AD1953
–18–
was not so sensitive to overload, then the compressor would be
too pessimistic and the volume of the woofer would be reduced.
If, on the other hand, the biquad filter were designed to follow
the woofer excursion curve of the speaker, then the volume of
the woofer could be maximized under all conditions. This is
illustrated in Figure 15.
20Hz 200Hz
FREQ
WOOFER EXCURSION
BIQUAD RESPONSE
20Hz 200Hz
FREQ
Figure 15. Optimizing Woofer Loudness Using the
Subwoofer RMS Biquad Filter
When using a filter in front of the detector, a confusing side-
effect occurs. If one measures the frequency response by using a
swept sine wave with an amplitude large enough to be above the
compressor threshold, the resulting frequency response will not
look flat. However, this is not real in the sense that, as the sine
wave is swept through the system, the gain is being slowly
modulated up and down according to the response of the biquad
filter in front of the detector. If one measures the response using a
pink-noise generator, the result will look much better, as the
detector will settle on only one gain value. The perceptual effect
of the swept-sine-wave test is not at all what would be pre-
dicted by simply looking at the frequency response curve; it is
only the signal-path filters that will affect the perception of fre-
quency response, not the detector-path filters.
De-emphasis Filtering
The standard for encoding CDs allows the use of a pre-emphasis
curve during encoding, which must be compensated for by a
de-emphasis curve during playback. The de-emphasis curve is
defined as a first-order shelving filter with a single pole at
(1/(2 × π × 50 μs)) followed by a single zero at (1/(2 × π × 15 μs)).
This curve may be accurately modeled using a first-order digital
filter. This filter is included in the AD1953; it is not part of the
bank of biquad filters, and so does not take away from the num-
ber of available filters.
Since the specification of the de-emphasis filter is based on an
analog filter, the response of the filter should not depend on the
incoming sampling rate. However, when the de-emphasis filter is
implemented digitally, the response will scale with the sampling rate
unless the filter coefficients are altered to suit each possible input
sampling rate. For this reason, the AD1953 includes three separate
de-emphasis curves; one each for sampling rates of 32 kHz, 44.1 kHz,
and 48 kHz. These curves are selected by writing to Bits <5:4> of
Control Register 1 over the SPI port.
Using the Sub Reinjection Paths for Systems with No Subwoofer
Many systems will not use a subwoofer, but would still benefit
from 2-band compression/limiting. This can be accommodated
by using sub reinjection paths in the program flow. These
parameters are programmed by entering two numbers (in 2.20
format) into the parameter RAM. Note that if the biquad filters
are not properly designed, the frequency response at the cross-
over point may not be flat. Many crossover filters are designed to
be flat in the sense of adding the powers together, but nonflat if
the sum is done in voltage mode. The user must take care to
design an appropriate set of crossover filters.
Interpolation Filters
The left and right channels have a 128:1 interpolation filter
with 70 dB stop-band attenuation that precedes the digital Σ-Δ
modulator. This filter has a group delay of approximately 24.185/f
S
,
where f
S
is the sampling rate. The sub channel does not use an
interpolation filter. The reason for this (besides saving valuable
MIPS) is that it is expected that the bandwidth of the sub output
will be limited to less than 1 kHz. With no interpolation filter, the
first “image” will therefore be at 43.1 kHz (which is f
S
– 1 kHz,
for CD audio). The standard external filter used for both the
main and sub channels is a third-order, single op amp filter. If
the cutoff frequency of the external subwoofer filter is 2 kHz,
then there are more than four octaves between 2 kHz and the
first image at 43.1 kHz. A third-order filter will roll off by
approximately 18 dB/oct × 4 octaves = 72 dB attenuation. This is
approximately the same as the digital attenuation used in the main
channel filters, so no internal interpolation filter is required to remove
the out-of-band images.
Note that by having interpolation filters in the main channels but
not the subwoofer channel, there is a potential time-delay mis-
match between the main and sub channels. The group delay of
the digital interpolation filters used in the main left/right channels
is about 0.5 ms. This must be compared to the group delay of
the external analog filter used in the subwoofer path. If the
group delay mismatch causes a frequency response error (when the
two signals are “acoustically added”), the programmable delay
feature can be used to put extra delay in either the subwoofer path
or the main left/right path.
SPI PORT
Overview
The AD1953 has many different control options. Most signal-
processing parameters are controlled by writing new values to
the parameter RAM using the SPI port. Other functions such as
volume and de-emphasis filtering are programmed by writing to
SPI control registers.
The SPI port uses a 4-wire interface, consisting of CLATCH,
CCLK, CDATA, and COUT signals. The CLATCH signal
goes LOW at the beginning of a transaction and HIGH at the
end of a transaction. The CCLK signal latches the serial input
data on a low-to-high transition. The CDATA signal carries the
serial input data, and the COUT signal is the serial output data.
The COUT signal remains three-stated until a read operation is
requested. This allows other SPI compatible peripherals to
share the same readback line.
The SPI port is capable of full read/write operation for all of the
memories (parameter and program) and some of the SPI registers
(Control Register 1 and data capture registers). The memories
may be accessed in both a single-address mode or in burst mode.
All SPI transactions follow the same basic format, shown in Table I.
The Wb/R bit is low for a write, and high for a read operation.
The 10-bit address word is decoded into a location in one of the
two memories (parameter or program) or one of the SPI regis-
ters. The number of data bytes varies according to the register
or memory being accessed. In burst-write mode (available for
loading the RAMs only), an initial address is given followed by a
continuous sequence of data for consecutive RAM locations.
REV. A
