SAA7715 Datasheet, PDF (21/36 Pages) NXP Semiconductors – Digital Signal Processor

Philips Semiconductors
Digital Signal Processor
Preliminary specification
SAA7715H
Ironically, most people think in terms of boosting the low
signals when talking about dynamics compression. In fact,
this is what actually happens after the output is rescaled to
account for the gain reduction imposed by the current
settings. By doing this the output signal can be forced to
carry more power than the input. This is what gives the
compressor its ‘punch’ quality, for a more ‘in your face’ sort
of sound. Figure 8 shows an example of the transfer
curves before and after application of output gain. Users
should be aware, however, that abuse of output gain can
amplify system noise to intolerable levels.
handbook, halfpage
output
level
(dB)
no compression
slope =
1/2 _{2:1 compression}
4:1 compression
10:1 compression
(limiting)
threshold
input level (dB)
MGT829
Fig.7 Gain reduction is applied only when the
signal exceeds the set threshold level.
handbook, halfpage
output
level
(dB)
output
gain
4:1 compression
threshold
input level (dB)
MGT830
10.1.1.1 Control parameters
Common to most compressors are five control parameters
used for adjusting the behaviour of the compressor. These
are typically labelled as threshold, ratio, attack time,
release time, and output. By careful adjustment of these
controls a skilled user can produce very pleasing results
for a wide variety input source material. In the following
subsections, functionality of each control is described.
10.1.1.2 Fixed versus variable mode
The compressor module can be operated in so-called
‘fixed’ mode or ‘variable’ mode. When in variable mode,
the user has full control over both the threshold and ratio
controls. In fixed mode, controls are frozen and the effect
operates at a fixed ratio of 2:1, with a threshold setting of
−36 dB(FS). These settings were chosen as a good
compromise for a wide variety of source material.
10.1.1.3 Threshold
Threshold determines the level at which gain reduction
begins. For example, if the threshold is set at −10 dB(FS),
this means that all signals below −10 dB(FS) will be
passed unaltered. Only when the input level exceeds this
threshold is gain reduction (compression) brought to bear.
Many times a dramatic change in the threshold setting will
call for a ratio adjustment. Experiment with these two
controls to find what works best for your system, your
music, and most importantly, your ears.
10.1.1.4 Ratio
The ratio control sets the desired compression ratio.
Settings are traditionally expressed in ratios such as 1.5:1,
2:1, 4:1, 10:1, etc. An explanation of how to interpret these
settings is best served by example. Say we are dealing
with a ratio of 1.5:1. This means that for every 1.5 dB
increase in input level beyond the threshold, only 1 dB is
passed to the output. Another way of explaining this is in
terms of gain reduction. In this particular case a 0.5 dB
gain reduction is imposed for every 1.5 dB increase
beyond the threshold level.
Compression ratio is changed by selecting one of the
values in the drop-down list labelled ‘Ratio’. To increase
the amount of compression, select one of the higher ratios.
For a more subtle effect, select a lower setting, such as
1.5:1.
Fig.8 Output gain can be used to restore the peak
level to its maximum.
2001 May 07
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