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| 型號： | AD1954 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | CONNECTOR COAXIAL BNC |
| 中文描述： | SigmaDSP的⑩3通道，26位DAC的信號處理 |
| 文件頁數(shù)： | 17/36頁 |
| 文件大?。?/td> | 1470K |
| 代理商： | AD1954 |
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AD1954
–17–
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 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.
LOOK-UP
TABLE
LINEAR
INTERPOLATION
MODIFIED RMS
DETECTOR WITH
LOG OUTPUT
HIGH BITS (1LSB = 3dB)
LOW BITS
TIME
CONSTANT
HOLD
RELEASE
DELAY
DELAY
SPI-PROGRAMMABLE
LOOK-AHEAD DELAY
POSTCOMPRESSION
GAIN, SPI-
PROGRAMMABLE
UP TO 30dB
2
(L+R)
Figure 13. Complete Dynamics Flow, Main Channels
The detector path works from the sum of the left and right channels
((L + R)/2). This is the normal way that compressors are built and
counts on the fact that the main instruments in any stereo mix are
seldom recorded deliberately out of phase, especially in the lower
frequencies that 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-
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.
rms_tconst_parameter
–
_
×
=
)
1.0
10
.
release rate
10 0
f
S
where
= the fractional number to enter into
rms_tconst_parameter
the SPI RAM (after converting to 22-bit 2.20 format), and the
release_rate = the
release rate
of the raw rms detector in dB/sec.
This must be negative, and
f
f
= the audio sample rate.
S
RMS Hold Time
rms_hold_time_parameter
int
hold_time
=
×
(
)
f
S
Where
= the integer number to enter into
rms_holdtime_parameter
the SPI RAM,
f
f
= the audio sample rate,
S
lute time to wait before starting the release ramp-down of the
detector output, and
int
() = the integer part of the expression.
RMS Release Rate
rms_decay_parameter
=
where
= the decimal integer number to enter
rms_decay_parameter
into the SPI RAM,
rms_decay
= the decay rate in dB/sec, and
int
() = the integer part of the expression.
Look-Ahead Delay
lookahead_delay_parameter
hold_time
= the abso-
int rms_decay /
)
0.137
lookahead_delay
f
=
×
S
where
lookahead_delay
= the predictive compressor delay in
absolute time,
f
f
= the audio sample rate, and the maximum
S
value is 100.
lookahead_delay_parameter
Postcompression Gain
post_compression_gain_parameter
post_compression_gain_linear
=
)
(
∧
1/5
where
is the linear post-compression
post_compression_gain_linear
gain and ^ = the 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 the
left and right inputs (aa
Left + bb
Right), where aa and
bb are both programmable.
2. The detector input has a biquad filter in series with the input
in order to implement frequency-dependent compression
thresholds.
3. There is no predictive compression since presumably the input
signals are filtered to pass only low frequencies and therefore
transient overshoots are not a problem.
The subwoofer compressor signal flow is shown in Figure 14.
LOOK-UP
TABLE
LINEAR
INTERPOLATION
MODIFIED RMS
DETECTOR WITH
LOG OUTPUT
HIGH BITS (1LSB = 3dB)
LOW BITS
TIME
CONSTANT
HOLD
RELEASE
V
IN
_SUB = k1
LEFT_IN + K2
RIGHT_IN
POSTCOMPRESSION
GAIN, SPI-
PROGRAMMABLE
UP TO 30dB
BIQUAD
FILTER
Figure 14. Signal Flow for Subwoofer Compressor
REV. A
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| AD1954YS | RESISTOR |
| AD1954YSRL | 16-bit fixed point DSP with Flash |
| AD1954YST | RESISTOR |
| AD1954YSTRL | Hook-Up Wire; Conductor Size AWG:12; No. Strands x Strand Size:65 x 30; Jacket Color:Yellow; Approval Bodies:UL, CSA; Approval Categories:UL AWM Style 1015, CSA Type TEW, JQA-F-, Passes VW-1 Flame Test; Conductor Material:Copper RoHS Compliant: Yes |
| AD1954YSTRL7 | 16-bit fixed point DSP with Flash |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| AD1954YS | 制造商:Analog Devices 功能描述:DAC 3-CH Delta-Sigma 24-bit 44-Pin MQFP 制造商:Rochester Electronics LLC 功能描述:THREE CHANNEL, 24-BIT SIGMADSP DAC - Bulk |
| AD1954YSRL | 制造商:Analog Devices 功能描述:DAC 3-CH Delta-Sigma 24-bit 44-Pin MQFP T/R |
| AD1954YST | 制造商:Analog Devices 功能描述:DAC 3-CH Delta-Sigma 24-bit 48-Pin LQFP 制造商:Rochester Electronics LLC 功能描述:THREE CHANNEL, 24-BIT SIGMADSP DAC - Tape and Reel |
| AD1954YSTRL | 制造商:Analog Devices 功能描述:DAC 3-CH Delta-Sigma 24-bit 48-Pin LQFP T/R |
| AD1954YSTRL7 | 制造商:AD 制造商全稱:Analog Devices 功能描述:SigmaDSP⑩ 3-Channel, 26-Bit Signal Processing DAC |
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