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Analog vs. digital modulation

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ZenithCKLW
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Joined: Sat Nov 13, 2004 11:21 am
Location: Livonia, MI

Analog vs. digital modulation

Post by ZenithCKLW » Tue Dec 14, 2021 11:36 am

Out of curiosity, I'm looking for someone to explain modulation and its terminology. "AM radio" as I understand it is a method of analog modulation, amplitude modulation, frequently used in the MW and SW bands. "FM radio" as I understand it is a method of analog modulation, frequency modulation, frequently used in the VHF bands.

A few questions:
  • Let's say someone breaks some FCC rules and can do whatever they want. Can an FM broadcaster change its transmitter frequency from the VHF band (example, 100.1 MHz) and put itself down in the MW band (example, 540 kHz) while still remaining FM, and still be receivable by analog AM radios, and listenable? And vice versa with an AM station increasing its frequency into the FM band with still remaining AM?
  • I am hearing things about "all digital AM." Is this description a misnomer? The way I understand it, "amplitude modulation" is an analog process, whereas digital doesn't use amplitude modulation to decode a signal, although digital signals still travel on the same carrier signal. Is it more accurate to refer to all digital AM as "digital radio on the radio band we typically allocated for AM broadcasts"? (I'm not referring to anything hybrid-digital with my questions.)



km1125
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Joined: Thu Mar 14, 2019 3:09 pm

Re: Analog vs. digital modulation

Post by km1125 » Tue Dec 14, 2021 1:10 pm

Technically speaking, you can do AM or FM modulation at any frequency. Legally, maybe not. Also, AM or FM can be digital or can be analog.

If you're going to do digital transmission, you have to take the original signal and encode it to a digital form (all 1's and 0's). Then you can modulate a carrier either using AM or FM to transmit those 1's and 0's to the other end. There are numerous ways to do that. You could do them individually, which means the carrier would send a 1 and a 0 in two different forms, and send them in whatever sequence represented the original signal. Then, when you modulate the signal, if it's AM then the amplitude of the carrier would be at one level for a "0" and another level for a "1". For FM, the carrier frequency would be at one frequency for a "0" an another for a "1". For example, if the carrier was exactly at 100Mhz, then you might change it to 100.1 for a "0" and 99,9 for a "1". That is the simplest way to do it but realistic ways are a bit more complicated, typiecally sending many bits (1's or 0's) at a time. There is also a thing called phase-shift modulation that many digital signals use.

Yes, it would be accurate to say "digital AM as "digital radio on the radio band we typically allocate for AM broadcasts""



k8jd
Posts: 603
Joined: Fri Jun 28, 2013 3:35 pm
Location: Commerce, MI

Re: Analog vs. digital modulation

Post by k8jd » Wed Dec 15, 2021 10:48 pm

You want a course in Broadcast and RF transmission principles . Try you Tube videos /
It would take a lot of typing to explain whe a FM raido can not recover audio from an AM Signal. Why FM transmission sounds like raw raspy noise when tuned to with an AM radio. Digital broadcasting it a totally different thing than AM or FM and takes up more bandwidh ! I have seen it on my band scope !



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