[sdiy] AM and FM conventions

[Michael Boracci]

Magnus,
Thanks. I was looking too deeply for a simple answer. I appreciate the time
that you took to write this explanation.

Michael

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Magnus Danielson
Sent: Saturday, December 06, 2003 9:29 AM
To: mboracci at nfpcomputer.com
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] AM and FM conventions


From: "Michael Boracci" <mboracci at nfpcomputer.com>
Subject: [sdiy] AM and FM conventions
Date: Sat, 6 Dec 2003 09:14:42 -0500
Message-ID: <FPEKLNJGOOPGCIMPEHLCIEDGCHAA.mboracci at nfpcomputer.com>

> Friends,
> I have a few questions regarding AM and FM.
>
> I need to know what the standard is for determining if a signal( carrier )
> is being FM'ed or AM'ed. Here are the examples.
>
>
> F of carrier is modified by the A of the modulator.
> F of carrier is modified by the F of the modulator.
> A of carrier is modified by the A of the modulator.
> A of carrier is modified by the F of the modulator.
>
> These are the composite versions:
> F and A of carrier are modified by the F and A of the modulator
> respectively.
> A and F of carrier are modified by the F and A of the modulator
> respectively.
>
> Out of these which one is actually FM and which is AM in synthesizer
world?

For both cases, it is the amplitude of the modulator which modifies the
carrier
and depending on if it is a Frequency Modulator or Amplitude Modulator it
affects either of the Frequency or Amplitude respectively.

However, it is true that the frequency of the modulator plays a role.

In an amplitude modulation where the carrier has the frequency of fc and the
modulation has the frequency of fm, then will the carrier frequency has two
sidebands at fc - fm and fc + fm respectively. Those sidebands will have the
same amplitude and usually the amplitude of the carrier lowers as the
sideband
amplitude increases. You can also make an AM in which the carrier is fully
suppressed (4-quadrant multipliers can do this). You can also reduce the
width
of the required frequency band by only having either of the side bands, i.e.
only fc - fm or fc + fm. This is then called Single Side Band (SSB) and in
music speak it is called a frequency shifter.

In a frequency modulation you will have a carrier at fc again, and a whole
bunch of sidebands having the frequency of fc +/- n*fm where the amplitude
of
the carrier and sidebands is determined by the Bessel function for the
n:side
band (0 = carrier) and the modulation depth. Those are a bit messy. In
broadcast radio (and indeed any other radio use) a bandpass filter is used
to
limit the number of sidebands being in practical use.

So, frequency modulation (and indeed so its relative phase modulation)
creates
a much richer suite of sidebands than amplitude modulation.

> FM and AM Radio are not these exactly ( I thought ), but what are they,
and
> do they relate to synthesizers at all?

Where do you think the synthesizer world got the terms my friend? Of the
radio
world of course! Stuff like AM, FM, PM, ring-modulator etc. in radio
predates
their use in the music/synthesizer world. Some of the early experimental
studios really used the telecommunication and radio-oriented lab equipment
which was available at the time.

In the music world we have since chosen specific variants (like exponential
input) which is useable for most people, a result of product development
with a
customer in mind (I do think about Bob Moog here, but Don too).

Cheers,
Magnus



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