At 02:47 PM 7/2/2004, Jim Brown wrote... >On Fri, 02 Jul 2004 12:47:44 -0400, Mike S wrote: > >>Analog phone circuits are indeed transmission lines. > >ONLY if they are long enough... > At 3 kHz, the limit of baseband audio on POTS, >1/20 wavelength is nearly two miles.
According to Verizon, over 20% of US subscriber loops are more than 18,000 feet long. The balance of your post continues to demonstrate a lack of familiarity with real world audio transmission lines. For instance, you're apparently unaware that loading coils are inserted on a long subscriber loops to offset the parallel capacitance. Nonetheless, throwing out non-sequiturs in defense of an incorrect statement is pointless. Your original statement was "forget all that ancient stuff about 600 ohms. Pro audio hasn't used a 600 ohm reference for at least four decades," which is demonstrably false. "Line level" is a telco term which has well established meaning. "Line level" had meaning before broadcasting or electronic sound recording even existed. You, in your response, gave an ambiguous answer of at least two significantly different values, which illustrates that the term has lost any real meaning in your environment through imprecise use and ambiguity. The original questioner deserved a correct answer. For all you know, he was trying to build a phone patch, for which your "impedance doesn't matter" response would cause him no end of grief, as would your ambiguous description of what constitutes "line level". >Modern telephone lines (from a central office to a home) are NOT 600 ohm lines >because they don't use 600 ohm cable. Your implication is incorrect. By your own statements, what matters isn't what's on paper, but what's measured. The measured "impedance is in the region of 500 to 1000 ohms" for normal telco twisted pair cable - http://www.qsl.net/vk5br/TransLines.htm Real world and by design, telco twisted pair is functionally 600 ohm. There's no doubt a Bellcore standard for all of this, too. And yes, this does matter beyond formal transmission line theory. You're used to unidirectional signal transfer (4 wire, in the telco world). A telephone uses two wires full duplex. The hybrids at each end of that line which make this possible must be impedance matched, or you get problems like echo, bad sidetone or improper levels. I'm done. My intent was to correct an obvious error, not get into a pissing match. _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
