AD1954 Analog Devices, AD1954 Datasheet - Page 16

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

AD1954
The AD1954 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 8. This figure shows two enve-
lopes—one with the harmonic distortion, as seen in the previous
figure, and the other, flatter envelope is the one produced by
the AD1954.
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 9. The
RMS detector is controlled by three parameters, stored in
parameter RAM: the RMS time constant, the hold time, and the
release rate. The LOG output of the RMS detector is applied to
a lookup 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 lookup table resides in the Parameter RAM and
is loaded by the user to give the desired curve. The lookup table
contains 33 data locations, and the LSB of the address into the
lookup 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 com-
pressor, the table resides in locations 110 to 142 in the SPI
Parameter RAM.
CONSTANT
TIME
DETECTOR WITH
MODIFIED RMS
LOG OUTPUT
HOLD RELEASE
HIGH BITS (1 LSB = 3dB)
LOOKUP TABLE
LOW BITS
PROGRAMMABLE
HOLD TIME, SPI-
PRELIMINARY TECHNICAL DATA
INPUT WAVEFORM
INTERPOLATION
LINEAR
RELEASE TIME, SPI-
PROGRAMMABLE
GAIN STAGE
OUTPUT TO
One subtlety of the table lookup involves the difference between
the RMS value of a sine wave at 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 top table entry (location 142 for main
channel compressor) has been set to correspond to the RMS
value of a 0 dBFS square wave. Since we would prefer to cali-
brate ourselves to sine wave amplitudes, we will refer to this
table entry as 3 dB. Therefore, the table will range from +3 dB
(location 142) to –96 dB (location 110).
The entries in the table are linear gain words in 2.20 format.
Figure 10 shows an example of the table entries for a simple
above-threshold compressor.
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.
1.0
INPUT LEVEL – 3dB/TABLE ENTRY
INPUT LEVEL – 3dB/TABLE ENTRY
COMPRESSION
DESIRED
CURVE