Loudness and Metering (Part 2)

Guest contribution by Owen Green

[This is the second of a two part series on loudness, mixing, metering and the new ITU loudness spec]

The New Loudness Standards, and What They Might Mean for Us

Depending on the field in which one worked, standard production practices have tended to be orientated around peak normalisation of material. That is, lining up recordings by their peak level; in music, recordings are now often lined right up 0 dBfs; in broadcast a common specification is to peak at -9 dBfs (which allows some headroom in recognition of the fact that analogue QPPMs don’t truly measure signal peaks).  As is hopefully clear from last week’s article, this has almost no bearing on how loud we perceive the material to be.

A set of new recommendations has been developed since 2006, and are now becoming standard or legally stipulated practice in some industry areas and countries. It is therefore worthwhile to get a handle on what they mean, and how to use them as soon as possible.

The core proposal comes in the form of a recommendation from the ITU (International Telecommunications Union) on how to measure the loudness of a signal (ITU BS.1770-2). Local bodies, such as the EBU in Europe and ATSC in the US, have then published (pretty similar) guidelines on working practices derived from the ITU spec. The common thrust of these proposals (actually, now rules in some places!) is that they enable a move away from normalising by peaks to normalising by (approximate) loudness:

So, material that has been more compressed in dynamic range (so as to appear louder when peak normalised) loses its ‘advantage’ by being reduced in gain so that its perceived loudness is on par with more dynamic material.

ITU BS.1770: Measuring Loudness

Measuring perceived loudness has the potential to be a highly complex affair. One of the things that is remarkable about ITU 1770 is that the scheme it proposes is very simple! It turned out from the ITU’s (apparently quite rigorous) research that this simple model performed about as well as considerably more complex models based on psychoacoustics. Simplicity has a number of advantages, particularly insofar as the resulting scheme is extremely easy to implement, and computationally cheap.

Weighting

The first thing that happens is that a frequency weighting curve is applied to the signal by filtering. It looks like this, and is called the ‘k-weighting’ (but has nothing to do with Bob Katz):

So what happens is that low frequencies are de-emphasised (register as less loud) and high frequencies register as louder.  This weighting is followed by an averaging process similar (as with a VU meter). The respective loudnesses from different channels are then summed and converted to a logarithmic scale (like dB) to give a loudness figure.

Loudness Units and Programme Loudness

That means we have yet more units of measurement! The ITU define a relative unit of loudness called the LU (very much like VU), which can be used for setting reference points on meters and describing ranges or differences in loudness. There is also an absolute unit defined relative to 0 dBfs; the ITU and the US standard currently call this LKFS (Loudness, k-weighted, relative to 0dBfs), whereas the EBU use LUFS. They are completely equivalent (and hopefully one or the other will be dropped for clarity’s sake).

The idea is that adjusting the gain of your mix by n dB should result in a corresponding change of n LU.

The fundamental measurement that the recommendation is concerned with is the loudness of a whole item – a programme, a commercial, a trailer, a movie, etc. This measure is derived by tracking the average LU over a whole programme (called the integrated measure).

Gating: Dealing with Dynamics

There remained a problem, however. Material with a high dynamic range (i.e. lots of quiet with a few noisy episodes) would end up being measured as artificially quiet, and then would not line up properly when loudness-normalised. The solution to this problem was to introduce a mechanism such that relatively quiet sections would not contribute to the loudness measure for the whole segment. What has been (quite recently) agreed upon is that material that is 10 LU or more below the average loudness of the programme will not be measured (so it is adaptively gated out). Furthermore, there is an absolute gate at -70 LUFS that stops extraneous background noise contributing to the loudness figure.

True Peak Levels

0 dBfs is widely understood at the top of the digital scale. Well that’s only kind of true. Sorry.

It turns out that it is possible, upon signal reconstruction (digital to analogue conversion) to end up with levels that are effectively above this nominal maximum. This is because digital audio is just a kind of model of the eventual signal that will come from the loudspeakers – consider that it would be possible to sample either side of a peak, and thus get digital peak readings that were actually too low.

This means that the peak meters in our DAW are even less useful than we thought, as they could miss situations that would actually cause clipping upon playback. Whether this happens or not is largely down to whether or not the equipment manufacturer has built-in headroom for signals that overshoot. As normal, more expensive equipment tends to do this, cheaper gear doesn’t; so a signal peaking around 0dBfs could sound fine in the studio, but distort on consumer equipment.

It turns out that the lower our sample rate, the more inaccurate our DAW peak meters are at gauging the true peak level. Again, the ITU opted for a simple approach, and mandate the measuring of peak values with a meter that oversamples (converts the signal to a higher sample rate – 192kHz, in this case) and measures the peaks of this up-sampled signal.  The idea is that measuring peak values relative to this (plus a bit of headroom) will be more accurate, and cause less unintended distortion.

This gives us another unit! dBTP – dB true peak. This just describes a peak value measured with a compliant peak meter.

Working Practices: EBU R128

On the basis of the ITU recommendation, the EBU (as well as other organisations worldwide) have developed a set of working practices. These are principally aimed at broadcast, but elements are being considered in other areas and are of general value, so worth understanding!

The fundamental aspect of the EBU (and related) specifications is to establish a reference loudness level as measurable by a ITU 1770 compliant meter. In Europe this level is -23 LUFS. They also mandate that the peak level of a programme should be no higher than -1 dBTP, and suggest a statistical mechanism for determining the range of loudness of  a programme – i.e. how dynamic it is.

Furthermore, they establish a specification for metering, based on the ITU 1770 measurement. As well as an overall programme level and a true peak meter, EBU compliant meters have a ‘momentary’ loudness measure based on a 400ms averaging window (slightly slower than a VU meter), and a ‘short-term’ loudness based on a 3-second window.

There is a great deal more material available about this system:

• The EBU’s loudness page
• Florian Camerer: “On the way to Loudness Nirvana” (pdf)

So, like, what?

Some of you do work in broadcast or similar, and therefore, this will affect you directly. But those who do not may be wondering at the relevance of all this. Well, two things: even outside broadcast, this is quite likely to have a bearing on the distribution or production of your work; second, it is should be of intrinsic interest as it represents a wholehearted effort to develop coherent working practices that should make our audio sound better.

If you deal mainly with music production, it is worth bearing in mind that many media players are starting to adopt some form of loudness normalisation, based upon BS.1770. iTunes has its own proprietary (of course) ‘sound check’ system; others make use of ‘ReplayGain’ meta-data. They both work in the same way, however, which is to scan your library with a loudness measure, and then simply turn down material that is above reference (-18 LUFS for Sound Check, -16.5 LUFS seems to be standard in ReplyGain based players).

Suggestions

If you haven’t dived into the world of loudness metering yet, I have some suggestions for your studio practices that I hope will help you deliver optimal work, as well as get you acquantied with the ITU/EBU/ATSC workflows:

• Get hold of an EBU compliant meter. The Melda one is free in its basic form; there are other free cheap ones as well (as some very expensive ones with fancy logging facilities and what have you).
• Practice working at -23 LUFS.  Just get used to having -23 LUFS as your meter 0-point that you mix around – i.e treat -23 as a maximum level (or thereabouts).
• With this in mind, get to know your monitoring level at -23 LUFS – borrow a sound level meter if you don’t have one.
• If you’re producing music, then you might consider -18 or -16.5 as alternative references; it shouldn’t matter though, as you can always just turn it up ;-)

Owen Green is a sonic artist, designer and teacher based in Edinburgh.