Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-lcc!ptsfa!ihnp4!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: sci.electronics Subject: Re: Wireless Audio Link Message-ID: <1659@kitty.UUCP> Date: Thu, 19-Mar-87 13:44:37 EST Article-I.D.: kitty.1659 Posted: Thu Mar 19 13:44:37 1987 Date-Received: Sat, 21-Mar-87 05:45:48 EST References: <174@m10ux.UUCP> Organization: Recognition Research Corp., Clarence, NY Lines: 124 Summary: Transmission considerations... 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 ... >> You might be wondering "How do you relay the >> 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. ... > Remember that an equalized phone line from the phone company > will be considerably longer than 2000 feet. If this radio station is experiencing poor transmission quality on only 2,000 feet of shielded, twisted-pair cable, then something is seriously wrong! First of all, assuming your cable is 22 AWG (even if it's 24 AWG, it won't be more worse for this distance), the attenuation at 15 KHz will be around 2.0 dB. That ain't very much. In fact, since most radio program lines are equalized to +/- 1.0 dB, you might not even need equalization at all in this application. Should you wish to equalize, a simple series-resonant equalizer should be adequate. These devices are available from both telecommunication and broadcast suppliers. However, with no equalizer you claim to have a present problem. As I see it, there are four likely causes: 1. You have a defective cable, possibly with leakage from conductor-to- conductor and conductor-to-ground. Such leakage could be rather high resistance and caused by water permeating a damaged section of cable. Since this may be high-impedance leakage, it may not have been recognized as such; i.e., any attenuation and noise may have been attributed to equipment and not the line. Many installations of conduit which run underground will fill with water. This is an accepted fact-of-life and generally not a problem, unless the sheath of the cable is damaged. How did the cable get damaged in the first place? Most likely by the electricians who pulled it! I have had firsthand experience with electricians who yanked the hell out of communication cable, causing internal damage, by thinking it was just like a piece of (more robust) 250 MCM wire! 2. The audio transmission line is incorrectly connected at the audio control board and/or transmitter ends. 2000 feet of cable between two buildings is too far to run without full transformer isolation at both ends. Perhaps one or both ends have no isolation transformer; if so, one should be provided at _each_ end. (600:600 ohm 15 KHz line transformers are common ). Also, be certain that the cable shield is connected at ONE end only; this situation is ripe for _real_ ground loop trouble (unlike the "imagined" ground loops 5 feet away in the same room as claimed by some twits in rec.audio :-) ). 3. You have correctly connected, but improperly adjusted equipment at one or both ends of the transmission line. As an example, perhaps you are sending from the audio control board at an abnormally low level like -20 dBm, and the transmitter audio gain is cranked up enough to also pickup hum. Use a properly terminated audio sine wave oscillator to send a test signal into the audio control board, and measure the level at both ends of the line using a properly terminated (bridging impedance to start) transmission level meter (i.e., AC voltmeter calibrated in dBm with proper isolation and termination impedances). This should give you a clue as to your problem area. NOTE: My personal opinion is to forget about VU's here, don't rely on VU meters for this type of measurement, and deal strictly in dBm using telecommunications transmission measuring equipment. 3. You have correctly connected, but defective equipment at one or both ends of the transmission line. > The tricks used by the phone company to preserve the signal quality > are to make sure the line is correctly balanced (with a transformer > at each end) and that it is terminated with the correct impedance. > Also, equalization is added to correct the frequency response. Equalization, as in the case of a series-resonant equalizer, will add overall line attenuation, but flatten the attenuation-vs-frequency curve to an acceptable deviation (usually +/- 1.0 dB for program lines). > To improve S/N, increase the signal level. You must make sure > that the amplifiers driving the lines are not being overloaded. > 10 volts into 600 ohms is a reasonable signal level. Also > make sure the receiving circuits are not overloading. > Attenuation may be necessary. 10 volts into 600 ohms is a pretty large signal, like +22 dBm. Signals this large can cause crosstalk on adjacent circuits. A more reasonable level would be +8 dBm. > ... For a driving amp, you could certainly > use any reasonably good amp (>25 watts) meant to drive speakers. No, no, no! We're talking about SIX MILLIWATTS (+8 dBm) here. Your audio control board should certainly be able to provide this type of signal to drive this transmission line - even allowing for insertion loss through a transformer. Fewer interposed amplifiers will result in less distortion and fewer other problems. > In any case, you will have to fiddle a lot with the various resistance > values, gain, and equalization to get the best results. There should be little need for any "subjective" fiddling here. All you need is a variable frequency sine wave oscillator and a levelmeter with proper terminations and designed for telecommunications measurement. Start at say, 50 Hz, and send tones in 1,000 Hz increments at the _same_ reference level through the audio control board and over the transmission line. At the transmitter, measure the level at each frequency, and make a frequency/attenuation chart over the full 15 kHz range. If you don't like the slope, then crank in a series-resonant equalizer. For each equalizer setting, plot a new graph; quit when you like the slope. Don't forget, as you add equalization, the overall attenuation increases, but curve gets flatter. When you get the correct slope (i.e., correct equalizer settings), then you can make a final overall level adjustment. > The reliability will be better that a STL, of course. Agreed! <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"