October 13, 2008
Final Cut Pro Audio Filter Guide Part 2
By Jon Chappell
This is the second part of the Final Cut Pro Audio Filter Guide. Part 1 covered the Audio Units that ship with Mac OS X and is available here. This part will cover the audio filters that ship as part of Final Cut Pro.
This guide is intended to inform Final Cut Pro users of the options available to them for fixing bad sound.
Now, one thing you may have noticed is that there are often duplicate filters such as AUBandpass (an Audio Unit) and the built-in Band Pass Filter. So which should you use? Well, for a start, bear in mind that some of those filters have the same function but different parameters. I would generally advise you to pick the built-in filters over the Audio Units whenever possible, as they tend to use up fewer system resources.[Top]
3 Band Equalizer
This allows you to take three separate bands (low, med and high) and adjust the gain up and down individually. I personally prefer AUFilter (in the Apple folder) because it gives you a bit more control and allows you to adjust 5 bands.
This is useful when you need to boost the bass of a voice or improve a flat-sounding voice.
Band Pass Filter
This will allow a range of frequencies on either side of the center frequency to pass through and reduce (attenuate) frequencies outside this range.
This is identical to the AUBandpass filter (in the Apple folder) except that instead of the bandwidth parameter, it has a mysterious one called Q. Q stands for Quality Factor and is a different way of representing the bandwidth. There are numerous articles about it on the internet that get quite technical but all you need to know is that it's the relationship between the center frequency and the bandwidth (f/b) so Q is inversely proportional to the bandwidth (i.e. when the bandwidth goes up, Q goes down by a proportional amount). It is worth noting that Q is not the bandwidth itself but it is related to it. If you want to find out the bandwidth, just divide the center frequency by Q.
If you don't understand the above explanation just play it by ear or use the AUBandpass filter.
The Compressor / Limiter reduces the volume of sounds above the threshold amplitude. This is a useful way of minimizing the difference between two subjects talking at different volumes or making sure that the audio fits within the limits of the playback device.
Obviously this reduces the volume of the overall audio so Preserve Volume rectifies this (although I find it is often then too loud). Attack time refers to the time it takes for the filter to decrease the volume once it has detected a frequency with an amplitude above its threshold. Release time refers to the time taken for the filter to increase the volume again once the high amplitude frequency has finished playing. Higher values allow for a smoother and less noticeable response but set them too high and the compressor won't respond quickly enough. This is something inherited from live audio mixing where you don't know what's coming next. Setting the threshold to just under your preferred limit allows time for the compressor to lower the volume in anticipation for a louder sound once the threshold is reached.
Ratio tells the compressor by how much it should reduce the volume when a sound exceeds the threshold. If the ratio is set to 2 (2:1), then a 10 dB increase in volume above the threshold will be halved to a 5 dB increase. It is worth noting that the compressor lowers the volume of sounds above the threshold but does not necessarily reduce them to a value at or below the threshold. Be aware that very loud sounds could still theoretically peak.
Finally, it is also worth mentioning that the compressor reduces the volume difference between the subject and any background noise, so background noise will be more noticeable upon boosting the audio after applying the compressor.
Sometimes you may experience a DC current leakage through the mic, causing noise in the recorded audio. A DC Notch filter will remove the DC offset component which you probably need a degree in audio engineering to fully understand. It has no parameters.
This is one of the lesser-used filters FCP provides.
This one obviously adds an echo. Effect Mix allows you to mix the echoing audio with the original in order to better blend it in. Effect Level controls the volume level of the echoes (but not the original audio). Brightness controls the degree that the echoes will overlap. Feedback controls how long each echo will last and delay controls the spacing between each repeat.
It is worth noting that the echoes will be abruptly cut off unless you lengthen the audio clip or fade it out.
Expander / Noise Gate
The opposite of a compressor. A compressor takes high amplitude sounds and lowers them whereas an expander takes low amplitude sounds (sounds below the threshold) and lowers them. I know it probably seems strange to lower sounds that are already quieter than the rest of the audio but this is designed to increase the dynamic range of the clip.
Threshold is the amplitude below which the expander will kick in. Ratio controls the proportion of expansion - for example with a ratio of 2 (2:1), a 3 dB fall below the threshold will be adjusted to a 6 dB fall. Attack time is the time for the volume change to be applied once it falls below the threshold. Release time is the time for the volume to return to normal once the signal goes above the threshold again.
A noise gate is a more extreme expander that will completely eliminate frequencies below the amplitude threshold. This can be achieved with high ratios (e.g. 10).
This is a simple filter that raises or lowers the volume of the audio. It is added automatically to clips when you use the Modify > Audio > Apply Normalization Gain command.
High Pass Filter
This will attenuate (reduce) low frequencies below the threshold. This is the same as a Low Shelf Filter. Use the Q slider to modify the bandwidth (width of the frequency range).
Useful for cutting out low frequency noise such as the rumbling of traffic or very low notes in a deep voice.
High Shelf Filter
This will attenuate (reduce) high frequencies above the threshold. This is the same as a Low Pass Filter. Gain allows you to adjust the volume of audio that passes through the filter.
Useful for filtering out high frequency noise such as buzzing on the soundtrack.
This is similar to the Shelf / Pass filters but it has several extra parameters. Frequency, Q and Gain have been explained many times above. In order to explain what the harmonic check boxes mean, we need to delve into a little audio theory.
The fundamental frequency is the lowest frequency in a harmonic series (the Frequency value in this case). Harmonics are integer multiples of the fundamental frequency - e.g. if f=60, 2f=120, 3f=180, etc. These play at the same time as the fundamental frequency and contribute to the tone of a sound. The Hum Remover is more powerful than a Shelf / Pass filter because it allows you to remove these specific frequencies without removing any frequencies in between these. For example, if f=60 and you wanted all harmonics up to 5f removed, a Low Shelf or High Pass filter would remove ALL frequencies up to 300 Hz, potentially affecting the quality of your sound. The Hum Remover would not do this.
Despite being called Hum Remover, the use of harmonics makes it useful for other purposes such as enhancing or reducing a musical instrument on a soundtrack.
Low Pass Filter
The Low Pass Filter has the same effect as the High Shelf Filter - it will attenuate (reduce) frequencies higher than the specified frequency range, keeping lower ones intact.
Frequency refers to the center frequency and Q is a way of representing the bandwidth (the width of the frequency range).
Useful for reducing high frequency noise such as buzzing.
Low Shelf Filter
The Low Shelf Filter has the same effect as the High Pass Filter - it will attenuate (reduce) frequencies lower than the specified frequency range, keeping higher ones intact.
Useful for reducing low frequency noise such as air conditioner hums.
The opposite of a Band Pass filter. Instead of only allowing frequencies within a certain range, this cuts out frequencies within a certain range.
Frequency refers to the center frequency and Q is a way of representing the bandwidth (the width of the frequency range).
Useful if you have noise of a constant frequency on your soundtrack (such as a buzzing sound).
This is a combination of Band Pass, Notch and Shelf filters combined into in a single filter. Frequency is the center frequency, Q is related to the bandwidth (see the explanation above) and gain allows you to boost or cut the frequencies passing through the filter.
This is similar to the Echo filter but considerably more sophisticated. Rather than simply repeating sounds with a delay, it allows you to mimic the characteristics of echoes within various locations. This is incredibly useful when performing ADR (Automated Dialogue Replacement) because it is highly likely that the sound booth you record the ADR in will sound nothing like the original location. This allows you to mimic the effect of sound waves bouncing off walls, with some canceling each other out and some increasing in intensity. It can also be used sparingly to improve a flat-sounding voice.
Subtlety is often the key with this filter and Effect Mix allows you to mix the reverb with the original sound to help blend it in. Effect Level controls the intensity, Brightness controls the degree that the echoes overlap and Type allows you to specify various preset locations.
It is worth mentioning that the reverberation will end abruptly unless you extend your audio clip or fade it out at the end.
This helps to reduce the intensity of "s" sounds, most noticeable if the actor has a lisp. The controls are similar to a compressor but it is optimized for reducing sibilant ("s") sounds. Ratio controls the amount of reduction - e.g. if the "s" sound is 6 dB and the ratio is 2, it will be reduced to 2 dB.
Emphasis controls the sensitivity of the filter and Broad Band Mode widens the bandwidth so that more frequencies around the center frequency are affected.
Sometimes if a microphone is directly in front of an actor's mouth they will accidentally breathe into it while speaking, causing a wind-type noise to be generated. This filter aims to minimize these.
The parameters are similar to a compressor - ratio controls the ratio of reduction proportional to the intensity of the sound. Broad Band Mode widens the bandwidth so that more frequencies around the center frequency are affected.
One Final Note
These filters do a decent job of repairing troublesome audio but they are not miracle cures. Sometimes (and this is never popular with producers) it just isn't cheaper to fix it in post. Even big Hollywood movies with access to multi-million dollar sound studios re-record a lot of their audio. It is important to always think realistically.
However, I hope this two-part guide has been useful in showing you just what can be improved. I noticed a significant improvement in the sound quality of my projects once I understood more about the audio filters available to me, and I hope you'll be able to say the same.
Part 1 can be found here
Copyright 2008 Digital Rebellion, LLC.
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 applications FCS Remover and Preference Manager. His film credits include Perfect Sport.
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