AD1954 Analog Devices, AD1954 Datasheet - Page 17

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

The AD1954 compressor/limiter also includes a “look-ahead
compression” 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 detector 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 11.
In the look-ahead compressor, the gain has already been reduced
by the time that the tone-burst signal arrives at the multiplier
input. Note that when using a look-ahead compressor, it is
important to set the detector hold time to a value that is at least
the same as the look-ahead delay time, or else the compressor
release will start too soon, resulting in an expanded “tail” of a
tone burst signal. The complete flow of the left/right dynamics
processor is shown in Figure 12.
The detector path works from a sum of left and right channels
((L+R)/2). This is the “normal” way that compressors are built,
and it counts on the fact that the main instruments in any stereo
mix are seldom recorded deliberately out-of-phase, especially in
the lower frequencies, which tend to dominate the energy spectrum
of real music.
The compressor is followed by a block known as “post-compression
gain.” Most compressors are used to reduce the dynamic range
of music by lowering the gain during loud signal passages. This
results in an overall loss of volume. This loss can be made up by
introducing gain after the compressor. In the AD1954, the coef-
CONSTANT
CONVENTIONAL COMPRESSOR GAIN
LOOK-AHEAD COMPRESSOR GAIN
(L+R)
TIME
2
DETECTOR WITH
MODIFIED RMS
LOG OUTPUT
HOLD RELEASE
HIGH BITS (1 LSB = 3dB)
SPI-PROGRAMMABLE
LOOK-AHEAD DELAY
LOOKUP
TABLE
LOW BITS
DELAY
DELAY
PRELIMINARY TECHNICAL DATA
INTERPOLATION
LINEAR
HOLD TIME
POST-COMPRESSION
PROGRAMMABLE
UP TO 30dB
GAIN, SPI-
ficient format used is 2.20, which has a maximum floating-point
representation of slightly less than 2.0. This means that the maxi-
mum gain that can be achieved in a single instruction is 6 dB.
To get more gain, the program in the AD1954 uses a cascade of
five multipliers, to achieve up to 30 dB of post-compression gain.
To program the compressor/limiter, the following formulas may
be used to determine the 22-bit numbers (in 2.20 format) to be
entered into the parameter RAM.
RMS Time Constant
This can be best expressed by entering the time constant in
terms of dB/sec “raw” release rate (without the peak-riding circuit).
The attack rate is a rather complicated formula that depends on
the change in amplitude of the input sine wave.
Where Rms_tconst_parameter = fractional number to enter
into the SPI RAM (after converting to 22-bit 2.20 format)
Release_rate = release rate of the raw RMS detector in dB/sec.
This must be negative.
f
RMS Hold Time
Where rms_holdtime_parameter = integer number to enter into
the SPI RAM
f
Hold_time = absolute time to wait before starting the release
ramp-down of the detector output.
int() = integer part of expression
RMS Release Rate:
Where RMS_decay_parameter = Decimal integer number to
enter into the SPI RAM
Rms_decay = decay rate in dB/sec
int() = integer part of expression
Look-ahead Delay
Where Lookahead_delay = predictive compressor delay in
absolute time
f
The maximum lookahead_delay_parameter value is 100.
Post-Compression Gain
Where Post_compression gain_linear is the linear post compression
gain ^ = raise to the power
S
S
S
= audio sampling rate.
= audio sample rate
= audio sample rate.
rms holdtime parameter
Rms tconst parameter
RMS decay parameter
Lookahead delay parameter
Post compression gain parameter
post compression gain linear
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Lookahead delay
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release rate
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AD1954
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