SAA7715 PHILIPS [NXP Semiconductors], SAA7715 Datasheet - Page 21

• Manufacturer Part Number: SAA7715
• Description: Digital Signal Processor
• Manufacturer: PHILIPS [NXP Semiconductors]

Philips Semiconductors
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.
2001 May 07
handbook, halfpage
handbook, halfpage
Digital Signal Processor
output
Fig.7
Fig.8
level
(dB)
output
level
(dB)
Gain reduction is applied only when the
signal exceeds the set threshold level.
Output gain can be used to restore the peak
level to its maximum.
threshold
threshold
slope =
input level (dB)
input level (dB)
1/2
4:1 compression
output
no compression
gain
10:1 compression
2:1 compression
4:1 compression
(limiting)
MGT829
MGT830
21
10.1.1.1
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
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
compromise for a wide variety of source material.
10.1.1.3
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
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.
36 dB(FS). These settings were chosen as a good
Control parameters
Fixed versus variable mode
Threshold
Ratio
Preliminary specification
SAA7715H