[Al Skierkiewicz]

Interesting story here. Belden had made and marketed 8451 shielded twisted pair as a 600 ohm audio cable and everyone used it in terminated 600 ohm audio applications for years. Many guys in remote TV production noticed that it behaved oddly in long runs, over 1000 ft. So ABC did some testing and determined that the impedance of the cable was actually more like 120 ohms. The cable had originally been intended to replace standard telephone wiring which is 19 gauge unshielded twisted pair and always terminated in 600 ohms. Audio wiring and equipment had been designed for using a terminated 600 ohm line until this became known. So in an attempt to modify designs, many manufacturers started using a low impedance source or line driver. Benchmark Audio did some extensive testing and decided on a 30 ohm source impedance using this cable. That is the typical equipment output these days for analog audio.
Please remember that while the wiring is intended to act as a transmission line, termination is important for noise immunity and signal level in our application. Even at 1 Mb/s data rates, a few feet does not begin to exhibit true transmission line characteristics. A half wave length is approaching 500 feet at that frequency.
When I mentioned the problems that teams have had in the past with terminations, most of the problems were related to poor soldering, using resistors other than 120 ohms, insulation and other quality control issues.