In message <[email protected]>, dated Mon, 24 Apr 2006, Robert A. Macy <[email protected]> writes
>I stand corrected. yes, 600 ohm must be balanced because it is less >than 377 for each line to infinite ground. > However, it is universally shoved through coax. Almost no-one uses 600 ohm audio these days, and those that do use it balanced. What is put through single-screened cable (not usually called coax) is consumer audio, which isn't 600 ohms; it comes from a 2 kohm or less source into a 10 kohm or greater load. Or at least, it should do. > > - Robert - > >PS: I still think "voltage" came first. <g> Even the BBC may have been a bit confused about that 30 years ago, but once you see that the source and load impedances [1] form a balanced bridge, so that the common-mode signal does not appear in the differential signal, you wonder why anyone ever thought that the signal voltages mattered. Another way of looking at it is to consider whether a circuit ceases to be balanced if the signal is turned off. [1] To see this, you need to put an equal impedance in each leg at the sending end and an equal impedance in each leg at the receiving end. The impedances need not, indeed should not, be the same at both ends; the sensitivity of the balance to impedance tolerances is less if the impedances are very different, e.g. 37.5 ohms in each leg at the sending end and 10 kohms in each leg at the receiving end. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
