why FM is not used in shortwave?
This is a long question I have. Whenever I switch the HF radio to FM, AM stations carriers are received more clear. Of course I am only hearing very low demodulated audio (slope detection probably). But the carriers are strong and with no all the noise when listening on AM or SSB stations.
So why FM is not used in shortwave?
FM can be used anywhere, historical reasons - as AM has always been used on short wave - the cost of changing over would be very high ...
Narrow band FM has much the same spectrum as modulated AM - this is why you can hear either on either ...
When you say "more clear" you are probably hearing less background 'mush' because of the limiting applied in FM amplifying stages and depending on the kind of demodulator, possibly other factors reducing random noise.
FM is used extensively on short waves, it is quite commonly used around 27MHz for CB radio and for aircraft to ground on lower frequencies. In fact digital modulation is now becoming commonplace on short waves with many stations broadcasting in high quality stereo! If you didn't know they were there, you probably heard an increase in background noise and thought it was interference! Read up on DRM (Digital Radio Mondiale) https://www.drm.org
It is also fairly easy to build a DRM receiver and use a PC to decode the audio, you would be surprised at what you can do, stereo sound and web pages in a single SW broadcast.
Brian.
Yes I should have corrected the topic title, I meant except CB radio.
Is DMR FM modulated? I thought it was AM. Something like QASK or similar.
It is a variation on QAM called COFDM (Coded Othogonal Frequency Division Modulation) the same system as used for terrestrial digital TV in most countries. It isn't FM or AM but a way of using lots of closely spaced carriers to carry digital bits with a lot of error protection. It has the ability to correct multiple errors although at the expense of reduced data rates. When the data stream has been recovered, it is decoded in much the same way as MP3 but it also carries some frames of pure data which carry HTML code to produce web-like schedule information and program information. The carriers are generated using mathematical techniques rather than lots of oscillators!
Brian.
I see. It reminds me the digital voice ham radio modes. It uses also multiple carriers. It seems that digital signals go that way (using multiple digital data streams within the same "channel").
Digital Voice (DV) mode is slightly different because as I understand it, the speech is reconstructed rather than just encoded. The digital information carries pre-determined sound code (phonemes and instead of sound samples). At low data rates it sounds very artificial and robotic but can be recognized as an individuals voice at higher rates. It is an interesting mode and can still carry reasonably good 'live' speech in a 700Hz bandwidth. I have only done limited experiments with it because when I transmit I get some feedback into the audio circuits in my PC, maybe I will do more when I have the time.
Brian.
the trouble with FM is that it is a little hard to bandlimit its transmit spectrum. The harder you modulate, the bigger the spectrum spread is....the besel functions just keep going out from the carrier. Other types of modulation...are more "compact", and a little easier to filter.
not a great answer, but probably why it is not used in shortwave broadcast systems