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Audio Signal Visualisation and Measurement Audio Signal Visualisation and Measurement Robin Gareus Chris Goddard Universit´eParis 8 Freelance audio engineer Paris, France Woodstock, Oxfordshire, UK [email protected] [email protected] Abstract Similarly for technical engineers, reliable The authors offer an introductory walk-through of measurement tools are indispensable for the professional audio signal measurement and visuali- quality assurance of audio-effects or any pro- sation. fessional audio-equipment. The presentation focuses on the SiSco.lv2 (Sim- ple Audio Signal Oscilloscope) and the Meters.lv2 2 Meter Types and Standards (Audio Level Meters) LV2 plugins, which have been For historical and commercial reasons various developed since August 2013. The plugin bundle measurement standards exist. They fall into is a super-set, built upon existing tools with added three basic categories: novel GUIs (e.g ebur128, jmeters,..), and features new meter-types and visualisations unprecedented • Focus on medium: highlight digital num- on GNU/Linux (e.g. true-peak, phase-wheel,..). Various meter-types are demonstrated and the mo- ber, or analogue level constraints. tivation for using them explained. • Focus on message: provide a general indi- The accompanying documentation provides an cation of loudness as perceived by humans. overview of instrumentation tools and measurement standards in general, emphasising the requirement • Focus on interoperability: strict specifi- to provide a reliable and standardised way to mea- cation for broadcast. sure signals. The talk is aimed at developers who validate DSP For in-depth information about metering during development, as well as sound-engineers who standards, their history and practical use, mix and master according to commercial constraints. please see [Brixen, 2010] and [Watkinson, 2000]. Keywords 2.1 Digital peak-meters Audio Level Metering, Visualisation, LV2, DSP A Digital Peak Meter (DPM) displays the abso- lute maximum signal of the raw samples in the 1 Introduction PCM signal (for a given time). It is commonly used when tracking to make sure the recorded Audio level meters are very powerful tools that audio never clips. To that end, DPMs are are useful in every part of the production chain: calibrated to 0dBFS (Decibels relative to Full Scale), or the maximum level that can be rep- • When tracking, meters are used to en- resented digitally in a given system. This value sure that input signals do not overload and has no musical connection whatsoever and de- maintain reasonable headroom. pends only on the properties of the signal chain or target medium. There are conventions for • Meters offer a quick visual indication of ac- fall-off-time and peak-hold, but no exact spec- tivity when working with a large number of ifications. Furthermore, DPMs operate on raw tracks. digital sample data which does not take inter- • During mixing, meters provide a rough es- sample peaks into account, see section 2.7. timate of the loudness of each track. 2.2 RMS meters • At the mastering stage, meters are used to An RMS (Root Mean Square) type meter is an check compliance with upstream level and averaging meter that looks at the energy in the loudness standards, and to optimise the dy- signal. It provides a general indication of loud- namic range for a given medium. ness as perceived by humans. dBu dBFS VU BBC EBU DIN Nordic For home recording, there is no real need for +20 this level of interoperability, and these meters +18 0 are only strictly required when working in or +14 +5 −4 +12 7 +12 +12 −6 with the broadcast industry. However, IEC- +10 −8 type meters have certain characteristics (rise- 0 +9 +8 6 +8 −10 time, ballistics) that make them useful outside +6 3 +6 −12 the context of broadcast. 2 +4 1 5 +4 −5 −14 Their specification is very exact [IEC, 1991], 0 +3 +2 1 −16 and consequently, there are no customisable pa- 2 0 3 4 Test Test −18 rameters. 5 −2 −10 −20 7 −3 −4 3 −4 −22 −6 10 −20 −6 −24 2 −8 −9 −10 20 1 −12 −30 −28 −15 −20 −40 −38 −30 −50 −48 −40 −42 −58 −50 −68 dBu IEC 60268−17 IEC 268−10 IIB IEC 268−10 I dBFS IEC 268−10 IIA IEC 268−10 Figure 1: Various meter alignment levels as spec- ified by the IEC. Common reference level is 0dBu calibrated to -18dBFS for all types except for DIN, which aligns +9dBu to -9dBFS. dBu refers to voltage in an analogue system while dBFS to digital signal full-scale. Bar-graph RMS meters often include an ad- ditional DPM indicator for practical reasons. The latter shows medium specifics and gives an Figure 2: Various meter-types from the meter.lv2 indication of the crest-factor (peak-to-average plugin bundle fed with a -18 dBFS 1 kHz sine wave. power ratio) when compared to the RMS me- Note, bottom right depicts the stereo phase correla- ter. tion meter of a mono signal. Similar to DPM's, there is is no fixed stan- dard regarding ballistics and alignment level for 2.4 EBU R-128 a general RMS meter, but various conventions The European Broadcast Union recommenda- do exist, most notably the K-system introduced tion 128 is a rather new standard, that goes by Bob Katz [Katz, 2000]. beyond the audio-levelling paradigm of PPMs. It is based on the ITU-R BS.1770 loudness al- 2.3 IEC PPMs gorithm [ITU, 2006] which defines a weighting IEC (International Electrotechnical Commis- filter amongst other details to deal with multi- sion) type Peak Programme Meters (PPM) are channel loudness measurements. To differenti- a mix between DPMs and RMS meters, cre- ate it from level measurement the ITU and EBU ated mainly for the purpose of interoperability. introduced a new term `LU' (Loudness Unit) Many national and institutional varieties exist: equivalent to one Decibel1. The term `LUFS' is European Broadcasting Union (EBU), British then used to indicate Loudness Unit relative to Broadcasting Corporation (BBC), Deutsche full scale. Industrie-Norm (DIN),.. [Wikipedia, 2013]. In addition to the average loudness of a pro- These loudness and metering standards pro- gramme the EBU recommends that the `Loud- vide a common point of reference which is used ness Range' and `Maximum True Peak Level' by broadcasters in particular so that the inter- be measured and used for the normalisation of change of material is uniform across their sphere audio signals [EBU, 2010]. of influence, regardless of the equipment used to 1the ITU specs uses `LKFS', Loudness using the K- play it back. See Fig.1 for an overview of ref- Filter, with respect to to Full Scale, which is exactly erence levels. identical to 'LUFS'. The target level for audio is defined as -23 logarithmic amplifiers were physically large). LUFS and the maximum permitted true-peak The meter's designers assumed that a record- level of a programme during production shall ing medium with at least 10 dB headroom over be -1 dBTP. 0 VU would be used and the ballistics were de- The integrated loudness measurement is in- signed to \look good" with the spoken word. tended to quantify the average program loud- Their specification is very strict (300ms rise- ness over an extended period of time, usually a time, 1 - 1.5% overshoot, flat frequency re- complete song or an entire spoken-word feature. sponse), but various national conventions ex- [Adriaensen, 2011], [EBU, 2011]. ist for the 0VU alignment reference level. The Many implementations go beyond displaying most commonly used was standardised in 1942 range and include a history and histogram of in ASA C16-5-1942: \The reading shall be 0 the Loudness Range in the visual readout. This VU for an AC voltage equal to 1.228 Volts RMS addition comes at no extra cost because the al- across a 600 Ohm resistance"2 gorithm to calculate the range mandates keep- ing track of a signal's history to some extent. 2.6 Phase Meters Three types of response should be provided A phase-meter shows the amount of phase dif- by a loudness meter conforming to R-128: ference in a pair of correlated signals. It al- lows the sound technician to adjust for opti- • Momentary response. The mean squared mal stereo and to diagnose mistakes such as level over a window of 400ms. an inverted signal. Furthermore it provides • Short term response. The average over 3 an indication of mono-compatibility, and pos- seconds. sible phase-cancellation that takes place when a stereo-signal is mixed down to mono. • Integrated response. An average over an extended period. 2.6.1 Stereo Phase Correlation Meters Stereo Phase Correlation Meters are usually needle style meters, showing the phase from 0 to 180 degrees. There is no distinction between 90 and 270 degree phase-shifts since they produce the same amount of phase cancellation. The 0 point is sometimes labelled \+1", and the 180 degree out-of-phase point \-1". 2.6.2 Goniometer A Goniometer plots the signal on a two- dimensional area so that the correlation be- tween the two audio channels becomes visually apparent (example in Fig.8). The principle is also known as Lissajous curves or X-Y mode in oscilloscopes. The goniometer proves useful be- Figure 3: EBU R-128 meter GUI with histogram cause it provides very dense information in an (left) and history (right) view. analogue and surprisingly intuitive form: From the display, one can get a good feel for the au- 2.5 VU meters dio levels for each channel, the amount of stereo Volume Unit (VU) meters are the dinosaurs and its compatibility as a mono signal, even to (1939) amongst meters.
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