Xref: utzoo sci.electronics:7112 rec.ham-radio:12021 Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!uunet!ginosko!husc6!encore!cloud9!jjmhome!cpoint!die From: die@cpoint.UUCP (David I. Emery) Newsgroups: sci.electronics,rec.ham-radio Subject: Re: Tunnel Radio (was reception) Summary: comb Message-ID: <2463@cpoint.UUCP> Date: 25 Jul 89 21:07:51 GMT References: <81@vax87.auc.dk> <3890@phri.UUCP> <1566@unccvax.UUCP> Reply-To: die@cpoint.UUCP (David I. Emery) Organization: Clearpoint Research Corp., Hopkinton Mass. Lines: 29 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