Digital Signal Processing

Categories of DSP

DSP (Digital Signal Processing) is the technique of manipulating a sound and can be divided into 2 main categories depending upon the function.

1) Effects DSP: The purpose is to change the colour or timbre of a sound, e.g. Delay, Phaser, Distortion, Cut Off. Effects DSP is covered in this chapter.

2) Mastering DSP: The purpose is to enhance the audible quality, frequency balance, clarity and loudness, e.g. Compressor, Limiter, Reverb and EQ. Mastering is covered in Chapter 14

Most DSP effects can be used in both categories, e.g. an EQ can be used to ensure that the frequency range is balanced for mastering. An EQ can be used to add a sweeping motion to a synth.

Effects and Mastering DSP

The following classification of effects can be applied to any sound source.

Name Description

Compressor Reduces the dynamic range or the quietest point to the loudest point Distortion e.g. overdrive, distortion, but crusher etc

EQ band pass Equalization filter that passes frequencies within a certain range and rejects all other.

EQ hi pass Equalization filter that passes on frequencies above a certain range.

EQ low pass Equalization filter that passes on frequencies only below a certain range.

Filter - Stereo Spread a. Stereo Spread high frequencies - automation left to right c. As above with auto filter resonance for added squelch Fuzz Wah Cut off and distortion, similar to a Wah Wah pedal.

Reverb Create the illusion of space by adding the ambience of different spaces, e.g. small room, hall, and cathedral.

Gate Rhythmic amplitude modulation

Modulation e.g. Phase, Flanger, etc

Pan Left and Right or spatial surround (if you have the technology) Pitch Shifter Pitch Shifter

Time Stretch Tiny grains of a sound are looped to stretch the sound (old hardcore) Vocoder A vocal sound is processed though ring modulation.

Applying Filters to Instruments

Even the most simplistic sounds can be manipulated using filters to turn them into pro studio quality. Once a few core techniques have been established, they can be used for Bass, Leads, Vocals and SFX.

Fig 1: A simple Bass sequence is programmed. By itself, the Saw Wave timbre is very static and uninteresting.

Filters Bass 1 Raw.mp3

Fig 2: A Band Pass Cut Off filter is modulated using track automation.

Filters Bass 2 Modulation.mp3

Fig 3: A Stereo Delay filter is added.

Filters Bass 3 Stereo Delay.mp3

14. Mastering

The main purpose of mastering is to ensure that each instrument together with drums, vocals and SFX is at the maximum volume and can be heard clearly with the other sounds. The fewer layers you have, the easier the job of mastering becomes.

The main tools to assist include Compressor, EQ, Pan and Reverb.

Compressor

Compressing reduces the dynamic range or the quietest point to the loudest point. Heavy compression will increase the amplitude of quiet sounds.

There is five important features to a compressor.

1. Attack

• The release is how fast the compressor stops processing the sound after the level drops below the threshold.

• The ratio determines how much compression is being applied, so a ratio of 2:1, for every 2db that passes the threshold, the output will be 1db.

• The threshold describes the minimum volume level of sound to be compressed, so that all sound above the threshold will be compressed.

• Now the knee describes how sharp the attack is.

EQ

The frequency range of human ears is from 20 Hz to 20,000 Hz. There is no specific value to describe the perfect EQ balance at it can vary according to Genre and personal taste. However, a song must contain a relatively equal balance across the spectrum from low to high. frequencies which can be boosted or reduced by an EQ. Use your ears or a frequency spectrum to analyse the sound and use EQ to boost or reduce frequencies accordingly. EQ can enhance a mix by adding sparkle (higher frequency boost), power (lower frequency boost), and many other characteristics. Further details are listed below in the EQ and Mastering Table.

One technique is to EQ for mastering during the production, i.e. ensure that the drums sound perfectly balanced before assign the Bass and Synths. After the addition of each new instrument, ensure that the EQ on all previous drums and instruments is modified if necessary to maintain balance and clarity. When the song is finished, the final EQ Mastering and compression is easier to tweak as most of the work has been completed.

EQ Tips

-Use the sweep control to sweep the frequencies until your boost or cut has achieved the optimum results.

- To maintain a natural sound, apply a gentle boost and retain a narrow bandwidth.

Prevent Frequency Overlap

When too many frequencies overlapping in a mix, the result is “muddy”. To retain a crisp and full sound, ensure that instruments like Synths and Leads have a low frequency cut to remove the low end of the spectrum.

If you have two Leads playing at the same time which disappear beneath each other, ensure that you boost different frequencies for each to highlight them. Panning one left and one right will also help prevent their sound getting lost.

Pan

If two sounds occupy a similar frequency range, pan them slightly to opposite channels left and right.

Panning will make the 2 similar sounds more distinguishable and subsequently more prominent in the mix.

Reverb

Reverb create the illusion of space by adding the sonic ambience of different spaces, e.g. small room, hall, and cathedral. Use reverb with caution because clear and crisp sounds become too distant and muddy with overuse of reverb.

Step By Step Mastering

Before you start - Essential Mastering Tips

Prevent Ear Fatigue - Let your ears rest for at least 24 hours before completing the final mix down. Constant exposure to music will unbalance the ears natural response to over exposed frequencies.

Add a limiter to the master output to ensure that the track doesn’t clip or distort. To achieved 0db for the entire mix, add a compressor before the limiter and boost the gain. If too much boost is required because each individual instrument is too low, then unnatural warping and level bouncing will ruin the mix.

Add a Cut Off filter below 40Hz on all Bass sounds, even the Sub bass to prevent muddiness.

Generally, use the EQ to reduce frequencies to obtain a balance instead of boosting. Cutting one frequency can be perceived as a boost to another frequency. Each change that you make can affect the perception of the overall tonal balance.

Appendix 12 provides a chart of useful EQ information.

Drums

Mute all layers in your track except for the drums. Primarily, ensure that the Kick Drum or Snare Drum is prominent in the mix. In House music, the kick drum dominates the mix but in Breakbeat and Drum & Bass, the Snare Drum dominates the mix. Compare your song to a professionally mixed song, usually the intro or outro will contain only Drums.

If a Snare or percussion sound hurts your ears because of too much high frequency of over compression, use a parametric EQ to boost the high frequency and identify the offending frequency range. Move the boosted frequency value up and down to the frequency which is the most harsh and painful. Change the boost to a reduction.

Bass

Next, un mute the bass and adjust the EQ to ensure that both the Drums and Bass are clear in the mix.

Unfortunately, the Kick Drum and Bass share a similar frequency range. One technique is to add a low cut filter to the Bass and Bass Drums so that the frequencies don't overlap causing muddiness. Another technique is to apply EQ separation; use add a parametric EQ to boost the Kick. Adjust the frequency up and down until you identify the most powerful value. Reduce the same frequency in the Bass and boost a higher frequency range. For example, if you boosted 50-80 Hz in your Kick Drum, boost around 120 Hz for your bass. It's very important to avoid accumulation of the same frequencies when dealing with lows as these tracks control the overall output level of your mix.

Be wary of boosting the Bass of too many tracks. Low frequency sounds are particularly vulnerable to phase cancellation between sounds of similar frequency. This can result in a net cut of the bass frequencies.

Next Layer

Next, un mute the most important instrument, usually a lead, hook, vocal sample. Again adjust the EQ and compression until all 3 layers are clear.

Other Layers

Un mute every other layer one at a time from the most important to the least. Adjust the EQ and compression to ensure that each instrument can be heard clearly in the mix.

Don’t EQ an instrument in isolation as changes can affect the other instruments.

Mastering Throughout the Song

It is unlikely that the EQ and amplitude of each layer will remain static throughout the song. Instruments will sound louder when no Bass or Lead are playing as they don’t have to compete so compensate by reducing the amplitude accordingly.

Mastering Throughout an Album to Maintain Consistent Bass Energy

Listen to the bass end of each song to see how that differs and use the EQ to try to even things out. For example, one song might have all the bass energy bunched up at around 80 or 90Hz while another might have an extended deep bass that goes right down to 40Hz or below. Low Cut on the sub-bass and peaking up the 80Hz area slightly may bring the bass end back into focus.

15. Organising your Studio to Optimise your Workflow