die@cpoint.UUCP (David I. Emery) (07/26/89)
In article <1566@unccvax.UUCP> dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) writes: > > [The Lincoln Tunnel authorities can break into...] >> normal re-broadcast for emergency messages. They do this, I believe, on >> every AM channel. > > Does anyone know exactly **how** they accomplish this? The >"every AM channel" system is used in Boston (in the sort-of-tunnel >under Copley, and in the Callahan and Sumner tunnels).... > > I suspect that it is spread spectrum (of sorts) which hits >(swept) from 530 to 1600 kHz every 100 uS or so. My understanding is that done with a comb generator driven by 10 khz which produces harmonics every 10 khz. The relevent ones are selected with a bandpass filter that passes the AM band and then modulated with a balanced modulator carrying the AM audio. This effectively AM modulates every line in the comb. The signal is then amplified in a linear amplifier for distribution. -- David I. Emery Clearpoint Research Corp. 35 Parkwood Dr, Hopkinton Ma. 01748 1-508-435-7462 {decvax, cybvax0, mirror}!frog!cpoint!die {m2c}!jjmhome!cpoint!die
ISW@cup.portal.com (Isaac S Wingfield) (07/27/89)
> Does anyone know exactly **how** they accomplish this? The >"every AM channel" system is used in Boston (in the sort-of-tunnel >under Copley, and in the Callahan and Sumner tunnels).... Hmmm... I don't suppose it could be as simple as transmitting on the receiver's intermediate frequency, could it? Back when car radios used "FET's with wicks", just about all of 'em had the same IF (I think it was 262.5 kHz). I don't know if contemporary units are so standardized. In any case, if there is only one frequency in use, sending a signal on it should get through to everyone. Isaac isw@cup.portal.com