AUDIO MEASUREMENT HANDBOOK Bob Metzler Audio Precision, Inc. Copyright © 1993 Audio Precision. All rights reserved. Second edition for PDF, January, 2005. Audio Precision document number 0058.0003 r1 No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the publisher. Published by: AUDIO PRECISION 5750 SW Arctic Drive Beaverton, Oregon 97005 U.S.A. Telephone: (503) 627-0832 U.S. Toll-free telephone: (800) 231-7350 FAX: (503) 641-8906 Web site: audioprecision.com Printed in the United States of America V0127090352 ABOUT THIS BOOK The AUDIO MEASUREMENT HANDBOOK is intended as practical, hands-on assistance for workers in all phases of the audio field. Its treatment of topics does not involve mathematics beyond simple algebra. Although pub- lished by Audio Precision, the measurement techniques described generally are not specific to Audio Precision instruments, but apply to any audio testing equipment. The first section describes basic tools and techniques and includes guides to aid in the selection of appropriate audio test instruments. The second section discusses the common environments for audio testing. The third sec- tion applies these techniques to various commonly-used types of audio equip- ment and describes the typical ranges of performance to expect in that equip- ment. Section Four is a glossary of specific audio terminology and major speci- fications used in the audio measurement field. ABOUT THE COVER The classic audio test instruments on the cover are from the “museum” at Audio Precision. Most are from the personal collection of Wayne Jones; all are fully functional. The Model 200 oscillator was the first product developed in the legendary Palo Alto garage by Hewlett and Packard. The black crackle, wooden cabinet General Radio Model 1932A and the HP 330 provided a gener- ation of broadcast engineers with their first ability to measure distortion. The Tektronix TM 500 instruments were designed by the engineers who later founded Audio Precision. ACKNOWLEDGEMENTS The author wishes to thank the following individuals for key advice and as- sistance in the development of this handbook: Bruce Hofer of Audio Precision, especially for his understanding of the mys- teries of balanced and unbalanced device interconnection, ground loops, shield- ing, and various other analog considerations critical to obtaining accurate au- dio measurement results in the real world. Dr. Richard Cabot of Audio Precision for helping me understand digital au- dio and the FASTTEST and FASTTRIG multitone techniques. Wayne Jones of Audio Precision (and formerly Amber Electro Design) for many useful suggestions on content and format of this book. The late Hans Schmidt of ABC-TV for sensitizing me to the differences be- tween transducer gain and “schoolboy gain,” and the importance of those dif- ferences. And the entire Audio Precision Engineering Design Team for providing the marvelous System One hardware and software platform which made it possi- ble to demonstrate and illustrate all the concepts described in the pages which follow. Bob Metzler, Beaverton, Oregon, July 1993 Contents ABOUT THIS BOOK............................. 3 ABOUT THE COVER ............................ 3 ACKNOWLEDGEMENTS .......................... 4 Section 1 Tools and Techniques.........................1 Introduction and Basic Tools ......................... 1 Sine Wave Generation............................ 2 RC Oscillators ............................... 2 Function Generators............................ 2 Frequency Synthesizers.......................... 2 Direct Digital Synthesizers (Waveform Synthesizers) ...........3 Current Technology Trends in Signal Generation .............3 Complete Audio Signal Generation Instruments..............3 Amplitude Calibration ........................... 4 Important Generator Characteristics ...................5 Amplitude Measurements .......................... 5 Rms Detectors............................... 5 Average-Responding Detectors ...................... 6 Peak Detectors .............................. 6 Detector Response Times ......................... 7 Crest Factor ................................ 7 Amplitude Measurement Units ...................... 7 Power ................................... 8 Decibels .................................. 8 Absolute Decibel Units .......................... 8 dBm....................................9 dBu....................................10 dBV................................... 10 Complete Instruments .......................... 11 Termination and Load Resistors .....................11 Audio Measurement Handbook i Contents Balanced Inputs ............................. 12 Ranging ................................. 12 Autoranging ............................... 13 Accuracy and Resolution......................... 13 Response Flatness............................ 14 Noise Measurements ............................ 15 Weighting (Psophometric) Filters ....................15 Time Variance .............................. 17 DUT Input Conditions for Noise Measurements .............17 Signal-to-Noise Ratio .......................... 18 Important Meter Characteristics for Noise Measurement ........18 Signal Measurements in the Presence of Noise ..............19 Dynamics of Selective Measurements ..................20 Time Domain and Frequency Domain ...................20 Measurement of Non-Linearity ....................... 22 Output Versus Input Amplitude...................... 23 Harmonic Distortion ........................... 27 Individual Harmonic Distortion ...................... 27 Total Harmonic Distortion (THD) .....................29 Total Harmonic Distortion Plus Noise (THD+N) .............30 Noise Bandwidth for THD+N Measurements...............31 Interference Signal Effects on THD+N ..................31 Noise-Limited Devices .......................... 32 Relative vs Absolute Distortion Units...................33 Harmonic Order vs Bandwidth; Limitations on Usefulness of Harmonic Distortion Techniques with Band-Limited Devices ............34 Instrumentation Requirements for THD+N Measurements........35 Intermodulation Distortion (IMD) .....................35 SMPTE/DIN IMD ............................. 36 SMPTE/DIN Instrument Criteria .....................38 “CCIF,” Twin-Tone, Difference-Tone IMD.................38 “CCIF” IMD Instrument Criteria...................... 40 Dynamic/Transient Intermodulation Distortion (DIM/TIM) ........41 DIM/TIM Instrument Criteria ....................... 42 Other IMD Techniques .......................... 42 Frequency Measurements ......................... 42 Frequency Counter Instrument Criteria .................44 Phase Measurement ............................ 44 Input-Output Phase ........................... 45 Phase Meter Instrument Criteria .....................45 Wow and Flutter Measurements ...................... 46 Measurement Technique ......................... 48 Instantaneous vs Processed Readings .................48 Wideband (Scrape) Flutter ........................ 49 ii Audio Measurement Handbook Contents Spectrum Analysis of Wow and Flutter..................50 Measurements With Noise Stimulus ....................50 White Noise ............................... 51 Pink Noise ................................ 51 Analysis of White Noise ......................... 51 Analysis of Pink Noise .......................... 51 Frequency Resolution Detail in Noise Testing ..............52 Pseudorandom vs Random Noise Signals ................53 Sets of Measurements ........................... 55 Measurement Dynamics and Reading Settling ...............56 Estimating Input Impedance ........................ 59 Output Impedance Estimation ....................... 59 Characterizing a “Black Box” ........................ 61 Interface to the Device Under Test .....................64 Balanced Devices ............................ 64 Unbalanced Devices ........................... 66 Digitally-Implemented Advanced Measurement Techniques ........67 Waveform Display (Digital Storage Oscilloscope Mode) .........68 Fast Fourier Transform (FFT) ...................... 70 FASTTEST Technique .......................... 74 Digital Domain Testing ........................... 77 Digital Audio Formats .......................... 77 Generation of Digital Domain Signals ..................79 Analysis of Digital Domain Signals ....................80 Section 2 Major Testing Environments ....................81 Laboratory Research and Development ..................81 Production Test and Quality Assurance ..................82 Maintenance and Repair .......................... 83 Section 3 Testing Specific DUT Types .....................85 Introduction ................................. 85 Amplifiers and Similar Real-Time Linear Analog Audio Devices ......85 Standard Tests—Response and Distortion................86 Maximum Output Level ......................... 87 Noise and S/N Ratio ........................... 87 Crosstalk and Stereo Separation.....................87 Input vs Output Clipping ......................... 89 Equivalent Input Noise .......................... 89 Transducer Gain vs voltage Gain ....................90 Transducer Gain via Substitution.....................90 Transducer Gain via Proper Test Instrument Features..........91 Measuring Equalization and Emphasis..................91 Audio Measurement Handbook iii Contents