The bandpass filter allows a certain
range of frequencies to pass through and rejects any frequencies
that are outside of that range. So if you have a person talking
with rumbling traffic and high-pitched TV static in the background,
you could use the bandpass filter to isolate the center frequencies
of the person's voice.
This filter allows you to adjust the
dynamic range of the audio (the difference between the loudest
and softest sounds). Compression lowers the volume when it exceeds
a certain threshold (making loud sounds quieter and therefore
decreasing dynamic range). Expansion lowers the volume when it
is lower than the threshold (thus making quiet sounds quieter
and therefore increasing dynamic range).
Using a compressor and expander at the
same time allows you to avoid overly-loud peaks while still maintaining
dynamic range. The compressor will reduce the louder portions
of the audio in proportion to their distance from the threshold
(i.e. louder sounds will be reduced by a much larger amount),
helping to even out amplitude variations. The expander will then
take the quieter sounds and reduce their volume (again, proportionally),
thus evening out the amplitude variation of the quieter sounds
and increasing the dynamic range.
Imagine some music where you have loud
crashing drums, a relatively quiet triangle and a trumpet somewhere
in between. The drums are far too loud so you apply a compressor
to bring them down. The drums are now a lot quieter but you've
noticed that the subtlety of the triangle has been lost - it
is now far too prominent. So you apply an expander to lower the
volume of the triangle to an acceptable level. The AUDynamicsProcessor
allows you to do this with one filter instead of two, which is
a much more efficient use of system resources.
Attack time refers to the amount of time
it takes for the compressor to implement a change in volume,
and release time is the time taken to reduce the compressor back
to its original level afterwards.
There's a bit of debate over what "headroom"
actually means as it's not a standard term. To the best of my
knowledge, it refers to the number of dB the signal is permitted
to exceed the threshold of the compressor/limiter.
The Dynamics Processor is a great way
of compressing and/or expanding your audio to make sure that
it fits within the audible range of your listening equipment.
This repeats parts of your audio. Dry/wet
mix controls whether or not the repeated sounds overlap each
other. 100% dry means no overlapping and 100% wet means full
overlapping. Delay time is the delay before the repeat starts.
Feedback controls the number of repetitions. Lowpass cutoff frequency
stops certain low sounds from passing through the filter.
This filter is normally used for atmospheric
AUDistortion (Leopard only)
This is a very comprehensive distortion
filter offering a lot of control. I can't give much advice on
the various parameters as I don't actually know what they mean
and there's no documentation available. I would suggest adjusting
by ear (it is probably worthwhile lowering the render quality
while adjusting, and then raising it again once you're happy
with the results).
I use this one a lot. It allows you to
take 5 bands (5 ranges of frequencies) and adjust the gain up
or down for each one. If a person's voice is muffled, increasing
the gain on the higher frequencies can often improve this. I
also sometimes use it to add bass to make a person's voice sound
The bandwidth slider allows you to adjust
the size of each frequency range, with the frequency sliders
referring to the center of the range.
This is similar to the AUFilter above
but the size of the frequency ranges cannot be adjusted and you
can have up to 31 bands instead of just 5. This
site provides a guide to common frequency ranges.
I've used this a couple of times to boost
bass in a person's voice but I generally prefer AUFilter for
this. It's good for removing or reducing a specific frequency
This cuts off frequencies above the cutoff
frequency, allowing lower sounds to pass through. The gain slider
controls the amount that the frequencies passing through will
Good for reducing high-frequency noise.
This reduces lower frequencies and allows
higher ones to pass through. Resonance controls the intensification
of the higher frequencies that pass through.
Good for removing low rumbling sounds
such as traffic or for reducing very deep voices (I have had
to do this at times).
Reduces (attenuates) higher frequencies
and lets lower ones through, controlled by the cutoff frequency.
Resonance controls the intensification of the frequencies that
Good for removing high-pitched noise.
Reduces frequencies lower than the cutoff
frequency and lets higher ones pass through. This is similar
to the AUHighPass filter above but this offers the ability to
adjust the gain instead of resonance.
Good for removing low hums.
This allows you to compress multiple
bands (frequency ranges) individually for more control than a
Pre-gain boosts the signal before it
is processed which is useful if the signal is too low to be processed
adequately by the compressor, and post-gain reduces the gain
back to a normal level afterwards.
Crossovers 1, 2 and 3 define the point
at which the previous band ends and the next one begins. Threshold
refers to the amplitude level at which the compressor kicks in.
Headroom, as stated above, probably gives you extra leeway above
the threshold. Eq allows you to boost or lower each band.
Possible uses for the Multi Band Compressor
include lowering the dynamic range of bass sounds without interfering
with higher frequency sounds. This is different from the High
Shelf Filter which indiscriminately cuts all bass below a certain
AUNetSend is one of the most interesting
filters in that it's not actually a filter. It does absolutely
nothing to affect the way your audio sounds but what it does
do is allow you to send audio across a network.
You need to have an application that
implements audio generators. Audio editing applications such
as Logic implement these. Alternatively, there is a tool called
AU Lab in /Developer/Applications/Audio if you have the developer
tools installed. Soundtrack Pro does not support this. Add an
AUNetReceive generator to the track in your audio application,
add an AUNetSend filter to your FCP audio clip and hit play in
FCP. You should see "AUNetSend" pop up in the AUNetReceive
configuration dialog. Select it and your track should be receiving
the audio from Final Cut Pro. Note that it only appears while
the timeline is playing in FCP.
In my testing, the Status parameter appeared
to do absolutely nothing.
This allows you to boost or lower the
amplitude of a signal within a certain range. This is useful
if for example you have a high-pitched buzzing noise in the background
and you only want to eliminate that particular frequency and
keep your higher sounds intact.
The Peak Limiter differs from a compressor
in that a compressor reduces the volume of an entire track when
a frequency reaches a certain level whereas the Peak Limiter
reduces just that frequency. This is particularly useful if there
is background noise on the track that would produce a noticeable
fluctuation if the entire track were to receive a volume adjustment.
Attack and release time, as mentioned
above, control the amount of time it takes for the filter to
implement a change in amplitude, with longer times allowing a
smoother transition. Pre-gain allows you to boost the volume
before it reaches the filter in order to change the number of
frequencies being affected. The limiting amount allows you to
limit the amount that the filter will reduce the amplitude.
Adjusts the pitch of your audio, obviously.
There are a LOT of controls though, and I have to admit that
I don't have a clue what a Glb Trigger Thresh or a Loud Aggr
K is. Effect blend blends the pitch-shifted audio with the original
and is sometimes necessary to make voices sound natural. It is
worth mentioning that I've gotten perfectly acceptable results
by adjusting the first two parameters and not bothering with
any of the others. I'd imagine that most people wouldn't have
to adjust more than these.
I tried a Google search on some of these
parameters but they only turned up forum threads where people
were asking what on Earth they meant and no-one was able to solve
their problem. If anyone does know, I'd be interested to hear
This can be useful for making a male
actor's voice more masculine (yes, I have had to do this!).
AURogerBeep (Leopard only)
This emulates the "roger beep"
sound when someone lets go of the button on a walkie talkie.
It will automatically play the sound when the audio level drops
below a certain threshold for a certain amount of time (as if
the person stopped speaking).
In gate threshold and in gate time refer
to the time that the threshold amplitude must be maintained before
the sound kicks in. Out gate threshold and out gate time refer
to the amount of time that the threshold must be maintained before
it ends the roger beep (adjust this if there is background noise).
Roger level is the volume of the roger beep, and I think sensitivity
and roger type are self-explanatory.
This is an effects filter with quite
a narrow purpose so it's not something that gets used very often.
Similar to AUDelay except that the delay
time is set as a number of audio samples instead of a number
Two of the above filters are Leopard
only. If you use those filters and then transfer your project
to a Tiger machine, you will receive an error message and will
be unable to use those filters within the project.
for Part 2
Copyright 2008 Digital Rebellion,
This article was first published
on Digital Rebellion
and is reprinted here with permission from the author.
Jon Chappell is an editor,
VFX artist and software developer originally from the UK. He
is the owner of Digital
Rebellion LLC and is a regular contributor to the Final Cut
Pro community. He is well known for developing the popular troubleshooting
Remover and Preference
Manager. His film credits include Perfect Sport.