AD1954 Analog Devices, AD1954 Datasheet - Page 18

• Manufacturer Part Number: AD1954
• Description: SigmaDSP Digital Audio Processor
• Manufacturer: Analog Devices

AD1954
Subwoofer Compressor/Limiter
The subwoofer compressor/limiter differs from the left/right
compressor in the following ways:
1. The subwoofer compressor operates on a weighted sum of left
2. The detector input has a biquad filter in series with the input
3. There is no predictive compression, as presumably the input
The subwoofer compressor signal flow is shown in Figure 13.
The biquad filter before the detector can be used to implement a
frequency-dependent compression threshold. For example,
assume that the overload point of the woofer is strongly fre-
quency-dependent. In this case, one would have to set the
compressor threshold to a value that corresponded to the most
sensitive overload frequency of the woofer. If the input signal
happened to be mostly in a frequency range where the woofer
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 14.
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 the response of the biquad
V
IN
and right inputs (aa × Left + bb × Right), where aa and bb are
both programmable.
in order to implement frequency-dependent compression
thresholds.
signals are filtered to pass only low frequencies, and therefore
transient overshoots are not a problem.
_SUB = k1
BIQUAD
FILTER
20 Hz
CONSTANT
LEFT_IN+K2
FREQ
TIME
DETECTOR WITH
MODIFIED RMS
LOG OUTPUT
HOLD RELEASE
200 Hz
HIGH BITS (1 LSB = 3dB)
RIGHT_IN
PRELIMINARY TECHNICAL DATA
LOOKUP
LOW BITS
TABLE
20 Hz
INTERPOLATION
POST-COMPRESSION
FREQ
LINEAR
PROGRAMMABLE
UP TO 30dB
GAIN, SPI-
200 Hz
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 AD1954; 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 AD1954 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. Alternatively, the 44.1 kHz
curve can be called upon using the DEEMP/SDATA_AUX pin.
This pin is included for compatibility with CD decoder chips that
have a de-emphasis output pin.
Using the Sub Re-Injection Paths for Systems with no Subwoofer
Many systems will not use a subwoofer, but would still benefit
from two-band compression/limiting. This can be accommo-
dated 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 non-flat 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
75 dB stopband attenuation that precedes the digital sigma-delta
modulator. This filter has a group delay of approximately TBD
taps. 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 there-
fore be at 43.1 kHz (which is f
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 4
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, and so no internal
interpolation filter is required to remove the out-of-band images.
S
– 1 kHz, for CD audio). The