Recording Techniques and Mastering Audio Recording :
Also known as equalization or EQ, filters are used to increase or decrease the level in a specificrange of audio frequencies. The most common filters are the simple bass and treble controls
found on inexpensive stereo systems, which act on a broad range of frequencies. But other filters
are designed to surgically boost or cut very narrow bands of the audio spectrum. A twelve band
equalizer is very common in studios.
SHELVING FILTERS
As the simplest form of filter, shelving EQ boosts or cuts all frequencies above or below a fixed
frequency. A bass shelving filter, also called a low-pass filter, boosts or cuts everything below its
fixed center frequency. Likewise a treble shelving filter, also called a high-pass filter, boosts or
cuts everything above its fixed center. A single control typically adjusts the amount of boost or
cut.
These filters are useful for making broad changes like reducing boomy bass and wind noise. But
encoders can easily be overloaded by too much bass or treble, so it"s often wisest to use these
filters to cut high and low frequencies to prevent artifacts.
BANDPASS FILTERS
These filters can be used to boost or cut audio on both sides of a center frequency. Bandpass
filters are commonly used as midrange filters, because they have little effect on either high or
low frequencies. The familiar graphic equalizer is just a set of bandpass filters tuned to different
center frequencies.
More sophisticated versions, called sweepable bandpass filters, have an additional control
allowing you to change the center frequency. Bandpass filters are useful for increasing the
intelligibility of a speaker without increasing hiss or background noise. A variation of the
bandpass filter is the notch filter, which boosts or cuts all frequencies except those around the
center frequency.
PARAMETRIC FILTERS
A parametric filter is a bandpass filter with an additional control to adjust the width of the
frequency band being effected . These are the surgical tools of audio editing. They can be used to
eliminate just the noise from an air conditioner, while having a minimal effect on the rest of the
audio.
With all filters it"s important to follow the audio engineer"s first rule of EQ -- cut rather than
boost wherever possible. Cutting undesired sounds is always less obtrusive, and boosting too
much can make a track too loud and lead to distortion and artifacts when encoding.
COMPRESSORS
A compressor"s basic function is to reduce the dynamic range of an audio recording, which is the
difference between the loudest and softest sounds that pass through the recording chain. Simply
put, a compressor is a processor whose output level increases at a slower rate as its input level
increases.
By reducing the volume of the loudest sounds, a compressor lets you raise the level of the entire
audio track, making it all sound louder than it actually is. Compression can be a big help in
achieving intelligible audio tracks with a more uniform volume that will survive the encoding
process. A compressor consists of a level detector that measures the incoming signal, and an an
amplifier whose gain is controlled by the level detector.
A Threshold control sets the level at which compression begins. Below the threshold, the
compressor acts like a straight piece of wire. But when the input level reaches the Threshold,
then the compressor begins reducing its output level by an amount determined by the Ratio
control.
The Ratio control establishes the proportion of change between the input and output levels. If
you set the compression Ratio to 2:1, then when the input signal gets twice as loud, the output
signal will increase by only half.
If you set the Ratio to its maximum (10:1 or more), the the compressor becomes a "limiter" that
locks the maximum level at the Threshold.
While a compressor can level out a recording, high levels of compression can also introduce
artifacts including "pumping", in which there is an audible up and down change in volume of a
track, or "breathing", which sounds like someone breathing as the back ground noise level goes
up and down.
EXPANDERS
An expander is the opposite of a compressor. As the level of the audio signal gets louder, the
expander"s amplifier turns up further making loud signals even louder. An expander can be used
to reduce noise in a process called downward expansion. In this case you set the Threshold just
above the level of background noise. The expander will then raise the volume of everything
above the Threshold, but won"t change anything below the Threshold, thereby lowering the
perceived background noise.
NORMALIZING
Normalizing increases the gain of the audio file until its loudest point (or sample) is at maximum
level. The overall signal level is now higher, which makes for clearer audio, and also gives the
encoder more bits of data to work with and reduces encoding artifacts. The only downside of
normalizing is that it increases the noise as well as the audio signal so it should be used carefully.
It should be your last step before encoding, and you may not need it at all.
MASTERING
Mastering your finished audio product is considered somewhat of an art. There are audio
mastering engineers who get paid a hefty sum of money to master an audio project for you.
With the equipment available today you can do a decent job of mastering your project yourself if
you are careful. Some important parameters as explained above help professional style mastering
and are summed up below.
Dynamics: This important step is also termed as compression. Your music needs to breath and
flow. You do not want to compress it so hard that it roars back at you. Find a good medium
where your music has that fine line between volume and dynamics. The most agreeable
compression ratio is 2:1.
Equaliser: Your songs were probably recorded over a period of time therefore they most likely
will have a slightly different feel to them. You need the songs to sound enough alike that they
feel like they belong on the same CD. You don"t want the bass pumping loudly in one song and
the next track finds the listener turning up the bass knob on his stereo. Try to get an even feel for
the whole CD.
Same Volume: The volume of each song needs to be about the same. The general thought on this
is you don"t want the person listening to your CD to have to turn the volume up or down. Your
listener should be able to listen to your CD at whatever volume they are comfortable with. The
answer is normalization across all the tracks on a CD.
Order of Songs: The order you choose for your songs is important. You need to be aware of the
feel of each song. You want to take your listener on an emotional roller coaster and the order of
the songs has everything to do with this ride.
Burn to CD: This final and very important step is to burn your master to a CD and listen to it on
as many different types players as you can. Listen to it on everything from a cheap boom box to
a high dollar stereo. Make sure it sounds as good as possible on each one. The bass is especially
troublesome to get right. Donò€™t get tempted to boost low notes too much.
Conclusion
Mastering your recording is one of the most misunderstood things in computer recording. No one
seems to really know what mastering is. The steps you take to master your audio are not all that
hard, you just need to go slow, use common sense and most important of all is to listen to your
results. Be careful and take it a step at a time and you can come out with a decent master.