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      The  Black  Box   Chris  Nottoli   With:  Melissa  King,  Joshua  Roberts,  Nathan  Sibon,  Zach  Bruin,  Eric  Moskus   Columbia  College  Chicago                 Acoustical  Testing  I   Attn:  Dr.  Dominique  Chéenne,  Dr.  Lauren  Ronsse   November  27th  2013                


Table of  Contents   Abstract  ....................................................................................................................................................  3   Equipment  ...............................................................................................................................................  3   Initial  Testing  of  the  Black  Box  ........................................................................................................  4   Input  “C”  to  output  “B”  .........................................................................................................................  4   Input  “D”  ...................................................................................................................................................  7   Output  “A”-­‐Testing  for  the  Antenna  ................................................................................................  8   Conclusion  ...............................................................................................................................................  8   Appendix  ...............................................................................................................................................  10      

                         

2


Abstract  

The objective  of  the  Black  Box  was  to  accurately  describe  unknown  audio/electrical  

components within  a  locked  box.  It  was  discovered  that  Input  “C”  to  output  “B”  contained  a   graphic  equalizer,  a  downward  expander  with  a  4:1  ratio,  and  a  limiter  with  the  threshold   set  at  -­‐29dBV.  Input  “D”  was  shorted  and  output  “A”  acted  as  an  antenna.   Introduction   The  Black  Box  was  tested  at  Columbia  College  Chicago  in  room  LL01  as  a   requirement  for  Acoustical  Testing  I.  Josh  Roberts,  Melissa  King,  Nathan  Sibon,  Zach  Bruin,   Eric  Moskus,  and  the  author  of  this  report  collaborated  on  the  study.    The  objective  of  the   study  was  to  accurately  describe  the  combination  of  audio  devices  inside  a  locked  box   using  any  necessary  equipment  and  software.   Equipment  

Black Box  

SpectraPLUS and  TEF  software  

TEF 6.0  Interface  

Leader LMV-­‐186A  AC  Millivolt  meter  

TerraSonde Audio  Toolbox  

Millivolt Meter  

Signal Generator  

Frequency Counter  

QSC-­‐K8 Loudspeaker  

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Initial Testing  of  the  Black  Box    

The Black  Box  contained  two  inputs  labeled  “C”  and  “D”  along  with  two  outputs  

labeled “A”  and  “B”  (shown  in  figure  1).  The  frequency  responses  of  all  inputs  to  outputs   were  tested  to  understand  the  behavior  of  each  path.  The  most  conclusive  paths  were   noted  and  investigated,  which  included  input  “D”,  input  “C”  to  output  “B”,  and  output  “A”.   The  frequency  responses  found  in  these  initial  tests  can  be  viewed  in  the  Appendix,  Fig.  A1-­‐ 3.    

Fig. 1:  The  picture  shows  the  casing  of  the  Black  Box,  which  contains  a  number  of  unknown  devices.   The  power  cable  is  plugged  in  the  far  left.  Located  in  the  middle  are  the  outputs  labeled  “A”  and  “B”.   The  inputs  are  to  the  right  labeled  “C”  and  “D”.    

Input “C”  to  output  “B”    

When sending  signal  into  “C”  and  out  of  “B”,  it  was  clear  that  the  signal  had  been  

altered both  in  amplitude  and  frequency.  Using  TEF,  frequency  responses  were  captured  at   different  gain  stages.  The  first  test  captured  the  least  amount  of  gain  provided  by  TEF  and  

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was increased  for  each  consecutive  test.  The  overall  goal  was  to  determine  any  amplitude-­‐ dependent  devices  (See  Fig.  2).     It  was  determined  that  the  equalizer  occurred  first  in  the  signal  chain  since  the   expander  and  limiter  that  followed  had  affected  the  contour  of  the  equalizers  parameters.   The  center  frequencies  from  Fig.  2,  where  all  peaks  and  dips  occurred  throughout  the   spectrum,  were  noted  and  can  be  found  in  the  Appendix,  Table  A-­‐1.  The  contour  of  the   equalization  remained  consistent  until  the  signal  reached  the  threshold  of  the  limiter   where  it  then  became  flattened.  The  relative  gain  of  each  center  frequency  could  not  be   found  due  to  the  complex  gain  structure  of  the  components  on  this  channel.  When  the   output  of  the  signal  generator  to  the  output  of  “B”  at  the  center  frequencies  were  measured   (Appendix  Fig.  A-­‐2),  the  output  of  “B”  was  consistently  lower  in  amplitude  than  that  of  the   input,  despite  the  boosts  and  cuts  are  seen  throughout  the  spectrum.  Upon  completion  of   the  study  after  the  Black  Box  was  opened,  the  relative  gain  was  recorded  and  can  be  found   in  the  Appendix,  Table  A-­‐2.  

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Fig. 2:  Frequency  response  of  input  “C”  to  output  “B”  is  depicted  with  varying  amplitude.  To  obtain  this   response  the  output  gain  on  the  TEF  interface  was  increased  by  even  intervals  until  maximum  output  was   reached.  

 

It was  then  hypothesized  that  the  downward  expander  followed  the  equalizer  in  the  

signal chain.  Fig.  A-­‐5  in  the  Appendix  shows  the  signal  chain  used  to  determine  the   parameters.  For  each  frequency  tested,  the  attenuation  knob  of  the  signal  generator  began   at  full  attenuation.  The  dBV  value  for  the  tone  that  bypassed  the  Black  Box  was  recorded.   The  attenuation  knob  was  then  manipulated  until  the  signal  from  the  black  box  began  to   fluctuate,  and  the  result  was  recorded.  This  process  was  repeated  for  every  one-­‐half  dBV   increment.  Fig.  3  shows  the  plotted  data  (found  in  Appendix  Table  3)  obtained  from  this   test.  It  was  concluded  that  the  device  was  in  fact  a  downward  expander  by  the  slope  of  the   graph,  which  showed  a  4:1  ratio.  

Fig. 3:  Transfer  functions  of  input  “C”  to  output  “B”.        

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The limiter  was  the  last  device  in  the  signal  chain.  From  the  results  obtained  in  the  

investigation of  the  downward  expander,  Fig.  3  also  shows  that  the  limiter  had  an  infinite   ratio.  As  the  signal  reached  the  limiter,  it  allowed  nothing  to  pass  through  it.  This  explains   the  flat  line  in  Fig.  2  from  400Hz  –  20  kHz.  It  was  theorized  that  a  low-­‐cut  shelving   equalizer  was  applied  after  the  limiter,  as  there  is  an  increase  in  amplitude  occurring  in  the   low  frequencies  exceeding  the  threshold  of  the  limiter.  An  impedance  mismatch  is  most   likely  the  cause  of  the  increased  amplitude  below  400Hz.    Lastly,  the  threshold  was  approximated  at  -­‐29dBV.  This  was  determined  by  using   the  millivolt  meter  to  compare  white  and  pink  noise  through  the  box  with  the  signal  that   bypassed  the  box.  By  determining  the  point  at  which  the  test  signal  through  the  box  would   not  longer  increase,  was  recorded  as  the  threshold.  The  measurement  was  also  confirmed   by  analyzing  the  limiter  region  on  Fig.  3  showing  that  frequencies  above  500Hz  reached  the   limiters  threshold  near  -­‐29dBV.     Input  “D”   To  determine  the  function  of  input  “D”,  a  voltage  test  was  used  to  compare  the  input   voltage  with  respect  to  output  voltage.  A  sine  tone  from  Audio  Toolbox  was  split  into  two   paths.  The  first  path  was  patched  directly  into  input  1  on  the  millivolt  meter  using  an  XLR   to  a  banana  cable,  and  the  second  was  patched  through  input  “D”  to  output  “B”  and  into   input  2  on  the  millivolt  meter.  Across  the  frequency  spectrum,  it  was  found  that  input  “D”   was  not  allowing  voltage  to  be  sent  through  “B”.     Additionally,  to  confirm  these  results  that  Input  “D”  had  been  shorted,  an  ohmmeter   was  used  to  measure  the  DC  resistance.  The  Black  Box  was  first  disconnected  from  the   power  supply.    Input  “D”  was  then  connected  to  a  “male  to  male”  XLR  cable  to  expose  the    

7


three conductors  so  that  measurements  could  be  taken.  By  testing  every  combination  of  the   pins,  the  ohmmeter  showed  infinite  resistance,  concluding  that  input  “D”  had  been  shorted.   Output  “A”-­‐Testing  for  the  Antenna    

Initially thought  to  be  a  ground  loop,  output  “A”  appeared  to  have  an  absence  of  any  

signal, only  providing  electronic  noise.  From  the  Audio  Toolbox,  a  sine  tone  was  sent  to   input  “C”  on  the  Black  Box.  From  there,  output  “A”  was  sent  to  a  QSC-­‐K8  speaker.  As  the   gains  of  the  speaker  and  Audio  Toolbox  were  increased  to  their  fullest,  the  sine  tone  could   be  faintly  heard  behind  the  hum  of  the  60Hz  tone  produced  within  the  box.  It  was  then   hypothesized  that  output  “A”  was  acting  as  a  receiving  antenna,  allowing  for  crosstalk   between  output  “B”  and  itself.  To  prove  this  hypothesis,  a  radio  frequency  transmitter   broadcasting  at  87.3MHz  was  placed  next  to  the  Black  Box.  By  sending  a  sine  tone  into  the   radio  frequency  transmitter  and  watching  the  response  of  the  output  “A”  in  SpectraPLUS,   the  sine  tone  was  represented  flawlessly;  this  lead  to  the  conclusion  that  output  “A”  was  in   fact  acting  as  an  antenna  (see  Appendix  Fig.  A7-­‐8).  This  antenna  was  created  by  connecting   an  XLR  to  the  output  of  a  device  in  which  nothing  was  connected  to  the  input.  The   impedance  mismatch  between  the  input  and  output  caused  the  component  to  act  as  an   antenna.   Conclusion    

It was  concluded  that  input  “C”  to  output  “B”  contained  a  graphic  equalizer,  a  

downward expander  with  4:1  ratio,  and  a  limiter  with  an  approximate  threshold  at  -­‐29dBV.   Input  “D”  had  been  shorted  as  an  absence  of  voltage  was  observed  and  output  “A”  was   configured  to  act  as  an  antenna.      

8


Further testing  can  investigate  the  length  of  the  antenna  by  determining  the  

standing wave  of  the  carrier  signal.  Additionally,  by  finding  the  transmitting  frequency  of   the  antenna,  a  signal  can  be  broadcast  onto  a  receiver  to  be  heard.  To  obtain  more  accurate   results  on  the  parameters  of  the  graphic  equalizer,  calculations  could  be  made  to  determine   the  Q.    Lastly,  by  connecting  the  600Ω  resistor  to  the  millivolt  meter,  more  accurate  results   of  the  limiters  low  frequency  data  can  be  determined,  as  the  impedance  mismatch  will  no   longer  occur.            

9


Appendix     Figures:

Fig. A-­‐1:  Frequency  response  of  Input  “D”  to  output  “B”.    

Fig. A-­‐2:  Frequency  response  of  input  “C”  to  output  “B”.  The  equalization  is  evident  as  center   frequencies  are  very  prominent.    

10


Fig. A-­‐3:  Frequency  response  of  output  “D”.  Note  the  electronic  noise  and  its  harmonics.  

Fig. A-­‐4:  Signal  flow  of  the  Black  Box.      

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Fig. A-­‐5:  Due  to  calibration  errors  within  the  signal  generator,  a  frequency  counter  was  used  to  determine  the   exact  frequency  being  produced.  Input  1  of  the  millivolt  meter  received  the  sine  tone  straight  from  the  signal   generator.  Input  2  received  the  sine  tone  after  going  through  the  Black  Box.        

Fig. A-­‐6:  The  signal  generator  outputs  a  clean  signal  into  Input  1  on  the  millivolt  meter.  A   banana  to  XLR  piggybacks  the  output  of  the  signal  generator  and  is  sent  to  input  “C”  on  the   Black  Box.  From  there,  the  signal  from  output  “B”  of  the  Black  Box  in  sent  to  input  2  of  the   millivolt  meter  

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Fig. A-­‐7:  Tesing  for  the  antenna.  Sine  tones  from  the  signal           generator  were  played  through  the  RF  transmitter.  Placing   the  transmitter  next  to  the  box  and  connecting  output  “A”  to   SpectraPLUS,  Fig.  A-­‐8  below  shows  an  image  of  the  output   response  as  frequencies  are  sweeped.      

Fig. A-­‐8:  Image  of  sweeping  between  frequencies  on  SpectraPLUS.  This  image  proves  that  output  “A”  is  acting   as  a  receiving  antenna.    

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Fig. A-­‐9:  Inside  view  of  the  Black  Box.    

Tables:    

Center Frequencies   Frequency  (Hz)   35   75   110   164   208   618   825   1560   2056   4000   4900   8000  

Boosted

Center Frequencies   Frequency   (Hz)  

Relative Gain  

Attenuated

20

+ 12  

Boosted Boosted  

80

-­‐

100

+ 12  

Attenuated

125

+ 3  

Boosted Attenuated  

160

+ 12  

200

-­‐

Boosted

250

+ 3  

Attenuated Boosted  

630

+ 12  

800

-­‐

Boosted

1.6K

+ 12  

Attenuated

Table A-­‐1:  General  observation   of  center  frequencies  obtained   from  Fig  A-­‐2.      

2k 4k   5k   6.3k   8k  

-­‐

12

12

12 12  

+ 12   +   6   -­‐   3   -­‐  

12

Table A-­‐2:  Relative  gain  of  the           equalizer  seen  in  Fig.  A-­‐9.    

14


Transfer Functions from C->B (Note: All Values are Represented in dBV)

Â

Linear Response

63Hz

110Hz

300Hz

500Hz

800Hz

1kHz

2kHz

4kHz

-59.5

Omitted

-59.25

Omitted

-59.5

-59.5

Omitted

Omitted

Omitted

-59

Omitted

-59.25

Omitted

-59.5

-59.5

Omitted

Omitted

Omitted

-58.5

Omitted

-59.25

Omitted

-59.5

-59.5

Omitted

Omitted

Omitted

-58

Omitted

-59.25

Omitted

-59.5

-59.5

Omitted

-58

Omitted

-57.5

Omitted

-59.25

Omitted

-59.5

-59.5

Omitted

-58

Omitted

-57

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-56.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-56

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-55.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-55

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-54.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-54

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-53.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-53

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-52.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-52

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-51.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-51

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-50.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-50

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-49.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-49

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-48.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-48

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-47.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-47

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-46.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-46

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-45.5

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-45

-58

-59.25

Omitted

-59.5

-59.5

-58

-58

-58

-44.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-44

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-43.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-43

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-42.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-42

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-41.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-41

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-40.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-40

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-39.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-39

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-38.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-38

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-37.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-37

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

15 Â


-36.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-58

-36

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-57.5

-35.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-57

-35

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-56

-34.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-54.75

-34

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-53.25

-33.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-51.25

-33

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-49

-32.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-46.75

-32

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-44.5

-31.5

-58

-59.25

-58.75

-59.5

-59.5

-58

-58

-42.75

-31

-58

-59.25

-58.75

-59.25

-59.5

-58

-58

-40.75

-30.5

-58

-59.25

-58.75

-59.25

-59.5

-58

-58

-38.5

-30

-58

-59.25

-58.75

-58.5

-59.5

-58

-58

-37.25

-29.5

-58

-59.25

-58.75

-57.5

-59.5

-58

-58

-35.25

-29

-58

-59

-58.75

-56

-59.5

-58

-58

-33.75

-28.5

-58

-59

-58.5

Skipped

-59.5

-57.5

-58

-32.5

-28

-58

-58.4

-58

-53.5

Skipped

-57.25

-58

-31.5

-27.5

-58

-57.75

-57.5

Skipped

-60

-57.8

-58

-30.75

-27

-58

-56.5

-57

-48.75

Skipped

-56

-58

-30.25

-26.5

-58

-54.8

-56.5

Skipped

-59.75

-54.75

-58

-29.75

-26

-58

-52.7

-54.5

-44

Skipped

-53

-57.5

-29.25

-25.5

-58

-50.5

-52.5

Skipped

-59.25

-50.75

-57.25

-29

-25

-58

-48.1

-51

-40

Skipped

-48.5

-57

-29

-24.5

-58

-46

-48.25

Skipped

Skipped

-46.5

-56.5

-29

-24

-58

-43.25

-46.75

-36.25

-57.75

-44.25

-55

-29

-23.5

-58

-40.9

-43.5

Skipped

Skipped

-42

-53.5

-29

-23

-58

-38.9

-41.5

-33.5

-53.75

-40

-51.75

-29

-22.5

-58

-36.6

-40

Skipped

-51.5

-38

-49.75

-29

-22

-58

-34.6

-38.5

-31

Skipped

-36.5

-47.25

-29

-21.5

-57.5

-32.7

-36

Skipped

-47.25

-34.75

-45

-29

-21

-57.5

-30.85

-33

-29.5

-45

-33.25

-42.75

-29

-20.5

-57

-29.5

-32.25

Skipped

Skipped

-32.25

-41

-29

-20

-55.75

-28

-30.25

-28.5

-41

-31.25

-39

-29

-19.5

-54.25

-26.55

-30

Skipped

Skipped

-30.5

-37.5

-29

-19

-52.5

-25.5

-29

-28

-36.25

-30

-35.5

-29

-18.5

-50

-24.75

-28.5

-28

Skipped

-29.5

-34

-29

-18

-47.75

-24.25

-28

-28

-33.25

-29

-32.5

-29

-17.5

-45.25

-23.7

-27.5

-28

Skipped

-28.75

-31.5

-29

-17

-42.75

-23.25

-26

-28

-31.5

-28.75

-30.75

-29

-16.5

-40

-22.7

-26

-28

Skipped

-28.75

-30.5

-29

-16

-38.25

-22.6

-26

-28

-29.75

-28.75

-30

-29

-15.5

-36

-22.6

-26

-28

Skipped

-28.75

-29.5

-29

-15

-33.5

-22.75

-26

-28

-29

-28.75

-29

-29

-14.5

-31.75

-22.8

-26

-28

Skipped

-28.75

-29

-29

-14

-29.75

-22.8

-26

-28

-28.5

-28.75

-29

-29

-13.5

-27.75

-22.8

-26

-28

Skipped

-28.75

-29

-29

-13

-26.25

-22.8

-26

-28

-28.5

-28.75

-29

-29

-12.5

-25

-22.8

-26

-28

-28.5

-28.75

-29

-29

-12

-23.75

-22.8

-26

-28

-28.5

-28.75

-29

-29

-11.5

-23

-22.8

-26

-28

-28.5

-28.75

-29

-29

16


-11

-22.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

-10.5

-21.75

-22.8

-26

-28

-28.5

-28.75

-29

-29

-10

-21.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

-9.5

-21

-22.8

-26

-28

-28.5

-28.75

-29

-29

-9

-20.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

-8.5

-20.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

-8

-20.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

-7.5

-20.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

-7

-20.5

-22.8

-26

-28

-28.5

-28.75

-29

-29

Table 3:  This  table  represents  the  data  obtained  while  testing  for  the  expander.  The  cells  that  read,   “omitted”  were  not  recorded,  as  no  signal  was  present.  For  those  that  read,  “Skipped”  were  not   recorded  as  the  sensitivity  of  the  millivolt  meter  made  it  difficult  measure.              

17

The Black Box  

By: Chris Nottoli

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