Time Delay Compensation of Distributed Multiple Microphones In
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Time delay compensation of distributed multiple microphones in recording -an experimental evaluation Theresa Ann Leonard Department of Theory Faculty of Music McGiII University, Montreal November, 1991 A Thesis Submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Music. J © Theresa Leonard, 1991 T ABLE OF CONTENTS Acknowledgements ...................................................................... ili LIst. 0 fD'lagrams ............... '" " ..... , .................. '" .......................... IV. Abst:ïd.ct ............................................................................. V Résumé ............................................................................. VI 1 OVERVIEW OF CLASSICAL MUSIC RECORDING PRACfICE: THE NEED FOR BLENDING MAIN MICROPHONES WITII SPOT MICROPHONES 1.1 Classical Music Recording Considerations ................................... ! 1.2 Spot Microphone Considerations ............................................. 5 1.3 Negative Effects of Spot Miking ............... .. ........ ................. 6 1.3.1 Acoustic Phase Cancellation ............................................. 7 1.3.2 Corn b-Filter Effects ....................................................... 8 1.3.3 Microphone Off-Axis Coloration ....................................... 8 1.4 Focus of Work .................................................................. 9 2 MIXING WITH DELA Y COMPENSATION 2.1 Time I:>elay Mixing ............................................................ Il 2.2 Previous Work in This Area ................................................. 12 2.3 Delayed Spot Microphones ................................................... 13 3 OVERVIEW OF FACfORS CONTRffiUTING TO GREA1ER UNDERSTANDING OF AUDffiLE EFFECTS OF DELA Y COMPENSATION 3.1 Psychoacoustic Comiderations ............................................. 16 3.2 The Role of First Reflections ............................................... 17 3.3 The Perception of Depth In Recording ...................................... 19 3.4 Musical Instrument Radiation Patterns ...................................... 21 l 4 RESEARCii PERFORMED: EXPERIMENTAL METHODS AND l IMPLEMENTATION 4.1 Procedure ..................................................................... 24 4.2 Methodology ................................................................... 25 4.2.1 A Calcuhttion Model .................................................... 2H 4.2.2 Temperature and Humidity Effects on the Speed of Sound. ..... 29 4.3 Listening Test Recordings .................................................... 30 5 LISTENING TEST RESULTS .................................................... 40 5.1 Analysis of Data .............................................................. 40 5.2 Imerpretation ofComments ................................................... 43 6 SUMMARY AND DISCUSSION ............................................... 48 6.1 'Time Delay Recording Technique: A Tooi For Balancing ................ 48 6.2 Potential Problems ............................................................ 49 6.3 Future Considerations ........................................................ 52 7 BIBLlOORAPHY .................................................................. 56 8 APPENDIX ......................................................................... 59 u ACKNOWLEDGEMENTS 1 would like 10 thank my advisor, Dr. Wieslaw Woszczyk, for his inspiration and advice in begmning this research and for taking the time 10 keep in contact with me from a distance. 1 also appn!Clate the efforts of Kevin Elliou for his helpful guidance through the final stages of this work and for his understanding 10 giving me time to complete this papl~r while at The Banff Centre. T'hank you 10 all who took part in the Listening Tests. To my mother for her continuous support of my work, 1 dedicate this thesis. iii LIST OF DIAGRAM~, 1 1. McGill Symphony Orchestra: Pollack concert hall; January 25,1989 ......... 32 2. McGill Symphony Orchestra: Pollack concert hall; April 1,1989 ............. 34 3. The Banff Centre: Project Studio Recording; December 2, 1989 ............... 36 4. The Banff Centre: Project Studio Recording; January 29, 1990 ............... 39 5. Potential effect of reverb sound with spot microphones ........................ 50 iv ABSTRACT In the search for improved reproduction of a classical music perfonnance, the technique of time delay compensation is shown to he a useful 1001 for balance in recording. ThIs paper investigates the importance and val1dIty of small rime adju),lments in recordIngs to compensate for vanaoon 10 distance between spot microphones and a mam stereo pair. Conventional recording techniques, psychoacoustic considerations and lechnological aspects of the use of delays are researched in order to determine therr validity in timbrai Improvement. Multiple microphone set-ups are used ta record hoth large orchestral works and smaller-scale cla'lsical compositions where auxiliary microphones may be net"Aied to ensure an optimum balance in the final mlx. Small Ume delays <Ut! denved from calculations involving the distances between microphones, the speed of sound, and humidity and temperature readIngs from the hall. Proper synchronization of these delays is deslrable w preserve phase coherence and combat comb-filter effects. Precise delay units are used to compile musical excerpls for listening tests. The results reveal any change in sound quality and provide a basis for investigating botJ-. the positive and negative effects through objective study of the value of time delay compensation in the live recording reproduction of classical music perfonnances. 1 v RESUME Dans le ca~ de recherches sur la façon d'améliorer la reproduction de la musiqut~ classique, la te:chnique de l'égalisation de temponsauon revêt une gmnde ullhté pour l'équilibre de l'enregistrement. Dans cet arucle. l'auteur étudie l'Importance et la valeur d.?' petits ajustements temporels visant à com~nser l'écan de distance entre le!'> mlcropholle .... isolés et la pJincipale paire stéréophomque L'auteur analyse les techniques d'enregistrement classiques, les paramètres psych~acoustiques et lc~ aspects technologIques de l'emploi des temporisatiom pour déterminer leur valeur dans ['amélioraüon du tImbre. La mif~e en place de microphones multiples sen à enregistrer les oeuvres pour grand orchestI'l~ et les compositIons claSSiques pour formanon plus restremte où Il ~e peut qu'on ait besoin de microphones auxilialre~ pour assurer un meilleur équIlibre dan .. le mixage final. Les petits retards temporels sont urés des calculs ~ur la dl'ltance cntre lc'i microphones. la vItesse du son et les relevés d'humidité et de température dan~ la 'Ialle La bonne synchromsation de ces retards est souhaitable pour préserver la cohérence de phase et lutter contre les effets des filtres en peigne. Les dlsposmf'l de tempon'iatlon précis servent à compIler des extraits musicaux pour les es'mls d'écoute. Les résultat" révèlent le moindre changement dans la qualIté du 'Ion et 'icrvent de fondement à l'étude des effets positifs et négatifs par l'étude objective de la valeur de l'égalisation de temporisation pour l'enregistrement en direct de concens de musique classique. vi Chapter 1 OVERVIEW OF CI.ASSICAI. MUSIC RECORDING PRACTICE: THE NEED FOR BLENDING MAIN MICROPHONES WITH SPOT MICROPHONES 1.1 Classical Music Recording Considerations The recording of a classical music perfonnance is generally most successful when Ibtcners are least aware of any signal processing by the recording technicians. There are argument!'> as to whether the reproduced sound should be as close as possible a replica of the perfonnance, or whether the recording engineer should improve on the performance where feaslble via si? 'al processmg. AIthough there is no speclfic recording technique for classical music, the microphone technIque wIll have a great effect on the reproduced sound. The choice of a particular [echmque wIll depend on vanous Clfcumstances. Three basiC philosophies developed regarding microphone placement Wlth the mtroduction of stereophonie recording: 1) l'oinCldent stereo, ~) spaced omni, and 3) multiple microphones. Each technique has its own uses in the recording studio, sorne with advantages over others 11. certrun sItuations, '\uch as a lIve event when the room IS not farniliar. There are three basic categories into whit:h lhe~e philosophies faB: single-point, main microphone pickup (mis includes ..:oincldent, near coincident and spaced microphone techniques); smgle point with auxiliary :ll1crophones, and muln-microphone technique. In the flfSt case, one stereo microphone is _l~cd to record the sound as accurately as pos:,.ti)le. This is considered to he the minimalist .tpproach. The second case uses one main stereo microphone as well with addltlonal spot mil:rophones to support '.lJeak instruments or groups. This require:; mixmg, as does the thtrd case where each m~trumental group is recorded with its own rmcrophone. Case two may offer an advantage over case one and three, especially where the hall tS not familiar, by using spot microphones for emphasis. Also, by adding an appropriate lime delay to the spot mIcrophones. whlch wIll be dISl'U~~cd ln Chapter 2. thl:! ~ense of depth can he Improved. 111lS mve~rigatlon will bc regarded a~ another microphone techmque to he explored In the same manner a~ more convcntlonal recording techmq ues. A proper under~tandmg of the effects of miCrophone