INPUTWAVEFORM
DESIRED
COMPRESSION
CURVE
RELEASETIME, SPI-
PROGRAMMABLE
HOLDTIME, SPI-
PROGRAMMABLE
INPUT LEVEL – 3dB/TABLE ENTRY
Figure 9. Using the Hold and ReleaseTime Feature
Using this idea of a modified rms algorithm, the true rms value
is still obtained for all but the lowest frequency signals, while the
distortion due to rms ripple is reduced. It also allows the user to
set the hold and release times of the compressor independently.
The detector path of the AD1954 is shown in Figure 10.The rms
detector is controlled by three parameters stored in the parameter
RAMs: the rms time constant, the hold time, and the release rate.
The log output of the rms detector is applied to a look-up table
with interpolation.The higher bits of the rms output form an
offset into this table, and the lower bits are used to interpolate
between the table entries to form a high-precision gain word.The
look-up table resides in the parameter RAM and is loaded by
the user to give the desired curve.The look-up table contains 33
data locations, and the LSB of the address into the look-up table
corresponds to a 3 dB change in the amplitude of the detector
signal.This gives the user the ability to program an input/output
curve over a 99 dB range. For the main compressor, the table
resides in Locations 110 to 142 in the SPI parameter RAM.
INPUT LEVEL – 3dB/TABLE ENTRY
Figure 11. Example ofTable Entry for a Given
Compression Curve
Note that the maximum gain that can be entered in the table is
2.0 (minus 1 LSB). If more gain is required, the entire compres-
sion curve may be shifted upward by using the post-compression
gain block following the compressor/limiter.
The AD1954 compressor/limiter also includes a look-ahead com-
pression feature.The idea behind look-ahead compression is to
prevent compressor overshoots by applying some digital delay to
the signal before the gain-control multiplier but not to the detec-
tor path. In this way, the detector can acquire the new amplitude
of the input signal before the signal actually reaches the multiplier.
A comparison of a tone burst fed to a conventional compressor
versus a look-ahead compressor is shown in Figure 12.
HIGH BITS (1LSB = 3dB)
MODIFIED RMS
DETECTOR WITH
LOG OUTPUT
OUTPUT TO
GAIN STAGE
LOOK-UP TABLE
LOW BITS
LINEAR
INTERPOLATION
RELEASE
HOLD
TIME
CONSTANT
CONVENTIONAL COMPRESSOR GAIN
Figure 10. Gain Derived from Interpolated Look-UpTable
One subtlety of the look-up table involves the difference between
the rms value of a sine wave and that of a square wave. If a full-
scale square wave is applied to the AD1954, the rms value of this
signal will be 3 dB higher than the rms value of a 0 dBFS sine
wave.Therefore, the table ranges from +9 dB (Location 142) to
–87 dB (Location 110).
LOOK-AHEAD COMPRESSOR GAIN
The entries in the table are linear gain words in 2.20 format.
Figure 11 shows an example of the table entries for a simple
above-threshold compressor.
HOLDTIME
Figure 12. Conventional Compression vs. Look-Ahead
Compression
REV. A
ADI [ ADI ]
