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From: dsi@unccvax.UUCP
Newsgroups: sci.electronics
Subject: Re: Wireless Audio Link
Message-ID: <646@unccvax.UUCP>
Date: Fri, 20-Mar-87 11:28:08 EST
Article-I.D.: unccvax.646
Posted: Fri Mar 20 11:28:08 1987
Date-Received: Sun, 22-Mar-87 18:21:55 EST
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 102

In article <174@m10ux.UUCP>, braun@m10ux.UUCP (MHx7079 mh) writes:

> Our studio is located 600 feet (line
> of sight) from our transmitter, which is located atop a near by building. I
> would like to devise a way of relaying audio at 15-16khz between the studio
> and transmitter. I have looked at STL's ...
> audio now ?". Unfourtunatly our audio is run through 2000' of  9 pair
> shielded "mic" cable (through a conduit along with the power feed for the
> building (none of this was my idea it was like this when I got there)). As
> you may imagine our air signal is less then optimal (read not very
> transparent). Another option is the use of two 15khz balanced broadcast lines
> rented from the phone company. ...

    No. No. No. No. No.

    Do ** NOT ** rent two broadcast grade telephone lines from the phone 
company under any circumstances.  Their program equipment is unbelieveably
archaic (although BellSouth have finally gotten with the program, so to 
speak, and are now providing active equalisation on their program audio
circuits).

    I am Director of Engineering for a nationwide radio network, and
have more problems with program audio circuits from BellSouth by far than
any single other thing.  95 % of our network failures are caused by 
deterioration or loss of our 15 kHz and 8 kHz loops.  Even double
redundant circuits to our uplink still gives us unacceptable downtime
(about 24 hrs/year).

     We are installing microwave feeds as a result.

     Now, over 2000 feet of twisted pair, even as the original poster 
described, it should be entirely possible to be flat within 2 dBm from
30-15,000 kHz PROVIDED THAT THE LINES ARE PROPERLY BALANCED (the usual
Western Electric repeat coils are very good for this purpose, but not
as good as say, Jensen Transformers repeat coils) AND TERMINATED. We 
have 12,000 feet of copper to the first CO, and about 8000 feet from 
there to our uplink facilities, and can guarantee a flat frequency
response to our Board of Directors within 0.5 dBm from 30 to 15,000 Hz.
Total harmonic distortion is 1.0 percent, though, at an injection level
of 8 dBmV.  Principally, we feel that this is due to the equipment at
the central office.

     I prefer an STL, even over such short distances.  The problems of
pulling cables through a college campus are unbelieveable. Mechanical
mishandling is exceedingly common in this environment, as are dufuses
who habitually bridge everything on a punchblock to see what is going
on.

     Also, you are using remote control (presumably), and I find the
implementation of remote control over the SCA to be desirable as well.
However, over 600 feet, this may not be a consideration.

     If you are not getting "transparent" audio quality over 2000 feet,
even without equalisation, you are probably experiencing grossly 
mismatched levels in the program audio equipment (again, having a composite
STL, when engineered correctly, allows you to virtually remote the
connection to the transmitter modulator into your studios). Try using
the station's modulation monitor, an ordinary oscillator, and remove
** all ** the program audio equipment and make a frequency response
check.  Be sure that the 75 uS preemphasis curve is correct (Royal
and Aincent transmitters used LCR circuits for this with -- gakk --
waxed paper capacitors).  If you don't have a decent modulation monitor,
get one.

     Put the station on monaural for a few days and see if the problems
change or go away.  Linearizing circuitry in some direct-FM exciters
needs periodic calibration for optimum THD and IM distortion characteristics.
This should be done by an experienced consulting engineer, however.

     Are you sure that the VSWR at the final amplifier plate circuit
is acceptably low, and that you don't have gross antenna problems, 
excessively low loading in the driver stage (meaning high circuit
efficiency but also high "Q") or high incidental AM? Find the manual
on your modulation monitor and learn how to use it (the McMartin and
Belar monitors are good, and the QEI is exceptional).

     Are the intermediate stages in your transmitter tuned correctly
(yes, people try to yank 110 kHz-ish crystals around still and multiply
yp to the carrier frequency) with correct stage loading and tuning?

     Does the modulated 38 kHz subcarrier cross the time axis with
a rising slope at the same time as the 19 kHz pilot ? Just 5 degrees
of change will cause stereo distortion to rise sharply and separation
to decrease.  

     If it were ** my ** station, the first thing I would do is take
my trusty CD player (no flames from the analog recording tech weenies,
as I am an analog engineer by profession), and connect it to the monaural
input of the transmitter with no compression or limiting. Try this also
at the input of the stereo generator, and listen critically. This will
at least give you an indication of how rotten your transmitter is.

     If you have older equipment, replace all of the electrolytics,
particularly if they were made by Sprague. Parallel them with some 
decent polypropylene caps for better transient response.

I hope this helps...nothing could be more satisfying than taking an 
underdog station and making it the best in town.

David Anthony
DataSpan, Inc
.l