By Dev Gualtieri
BUILD A STEREO
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One consequence of the 2009 transition to digital television in the US is that
I lost most of the New York City television stations I had watched for the last
30 years. Since I live in a far New Jersey suburb, I wasn't able to get strong,
multipath-free signals on the new UHF channels used for broadcasting the
digital signals. I had never felt a need for cable television but it suddenly
became a necessity. So, I opted for the fiber optic service available in my area.
The video reception on cable was terrific — since I
bought an HD television at the same time — but the audio
was an enigma. There was a tremendous variation in
volume going from one channel to another, and the
commercials seemed too loud. We never had such
problems with off-the-air signals. That's because
broadcasters can't risk transmitter over-modulation. At the
same time, they want to keep their signals as high as
possible to cover a wide audience. These competing
demands force them to keep firm control of their audio
28 January 2012
■ FIGURE 1.
of the circuit
signals. That doesn't seem to be the case for cable.
What does an electronic hobbyist do when faced with
a technical challenge like this? He heads for the parts
drawer and a soldering iron! Fortunately, I had a short
career as a broadcast engineer so I knew a lot about the
technology of automatic volume control. I also knew that I
didn't want to build a compressor or a limiter. I really
wanted a platform gain controller.
Circuits to regulate the amplitude of audio signals have
been around since the vacuum tube days. They were
patented as early as 1932 (H. A. Wheeler, "Volume
Control," U.S. Patent No. 1,879,863, September 27, 1932).
The most common types of audio gain controllers are
compressors and limiters. A compressor modifies the
dynamic range of an audio signal to make quiet passages
louder and loud passages softer. As a side effect, it makes
the audio signal sound louder overall. A limiter functions
somewhat like a compressor, but it's designed to pass
signals unchanged up to a threshold and then completely
block signals from rising in amplitude above the threshold.
Figure 1 is a block diagram of the circuitry involved for
both compressors and limiters. The differences between
these is the threshold at which the device starts to control
and the gain of the amplifier that drives the control signal