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Amateur Radio Association of Nebraska
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Author Topic: AUDIO DB LEVELS, MICROPHONES, ETC  (Read 3677 times)
K0YNE
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« on: April 29, 2009, 10:00:25 PM »

WHEN I WORKED FOR THE UNIVERSITY OF NEBRASKA AS A BROADCAST ENGINEER, THE JOB TOOK ME INTO AREAS THAT I BARELY TOUCHED UPON IN MY CABLE TV AND TELEVISION REPAIR BUSINESS.
MOST NOTABLY, WAS RECOGNIZING THE DIFFERENCE OF VOLTAGE OUTPUT FROM A MICROPHONE
COMPARED TO "LINE LEVEL". I GUESS I ALWAYS KNEW THAT A TREMENDOUS DIFFERENCE EXISTED,
BUT THE VOLTAGE COMPARISON IS MONUMENTAL.
SOMEWHERE I READ AND REMEMBERED THAT LINE LEVEL WAS A 600 OHM IMPEDANCE WITH ABOUT
ONE MILLIWATT OF POWER. HOPE MEMORY SERVES HERE....
THE LINE LEVEL SEEMS TO VARY SOME FROM EQUIPMENT I HAVE CHECKED OVER THE YEARS IN OUR RADIO STATION, WITH CD PLAYERS, TAPE RECORDERS AND OTHER AUDIO EQUIPMENT THAT PUT OUT
 A LINE LEVEL OF AUDIO. SOME LINE LEVELS ARE  A PLUS 5 OR 6 DB ON A VU METER.
THEN AGAIN, SOME MEASURE SLIGHTLY LOW BY A MINUS 3 DB.
IN DETERMINING AUDIO SIGNAL DOUBLING IN VOLTAGE, MEANS A 6 DB INCREASE, AND A HALVING
OF AUDIO VOLTAGE WOULD MEAN A MINUS OF 6 DB'S..

MICROPHONES HAVE OUTPUTS OF MINUS -40 DB AND AS MUCH AS -70 DB.  CONSIDERING THAT
WHEN YOU COMPARE ZERO DB ON THE DB VU METER, AT 1 MILLIWATT, AND EVERY -6 DB HALVES
THAT AMOUNT, THEN YOU CAN SEE BY WORKING THIS OUT ON PAPER THAT WHEN YOU GET DOWN TO A MINUS 70 DB, THE SIGNAL POWER AND VOLTAGE IS VERY MICROSCOPIC IF THAT CAN BE SAID.


WHEN HOOKING UP A COMPUTER FOR DIGITAL WORK, AND USING THE SOUND CARD PORTS, LINE LEVEL
ON THE COMPUTER WOULD BE MORE APPROPRIATE SINCE IT IS MORE ABLE TO HANDLE THE LINE LEVEL
COMING FROM THE FIXED AUDIO OUT OF THE ACCESSORY PORT OF THE TRANSCEIVER. EVEN THOUGH
THE LEVELS MAY BE MATCHING, AN ISOLATION RESISTOR IN LINE FROM THE RECEIVER AND ONTO THAT
RESISTOR, ANOTHER RESISTOR SHUNTING TO GROUND TO MINIMIZE SOMEWHAT THE AUDIO ENTERING
THE COMPUTERS" LINE IN PORT". THE MORE SENSITIVE MICROPHONE PORT MIGHT NOT LIKE THE AUDIO
OVERLOAD OF THE LINE LEVEL. WHEN THE TIME COMES, THERE ARE WEBSITES ILLUSTRATING THE WIRING OF THE TRANSCEIVER TO INTERFACE WITH THE COMPUTER AND WITH PUSH TO TALK.
A FINAL NOTE ABOUT SOUND CARDS. I USE THE SOUNDBLASTER X-FI EXTREME AUDIO IN BOTH OF MY COMPUTERS. THE REASON IS, THAT THE QUALITY OF THE SOUND CARD PLAYS AN IMPORTANT PART IN THE DECODING OF THE SIGNAL RECEIVED. THE CHEAPER MORE NOISY SOUND CARD WILL NOT DECODE OR ENCODE THE SIGNALS YOU SEND AS CLEARLY AS THEY COULD BE  SENT OR RECEIVED.
IF LOOKING FOR A SOUND CARD, THE SIGNAL TO NOISE RATIO IS THE QUALITY OF THAT CARD AND
A S/N OF AT LEAST 106 DB WILL AIM YOU IN THE RIGHT DIRECTION. I CONDUCTED A SOUND CARD
TEST WITH MY TWO COMPUTERS AND DECODED WITH MY 3 CARDS HAVING 100 DB, 106 DB AND THE X-FI WITH 109 DB.... I RECEIVED PSK 31, RTTY, AND THE DIGITAL SSTV FROM EASY PAL FOR THESE
COMPARISONS. IT IS SAFE TO SAY THAT THE X-FI WITH S/N RATIO OF 109 DB WON EVERY TIME IN
DECODING MORE LETTERS CORRECTLY OR MORE PICTURE SEGMENTS THAT WERE SENT.

PAUL K0YNE

 
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