TCM MASTERING: HOME MUSIC STUDIO PART 26 – SIGNAL PROCESSING

TCM Mastering: Home Music Studio Tips and Information

Part 26 Signal Processing – Dynamics

We covered EQ (equalisation) in last week’s blog. Continuing with our look at signal processing, we’ll consider dynamics this week and effects next week.

Don’t forget, if you have any questions about the blog series so far, or any queries about the whole recording/mixing process – get in touch, by clicking here.

Or if you have a project that needs a session musician, producing, mixing or mastering – TCM Music Group are currently offering some really incredible recording and online mastering packages.

Note – Signal processing can be added at the recording, mixing and mastering stages. Any signal processing you add to an instrument or voice that is recorded to the same track, cannot be undone. If possible (where appropriate), record the clean instrument and the processed instrument to separate tracks on your recorder. You will definitely want to add some signal processing in the mix stage. 

A Large Dynamic Range – From The Quiet African Kalimba (Thumb Piano), To The 26,000 Watts Grateful Dead’s ‘Wall Of Sound’.

Dynamics, broadly speaking, describes how loud or soft a signal is. The dynamic range is measured in decibels (dB). Using dynamic processors – compressor, limiter, expander and gate – will allow you to add some punch to a kick drum, smooth out levels and transients, even out a particularly erratic performance or reduce unwanted noise.

Be very careful…..these are some of the most useful processors available to the musician and recording engineer, but also tend to be the most abused. Set them up incorrectly and your music will sound worse, not better. Every voice and instrument will react differently to the same settings. So if you have never used them before, take some time to get to know what each parameter or control knob does, before you have to use them in a recording session.

Compressors/Limiters reduce the dynamic range. They reduce the difference between the loudest and softest parts of a recording. The compressor limits how loud a sound can be. The limiter is similar in that it limits the highest level of a signal. But whereas the compressor subtly compresses signals above a specific threshold, the limiter essentially chops off the signal at the threshold level.

So a vocalist who has little control over his/her volume can have their performance smoothed out by using a compressor to take care of the peaks and troughs, or a guitar can be compressed/limited to give it more sustain and power in a mix.

The various makes on the market will look slightly different but essentially have the same controls – Threshold, Ratio, Attack, Release, Gain and Hard/Soft Knee.

Compression and Limiting are basically the same process, but are different in degree and perceived effect. Limiters are essentially Compressors with a very high ratio and fast attack time. Signal peaks are arrested at the threshold and for the most part, prevented from exceeding it.

Threshold is the level in decibels above which the compressor acts. Unless you’re aiming for a particularly audible effect, set the threshold so that it only acts on the highest transients.

Important – Signals below the threshold level are not affected. Only signals that exceed the threshold level have their gain reduced.

The Ratio is the amount the compressor affects the incoming signal. In the picture above, the 45 degree line (1:1) represents no compression. By applying a 2:1 ratio the output level above the threshold, is reduced by a half. Or, for every 1dB a signal goes above the threshold setting a 1/2 dB is output. A 5:1 ratio means for every 1dB going into the compressor that is over the threshold level, a 1/5 dB comes out.

Now the ∞:1 ratio is a special case. Basically, it’s now acting as a limiter eg. 60:1 ratio. In this case, any signal above the threshold will be limited to the threshold level.

 

 Pro Tools Limiter Plug-In Includes Phase Reversal And External Key Option.

The Attack parameter controls how quick the compressor acts on a signal (usually calibrated in milliseconds)…..in other words, how long it takes to turn down the gain on the signal. A lot of the time you will be trying to control fast transients eg. a drum hit, so a fast attack should be used.

The Release control affects how long it takes for the signal gain to return to normal, once the input signal has dropped back below the threshold.

The Gain (measured in dB) adjusts the level of the signal leaving the compressor. As you can see from the diagram above, applying compression results in gain reduction. So, the gain control allows you to increase the output level to make up for the gain reduction.

Hard Knee/Soft Knee refers to how severely the compressor reacts when a signal passes the threshold. The Soft Knee setting makes the change from non-compressed to compressed less noticeable (than the Hard Knee setting), especially when higher ratios are used.

Pro Tools Stereo Compressor Plug-In.

When working on a stereo track, it’s important to use a stereo compressor. By doing this, both channels receive the same gain reduction at the same time. If the two channels were treated independently, the channel with the loudest signal would get the most compression, resulting in a stereo image shift of the sound. Two mono compressors set up identically will not achieve the correct result for a stereo track.

Expanders/Gates – can be considered the opposite of compressors and limiters. Expanders can be used to gently remove noise from a track. A gate is more abrupt.

Limiters limit how loud a sound can be. Gates limit how soft a sound can be.

A gate can be set up to close when the wanted signal is not present. But in order for it to work effectively there needs to be a good signal to noise ratio. If you don’t have a good signal to noise ratio, you may still get unwanted noise getting through the gate along with the wanted signal.

A typical use of gates would be on drum kit mics. You can set up a gate so that the mic on the snare is ‘open’ only when the snare is hit and closed when the other drums are struck.

Phil Collins’ – In The Air Tonight – Engineered/Produced By Hugh Padgham Used A Gate On The Drums To Good Effect.

Phil Collins’ song ‘In The Air Tonight’, made good use of a gate to cut off the added reverb on the drum sound, which gave the song a very distinctive quality.

Compared to Pro studios, many Home Music Studios do not have the budget to spend on soundproofing. Consequently, stray noises from outside the recording room will find their way onto recordings. Also, the process of ‘multitracking’ always adds unwanted noise to a recording.

Let’s consider a vocalist recording with a guitar, bass and drums. At the very least, you would use 5 tracks. One for each instrument and voice and two for the drum kit.

In reality you could end up using many more tracks, especially if you double tracked guitar and vocals, added backing vocals and used more mics on the drums.

When wanted audio is present it will normally be loud enough to mask most unwanted noise. However, with each track that is recorded, there will be sections that have no wanted signal…..for example, gaps between vocal lines or drum hits.

The Section Highlighted In Yellow Between The Two Waveforms Is Unwanted Noise, Which Can Be Significantly Reduced By Using An Expander Or Gate.

Even though there would be no wanted signal, there could still be audible, unwanted noise…..air conditioning rumble (if you have it in your room), distant traffic, hiss or hum from an amp or effects unit, creaks and squeaks from the drum kit. All these sounds when added together over several tracks can become quite annoying and distracting in a recording.

By using an expander or gate effectively, you can substantially reduce or remove most of these and other annoyances from your recordings.

But remember, it’s always better to try and get a clean sound to start with, rather than fix problems later, so an alternative remedy might be to use acoustic panels to reduce the level of unwanted sounds reaching the mics.

Pro Tools Expander/Gate Plug-In.

There are similar controls for an expander/gate to those used for a compressor and limiter…..Threshold, Ratio, Attack, Hold, Decay or Release and Range.

Left – Shows A Gate In Operation, Right – An Expander.

The Threshold sets the level in dB that the gate or expander opens. Any signal below the threshold is reduced by the amount set by the range control. Or if using gate settings, silenced completely. Signals above the threshold pass through the processor unchanged.

If a Ratio of 2:1 is used, the expander will reduce a signal’s level by a factor of 2. In other words a signal 5dB under the threshold would be reduced to 10dB below it. A gate usually operates at much higher ratios. Typically 20:1 to 60:1 or more.

The Attack measured in milliseconds, sets the speed at which the gate or expander opens. Instruments with fast transients eg. drums, require very fast attack settings.

Attack, Hold And Release Functions Of A Gate.

The Hold control determines how long the gate remains open after the signal falls below the threshold (before it reaches the decay/release phase). Once the hold time has passed, the gate closes abruptly. Manipulating the hold time allows you to keep the gate open when there’s no signal eg. this can be useful during short pauses between words or phrases.

The Decay/Release time determines the rate that the gate closes once the signal has fallen below the threshold level and the hold time has expired. The decay/release setting closes slower than the hold setting producing a more natural sound.

The Range control allows you to choose the amount of attenuation to the signal when the gate is closed or remove unwanted signals entirely. For example, a 30dB setting drops signals below the threshold by 30dB.

Drawmer Combination Compressor/Expander.

At the risk of repeating myself…..remember you can always add compression or any other signal processing to a recorded track, but you can never remove it once it’s embedded with the signal on the same track. So if you have any doubts, use sparingly (or not at all) until the mix stage.

Dynamic processors are not the easiest tools to use. The only way to truly understand how they work is to play with them using various instruments and voices, without being under the pressure of a formal recording session.

And use those ears…..listen very carefully to how the processor affects the sound. As with all processing, the aim is to improve the sound. So if the recorded track sounds worse, don’t use it or change the settings until it sounds better.

There are other things to consider with dynamic processors eg. side chain, hysteresis, external key and ducking. We’ll return to the practical applications and suggest specific settings for dynamic processors in a future blog.

Next week we’ll discuss effects processors.

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3 Comments on “TCM MASTERING: HOME MUSIC STUDIO PART 26 – SIGNAL PROCESSING”


  1. […] Processing. We then went on to consider in some detail, various types of EQ (graphic, parametric), dynamics (compressors/limiters, expanders/gates) and effects (reverb, echo, chorus etc). So feel free to refer back to these earlier blogs for more […]


  2. […] Processing. We then went on to consider in some detail, various types of EQ (graphic, parametric), dynamics (compressors/limiters, expanders/gates) and effects (reverb, echo, chorus etc). So feel free to refer back to these earlier blogs for more […]


  3. […] that approach recording and mixing in exactly the same way. So opinions vary on the use of dynamic processing in the mix stage. A major part of the debate centres around whether to EQ first then Compress or […]


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