Brian Donegan said:
I am sorry of this question has been asked before. What is the affect of AM stations with HD radio. Does this mean that AM stations will not have to power down anymore to protect the Clear Channel AM's (read traditional 50,000 watt stations) when the whole industry makes the jump to Digitial?
Right now, AM HD operation is not permitted at night due to interference concerns. Rumor has it the FCC is
very close to lifting the nighttime ban. Right now, there are no plans to change relative day and night power/pattern assignments; i.e., AM stations that must reduce power, go directional, or sign off at sunset right now will need to continue to do so after HD is implemented. (it should be noted that right now, there are no plans to make HD obligatory; presumably some - possibly many - stations will never go HD) Rumor also has it that the FCC will drop protection of nighttime "skywave" signals when they authorize nighttime AM HD. (to most tech types it seems impossible to implement nighttime AM HD without doing so) This
might make it possible to relax nighttime power reductions, at least for stations within 20KHz of a clear channel. (clear channel the frequency, not Clear Channel the company)
editorial mode on: HD Radio is
very dangerous for the AM service. While touted as "in band on channel" it is actually "in band adjacent channel", transmitted in the two channels either side of the existing analog signal. Theory is that the HD carriers are weaker than the "modulation splash" allowed for analog stations, but "modulation splash" isn't there anywhere near 100% of the time. HD carriers are. DXers believe the widespread deployment of HD on AM at night will cause ruinous interference across the dial. In part because they don't want to lose their hobby -- but in part because they're pretty much the only even remotely technically competent people who both care about the AM service and don't have a financial interest in seeing HD succeed.(editorial mode off)
Also on the FM Dial will the transition over to digital signals open up frequencies that can't be used currently on FM because it is too close to another station? If so would it be possible for NOAA to start putting Weather Radio on a digital frequency and phase out the special radios once everyone goes digital?
No, at least in the interim hybrid system (while old analog radios must still be supported) HD Radio uses
more bandwidth than analog, and when (if!) analog is ever completely replaced with HD it will still use at least as much space as analog. No space is being freed for new stations.A FM HD station does support more than one program stream -- a single transmitter can transmit at least three different stations on the same frequency at the same time. In theory, "Jack FM" could transmit NOAA Weather Radio on a substream, or something like that. To be honest I wouldn't count on that happening - I would expect stations to want to carry commercial programming on their subchannels. One might think "NPR" but public stations seem to be leading the way in multicasting already.
