Ynot Posted March 25, 2006 Share Posted March 25, 2006 OK, so this MAY not be the right section to discuss the quality of radio signal - maybe general tech may be better, but I'm sure the mods will put me right if not. So - the question is this: Has anyone else noticed anything odd about the signal from local radio? I've noticed over the past couple of months that behind the presenters' chattering I occasionally hear what sounds like a high frequency whistle - the sort we older techs (<_<) used to get when trying to tune one of those old-fashioned dial radios in on AM... It's not loud, and doesn't tend to come over the music, but it's definitely there. I've heard it on at least 2 stations, and on a couple of receivers - one portable, the other in the car. Part of me (the cynical part!) wonders whether it's something that's being injected on the QT over FM stations in order to make punters feel the need to go out and but a spanking new DAB digital set..... TD Link to comment Share on other sites More sharing options...
Guest lightnix Posted March 25, 2006 Share Posted March 25, 2006 Maybe not quite on topic, but if anything I found that our new DAB set was the most sensitive tuner I've ever had to set up when it came to aerial positioning. It didn't have to be that far out of kilter for the whole signal to break up into something that sounded like a Dalek on acid; apart from that, most of the programs only seem to be being broadcast at 128 kB/s - hardly the best quality digital sound :( :) PS - Topic moved from Sound forum, as suggested :) Link to comment Share on other sites More sharing options...
J Pearce Posted March 25, 2006 Share Posted March 25, 2006 The breakup is due to digital transmission. It will reconstruct the signal if bits are missing, but once you cross a certain line it can't reconstruct it so it cuts out. No fading like analogue, just a cut out. Also it runs at mpeg layer 2, not the layer3 we are all familiar with. DAB has the capability to be very good. However they are running the multiplexes too hard, and quality suffers. There was a good article in sound on sound about DAB. I might be back with an edit to link to it. [edit] Here you go Scroll down to the digital airways. Link to comment Share on other sites More sharing options...
GreyBadger Posted March 26, 2006 Share Posted March 26, 2006 FM stereo works by transmitting a M-S signal, the M being transmitted as baseband audio in the 50Hz -> 15kHz range. The S signal is tranmitted by amplitude modulating it onto a 38kHz pilot tone. This handily (do the fourier transform and find out why :) ) surpresses the carrier, leaving a double sideband signal higher in the allowed frequency range than the M signal. A 19kHz tone, phase locked with the 38kHz pilot tone (and note, also in phase with the RDS carrier at 57kHz) is transmitted at a lower level (10% IIRC) than the overall modulation amplitude to allow the 38kHz tone to be reconstructed by the receiver. There is a band gap between all these bits, but a dodgy receiver could allow this pilot tone to bleed. Perhaps that's what happening. Link to comment Share on other sites More sharing options...
Tomo Posted March 26, 2006 Share Posted March 26, 2006 Well, that clears that up! Link to comment Share on other sites More sharing options...
Ynot Posted March 27, 2006 Author Share Posted March 27, 2006 There is a band gap between all these bits, but a dodgy receiver could allow this pilot tone to bleed. Perhaps that's what happening.Hmmm...Not sure this is a likely answer - as I said, I picked upthe same inter-mod type interference on two different sets.... TD Link to comment Share on other sites More sharing options...
Solstace Posted March 27, 2006 Share Posted March 27, 2006 In my case, I've put poorer FM reception down to the amount of electronic and computer equipment we have around us. In my house alone there are two desktop PC's and three laptops, all using wireless networking. We have a wireless router to bring an internet connection to these. We also have two freeview boxes and a cable receiver. This count-up of electronic devices doesn't take into account the things like CD / DVD players and the like, all ringing out their own brand of radio emissions. So even without knowing the physics behind FM broadcasting, I don't find it surprising that quality FM reception is getting more and more difficult. Even with such a cynical outlook as mine, I find it hard to believe that local and national broadcasters are turning down their FM transmitters in an effort to divert everyone to the new technology. For example, one area where this attitude would not work is in cars. Yes, DAB in-car receivers are available, but at a cost and the vast majority of cars are still using FM receivers, with RDS at best. Link to comment Share on other sites More sharing options...
Bobbsy Posted March 27, 2006 Share Posted March 27, 2006 Hmmm...Not sure this is a likely answer - as I said, I picked upthe same inter-mod type interference on two different sets.... It's worth checking the frequency of the offending tone. If it's 19kHz, then it's almost certainly pilot tone; any other frequency and we need to look for other causes. This may be one of the advantages of getting old....a tone at 19K would be inaudible to me these days! Bob Link to comment Share on other sites More sharing options...
Ynot Posted March 27, 2006 Author Share Posted March 27, 2006 It's a varying tone - if I had accss to the kit I used to at work about 20 years ago, I'd see about measuring it, but .......:) Link to comment Share on other sites More sharing options...
Bobbsy Posted March 27, 2006 Share Posted March 27, 2006 If you have access to an form of computer DAW (I use Audition for example) they almost all allow you to do a frequency analysis. Or, if you can record a second or two an email me a .wav file (not MP3 since these roll off steeply before 16K) I could have a quick look. Bob Link to comment Share on other sites More sharing options...
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