Audio Signal Visualisation and Measurement

Robin Gareus

linuxaudio.org
CiTu, Université Paris

• When tracking, meters are used to ensure that input signals do not overload and maintain reasonable headroom.

• When tracking, meters are used to ensure that input signals do not overload and maintain reasonable headroom.
• Meters offer a quick visual indication of activity when working with a large number of tracks.

• When tracking, meters are used to ensure that input signals do not overload and maintain reasonable headroom.
• Meters offer a quick visual indication of activity when working with a large number of tracks.
• During mixing, meters provide a rough estimate of the loudness of each track.

• When tracking, meters are used to ensure that input signals do not overload and maintain reasonable headroom.
• Meters offer a quick visual indication of activity when working with a large number of tracks.
• During mixing, meters provide a rough estimate of the loudness of each track.
• At the mastering stage, meters are used to check compliance with upstream level and loudness standards, and to optimise the dynamic range for a given medium.

• Focus on medium: highlight digital number, or analogue level constraints.

• Focus on medium: highlight digital number, or analogue level constraints.
• Focus on message: provide a general indication of loudness as perceived by humans.

• Focus on medium: highlight digital number, or analogue level constraints.
• Focus on message: provide a general indication of loudness as perceived by humans.
• Focus on interoperability: strict specification for broadcast.

• DPM (digital peak meter) displays the absolute maximum signal of the raw audio PCM signal. Focus on medium. ie. digital numbers, and Analog/Digital converters.

• DPM (digital peak meter) displays the absolute maximum signal of the raw audio PCM signal. Focus on medium. ie. digital numbers, and Analog/Digital converters.
• RMS (root mean square) type meters provide a general indication of loudness as perceived by humans. It emphasizes on the message as opposed to technical, medium related parameters.

• DPM (digital peak meter) displays the absolute maximum signal of the raw audio PCM signal. Focus on medium. ie. digital numbers, and Analog/Digital converters.
• RMS (root mean square) type meters provide a general indication of loudness as perceived by humans. It emphasizes on the message as opposed to technical, medium related parameters.
• IEC type meters are a mix between DPM and RMS, created mainly for the purpose of interoperability. (broadcast instustries: BBC, EBU)

The whole point of measuring things is to be able to meaningfully compare the reading of the meter to that of another [similar] meter.

The IEC specs are designed such that all meters complying with the specs, even when using completely different implementations, will produce identical results.

• Audio Level Meters are standardized (IEC, ITU, IEEE, DIN, BBC,…)
• Due to historical and commercial reasons various standards exist
• The scale of [most] meters is logarithmic (dB)
• Different types - Medium vs. Content.

• Alignment or Reference Level and Range
• Ballistics (rise/fall times, peak-hold, burst response)
• Frequency Response (filtering)

• “The reading shall be 0 VU when for an AC voltage equal to 1.228 Volts RMS across a 600 Ohm resistance”
• “The rise time, defined as the time it takes for the needle to reach 99% of the distance to 0 VU when the VU-meter is submitted to a signal that steps from 0 to a level that reads 0 VU, is 300 ms.”
• “The fall time is the same as the rise time”
• “The overshoot must be within 1 to 1.5%.”
• “The reading must not depart from the reading at 1kHz by more 0.5 dB between 25 Hz and 16 kHz.”

• incorrect deflection (not to scale)
• not useful for measurements (but it looks cool :)

• LSR Audio – incorrect ballistics (overshoot, rise/fall times, jitters at lower freq)
• PSP Audioware – incorrect frequency response

• IEC 60268-17 / VU
• IEC 60268-10 Type I / DIN
• IEC 60268-10 Type I / Nordic
• IEC 60268-10 Type IIa / BBC
• IEC 60268-10 Type IIb / EBU
• Stereo Phase Correlation Meter (Needle Display)
• EBU R128 Meter with Histogram and History
• Digital True-Peak Meter (4x Oversampling)
• Goniometer (Stereo Phase Scope)
• IEC61260 30-Band Spectrum-Analyzer
• Phase/Frequency Wheel
• Stereo/Frequency Monitor
• DR14 (pleasurizemusic.com, specs by tischmeyer/algorithmix)

• Source-code at https://github.com/x42/meters.lv2
• Licensed under GPLv2
• Binaries available for OSX, GNU/Linux

Contributors:

• Fons Adriaensen – DSP-expert (jmeters)
• Jaromír Mikeš – packages for Debian/Ubuntu
• Alexandre Prokoudine – P&R; sponsored book
• Chris Goddard and Axel Müller – testing
• Joern Nettingsmeier – inspiration & motivation

• VU_meter
• Peak_programme_meter
• How To Make Better Recordings in the 21st Century - An Integrated Approach to Metering, Monitoring, and Leveling Practices by Bob Katz.
• “Audio Metering: Measurements, Standards and Practice: Measurements, Standards and Practics”, by Eddy Brixen. ISBN: 0240814673
• “Art of Digital Audio”, by John Watkinson. ISBN: 0240515870

The oscilloscope is the jack of all trades of electronic instrumentation tools. It produces a two-dimensional plot of one or more signals as a function of time.

For an an oscilloscope to be useful for engineering it must

• reproduce the signal (regardless of sampling rate)
• be calibrated - both in time and level
• allow acquisition of particular events (triggering, signal history)
• provide numeric read out

An oscilloscope is not just a waveform display!

Audio support:

• OSS
• ESD

(last update 5 years ago)

• classic audio oscilloscope with variable time scale
• triggering and buffering
• cursors
• numeric readout
• LV2 plugin format and standalone jack application

It is feature complete for an audio-scope but it is rather simplistic compared to contemporary hardware oscil loscopes..

• Source-code at https://github.com/x42/meters.lv2
• Licensed under GPLv2
• Binaries available for OSX, GNU/Linux

Thanks to:

• Fons Adriaensen – LAC'13 paper
• Jaromír Mikeš – packages for Debian/Ubuntu
• Aurélien Leblond – video tutorial (w/ingen & AMS)
• Damien Zammit – power user & tester
• Tomas Brand – for insisting on making it scalable

Thank you for your attention.