Decibels. Frédéric Lanteigne

Transcription

1 Decibels Frédéric Lanteigne

2 Introduction l If you re in the professional audio field, you can at least give me one definition of what is a decibel. l For example, if someone asks you for an audio mix destined for television, radio, cinema or musical recording, you got to be aware of the levels to respect. 2

3 Agenda l Decibels definitions l Gain l db calculation l dbv l dbu l dbfs l dbspl l dbm l Charts and standards l Reading and calibration of VU meter l How do we measure a RMS signal, studio application and examples 3

4 The decibel l Mathematically, the decibel is simply a unit of measurement used to know the power of sound. It represents the ratio of a known power or the relation between a known reference power. Ex; a dbw is the decibel above a Watt. The reference is one (1) Watt. Ex; a dbm is a decibel above a milliwatt. The reference power is one (1) mw. 4

5 Gain (G) l The gain is the ratio of the output value diveded by the input value before being converted to db. The gain has no unity variable.. Input signal 2V Output signal 10V The gain is 10V/2V = 5 (without unity variable) G = V(output) / V(input) G = P(output) / P(input) 5

6 db calculations with power (Watts) 2W P (input) G= 100W / 2W = 50 G P (output) 100W db = 10 log G db = 10 log (100W / 2W) db = 10 log ( 50) = 17 db 6

7 db calculations with tension (volts) 2V V (input) G= 100V / 2V = 50 G V (output) 100V db = 20 log G db = 20 log (100V / 2V) db = 20 log ( 50) = 34 db 7

8 The dbv Ex; a measured output of V in relation to a reference of 1V 1V V (reference) V(output) G V dbv= 20 log G dbv= 20 log ( V / 1V) dbv= 20 log = -10dBV 8

9 The dbu It s the relative tension measured in relation to a reference of 0.775V 600Ω) 0.775V V (reference) V(output) G 1.228V dbu= 20 log G dbu= 20 log ( 1.228V / 0.775V) dbu= 20 log (1.585) = + 4dBu 9

10 dbfs l It s an acronym meaning a decibel relative to full scale. l It s the measurement of audio power in a digital universe that has a maximum level of 0 dbfs. l Above this value, there will be what we call a clipping effect. It will distort. 10

11 dbspl l It s an acronym meaning decibel relative to Sound Pressure Level l It s the unity representing the acoustic pressure. l Reference is Pa (Pa = pascal) A pascal is a unit used to measure atmospheric pressure. l We can only measure a dbspl with a decibel meter. JL Audio provides a free application for your ipads. l We have no need to convert this unit of measurement in audio. It is used as is to calibrate speakers in a controlled studio environment or in live settings." 11

12 The dbm l It s the relative power (Watt) measured in relation to a reference of 1mW (0.001W) l 1mW 1mW (reference) G P(output) 100W dbm= 10 log G dbm= 10 log ( 100 / 0.001) dbm= 10 log ( ) = 50dBm 12

13 Standards and charts This is the signal to noise ratio of the equipment which brings us to establish benchmarks and audio standards. For example, on the VU, how is 0VU in dbfs, dbu??? 13

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15 Résumé l Récapitulez les compétences acquises lors de la formation l Définissez les modes d'application de la formation l Demandez des commentaires sur la formation dispensée 15

16 EBU R68 is used in most European countries, specifying +18 dbu at 0 dbfs In Europe, the EBU recommend that -18 dbfs equates to the Alignment Level European & UK calibration for Post & Film is 18 dbfs = 0 VU UK broadcasters, Alignment Level is taken as 0 dbu (PPM4 or -4VU) US installations use +24 dbu for 0 dbfs American Post: 20 dbfs = 0 VU = +4 dbu " 16

17 The American SMPTE standard defines -20 dbfs as the Alignment Level In Japan, France and some other countries, converters may be calibrated for +22 dbu at 0 dbfs. " BBC spec: 18 dbfs = PPM "4" = 0 dbu " " German ARD & studio PPM +6 dbu = 10 ( 9) dbfs. +16 (+15)dBu = 0 dbfs. 17

18 How to measure an RMS output. With a digital multimeter, simply place the switch on the AC position to measure the potential difference between pins 2 and 3 of an XLR or Tip Sleeve of a ¼ TRS jack. 18

19 l With the known formulas, we can now calculate the value of a measured signal in dbu. l To calibrate a VU meter, we need a sound generator and a sine wave of 997Hz or 1KHz 1.228V (rms or ac) l Without the EQ engaged and a normalised mixer, we need to send the signal in the line input with all the faders to 0dB. l We now need to trim the adjustment on the VU meter. 19

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22 En application 22

23 PPM l PPM comes from the acronym (Peak Program Meter) and is used to measure transients. l It is important to observe this value which is just as important as the Meter. l It is easy to reach between 5 and 15dB with dynamic percussive instruments. 23

24 External VU l VU meter of high quality are prefered, especially when it is time to deliver a mix for the CRTC (the Canadian Radio-television and Telecommunications, EBU (European Broadcast Union) or SMPTE (Society of Motion Picture and Television Engineers) l You can send a stereo mix from stereo output of a converter and plug into any external meter. 24

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