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Digital Signals Spark Static From AM Radio
Carl Bialik writes "Digital radio is touted as broadcast radio's golden ticket, but the transition to digital broadcasts is creating static and interference for many smaller AM stations that are still analog-only, the Wall Street Journal reports: 'The AM stations most affected are those whose neighboring stations -- nearby on the dial -- add a digital signal.' The WSJ adds, 'For some small AM operators, it adds insult to injury that the only company licensing the digital broadcast technology is one backed by the small stations' deep-pocketed competitors.' Critics question why the FCC only approved the technology from that big radio-backed company, Ibiquity."
So, that must mean you've never acutally watched HDTV signals on an HDTV set. If you had, there's no way you would think it is not perceptibly different.
I currently have a tv with built-in HDTV tuner and a plain-old VHF antenna. I also have cable from comcast. Whenever I can, I watch over-the-air HDTV instead of the cable (which I'm paying for) just because the quality is so much better.
(Why do I pay for cable then? 2 reasons, bundled with internet service, it's pretty cheap, and I get more stations than currently available over-the-air)
I believe the digital signal to which you are referring is the Radio Data System. It's nice when you don't know the name of a song or the artist and some stations allow you to keep your radio's clock sync'd.
Mr. Universe: "They can't stop the signal, Mal. They can never stop the signal."
I recently purchased a car stereo with digital support and can say that the AM digital stuff sounds absolutely terrible! You really hear the digital artifacts... tinny, cellphone-ish sounds. Also, the analog signals aren't in sync with the digital ones so that when the stereo goes from one to the other, you either miss something, or hear the same thing over again... typically around 4 seconds worth. Get into an area where the AM signal isn't too great, and the stereo repeately goes back and forth between analog to digital. It's enough to drive you insane.
Hopefully digital AM will get dropped as people realize how terrible it sounds.
FM, on the other hand, sounds great and doesn't have the sync problem.
If I can only figure out how to turn off the digital AM support in my stereo!!!
In case this is just a fluke, here are the details:
AM 740 in San Francisco.
Headunit: Kenwood KDC-890
Digital Box: Kenwood KTC-HR100
I am a broadcast engineer.
This "HD Radio" is actually called IBOC (in-band, on-channel). Radio works by changing the amplitude (AM) or the frequency (FM). You're only permitted to change it by a certain amount--your bandwidth--otherwise it will interfere with adjacent stations. By adding more information, the digital data stream, you're using increased bandwidth.
The other big issue with AM is overnight. Most AMs power down or sign off at sunset because the atomosphere changes overnight and AM travels much farther--for example, I get ESPN 1000 out of Chicago 1,200 miles away and the 1000khz station here shuts off overnight. The sky is already being polluted with IBOC hiss (sound just like a modem) and it's DEFINITELY hurting the smaller AM stations who have special authorizations to stay on overnight, but at a significantly reduced power.
Surf the spectrum and you'll see. This has been a HUGE issue in the broadcasting community and it's about time it gets mainstream attention.
I love it, another untested closed standard.
l e
The AM MW mode should be left alone. The band that it is used in the US is not that reliable for digital modes, but it is very reliable for AM transmision to local areas.
This band/mode is essential for emergencies. Everyone in the US has an "AM" radio somewhere in the house or their car. It will just work.
Has the FCC rated this HD radio as an actual mode?
I have started to experiment with drm on my HF equipment and in fact i am upgrading to a SDR-1000 in a month or so.
With my existing equipment, I have heard a few of the DRM broadcasts on HF Shortwave and they are impressive. I have heard that you can broadcast it with a SDR-1000.
DRM is an open standard based on mpeg4. In fact this is the standard for european countries. There is GPL code for both listening and transmitting it.
http://en.wikipedia.org/wiki/Digital_Radio_Mondia
KB0FHF
The problem is that the digital signals are added to the analog AM signal as sidebands that exceed the allocated bandwidth of the 10 KHz channel spacing of AM radio. Even analog AM, being 20 KHz bandwidth overlaps the 10 KHz channels, that is why you can hear a 10 KHz "squeel" on some stations when there is another station 1 channel away.
Digital AM radio uses over 50 KHz of bandwidth, so it is possible for a digital AM station to wipe out weaker stations 2 or even 3 channels away on either side of thier dial position.
I design radio test equipment for a living - so welcome to Modulation Theory 101.
Amplitude modulation, or more correctly double-sideband non-suppressed carrier amplitude modulation (FCC emission type A3E), results in an RF spectrum that is twice as wide as the highest frequency component of the modulating signal. In other words, if the signal you are modulating has as its highest frequency component 3kHz (normal voice signals), then the resulting AM signal will occupy 6 kHz of RF spectrum - from 3kHz below the nominal carrier frequency to 3 kHz above the nominal carrier.
Now, we have to consider the concept of "receiver bandwidth". A properly designed radio receiver will only pick up signals within a given frequency difference of where it is tuned (the "tuned frequency" or TF) - this is the receiver bandwidth (sometimes referred to as "IF bandwidth" since in modern superheterodyne receivers it is the bandwidth of the narrowest intermediate frequency section that determines the overall receiver bandwidth).
Now, consider the case of 2 radio stations spaced such that their carriers are 10 kHz apart - the normal spacing for AM radio stations. Assume your radio is tuned to one of the stations. If your radio has a receiver bandwidth of 20 kHz (in other words receiving signals from 10 kHz below tuned frequency to 10 kHz above tuned frequency), you would hear the station you *weren't* tuned to as a 10 kHz whine on your radio (the carrier of the other station, 10 kHz off your tuned frequency), plus the audio of the other station inverted in frequency (low tones become high tones and vise versa).
So, your radio has to have a narrow filter to receive only those signals within 5 kHz of tuned frequency (total 10 kHz). Now, a perfect "brick wall" filter would allow, say, 4 kHz through, but stop 4.00001 kHz dead. Now, filters are not perfect, and so if your filter allows signals from, say, 4 kHz away from TF, then it will not totally block signals until they are, say, 6 kHz from TF. So, radios are designed to allow signals +/- 3kHz from TF in (receiver bandwidth of 6kHz), and block signals more than 5 kHz from TF.
OK, now, how do we add any new signals to the A3E signal so that we can put the digital signal in place? We cannot place those signals within 3kHz of the carrier without going to a LOT of trouble, otherwise analog radios will "hear" the digital signal as noise. So what IBOC does is exploit that "no man's land" from 3kHz away from carrier to 5kHz away from carrier to put the digital signal in. Now, your old analog radio will still "hear" these signals to an extent, but between the attenuation of the receiver bandwidth and the attenuation of the audio chain, this noise will not be very perceptible.
HOWEVER - remember how I said there were no perfect "brick wall" filters? Well, that applies to transmitters too. The transmitter may be putting signal into the 3 kHz to 5 kHz region, but it will put some unwanted signals beyond 5kHz (they will just be very weak compared to the desired signals) - and that means into the frequency band of the next guy on the dial. However, if the next guy is far enough away in space, your signals that are in his band (which are already weak) will be weakened further by distance, and won't be perceptible by the other guy's listeners. Also, your signals that are in the 3kHz to 5kHz area will be weakened by distance, and attenuated by the receiver's filters, and so they, too, won't be very objectionable.
Except in the fringes between where your spatial region stops and his starts. That's what is happening here - if you are close to "the other guy" you won't hear the interference, but if you are far from him, and close to the digital station - you get noise where there was none before.
Add to this the fact that the stations that are going digital are the stations with money - and how do you get money? By having lots of listeners. How do you get lots of listeners? Among other things, by having lots of POWER <Tim Allen Grunt>. The little station
www.eFax.com are spammers
I believe its called amateur radio!
c ate.html
(frequencies)
http://www.arrl.org/FandES/field/regulations/allo
(general)
http://www.arrl.org/hamradio.html
IBOC (In Band On Channel) is both an analog and digital service. no changes in frequency, no adapters required for older radios.
It is efficient and higher quality and still free to the user, which is why even the shortwave broadcast bands are going digital.
It is in Ibiquity's interest to encourage diversity in programming the multi-channel digital service and enhancements such as pro-logic surround sound.
The real question is - why is it only in the US that these odd technology hacks are being used - the rest of the world is using DAB for digital broacast radio. It seems like the US is the only place that WON'T be using DAB.
Oolite: Elite-like game. For Mac, Linux and Windows