Mark, to keep this response to a readable length, let me address only the
first of your questions.
The "other sideband" and the carrier contribute nothing to the transfer of
information from A to B. The carrier is required to maintain proper phase
relationship between the two sidebands. Both sidebands are "saying the same
thing" since one is simply frequency-inverted mirror-image of the other. The
carrier conveys no information at all other than the fact that an AM station
is on the frequency. The carrier presence does, however, simplify AGC and
S-meter circuitry. It represents an expenditure of half the power output of
the transmitter in the simplest case, with each sideband doling out the
remaining half in equal measure.
So, with SSB, we eliminate the 50% power waste in the carrier and the 25%
power waste in the other sideband. Additionally, by requiring that only one
sideband be processed, we reduce the receiver bandwidth by a half, thereby
halving the noise power. And, of course, the occupied bandwidth of the
transmitted signal is cut in half, conserving spectrum.
What do we lose? Simplicity of transmitter and receiver design and
implementation, for one. Another is the need for critical tuning of the one
sideband by selection of the proper frequency of the inserted carrier at the
receiver. Finally, without a transmitted carrier, we lose phase reference
for the components in the remaining sideband. For voice and some data modes
this is of little or no importance. For the others, a low-amplitude pilot
carrier can be transmitted to facilitate the demodulation process.
As you know there are variants of conventional AM that have been popular in
amateur radio over the years. Most common has been the DSB approach: double
sideband with carrier suppressed.
Modern AM broadcast stations use complex digital modulation schemes to
achieve well in excess of 100% positive modulation with minimal distortion.
Peak power is increased accordingly.
Commercial AM stations have the capability for ISB, independent sideband,
operation with different program material in each sideband. During WW2, The
BBC continued broadcasting as usual, but some of the carrier power and
frequency was devoted to very high speed CW transmissions during pauses in
the normal program material.
A more detailed analysis of AM and SSB, which can be found in early QST
issues in the late 40's and early 50's when SSB was thoroughly analyzed and
dissected, reveals an overall gain of about 9 dB over a corresponding
conventional AM signal with both limited to the same peak power. There are
of course substantial reductions in weight, size, input power required, heat
dissipated, etc. with SSB.
And, no, Mr. Nyquist is quite content with SSB. The situation is not that we
bending his rule with SSB, rather we are using twice the bandwidth necessary
per his criterion by using AM instead of SSB. The information rate per total
bandwidth for AM is about one-half what it is for SSB. Using the same full
bandwidth, SSB can transmit at twice the information rate or, by limiting
bandwidth to conserve spectrum and be better neighbors, we get the same
information rate in half the bandwidth with SSB.
Hope this short overview is of some interest and helps to answer your first
Amateur Radio W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
----- Original Message -----
From: "Mark Erbaugh" <firstname.lastname@example.org>
Sent: Tuesday, April 22, 2003 10:43 AM
Subject: [TenTec] Slightly OT: SSB vs AM
> This is sort of related to the discussion on the list of late about HiFi
> I understand at a basic level the reason for improved performance of SSB
> over AM, i.e. only one sideband and no carrier. My question is: Compared
> SSB, is the energy in the other sideband and carrier truly 'wasted' or
> it convey some intelligence or noise immunity? If it is wasted, it seems
> that we are somehow violating Nyquist's rule. We have that same data rate
> with half the band width and 1/4 the power. If that truly is the case, is
> there the possiblity of another way further reduce bandwith and power and
> keep the same data rate (short of digital encoding)?
> Could some sort of frequency 'compression' be used. Assume that the audio
> signal is 300 to 3000 Hz. Each frequency component in the signal would be
> represented by a similar component, but at half the frequency difference
> from 300 Hz. For example, the 300 Hz component would still be at 300 Hz,
> the 350 Hz component would be at 325 Hz, the 400 Hz component and 450 Hz,
> and the 3000 Hz component at 1750 Hz (( 3000 - 300 ) / 2 + 300 ). The new
> bandwidth would now be 300 - 1750 Hz. On the receiving end each frequency
> component would be 'un-compressed' to 300 - 3000 Hz.
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