SSB

[Gene & Debby Stopp]

----------
> From: Martin Czech <martin.czech at itt-sc.de>
> Date: Wednesday, July 30, 1997 1:31 AM
> 
> Another book of interest dropped into my hand:
> 
> Amateur Radio ...
> 
> Everyone knows what modulation is, with a "ringmodulator"
> you'll get upper and lower sidebands, with the typicall "beating"
> sound, if the carrier frequency is quite low.
> 
> Now, those radio people always want to get rid of one of the sidebands
> for serveral reasons. This is called single side band modulation or ssb.
> 
> This could be very intersting for us e.music people and diyers, too.
> 
> How to get rid of the unwanted sideband:

Yes! There does exist such a device for musical applications - called a
"Frequency Shifter", and produced in the past by 360 Systems (can be heard
on John McLaughlin albums), by Moog (called the "Bode Frequency Shifter",
and heard on some Tomita albums), by Serge, and as a project in
Electronotes. There may be others out there, but these I know of.

Technically a Frequency Shifter is a single sideband modulator, as opposed
to a double sideband modulator which is the technician's name for a ring
modulator (or "balanced modulator", or even a "mixer" in some radio
circles). The circuit consists of a pair of 90 degree phase splitters, two
ring modulators, and an output mixer. The 90 degree phase splitters can
consist of either a quadrature sine wave source, or a live signal wideband
phase difference network. You can make a frequency shifter with both 90
degree splitter sources as quadrature oscillators, but this is really just
an expensive way to create sine waves. You can make one with two live
signal 90 degree phase difference networks, which would produce results
that may be a bit too complex for easy operation (and besides, expensive
since the 90 degree PDN's are quite parts-intensive). The most typical
application involves a quadrature sine wave source plus a live signal 90
degree PDN.

The Electronotes circuit (which is included in the Preferred Circuits
Collection BTW) has a fairly easy-to-build 90 degree live signal PDN which
covers the audio range and consists of only a few op-amps plus some
carefully selected parts. The quadrature since wave VCO is of the beat
frequency type which allows through-zero FM - it creates a sin and cosin
output by mixing a fixed 20 KHz sin/cos pair with a 15 KHz-25 KHz VCO sine
wave with two ring modulators. This method tends to drift a bit since
keeping a 20 KHz VCO to within a single cycle over time is not easy.
However, with this method the FM capabilites are maximized. The circuit is
more complex than most projects (including a total of four ring
modulators!) and is probably not for the beginner, but it is quite
satisfying for those who like to build.

Sound-wise, the highly technical designer of the circuit (the famous Bernie
Hutchins) refers to the effect as "dramatic", which in Electronotes
terminology means "pretty darn weird sounding". I've built this module and
it's true - the sound can be pretty out there. Basically the transformation
of a live signal (any sound source, such as an instrument or human voice)
will be warped so drastically as to be almost unrecognizeable, for large
modulation amounts. Major inharmonic stuff going on here, but not quite the
same as a ring modulator. I suppose I would call the output "less confused"
compared to a ring modulator, meaning that you can hear the shift effect
more clearly than with a ring modulator since there are only sidebands on
one side of the signal (either above or below the signal being modulated,
depending on the direction of modulation).

A rather obscure example of the effect on a human voice can be found in the
alien being vocal effects in some episodes of the old TV show "The Outer
Limits" (one such episode would be the pilot show, entitled "The Galaxy
Being").

All in all I think that it's worth the effort of either building one or
finding a used one. I suspect that the Electronotes version may be the
cleanest of all, due to the use of Analog Devices balanced modulator chips
plus a pair of noise gate VCA's on the output driven by a precision
envelope follower (jeeez I'm realizing just how good that circuit really
is!). A bit expensive (the chips cost me about $120 total, just for the AD
parts) but worth it.

- Gene