AUDIOWAVEFORMS &METERS

Mark Yonge MIBS attempts todemystify the subject of audiometering.

Typical spiky audio waveform

Youmay have noticed that audiometering is no longer a local matter,for local engineers. It hasn’t been

for years. The business of sound recordingis now increasingly international so thataudio metered using one convention mustbe acceptable when using another. Thereare already signs that, in the UK, deliveryrequirements are moving from comfyBritish tradition to newer frameworks.

Audio level metering tends to beaccepted as a fixed thing; it isn’t usuallyquestioned. Sound engineers rely on a fixedreference for what they do whileconcentrating their efforts on the moreinteresting variables of programmeproduction. Unfortunately, deeply-ingrainedtraditional practices in different parts of theworld are different, but their usersfrequently use similar words to describequite different phenomena. Inconsequence, terms like ‘peak’ and ‘zero-level’ are often used without qualification,despite the fact that their vagueness couldmake them worse than useless in anyinterchange discussion.

There’s another reason to be interestedin metering. Increasingly, we are expectedto use equipment that is built for ageneralised international market and simplydoesn’t have the metering that a UKprofessional, for example, might expect.Some considered translation and adaptionwill be necessary!

In what follows, we are considering theamplitude of a single audio channel. Otherforms of metering for stereo and surroundsignals, or special metering for ‘loudness’will have to wait for another article inanother edition of Line Up.

The AudioThere are two reasons for using aprogramme meter: firstly as a technical levelgauge, and secondly as a guide to consistentbalance. Most of us do a bit of both thesethings, but the applications are really quite

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separate. I believe that the usefulness of thesecond flows from the success of the first,not the other way round.

A real audio waveform is a naturallyspiky, and often asymmetric, thing. You cansee this on an oscilloscope or by looking atthe waveform display in your favourite harddisk editor. Even comparatively low-frequency sounds, like speech, can havemomentary waveform excursions towardsthe extreme.

Programme meters will typically show amore-or-less smoothed version of thewaveform and will not show extremes atall. The difference between the indicatedmeter peaks and the actual waveform peaksneeds to be consciously inferred, but is notwell understood.

On the other hand, a sine-wave tone isalmost entirely unlike an audio signal. It hasno spiky excursions and is very stable inamplitude. It’s useless for simulating anaudio programme, but handy to calibrateand confirm the gain of amplifier(s) in theaudio chain.

Audio BoundariesAll recording and transmission media foraudio – analogue or digital – haveboundaries limiting the maximum andminimum audio levels that can be carriedsuccessfully. The limits for each mediumare pretty well understood and aprogramme meter is intended to provide aguide to the available working space foraudio. It will indicate when signals are toohigh with distortion a likely outcome, andwhen the signals are so low they may beswamped by noise.

Programme meters traditionally relatedthese high and low limits to some nominalmid-point reference so that the referencesignal itself would not be corrupted bydistortion or noise. Today, digital alignmenttones can be at any level up to themaximumwithout distortion, but a mid-level alignment is still useful as a means tocompare meter dynamics without beingconfused by faster or slower meter types.

Interchanged AudioIn a single box, or a closed system, anymetering system can be used withoutchallenge provided it performs thenecessary service. The need for predictablecharacteristics arises when audio from onesystem is sent to another. It makesenormous sense for the source and thedestination to be able to discuss audio levelswith a common basis for understanding sothat the chain can be made predictable. Thegeneral term for sending audio from oneplace to another is ‘interchange’ and itreally doesn’t matter whether a digital audiosignal is passed from source to destinationas a recording on a physical medium (tape,CD-R, flash RAM), or streamed over asatellite, or uploaded to the client’s FTP site

via the Internet; the audio modulating thePCM coding range has the same need tointerchange predictably.

At one time, meters were calibrated interms of analogue line levels, expressed indBm, dBu, or millivolts. This made sensebecause all these meters were built usinganalogue technology and they couldmeasure an electrical signal in a way thatwas impractical with the recordedquantities actually interchanged.Consequently, for a long time it wasconvenient to use line levels as a proxyvalue for the real quantities that wereinterchanged.

However, outside the studio where thecalibration was performed, the proxyrelationship becomes unreliable. These in-house habits are hard to break and lead toerror in interchange discussions:

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A

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A

A

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D(

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C(ddUs

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“What’s the level of the tone on thistape?”“Zero level.”“Oh. What’s that?”“Err, 0.775 volt?”

alogue Interchange…

edium Interchange Method Interchange Units

nalogue tape and film Physical nWb/mmagnetic flux

M radio (and TV sound) Wireless transmission % FM deviation

nalogue optical film sound Physical % Variable-area optical track width

nalogue disc Physical cm/s groove velocity

A schematic programme chain. Programme metering relatesto Alignment Level throughout

See what I mean? The level on a tapecould have been correctly expressed innanoWebers per metre, or decibels relativeto full scale for a digital tape, but never involts. The misunderstanding shows a once-useful craft tool now sadly divorced fromengineering reality!

In recent years, analogue media havefaded and digital audio interchange is nowdominant. Unlike analogue interchangeunits, digital interchange units are easy tomeasure directly. It’s also convenient that,although the physical recording media canvary a lot, almost all digital audiointerchange is referenced to a single PCMcoding scale. Digital inputs and outputs,such as AES3 connections, use the PCMcoding range directly, while analogue-interfaced equipment uses PCM at theconvertors. Recorders using data-compressed formats, such as MP3, AAC orMPEG, are referenced to PCM external tothe codec. This means that we don’t haveto wrestle with the notion of special signallevels in differently encoded forms. It’ssufficient to measure the PCM signal.

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and Digital Interchange

edium Interchange me

igital audio tape:DAT, DTRS, DASH, ProDigi, etc.)

Physical

igital videotape:Many variants)

Physical

omputer files:BWF, Wave, AIFF) stored on hardrives, optical discs (CD-R, DVD),ata tape, flash memory cards,SB memory sticks, Internetervers, etc.

Physical, network

treams:AES3, S/PDIF, IEC 60958, AES47)

Connection, netw

All PCM levels are referenced to themaximum coding level of 0 dB FS (fullscale). This is the same whether using 16,20, or 24 bits of digital resolution – thehigher resolutions simply add more LeastSignificant Bits to the digital sample toimprove the noise floor. Because audio

signals are now interchanged almostentirely in the digital domain, I suggest thatthe primary reference for audio levelsshould clearly be a point in the PCM codingrange, expressed in dB FS. Proxy values indBu should only be considered as a localexpedient.

thod Interchange units

dB FS

dB FS

, Internet dB FS

ork dB FS

From time to time it’s good to ask thequestion, “What exactly are we measuring?”It’s easy to get suckered into believing thatwhat’s displayed on our meter is directlyrelevant to the job we’re trying to do. Weneed to be careful, because the quantitieswe meter may not be the quantities weinterchange in the recording.

All types of audio meter to date havemeasured the amplitude of the audiowaveform using various meters of definedcharacteristics. All of them provide thetechnical level gauge, but rely on expertinterpretation to complete the judgment ofaudio balance. Where all users share acommon set of rules to guide theirjudgement, the quality of interchange isgood. When programmes move acrosscultural boundaries, the quality ofinterchange tends to deteriorate.

The meters we use today derive frommostly-sane decisions made many decadesago. The fact is, though, that mostprogramme metering in current use wasoriginally designed using electro-mechanicalmovements and an analogue electricalinput. Because we still use those meter

characteristics (even if often within a digitalemulation) an understanding of thosemeters can be illuminating.

As we have already seen, a real audioprogramme is AC and can vary hugely inshape and symmetry. A moving-iron metercan measure an AC signal, but is too slow tobe responsive enough for audio. A moving-coil movement is lightweight and fast-responding, but can only measure a DCsignal. However, rectifying the audiowaveform produces DC to drive a moving-coil meter; either to make a needle waggle,or to make a mirror waggle a light beam ona ground-glass scale. In the early-to-mid

Term Abbr. Definition

Permitted Maximum Level PML The level of a sine-wave equivalent to thepermitted maximum programme-signalindication of a PPM. The programme soundshould be controlled by the broadcaster so thatthe amplitude of the peaks of the programmesignal rarely exceed the peak amplitude of asine-wave signal at the permitted maximumlevel. (This meter mark is traditionally used as aline-up reference level in DIN territories.)

Alignment Level AL (*) The level of a sine-wave signal 9 dB below asine-wave at permitted maximum level. Thislevel can be used to align sound-programmecircuits and equipment.

Measurement Level ML The level of a sine-wave signal 12 dB below asine-wave at alignment level (and 21 dB below asine-wave at permitted maximum level). Thislevel can be used for measurements at allfrequencies.

Maximum Coding Level The level of a sine-wave whose peaks can just beaccommodated by the full coding range of thedigital system in use.

Full Scale FS The full range of numbers available in a digitalcoding system. A sine-wave at maximum codinglevel is 0 dB FS

(*) For each type of programme meter, there is a scale marking corresponding to AlignmentLevel. This is the only metering point that is consistent for all programme meter types. It isdirectly related to a digital reference level expressed in dB FS.

1930s, rectifiers were a leading-edgetechnology and design choices were nottrivial. Two ways to produce a rectifiedaudio signal emerged.

A design based on a valve precisionrectifier followed by a valve logarithmicamplifier was clever but achievable.Thermionic valves were very expensive inthe 1930s when these meter designs werestandardised, but it was not expected thatyou’d need many of them – one or two perControl Room, probably – and thisapproach led to the BBC and DIN PPMs. Analternative technology was invented in theUSA in 1927 – the copper-oxide rectifier.This remarkable solid-state device hadlimitations in terms of voltage handling, butthey didn’t matter in audio applications andthe simplicity of manufacture offered acheap but rugged solution that could bebuilt in comparatively large numbers. This isthe SVI better known as the VU meter. Thehistorical development and thecharacteristics of these disparate metertypes is discussed at depth in a separatedocument available in the 2008 archivesection of the Line Upwebsite.

So, What Does ‘Peak’ Mean?Not much on its own. You need to be veryclear about what sort of peak you mean –other people’s assumptions will sometimesbe different. True waveform peaks areclearly different from PPM indications,which are different to the peak indicationson VU meters. For a typical, real-worldaudio signal, the true waveform peaks willbe 6 to 8 dB higher than the PPM peaks,and much higher than the VU meterindicates; all depending on the character ofthe audio in question.

It’s not widely known that the analoguewaveform can actually exceed the digitalcoding range. If the peaks of the waveformfall between two samples, then whenreconstructed the waveform will be higherthan the samples on either side. If thesesamples are already at 0 dB FS, thereconstructed waveform will exceedmaximum coding level. Clipping distortionwill result if that waveform is subsequentlyre-sampled, perhaps in a sampling-frequency converter or oversampling ordelta-sigma D-A converter. In practice, thisonly happens at high audio frequencies orwith impulsive sounds. (See AES-R7-2006).

What About Headroom?Headroom simply refers to the unused andavailable space above signal peaks, but theterm is almost meaningless without acareful definition of ‘peak.’ Headroom is ametering issue. Headroom above a meterindication does NOT imply that the

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‘unused’ space is empty – it will be more orless full of the fast-moving waveform peaksthat the meter is unable to indicate.

Trained ears provide us with the abilityto produce a consistent listening level withreference to our programme meterreadings, regardless of whether we use allthe dynamic range available or not. In CDmastering, by contrast, the headroom abovewaveform peaks is ruthlessly minimised by‘normalisation’ and/or the use ofcompression, regardless of whether aconsistent listening level is produced.

Note that not all transmission chainshave a flat maximum headroom frequencyresponse. Many analogue broadcastsystems, and some digital ones, use pre-emphasis to boost high-frequency (HF)signals for the transmission so that thereceiver can attenuate them again andthereby reduce HF noise introduced in thetransmission (TX) path. This has the effectof reducing the TX headroom at highfrequencies. A meter intended for use insuch systems should probably show this HFheadroom reduction in its overloadindication. National Geographic’s deliveryspecs, for example, currently require a 75-microsecond pre-emphasis in FMtransmitters to be respected. This will besomething to think about when specifyingmeters for a postproduction facility.

International Exchange ofRecordingsThe need to resolve different programme-level measurements is not new. The EBUand the ITU have been working for years toresolve such conflicts. The audio RosettaStone is to be found in ITU–RRecommendation BS.645, ‘Test signals andmetering to be used on international soundprogramme circuits.’ This sets out someformal definitions that pin down a workingrelationship between all conventionalprogramme meters in broadcast use, basedon a sine-wave line-up tone.

Digital Reference LevelUnfortunately, there are two of these. EBUR68 defines a digital audio reference level tobe -18 dB FS. SMPTE RP155 defines aslightly different digital audio referencelevel, -20 dB FS. It is worth pointing outthat, when these two figures were chosen,they were felt to represent the meter line-up level such that instantaneous waveformpeaks with real programme would be safelycontained within the PCM coding rangewithout clipping.

It’s my personal, if empirical, view thata faster-acting meter (PPM) with 18 dBheadroom and a slower-acting meter (VU)with 20 dB headroom will tend to fill thedynamic range in a very similar way. Thisview will be contentious for some who will

Meter scale comparison for line up

see the 2 dB difference between EBU andSMPTE reference levels as a systematic errorthat always needs fixing.Remember the golden rule:Alignment Level = PPM 4

= DIN -9= 0 VU= -18dB FS

Be ReadyDon’t assume that the metering techniquesyou learned at your mother’s knee will beall you need to know in your career.Metering in the wild may not use standardmeters at all. Don’t assume it’s a VU justbecause the scale says ‘vu.’ Don’t assumeit’s a PPM when it says ‘peak.’ Check itusing test signals. Where possible, use a

meter you know and trustand use the VTR metersfor line-up only.

When using newequipment, check that it’sset up as you would wantand expect. Carry a suiteof test tones on CD (or anyother medium that’sappropriate). If you’reusing an analogue meterwith a digital recorder, besure that the meterAlignment level is set tocorrespond to the correctdigital alignment level.

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