Audio For Broadcast: Metering

The capacity to accurately measure what is happening within an audio signal in terms of level, frequency, phase and loudness brings the solidity of science to knowing all is as it should be. Metering occurs at every stage of the production chain from capture through to playout and learning how to measure accurately dramatically improves results.

There are hundreds of audio tools which can help simplify broadcast workflows and sweeten signals. Probably thousands. But the most important audio tools in your arsenal aren’t plug-ins, processors or analysers. You don’t need to find valuable rack space to house them. In fact, you already own them.

You will know if your mix sounds good because you will hear it with your trusty, old-fashioned, analogue ears. If it sounds good, it’s a good mix, but that doesn’t mean that you can’t use some extra help.

There are many reasons why you might need a clearer view of what’s going on and metering can provide it: you’ll need it for confidence monitoring, to ensure QoS, to verify downmixes, or to check you have an active signal from a remote source. In a digital architecture you need reassurance that you have enough headroom to avoid distortion of the signal as well as guarantee your loudness levels adhere to legal requirements.

You’ve only got two ears. You can’t be across everything.

The Objective View

Metering makes use of objective data to enable an audio operator to ensure everything is as it should be and it does so at every link in the broadcast chain from input through to output.

Output metering is often about compliance and loudness, while metering within the chain helps ensure operators manage gain staging and signal to noise ratios (more on this in a later article).

Confidence metering is important throughout. At the input stage it’s good to check that you have audio where you expect it. For example, a sound operator at the console is more likely to be listening to an output mix rather than an individual input, so mixing consoles have an input bargraph to visually illustrate a live outside source with a PFL (Pre Fade Listen) button as an auditory double check.

Broadcast infrastructures will also have confidence monitors at input and output points throughout the chain to check audio levels and improve efficiency. These meters will gauge levels and also enable engineers to identify where any issues might arise and guarantee QoS.

Phase meters ensure you have phases aligned and show the phase relationship between the left and right channels of a stereo signal on a scale of -1 to +1, where +1 is completely correlated and -1 is completely out of phase. This is important at the channel input level to avoid issues further down the mix.

All these build confidence and help guarantee quality, but what are they measuring in the first place?

Decibels

Decibels (dB’s) are useful because they represent sound in the same way that humans perceive it. Human folks perceive loudness logarithmically rather than in a linear fashion, so if the amplitude of a signal is increased by 6dB it will be double the original amplitude. Another 6dB increase will at four times the original amplitude.

Meters measure decibel levels, but not necessarily in the way you might think. Decibels do not describe an absolute level but provide a means of comparing the amplitude of one audio signal with another, so they need a reference point to function.

On a broadcast console, the reference point is at 0dB and is clearly marked on the fader. This is called ‘Unity Gain’, and at unity the output signal is exactly the same level as the input signal. Pushing the fader above 0dB will increase the level of the signal, while pulling the fader below 0dB will reduce (or attenuate) the signal.

Broadcasters need to measure these levels for two reasons; firstly, to maximise the integrity of the signal, particularly when it is mixed with others where relative levels are important, and secondly to ensure consistency across a station’s output in accordance with ratified standards.

There are a variety of reference levels but in practical terms broadcast environments are more concerned with measuring dBFS, or ‘decibels relative to Full Scale’.

Meters provide the tools to keep track of this although different meters do this in different ways.

Ballistics

Ballistics is a fancy way to describe a meter’s reaction time. Meters don’t instantly react to the level of the audio signal which is feeding them, they rise and fall to illustrate those levels. These changes are referred to as attack (rise) and decay (fall).

In a broadcast environment there are two common meter types; VU (Volume Unit) and PPM (Peak Program Meters). When fed a signal with a constant level such as tone they both display exactly the same level, but live TV sound doesn’t deal with constants; Live broadcast audio is by nature unpredictable and transient.

VU meters struggle with this as they track the average volume level of an audio signal. They have a much slower attack and decay (around 300ms) and while they are too slow to react to fast peaks hey do a fabulous job of representing average audio levels.

Due to the unpredictable nature of the content a broadcast meter is more concerned with measuring peaks than measuring the average, and one way to track those pesky peaks is to use a meter which reacts faster to the sound. PPM meters have a much faster attack time (less than 10ms), which means they are much better at displaying quick, transient peaks in audio such as the sound of a gunshot or a drum hit. Because peak level meters react faster to these transients rather than an average they also tend to read several dB’s higher than a VU.

In fact, transients in broadcast can be so fast that bargraph meters on broadcast consoles will have a peak-hold where the peak level remains displayed whilst the rest of the bargraph decays so operators can keep track.

There’s A Limit

It is useful for meters to display peaks to ensure signal integrity because it’s those peaks and transients that are the most common culprits of distortion. Remember dBFS? Digital audio has an absolute level of 0dBFS, so dBFS values are always less than or equal to zero. At 0dBFS the audio runs out of quantising levels and distorts (also known as ‘clipping’).

Mixers need to ensure that the combined signal doesn’t overload (i.e. go above 0dBFS), and digital signals are referenced from this point down simply because a 0dBFS signal already contains the maximum amount of digital information it can deal with without distorting.

Reference Levels

How broadcasters visualise the meters and how much headroom they have to play with until it hits that limit is ratified in different territories with different reference levels. First issued in 1992, the European Broadcasting Union’s (EBU) R68-2000 Alignment level in digital audio production recommends a standard alignment for professional equipment where 0dBu is equal to -18dBFS. This gives operators 18dBFS of headroom to play with before the meter shows clipping. Meanwhile, the US uses SMPTE’s RP155 standard where +4dBu equals -20dBFS, which provides 6dB more headroom.

More headroom gives broadcast mixers more flexibility where the inputs are metered at the input, and broadcast consoles have to be able to adapt their reference levels depending on location.

Lots Of LUFS

Another vital meter is loudness. In 2010 the EBU introduced its Loudness Recommendation EBU R128 and the US Congress passed the Commercial Advertisement Loudness Mitigation (CALM) Act. Both are based on the International Telecommunication Union’s BS.1770-3 recommendation which was introduced as a universal standard to measure perceived loudness over time.

Loudness regulations were introduced to keep the volume of broadcast output and the volume of advertising consistent. Loudness metering extends the capabilities of VU or PPM meters and is measured in LUFS (‘Loudness Unit Full Scale’). In the simplest of terms it measures a broadcasts relative loudness over time. Because they’re referenced to full scale where 0dBFS is the maximum, readings are always negative.

BS.1770 loudness meters measure momentary (average loudness over the last 400ms), short term (averages over three seconds) and integrated (average over the complete program so far). The latter is used to ascertain whether the program passes international guidelines. It also measures the maximum True Peak, which is similar to a peak level measurement but also considers inter-sample peaks.

Broadcast standards around the world are actually very similar and are used to help normalize audio across all streaming platforms too. The main profiles for live OTA broadcast are EBU R128 (working to a Target Loudness (LUFS) of -23 and a Max True Peak of -1), and the ATSC A/85 profile in the US and Canada (working to -24 and -1 respectively).

Loudness meters provide a way to monitor and regulate average loudness levels over the duration of a broadcast and are usually part of the meter bridge on a broadcast console. If the transmission output runs too loud, then broadcasters are obliged to compress that output to fit, so mix operators are at pains to adhere to it to avoid on-air compression.

Ears Are Not Enough

Ears are a wonderful thing, but with so much to keep across, the two of them aren’t enough to measure what is happening within an audio signal in terms of level, phase and loudness. Metering brings the solidity of science to ensuring everything is as it should be, and it means that you can use your ears to concentrate on more creative endeavours.

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