM OF f S_OUT SAMPLING
SIN(X)/X OF ZER0-ORDER HOLD
S_IN
SAMPLED AT f S_IN
ORIGINAL SIGNAL
f S_OUT
ZERO-ORDER
sample and convolving this
20
HOLD
. Conceptually, interpolation by
S_IN
sample. Figure 10 shows
f S_OUT = 1/T2
20
.
20
20
−1) number of
S_OUT
involves placing
2 × f S_OUT
OUT
selects the
801-0008
Rev. Pr G | Page 16 of 54
In the frequency domain shown in Figure 11, the interpolation
expands the frequency axis of the zero-order hold. The images
from the interpolation can be sufficiently attenuated by a good
low-pass filter. The images from the zero-order hold are now
pushed by a factor of 2
of the zero-order hold, which is f
zero-order hold are the determining factor for the fidelity of the
output at f
the zero-order hold frequency response, maximum image = sin
(× F/f
image that would be 2
The following worst-case images would appear for f
kHz:
Image at f
Image at f
IN
f S_IN
S_INTERP
S_INTERP
S_INTERP
S_OUT
TIME DOMAIN OUTPUT OF THE LOW-PASS FILTER
TIME DOMAIN OF f S_OUT RESAMPLING
)/(× F/f
INTERPOLATE
TIME DOMAIN OF f S_IN SAMPLES
TIME DOMAIN OF THE ZERO-ORDER HOLD OUTPUT
. The worst-case images can be computed from
BY N
− 96 kHz = –125.1 dB
+ 96 kHz = –125.1 dB
S_INTERP
Figure 10. SRC Time Domain
Preliminary Technical Data
20
20
× f
closer to the infinite attenuation point
). F is the frequency of the worst-case
S_IN
LOW-PASS
FILTER
± f
S_IN
S_IN
/2 , and f
× 2
20
ZERO-ORDER
The images at the
HOLD
S_INTERP
S_IN
is f
f S_OUT
S_IN
= 192
× 2
OUT
20
.