Audio Processor Composite Baseband Connection

[Brian Bowers]

Situation - I was investigating some noise on our FM Sunday night/Monday morning. Depending on what mod monitor we were looking at, one mod monitor, an Inovonics (which only goes as low as -64 dB), it was showing a noise floor of -64 dB. On an old set of Belars, it was -53 (pilot on, analog XLR audio inputs shorted, RBDS encoder fed into audio processor). In the weeks prior to this troubleshoot session, I had been prepping for this investigation, and had noticed a balanced composite input, and an unbalanced composite input on our Harris THE-1 exciter. After more investigating, I noticed Harris made a triaxial composite cable to connect between the processor's composite output, and the input to the exciter. I decided to order one. The Harris THE-1 manual suggested, as a first choice for the lowest noise, to use this triaxial cable, and connect it via the processors unbalanced output, to the balanced input of the THE-1 (we eventually discovered most of our noise was actually coming from our RBDS encoder, it has been temporarily unplugged). But here is the part I question. Prior to replacing the basic standard BNC composite cable we had connected between the two devices, from a spectral balance, I pretty much knew what our station sounded like. After replacing that cable with the Harris Triaxial BNC cable, and ending our troubleshooting session for the night, I drove home. After listening to the station while driving home, and the next day, to me, the station sounded dramatically different. Am I imagining this? Or could replacing this standard 10' BNC composite baseband cable (at this moment, I don't remember what # RG it is), with a 3' 75 ohm triaxial BNC cable dramatically change the sound of my station? I was also inspecting a freeze up problem with our Orban Optimod 8500. So I had written down all of my settings, just in case there was a problem with the unit, and it didn't boot back up correctly. Just tonight I confirmed all settings were the same as before I booted down the 8500. The station's sound, to me, sounded much flatter, than before I installed the triaxial BNC cable. Could this be true, or is my hearing to blame?

 

[fm-engineer]

In most cases there is no need for a balanced composite connection unless extreme RF interference is suspected. There really should be no audible change from one to another unless a undesired termination load is presented to the signal.

 

[littlejohn]

Well, if you put a different cable in line, did you not change the shunt capacitance - by an admitetedly small amount - shuntiong the composite?
Of course, the simple test is plug the old wire back in and listen. If it reverts, then go looking as to why that should be.