What are the disadvantages of analog phase modulation?

(1) It generates hard homework questions for students.

(2) It isn't very easy to make just a phase modulator that is linear.

Mmm... compared to what? Digital phase modulation?

Yes. Compared to MPSK. MPSK [Multiple Phase Shift Keying] is digital phase modulation.

I'm guessing the analog phase modulation would be more vulnerable to random noise. Right?

Digital systems can ignore EMI/RFI more easily than analog systems.

Now, if PM is compared to QAM, then what would be the drawbacks of PM?

In what context?

Thanks, Rich

It is? There were certainly plenty of, e.g., QPSK and BPSK systems while analog still dominated communication systems. I expect there were even, e.g.,

16-PSK systems done in analog, although I can't personally give you an example of one.

Well... as a practical matter this may often be the case: If you look at "dollars spent" to "signal quality achievable," while DSP starts off higher the slope isn't generally nearly so bad as with analog. It's probably a fairly safe statement that for a given dollar amount, for any reasonably high-speed MPSK system you can potentially do better digitally than with analog. But there are a lot of big qualifiers in there -- plenty of otherwise solid digital communication systems end up having lousy performance because some digital guys had nowhere near the background in how to properly route a board and select an ADC as those old analog guys did. :-)

Lower data rate for a given occupied bandwidth. The drawback of QAM vs. PM though is that it requires a higher SNR to achieve the same bit rate, of course. In fact, for broadcast purposes or other scenarios where the you don't have a predictable propagation path (e.g., two high-gain antenna pointed at each other on moutain tops!) QPSK is still quite popular.

I realize now that what you're probably been discussing is communication systems that traditionally carried analog information and digital information, and not how those modulations were implemented (in analog vs. digital electronics). Sorry for the confusion there...

When it comes to analog vs. digital communications, here's the deal: If you're looking strictly for being able to communicate with another person -- meaning that it's OK if there's a lot of noise on the signal -- analog modes (FM, AM, etc.) almost always do better than traditional digital-based methods *given the same power and bandwidth to operate in*. On the other hand, if you're looking for "crystal clear" communications, this will persist in digital systems to much lower SNRs than it will in analog systems (at which point the "cliff edge" effect in digital systems means you'll hear little or nothing at all). Finally, if you're looking for the lowest possible signal strengths with which to communicate, in practice digital wins again, because it's quite straightforward to average (correlate) a bunch of signals when all you're after at the end is a "1" or "0" decision (this is what GPS does, where the sigal strength is well below the noise floor)... although you pay for this "codig gain" with bandwidth, of course. Conceptually you could build averagig analog systems as well, although to my knowledge this has never been done (it's likely that by the time you were done you'd have paid more than for the equivalent digital system). Note that "hybrid" systems such as analog FM being chipped with direct sequence BPSK spreading patterns (very much digital) is traditionally quite common as well.

Hence, as with most things, you really need to set up a problem scenario before it makes a lot of sense to discuss advantages and disadvantages. One common mistake people make is assuming they want crystal clear digital systems when, in actuality, for "critical" systems such as air traffic control and police/fire/ambulance, often "noisy but understandable" is much preferable to the "no signal at all" digital option.

Where I work we've been successful in convincing parts of the military and commercial entities that digital doesn't have as many advantages as you might initially think, particularly in shared spectrum usage/heavy multipath/difficult terrain situations (in other words... one of the reasons cell phones work as well as they do is that the service providers literally own the entire block of spectrum they're using and have very precise control over all the transmitters in it... CDMA capacity would be nothing compared to what it is if the towers couldn't precisely control your handset's transmit power, and didn't have the ability to support handing you off between multiple towers all within your phone's range!)

---Joel

Any PSK, FSK, ASK are digital. The use 1s and 0s. So they are digital.

--- The main disadvantage is that its very existence allows annoying goddam trolls like you to ask asinine questions which could be easily answered by perusing Google.

Piss off, s*****ad.

JF

Does that not depend on the level of digitisation?

On the other hand, if you're

One thing that seems to be overlooked is the signal-processing that occurs in the human ear-brain system. Has there been any research into the quantitative efects of this?

Yawn. Please people do a google on [Radium] before answering his posts.

--
Adrian C

Yawn! So why did YOU reply to his post?

Lower level of digitisation = more bits per sample = more bandwidth... which I constrained to be the same in both cases.

The most obvious traditional analog method of bandwidth expansion is something like wideband FM... commercial FM stations take up ~180kHz (channel spacing of

200kHz), after all -- an *incredible* amount of bandwidth for what you get in return. The more contemporary approach is what I mentioned in that last post -- FM that's BPSK chipped with a long spreading code.

Edwin Armstrong was motivated to invent FM in an attempt to improve the fidelity of (then) traditional AM broadcasts, which suffered from a lot of impulse noise as well as limited bandwidth. I have a suspicion that the first thought that popped into his head was, "Well, hey, let's just make this new form of modulation constant amplitude, that'll help!," and that it might have initially been coincidental that he used such a high modulation index which greatly improved the fidelity as well. (But being a very smart fellow, I'm sure this fact wouldn't have been lost on him for long... compare to, e.g., Lee de Forest, who tried to take a lot of false credit for early radio inventions when his technical skills were somewhat limited -- in stark contrast to his self-promotional skills, which were great...)

He also built the first "luggable" FM receiver (think about, mmm... 4 breadboxes...) to woo his girlfriend/future wife while out on the beach. Unfortunately for Armstrong, even though he was incredibly successful and well-recognized, he spent years (decades!) engaged in vicious legal battles and was quite the workaholic. Eventually his wife couldn't take it anymore -- wanting to retire to a simple life on a farm -- and left to move back in with her mother. This made Armstrong go a bit nuts, who killed himself via self-defenstration.

I'm sure there has been, although I can't point to a particular good source on the matter.

---Joel

For the same reason you responded to mine?

--
Adrian C