Dolby Laboratories 430 - Manual (Page 10)

2-2
4 High-frequency and Low-frequency Suppression Controls
The left-hand slider controls the amount of low level LF attenuation the unit applies; the right-hand slider
controls the low level HF attenuation. When the controls are at the top there is no processing, and at the
bottom there is maximum processing. The effect is gradual with about half scale giving enough processing
for most applications; only on severely noisy tracks will more than half scale be needed. Maximum
attenuation is approximately 16 dB at low-frequencies, and 18 dB at high-frequencies.
2.2 PRACTICAL OPERATION
If the amount of noise that needs to be suppressed is small, i.e., the noise itself is at a low level, then the
amount of processing needed will be small, and the change from expansion to linear (no processing) will be
so gentle and gradual that the default setting of the rotary level control (pushed in) will give good results on
virtually all types of dialog and interference. In fact, for small amounts of processing, the effect of the control
is almost inaudible.
On the other hand, if the unwanted background noise is relatively loud and there is only a small difference
between speech level and the background noise, then maximum or near maximum processing may be
required. The rotary level control should be pulled out and set so that normal voice is lighting the three
central LEDs more or less equally (there is little or no processing at
±
10 dB of nominal level). This gives a
visual indication that the dialog lies in the linear (no processing) band and the noise can then be suppressed
by drawing down the sliders.
If the level of the track is too high or the rotary level control too far clockwise, then noise can start to enter
the linear band and processing will be partially lost. If the track level is too low or the level control too far
counterclockwise, then the voice will take on an expanded character with poorly controlled dynamics. Noise
will be modulated by the voice more and the voice will be dulled by the HF control and dried by the LF
control.
In situations where there is too much reverberation, a combination of HF and LF processing increases
intelligibility and reduces the perceived reverberation time. Similar treatment is also useful in situations
where dialog has been overcompressed by ALC circuits in portable camcorders or similar recording
equipment. If the interference is broadband, clearly both the LF and HF bands can be used in varying
amounts. The expander sections are in series so their effects tend to sum. For this reason, the amount of
processing used can often be reduced when both bands are brought into play since they broadly overlap in
the range of 200 Hz to 2kHz.
2.3 CONSOLE SIGNAL PATH INSERTION GUIDE
The choice of Model 430 insertion point in the console chain is very important for best results. Where the
dialog already lacks presence, HF processing will tend to accentuate the problem and sibilants will become
more muffled except when they occasionally come through clearly giving an impression of poorer vocal
control than the original track. Under these circumstances, an equalizer should precede the unit to bring the
sibilants up into the linear region of operation in the Model 430.
As with other playback-only devices, a large amount of high-frequency low level attenuation may
subjectively lower the HF speech content. This effect can be compensated by judicious use of console
equalization after the Model 430 unit.
When there is a strong interfering tone such as AC power buzz or generator hum, this is best reduced by
high pass filtering before Model 430 processing so that the level difference between noise and dialog can be
maximized, reducing the amount of processing required. If intrusive noise includes spot tonal signals along
with ‘noise blanket’ components, the spot tones should be attenuated prior to suppression of the wide-band
noise components, e.g., dip filters should precede the Model 430 in the signal chain.
On the other hand, owing to the filtering nature of the treatment, it may be more pleasing to lightly equalize
the output of the Model 430 processor at the opposite end of the spectrum to that at which treatment has
been selected so that the effective passband is more balanced about the 200 to 2 kHz decade.
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