82 lines
5.9 KiB
Plaintext
82 lines
5.9 KiB
Plaintext
Phile 3 of 4 Audio Surveillance
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Audio is the most common surveillance method in use. Most listening devices depend on some form of electronics, and it is important to understand
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the usual steps to audio electronic surveillance. It is basically a 5 step process.
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1) Input- usually a microphone
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2) Preamplifier- used to boost the nominal signal of a mic to usable levels
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3) processing- eliminates excess noise and unwanted sounds from the output
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4) output- headphones, recorder, transmitter, etc.
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5) post-processing (sometimes)
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This phile deals with signal processing [steps 3 (processing) and 5 (post processing)]. Signal processing gets rid of as much unwanted noise as
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possible, while retaining and boosting human speech. Ideally, processing is done as the audio leaves the preamp, but that is not always possible due
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to size restrictions and personel availability, so we sometimes record the audio and process it later, but call it post-processing. Processing can be
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divided into 3 parts; speech passband, compression, and equalization.
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The first step to processing is the removal of sounds outside the speech band. This makes the rest of the processing go more smoothly because the
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sounds that are unwanted anyway aren't dealt with. The speech passband goes from 300-3000 Hz. By eliminating the sounds outside this range, we cut
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the unwanted noise considerably. Filters that eliminate the sounds above and below are very easy to build (an Op Amp and a few resistors and
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capacitors can be thrown together to make a passable filter), but, for surveillance, we sometimes make complex filters with high dB/octave slopes.
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Slope measures how quickly response drops below nominative level (3 dB below input level). Steepness is expressed in dB/octave, which occurs in
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multiples of 6. A 36 dB/octave filter eliminates all sound below about 150 hz, and sound above that is practically inaudible up to almost 300 Hz. A
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6 dB/octave filter would dampen the sounds, but they would be audible down to around 100 db, and still noticable down to around 50dB. The high end
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filters work the same way, only response is lower for higher signals instead of lower ones. A 24 dB slope at each end of the passband is a fair
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negotiation, and, to make design simpler, we could drop it to 18 dB/octave but raise the low end to 500 Hz and drop the high end to 2000 Hz and not
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miss much. A filter below 18 dB/octave is almost a waste of time because the filter would barely dampen the sounds that need to be removed.
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The next step is compression. It would be unessacary if the target would stand in one place and speak in a clear, medium voice. Unfortunately, if
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you ask someone to do this, they might get a teeny bit suspicious. We all have the tendancy to speak at various levels, from a whisper to a shout,
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and everyone tends to move around and change the direction that they're facing when they are speaking. In a surveillance recording, we want to hear
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whispers as if they had been spoken aloud, and we want to hear shouts at the level of a normal voice. That's where a compressor comes into play. It
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raises the level of a low sound, and lowers that of a loud one. With a compressor made from an IC compander, a -80 dB signal is boosted to -40, and a
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+20 signal is cut to +10. The chip I use is capable of double compression, which means that a -80 dB signal is boosted to -20 and a +20 signal is cut
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to +5. It is possible to use 2 compressors together to bring the range within 6.25 dB of each other, but that is really unnecessary and causes the
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component to be bulkier than it should be. A limiter can be used with or (shudder) instead of a compressor. A limiter suppresses signals above
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certain levels, so your recorder or ears won't be overloaded.
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The last step of signal processing is equalization. Equalization is the process of removing sounds within the speech passband that can be as
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annoying as those outside it. For example, if you are listening with a laser bug, your speech passband will remove 90% of the noise, and the
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compressor will make all the sounds audible without battering your eardrums, but the mark has a refrigarator next to the window you are useing as a
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reflector that is obscuring some of the sound. So you need to get rid of the narrow band that the refrigarator is on without obscuring the voices. A
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parametric equalizer can do the job. This is not the same as a "graphic" equalizer that you can find on a stereo system, although that can substitute
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if necessary. The graphic equalizer has set center frequencies and bandwidths, usually at octave points. If the sound you want to eliminate is
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between 2 frequencies, you have to adjust both and sacrafice some of the speech. A parametric lets you set the center frequency and bandwidth. A
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good parametric should operate from about 200 Hz to 4000 Hz. Anything below or above will be filtered by the passband filters. (from 200-300Hz and
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3000-4000Hz will be damped, but not eliminated) A parametric equalizer with 3 bands can run rings around a graphic equalizer in the same range with
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30 bands. You can also use a parametric to boost the high frequency sibilants to make speech more clear.
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That's about it for signal processing. Look for the last file (Output) in the series by,
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The Gaurdian
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-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==
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()()()()()()()()()()()()()(SANctuary)()()()()()()()()()()()()()()()()
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-==-==--==-==-==-==-==--===-=-=-=-=-=-====--=-=-=-==-=--==--====--==-
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Oi! Gaurdian is really kickin' some major tail!
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great stuff dude!
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Call HELLFIRE OR DIE!
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----===Red===----
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EAT THE RICH!
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=-=-=-=-=-=-=-=-==-=-=-=-=-=-=-=-=-=-==\-=\-=-=-=\-=\-=\-=\-=\-=\-=\-=\
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SANctuary
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=-\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\-=\
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