Measurements of electrical transfer characteristics of ...

Motivation Measurement Basics Results Conclusions

Overview

Measurements of electrical transfer characteristics of soundcards as classroom activity Stephan Paul and Pascal Dietrich Contributions from Eric Brand˜ ao, Bernardo Murta, J´ essica J. Lins de Souza and Sergio Aguirre

Undergrad. Program in Acous. Engineering Federal University of Santa Maria Santa Maria – Brazil

S. Paul, P. Dietrich

Institute of Technical Acoustics RWTH Aachen University Aachen – Germany

Measurement transfer characteristics of soundcards


Overview

1

Motivation Teaching Acoustics and Acoustical Signal Processing Goals

2

Measurement Basics Software Measurements with Audio Interfaces and ITA-Toolbox

3

Results Reference Cable Device under Test Post-Processing

4

Conclusions




Teaching Acoustics and Acoustical Signal Processing Goals

How to teach acoustics and acoustical signal processing? Teaching should be practical, but Around 30 students attend the same class in our undergraduate program How to pay and maintain acoustical equipment for such a large number of students? How to make 30 individual measurements within a classroom possible?

Students of acoustical engineering usually have laptop computers with MATLAB → develop a classroom activity to teach acoustics (transfer functions) and acoustical signal processing for 30 students simulteanously




Teaching Acoustics and Acoustical Signal Processing Goals

Goals

Measure transfer characteristics in classroom activity Aspects to be taught frequency response function (FRF) impulse response (IR) relationship – FRF IR linear time invariant (LTI) systems linearity and limited range signal to noise ratio (SNR) latency

Acoustical system substituted by electrical system Filter circuit




Software Measurements with Audio Interfaces and ITA-Toolbox

Theory of LTI-Systems s(t)

*

h(t) = g(t)

n(t)

LTI S(f) •

H(f) = G(f)

N(f)

Understand systems as LTI systems Understand the limits What happens outside these limits ?





ITA-Toolbox Open Source Kernel available – BSD License – ita-toolbox.org First introductory course for a selected group of undergraduate students and post-graduate held in January 2011 Plots/GUI DSP

Time/Frequency Balloon–Directivity Geometry/Mesh Parametric GUI GLE

Convolution Filtering/Windowing Levels dB(A)/dB(C) Transformations

STFT/cepstrum/hilbert

Im-/Export Measurement/Simulation LMS Virtual Lab, BK Pulse, ArtemiS, Sysnoise, MonkeyForest, ANSYS Universal File Format, ASCII

Knowledge Kernel Classes Operators Units

Data IO

Documentation in-line help/PDF

Tutorial

Application

Demos, test routines

SVN

optional: PortAudio/PortMedia Acoustic Measurement

MIDI, RS232

Turntable/Arm, X-Y-Bench, Measurement Hardware www.ita-toolbox.org





First Course on DSP, MATLAB and ITA-Toolbox January 2011 by the developers of ITA-Toolbox Attendee – different background Students and faculty members (Undergraduate Program) Post-graduate students (Mechanical Engineering)

Valuable but heavily packed 3 days Suggestion: simple and cheap hardware simulate system response connect directly to laptops hands-on approach





Simple Circuit with Frequency Response

easy to connect to onboard souncards external soundcards

provides bypass (direct) filter circuit

manufacturing cheap easy





Physical Set-up of hardware





Measurement Parameters exp. sweep, 20 − 20000 Hz stop margin, length and level varied all measurement parameters can be accessed and changed quickly by MATLAB command line, e.g. x = M.fftDegree M.fftDegree = x

influences can be experienced directly and quickly → perfect for classroom activity S. Paul, P. Dietrich



Reference Cable Device under Test Post-Processing

Spectra of Reference Measurements

Figure: Reference (bypass cable) measured with PreSonus Soundcard and different output levels

Student question: Where do the “ripples” come from? Noise? S. Paul, P. Dietrich




Impulse Response of Reference Measurement Exponential sweep and deconvolution Harmonic peaks visible [Farina AES 2000, M¨uller/Massarani JAES 2001]





Finding the Limits of Linear Range

latency: 6 ms MS = itaMSTF; MS.freqRange = [5 25000]; MS.fftDegree = 17; MS.stopMargin = 0.5; different output amplifications S. Paul, P. Dietrich

latency: 6 ms MS = itaMSTF; MS.freqRange = [5 25000]; MS.fftDegree = 17; MS.stopMargin = 0.5; MS.output amplification=-20; Measurement transfer characteristics of soundcards



Finding the Limits of Linear Range





STFT of Linear and Non-linear System Output Explanation of harmonic peaks Non-linearities cause harmonic frequencies of the sinusoidal input




Conclusions Classroom activity successful, requires good preparation of hardware and its settings better use RCA connectors on special blackbox cable → more easy to fit to different soundcards1

Several parameters and setting important, also in the operating system (e.g. latency) measurements with soundcards possible ITA-Toolbox is the perfect buddy for first contact: use GUI for quick changes: use command line tool

students developed a small ITA-Toolbox for dummies manual 1

center pin of 3, 5 mm jack is difficult to handle S. Paul, P. Dietrich
