AD1953 Analog Devices, AD1953 Datasheet - Page 15

• Manufacturer Part Number: AD1953
• Description: SigmaDSP Digital Audio Processor
• Manufacturer: Analog Devices

The spread_level is a linear number in 2.20 format that multiplies
the processed left-right signal before it is added to or subtracted
from the main channels. The parameter alpha_spread is related to
the cutoff frequency of the first-order low-pass filter by the equation
where EXP is the exponential operator, spread_freq is the low-pass
cutoff in Hz, and f
Note that the stereo spreading algorithm assumes that frequencies
below 1 kHz are present in the main satellite speakers. In some
systems, the crossover frequency between the satellite and
subwoofer speakers is quite high (> 500 Hz). In this case, the
stereo spreading algorithm will not be effective, as the frequencies
that contribute to the spreading effect will be coming mostly from
the subwoofer, which is a mono source.
Delay
Each of the three DAC channels has a delay block that allows
the user to introduce a delay of up to 165 audio samples. The
delay values are programmed by entering the delay (in samples)
into the appropriate location of the parameter RAM. With a
44.1 kHz sample rate, a delay of 165 samples corresponds to a
time delay of 3.74 ms. Since sound travels at approximately
1 foot/ms, this can be used to compensate for speaker place-
ments that are off by as much as 3.74 feet.
An additional 100 samples of delay are used in the look-ahead
portion of the compressor/limiter, but only for the main two
channels. This can be used to increase the total delay for the left
and right channels to 265 samples, or 6 ms at 44.1 kHz.
Main Compressor/Limiter
The compressor used in the AD1953 is quite sophisticated and
is comparable in many ways to professional compressor/limiters
used in the professional audio and broadcast fields. It uses rms/
peak detection with adjustable attack/hold/release, look-ahead
compression, and table-based entry of the input/output curve for
complete flexibility.
The AD1953 uses two compressor/limiters, one in the subwoofer
DAC and one in the main left/right DAC. It is well known that
having independent compressors operating over different frequency
ranges results in a superior perceived sound. With a single-band
compressor, loud bass information will modulate the gain of the
entire audio signal, resulting in suboptimal maximum perceived
loudness as well as gain pumping or modulation effects. With
independent compressors operating separately on the low and
high frequencies, this problem is dramatically reduced. If the
AD1953 is being operated in 2-channel mode, an extra path is
added so that the subwoofer channel can be added back into the
main channel. This maintains the advantage of using a 2-band
compressor, even in a 2.0 system configuration.
Figure 7 shows the traditional basic analog compressor/limiter.
It uses a voltage controlled amplifier to adjust gain and a feed-
forward detector path using an rms detector with adjustable
time constants, followed by a nonlinear circuit to implement the
desired input/output relationship. A simple compressor will have
a single threshold above which the gain is reduced. The amount
of compression above the threshold is called the compression
ratio and is defined as dB change in input/dB change in output.
REV. 0
Alpha spread
_
S
is the audio sampling rate.
=
1 0
. –
EXP



– .
2 0
×
π
×
spread freq
f
S
_



–15–
For example, if the input to a 2:1 compressor is increased by
2 dB, the output will rise by 1 dB for signals above the threshold.
A single “hard” threshold results in more audible behavior than
a so-called “soft-knee” compressor, where the compression is
introduced more gradually. In an analog compressor, the soft-knee
characteristic is usually made by using diodes in their exponential
turn-on region.
The best analog compressors use rms detection as the signal
amplitude detector. RMS detectors are the only class of detec-
tors that are not sensitive to the phase of the harmonics in a
complex signal. The ear also bases its loudness judgment on the
overall signal power. Using an rms detector therefore results in
the best audible performance. Compressors that are based on
peak detection, while good for preventing clipping, are generally
quite poor when it comes to audible performance.
RMS detectors have a certain time constant that determines
how rapidly they can respond to transient signals. There is always
a trade-off between speed of response and distortion. Figure 8
shows this trade-off.
In the case of a fast-responding rms detector, the detector enve-
lope will have a signal component in addition to the desired dc
component. This signal component (which, for an rms detector,
is at twice the input frequency) will result in harmonic distortion
when multiplied by this detector signal.
The AD1953 uses a modified rms algorithm to improve the
relationship between acquisition time and distortion. It uses a
peak-riding circuit together with a hold circuit to modify the rms
signal, as shown in Figure 9. Figure 8 shows two envelopes—one
with the harmonic distortion and another, flatter envelope,
which is produced by the AD1953.
Figure 8. Effect of RMS Time Constant on Distortion
FILTER
Figure 7. Analog Compressor
RMS DETECTOR
WITH dB OUT
THRESHOLD
LINEAR CIRCUITS
COMPRESSION
CURVE NON-
SLOPE
INPUT WAVEFORM
COMPRESSOR ENVELOPE –
COMPRESSOR ENVELOPE –
SLOW TIME CONSTANT
FAST TIME CONSTANT
VCA WITH EXP
CONTROL
AD1953
OUT