dbx 128 - Application Scenarios

range and, therefore, the excitement of the original per-
formance. The same circuits that do the expanding can be
used to compress a program for background music or for
making car stereo tapes, making the loud levels quieter,
and the quiet levels louder. Depending on front-panel
control settings, the linear decibel compression/expansion
circuitry can also “‘peak limit’ or “peak unlimit.” Peak
limiting is simply compression with the 128 set for “above
threshold” operation. This prevents program levels from
rising above an adjustable level (threshold), but has no
effect on levels below that point. Conversely, peak un-
limiting is simply expansion withthe 128 set for ‘‘above
threshold’”’ operation. This increases the amplitude
during the loud passages of the music, without affecting
the program below the set threshold level.
When to Use Expansion
Use the 128’s expansion capabilities to reduce the noise
present in the original source. For example, you can expand
a highly compressed and older recording and increase its
dynamic range to the point where it is comparable to your
best conventional phonograph discs and, at FM broadcast
time, significantly reduce the hiss. Or you can expand a
conventional disc and dramatically reduce surface noise
and turntable rumble at the same time you increase its
dynamics to recreate sound like a live performance.
When to Use Compression
Compression is used for recording a program which
must have a relatively constant level, as for example when
playing or recording background music. At the low listen-
ing levels of background music, quieter passages must be
kept loud enough to be heard, yet loud passages must not
be too loud. Similarly, if you are recording music for play-
back in your automobile or any other high noise area,
compression can keep the quiet sounds above the noise,
while it holds the loud sounds below distortion-causing
levels. Compression is also useful for a live recording of
several people with different voice levels. In this case, the
compressor acts like an automatic volume control and
saves you the task of “gain riding.”
When to Use Peak Limiting
If a tape must be recorded without the benefit of dbx Il
tape noise reduction (for later playback without dbx
decoding), then peak limiting is very useful because it can
prevent musical peaks from causing tape saturation. Peak
limiting can also be used to protect speaker systems from
extremely loud musical peaks, or from loud ticks or pops
on a damaged phonograph disc.
When to Use Peak Unlimiting
Many record manufacturers, and all radio stations use
some form of peak limiting in order to record or broadcast
high average program levels without having sudden musical
peaks saturate the tape or overmodulate the radio
transmitter.
“Unlimiting’’ (peak expanding) with the 128 can correct
the dynamic alteration created by peak limiting operations.
For example, suppose you have a particularly good record-
ing of a concert, one with excellent overall dynamics, how-
ever, some passages may lack the expected dynamic impact,
giving an unnaturally controlled feeling where instruments
should stand out. Such a recording probably had very little
original compression, but had peak limiting applied to high
level passages. The 128’s peak unlimiting action could make
this recording sound better.