AD1953_15 datasheet(17/36 Pages) AD

17 / 36 page

REV. 0

AD1953

–17–

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.

LOOK-UP

TABLE

LINEAR

INTERPOLATION

MODIFIED RMS

DETECTOR WITH

LOG OUTPUT

HIGH BITS (1LSB = 3dB)

LOW BITS

TIME

CONSTANT

HOLD

RELEASE

DELAY

SPI-PROGRAMMABLE

LOOK-AHEAD DELAY

POST-COMPRESSION

GAIN, SPI-

PROGRAMMABLE

UP TO 30dB

(L+R)

Figure 13. Complete Dynamics Flow, Main Channels

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.

rms tconst

parameter

release rate

f S

. –









10 10

10 0

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. fS = audio sampling rate.

RMS Hold Time

rms holdtime

parameter

hold time

int

=×

()

where

rms_holdtime_parameter = integer number to enter into the SPI RAM

fS = audio sample rate

Hold_time = absolute time to wait before starting the release

ramp-down of the detector output

int() = integer part of expression

RMS Release Rate

rms decay

parameter

rms decay

int

/ .

()

1 096

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

Lookahead delay

parameter

Lookahead delay

=×

Where Lookahead_delay = predictive compressor delay in abso-

lute time

fS = audio sample rate

The maximum Lookahead_delay_parameter value is 100.

Post-Compression Gain

Post

compression

gain

parameter

Post

compression

gain

linear

()

∧ 15

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

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, as 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

VIN_SUB = K1

LEFT_IN + K2

RIGHT_IN

POST-COMPRESSION

GAIN, SPI-

PROGRAMMABLE

UP TO 30dB

BIQUAD

FILTER

Figure 14. Signal Flow for Subwoofer Compressor

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

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