Audio For Broadcast: Dynamics Processors

Keeping audio levels under control is the foundation of audio mixing, and Dynamics Processors give us tools to automate level control in various ways.

In modern broadcast environments it’s good to know a little bit about everything. Knowing a little bit about what other people do and how they do it not only provides a better appreciation of what it takes to knit together a live production, but it also promotes empathy and builds confidence when we’re asked to pitch in.

In broadcast audio, where preset processing chains are often already in place, if everything is set up properly, there should be little to do outside of a tweak here and there. But sound is annoyingly unpredictable, so it’s important to understand some of the techniques sound engineers use to help manage sound as it reaches the control room.

Audio processors, whether they are standalone or built into a mixing console, are essential tools in helping to tame unruly signals and provide the ability to manipulate those signals in a multitude of ways and Dynamics Processors are a fundamental part of that tool kit.

Dynamics Processors help sound engineers protect against unpredictable spikes; they are used to adhere to loudness regulations, to ensure broadcasts are intelligible, to tailor broadcast output to fit a specific genre, or a specific medium, and are not as frightening as they first seem.

In fact, Dynamics can be summed up in ten words, and if you are pushed for time, you can skip the rest after this next line:

Compressors make big sounds quieter; expanders make quiet sounds bigger.

Home On The Range

Of course, in reality Dynamics aren’t just about volume; they provide operators with the ability to manage the dynamic range, which is the difference between the loudest and the quietest parts of an audio signal.

Compressor/limiters and expander/gates provide this functionality, and you can hear real-world examples of how these tools can affect broadcast output every time you turn on the radio in the car.

From the quietest whispers of a clarinet to the intensity of the brass section, classical radio has a very wide dynamic range; in a symphony, the range is integral to the storytelling. To maintain that, classical music broadcasters apply very little compression to their output.

But it’s no good for the school run when it’s competing with traffic, road noise and chattering children. To hear the full dynamic range, you really need to crank up the volume to hear the quiet bits, which makes the loud bits deafening.

To resolve this, drive-time commercial music stations will compress the dynamic range of their output to make it more audible. Compressing the peaks to create a narrower range means the content can be broadcast at a higher volume without the risk of encroaching on loudness regulations or distortion/clipping. In this kind of environment it makes the whole programme easier to listen to.

Putting On The Squeeze

A compressor reduces the dynamic range of an audio signal by attenuating (reducing) the loudest parts of the signal – usually the transient at the start of the waveform – while leaving the quieter parts untouched. This smooths out the signal, reducing the peaks and making the overall signal quieter.

Basic compressor.

Basic compressor.

Compression is controlled with a combination of four settings: threshold, ratio, attack and release.

Threshold and ratio settings work as a team; without each other they are useless.

The threshold sets the level at which the dynamics processing kicks in, and only signals which are higher than the threshold will be affected; by definition, any signal lower than the threshold is left alone. But in order to have any effect at all, the compressor needs to know what to do with those signals.

This is set by the ratio. When a signal exceeds the set threshold its input to output ratio is changed by a set ratio. A ratio of 1:1 will have no effect at all – it simply means that for every 1dB of input signal beyond the threshold setting, the output will also be 1dB. However, a ratio setting of 3:1 would mean that for every input signal 3dB above the threshold it would reduce to 1dB, bringing down the level of those troublesome transients by 2dB. A ratio of 5:1 reduces 5dB of input above the threshold to 1dB of output, resulting in a reduction of 4dB.

Ratios of 10:1 or more changes the game significantly, but we’ll get to that later.

On The Attack

The attack parameter defines how quickly the compressor kicks into action once the signal has exceeded the threshold. A shorter attack time will reduce the signal as soon as it hits the threshold, while a longer attack will allow some of the transient to pass through before any gain reduction is applied. Conversely, the release time defines when the compressor stops processing the input signal and how quickly it returns to applying no processing after hitting the threshold.

Attack & release.

Attack & release.

These settings can significantly shape the sound: for example, as a faster attack and release time lets less of the transient through, it can result in a more aggressive sound and boost the perceived loudness.

In some outboard gear, some of these parameters are already set, but the dynamics section of a broadcast console will all be user definable. There is no right or wrong way to set these up; like all things in live broadcast, the sound engineer’s primary consideration is always to meet the needs of the broadcast, and so compression levels and how aggressive they are will depend on the content, the output, and the broadcast medium.

If You Kneed It

Another element to dynamics settings is the knee setting. The knee determines the range around the threshold where gain reduction will be applied. A hard knee means that gain reduction is applied at the assigned ratio as soon as the input signal hits the threshold. A softer knee starts to apply gain reduction before the signal hits the threshold, from a lower point in the input signal, and will apply full reduction when the signal reaches a certain level above the threshold.

Compressor knee.

Compressor knee.

Knee settings can be used to produce a more transparent gain reduction and broadcast consoles will often have a graphical representation of the setting to help visualise what it will do to the signal. You’ll know it when you see it; it looks like a leg bending at the knee.

One consequence of using compression is that it makes the overall signal quieter, and some parts of its output will be at a lower level than its input. If the application of compression makes the overall signal too quiet, make-up gain can be applied to the signal just prior to output. Make-up gain shifts whole signal equally to bring it back up to the required level.

In this way, a signal can be processed with the right output level while avoiding distortion (or clipping, in a digital workflow) caused by rogue peaks which exceed the maximum level the equipment can handle.

Limiting

A compressor becomes a limiter when a minimum ratio of 10:1 (remember that?) is applied. This high ratio effectively compresses the signal so hard that it’s as if it stops it dead. In broadcast a limiter is often applied at the last stage of the signal processing chain to help ensure that the signal is within loudness regulations prior to broadcast, and to prevent it from being squashed by an automatic broadcast limiter prior to going to air.

Close The Gates

Now we understand the compressor it’s easier to understand how an expander works because it’s technologically similar to a compressor (while being completely the opposite).

Rather than reducing the dynamic range by reducing louder sounds, an expander increases the dynamic range of the signal. There are two types of expansion: upwards expansion - where signals above the threshold are increased (making loud sounds louder), and downwards expansion - where signals below the threshold are reduced (making quiet sounds quieter). Most expanders found in broadcast systems are downwards expanders.

A gate is like an extreme downwards expander because it filters out quiet parts of the signal and allows just the louder sections to pass through.

A threshold is set in the same way as with a compressor, but now signals which are above the threshold will be sent to the output of the gate. Signals below the threshold level will be attenuated (reduced) by a set amount of dB.

Attack and release settings work the same as a compressor. Gates also have a hold (or delay) setting to determine how long the gate stays open. Some gates have a hysteresis control which sets two thresholds; one for the opening level of the gate and the other for the closing level. This is usually controlled using an offset from the opening threshold to the closing threshold – which is generally set a few dB lower and is relative to the open threshold – this can help prevent oscillation if a gate is opening and closing rapidly.

An expander does a similar job to a gate, but instead of muting the signal below the threshold, it uses ratio settings in the same way as a compressor. It works in the same way, so a ratio of 2:1 means that for every 1dB below the threshold, the signal will be reduced to 2dB, and so on.

Expander/gates are super useful for cleaning up signals by filtering out unwanted low-level background noise from the signal such as crowd hubbub or machinery, or for removing any background spill which is caught on a nearby microphone, which can often happen with using multiple mics on a drumkit, for example. Reducing rather than completely removing this noise with a gate can produce a more natural sounding result.

Getting It Right From The Start

Getting the dynamic range right at the input stage helps protect against signal overloads later in the signal chain and is an essential element of the mixers’ tool kit. In our next section we’ll look at equalization (EQ) which is used to manipulate specific frequency components to shape the sound for aesthetic or correctional applications.

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