Audio Signal Visualisation and Measurement

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Robin Gareus

linuxaudio.org
CiTu, Université Paris

Part I - Audio Signal Meters

Why measure ?


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

Why measure ?


  • 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.

Why measure ?


  • 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.

Why measure ?


  • 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.

What to measure ?



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

What to measure ?



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

What to measure ?



  • 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.

Types of Meters


  • 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.

Types of Meters


  • 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.

Types of Meters


  • 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)

On Measurements - An analogy


On Measurements - An analogy


On Measurements - An analogy


On Measurements



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

On Specs (1/2)



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

On Specs (2/2)



  • 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.

Key Characteristics of Audio Level Meters



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

VU meter specs (ASA C16-5-1942)

  • “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.”

Presonus Tube Pre - a VU meter toy

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

Two Commercial Plugins - digital VU toys

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

meters.lv2

  • 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)

Look & Feel (1/3)

Look & Feel (2/3)

Look & Feel (3/3)

Real World Testing


Source Code & Documentation

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

References

Part II - Oscilloscope

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.

Requirements

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!

xoscope

Audio support:

  • OSS
  • ESD

(last update 5 years ago)

Signal vs. Waveform

Sisco.lv2 - Simple Scope

  • 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..

Sisco.lv2 - Screenshot

Source Code & Documentation

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

The End



Thank you for your attention.

 
wiki/lac14slides.txt · Last modified: 02.05.2014 07:08 by rgareus