AD1953_15 datasheet(16/36 Pages) AD

16 / 36 page

REV. 0

AD1953

–16–

INPUT WAVEFORM

HOLD TIME, SPI-

PROGRAMMABLE

RELEASE TIME, SPI-

PROGRAMMABLE

Figure 9. Using the Hold and Release Time Feature

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 AD1953 is shown in Figure 10. 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 look-up 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 look-up table resides in the parameter RAM and is loaded

by the user to give the desired curve. The look-up table contains

33 data locations, and the LSB of the address into the look-up

table corresponds to a 3 dB change in the amplitude of the detec-

tor signal. This gives the user the ability to program an input/

output curve over a 99 dB range. For the main compressor, the

table resides in locations 110 to 142 in the SPI parameter RAM.

LOOK-UP TABLE

LINEAR

INTERPOLATION

MODIFIED RMS

DETECTOR WITH

LOG OUTPUT

OUTPUT TO

GAIN STAGE

HIGH BITS (1LSB = 3dB)

LOW BITS

TIME

CONSTANT

HOLD

RELEASE

Figure 10. Gain Derived from Interpolated Look-Up Table

One subtlety of the table look-up involves the difference between

the rms value of a sine wave and that of a square wave. If a

full-scale square wave is applied to the AD1953, the rms value of

this signal will be 3 dB higher than the rms value of a 0 dBFS sine

wave. Therefore, the table will range from +9 dB (location 142)

to –87 dB (location 110).

The entries in the table are linear gain words in 2.20 format.

Figure 11 shows an example of the table entries for a simple

above-threshold compressor.

INPUT LEVEL – 3dB/TABLE ENTRY

1.0

DESIRED

COMPRESSION

CURVE

Figure 11. Example of Table Entry for a Given

Compression Curve

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 compression

curve may be shifted upward by using the post-compression gain

block following the compressor/limiter.

The AD1953 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 12.

CONVENTIONAL COMPRESSOR GAIN

LOOK-AHEAD COMPRESSOR GAIN

HOLD TIME

Figure 12. Conventional Compression vs. Look-Ahead

Compression

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