AD1953YSTRL7 Analog Devices Inc, AD1953YSTRL7 Datasheet - Page 17

• Manufacturer Part Number: AD1953YSTRL7
• Description: IC DSP DAC AUDIO3CH/26BIT 48LQFP
• Manufacturer: Analog Devices Inc
• Series: SigmaDSP®r

Specifications of AD1953YSTRL7

• Rohs Status: RoHS non-compliant
• Number Of Bits: 26
• Data Interface: Serial
• Number Of Converters: 3
• Voltage Supply Source: Analog and Digital
• Power Dissipation (max): 540mW
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• For Use With: EVAL-AD1953EBZ - BOARD EVAL FOR AD1953 3CH 24BIT
• Settling Time: -

In the look-ahead compressor, the gain has already been reduced
by the time the tone-burst signal arrives at the multiplier input.
Note that when using a look-ahead compressor, it is impor-
tant 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 13.
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 AD1953, the
coefficient format used is 2.20, which has a maximum floating-
point representation of slightly less than 2.0. This means the
maximum gain that can be achieved in a single instruction is 6 dB.
To get more gain, the program in the AD1953 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
REV. 0
CONSTANT
(L+R)
TIME
Figure 13. Complete Dynamics Flow, Main Channels
2
DETECTOR WITH
MODIFIED RMS
LOG OUTPUT
rms holdtime parameter
HOLD RELEASE
rms tconst
_
_
HIGH BITS (1LSB = 3dB)
S
SPI-PROGRAMMABLE
= audio sampling rate.
LOOK-AHEAD DELAY
_
LOOK-UP
_
TABLE
LOW BITS
parameter
DELAY
DELAY
INTERPOLATION
=
=
LINEAR
1 0 10
int
. –
POST-COMPRESSION
(
PROGRAMMABLE
f
S
UP TO 30dB
×



GAIN, SPI-
release rate
10 0
hold time
.
×
f
_
S



)
–17–
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 abso-
lute 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
Subwoofer Compressor/Limiter
The subwoofer compressor/limiter differs from the left/right
compressor in the following ways:
1. The subwoofer compressor operates on a weighted sum of left
2. The detector input has a biquad filter in series with the input
3. There is no predictive compression, as presumably the input
The subwoofer compressor signal flow is shown in Figure 14.
The biquad filter before the detector can be used to implement
a frequency-dependent compression threshold. For example,
assume that the overload point of the woofer is strongly fre-
quency-dependent. In this case, one would have to set the
compressor threshold to a value that corresponded to the most
sensitive overload frequency of the woofer. If the input signal
happened to be mostly in a frequency range where the woofer
V
S
S
IN
= audio sample rate
= audio sample rate
and right inputs (aa × Left + bb × Right), where aa and bb are
both programmable.
in order to implement frequency-dependent compression
thresholds.
signals are filtered to pass only low frequencies, and therefore
transient overshoots are not a problem.
_SUB = K1
Lookahead delay parameter
Figure 14. Signal Flow for Subwoofer Compressor
BIQUAD
FILTER
rms decay parameter
CONSTANT
LEFT_IN + K2
_
Post compression gain parameter
Post compression gain linear
_
TIME
DETECTOR WITH
MODIFIED RMS
LOG OUTPUT
_
_
_
HOLD RELEASE
_
HIGH BITS (1LSB = 3dB)
RIGHT_IN
_
_
=
=
int
LOOK-UP
LOW BITS
Lookahead delay
TABLE
(
rms decay
_
_
_
∧
INTERPOLATION
( )
_
AD1953
1 5
POST-COMPRESSION
LINEAR
/ .
PROGRAMMABLE
=
1 096
UP TO 30dB
GAIN, SPI-
×
f
)
S