AD1954 Analog Devices, AD1954 Datasheet - Page 15

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

The AD1954 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
AD1954 is being operated in two-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 6 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 non-linear 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 com-
pression ratio, and is defined as dB change in input/dB change
in output. 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.
FILTER
RMS DETECTOR
WITH DB OUT
THRESHOLD
PRELIMINARY TECHNICAL DATA
LINEAR CIRCUITS
COMPRESSION
CURVE NON-
SLOPE
VCA WITH EXP
CONTROL
OUT
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, and therefore using an RMS detector re-
sults 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, which determines
how rapidly they can respond to transient signals. There is always
a tradeoff between speed of response and distortion. Figure 7
shows this trade-off.
In the case of a fast-responding RMS detector, the detector
envelope 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.
INPUT WAVEFORM
COMPRESSOR ENVELOPE –
COMPRESSOR ENVELOPE –
SLOW TIME CONSTANT
FAST TIME CONSTANT
AD1954