B CUHILDAA NVOIGCEER
BY JIM STEWART
One day back in the 1970s, I tuned to a public
television station to see an oddly dressed man
fighting off a swarm of evil looking robots. It
was Tom Baker as Dr. Who saving the universe
from the Daleks. I’ve been a fan ever since.
For the uninitiated, Dr. Who is a sci-fi show
produced by the BBC. Daleks (shown here) are
half-robot aliens that roll around saying things
like “You will be exterminated!”
Recently, I came across a talking Dalek toy for sale
on-line. Of course, I had to get one. Daleks have a
bizarre voice, and I thought it would be fun to build a
voice-changer circuit to make me sound like one (see the
sidebar on voice changer circuits).
With a little research on the Internet (e.g., look up
“Daleks” on Wikipedia), I found out that the Dalek voice
■ FIGURE 1
■ FIGURE 2
is produced by mixing an actor’s voice with a fixed
frequency sine wave in a ring modulator (also called a
balanced modulator). I found different values for the
frequency of the sine wave in different sources; the range
was 30 Hz to 100 Hz. Using an oscilloscope to actually
look at the voice signal from my talking Dalek toy, I saw
the typical double sideband suppressed carrier (DSB-SC)
wave form produced by a balanced modulator. Figure 1
shows a simplified version. The outer envelope would be
the low frequency sine wave while the inner signal would
be the voice.
Balanced modulators are used in RF work to generate
and detect signals. (When used in a detector, it’s usually
called a mixer.) DSB-SC is basically an AM signal, but
without the carrier. Its output contains sum and difference
frequencies only, which it achieves by multiplying the
audio with a fixed frequency carrier.
A balanced modulator can be built in several ways.
You can use an IC such as the old LM1496 which was
designed for this purpose. Or, you could use a multiplier
IC, such as the AD633. The 1496 requires +12V, -8V, a fist
full of resistors and capacitors, and a carefully adjusted
trimpot. The 633 is relatively expensive ($8), but luckily,
there are other ways to do it.
Figure 2 shows a classic four-diode ring modulator.
While at first glance it looks like a bridge rectifier, closer
scrutiny shows the diodes are not arranged to rectify.