14.25 Why do I have hum when I connect cable to my VCR (or TV), which is connected to my audio system?
Description
This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
14.25 Why do I have hum when I connect cable to my VCR (or TV), which is connected to my audio system?
What you are experiencing is probably a "ground loop", caused
by multiple connections from your equipment chassis ground to
building ground. Since disconnecting the cable or building
antenna from the VCR eliminates the hum, the cure is simple.
The following info talks about "the cable" but works the same
with a coax from a master antenna system.
Go to Radio Shack and buy one each of:
15-1253, "300-ohm TV-VCR Matching Transformer"
This looks like a little box with two screw terminals
and a push-on male F (coax) connector.
15-1140, "75-ohm coax/300-ohm twin lead indoor/outdoor matching
transformer"
This is a longish box or tube, with a female F
connector on one end and a bit of twin-lead coming from
the other. The twin-lead ends in a pair of what are
called "spade lugs" (shaped like U's).
Note: each of these part numbers may have a "B" or other
letter at the end. These indicate slightly different details
of functionally equivalent parts. Don't worry about it.
Connect the two spade lugs on the -1140 to the two screw
terminals on the -1253. Make sure they don't touch each other;
this shouldn't be difficult to get right. This gives you the
"isolator", with a female coax connector on one end and a male
coax connector on the other.
Just insert the isolator "in line" in the incoming cable lead.
ie treat it as you would a (very short) extension cord. You
can do this right at the back of the VCR (or whatever the cable
is hooked to).
Only one of the two units called out here (15-1140) actually
provides isolation. Two of the 15-1253 units back to back will
NOT work. Two of the 15-1140 units back to back will work fine
but will be less convenient.
If you can't find these specific parts, and want to know if the
substitutes you've found will work, test them with an ohmmeter,
measuring from either the pin or shield of the coax side to
either wire of the twin-lead side. If it's not an autoranging
unit, set the meter to its highest resistance range. You want
to see no connection (ie: infinite resistance, an open circuit)
between them. As with the parts described above, only one of
the coax/twinlead adapters needs to pass the test.
This trick runs the signal through a PAIR of baluns. This is
more than is absolutely required to solve this problem, and may
weaken the signal slightly. This should not be a problem on
most cable systems. But, some audio stores are beginning to
carry a unit made expressly for this purpose. It contains a
single 75 ohm to 75 ohm isolation transformer. This should
introduce less signal loss. It will also be better shielded
than the two baluns (see next paragraph). Under $10 would be an
appropriate price.
The back-to-back baluns may allow "ingress". That is, if you
are near to a TV transmitter, the short length of twinlead may
pick up broadcast TV signals and mix them with the cable,
causing interference. If you can find a prepackaged 75 ohm
isolation transformer as described in the preceding paragraph,
it should be better in this regard.
Mondial is selling a unit dubbed the "Magic-1"; this does the
same job but with three capacitors instead of transformers. It
is said to cause less than 1 dB of signal loss. On the other
hand, it costs about $90.
Yet another solution is to attack the problem at the line-level
audio connection between the VCR and the rest of your stereo.
Radio Shack's stereo ground isolators (270-054) are made for
this purpose. These go in the line-level AUDIO connections
between the VCR (or TV) and the rest of your sound system. If
both the line in and line out jacks on the VCR are connected to
the sound system, you'll need two of these isolators. They are
audio frequency transformers and may add some distortion and
frequency response error.
14.26 Is Binaural better than stereo? What is Binaural?
Judge for yourself. There are samples of binaural recordings
available for free download at:
http://www.binaural.com
According to the Binaural FAQ (slightly edited to save space):
http://www.binaural.com/binfaq.html
"Binaural...record(s) music and sounds with two tiny
omnidirectional mikes at the entrance to the ear canals on an
artificial head...This includes even the fleshy ridges of the
outer ears which modify the frequency balance of sounds
depending on the direction from which they originate...
"...A stereophonic system...uses loudspeakers but requires an
infinite number of channels for perfect reproduction...
(Binaural) requires only two channels for perfect reproduction
but involves the use of a pair of head receivers [drivers] held
tightly to the ears for each listener. All listeners with such
a system can be given the illusion of sitting in the best seat
in the concert hall. Harvey Fletcher in the SMPTE Journal Vol.
61, September 1953."
"The binaural experience is striking, and requires no special
equipment besides stereo headphones and binaural recordings.
However, the 'perfect reproduction' mentioned by Fletcher is
not necessarily achieved by all listeners due to variations
in dummy heads, headphones and individual hearing. The
astonishing realism is heard by nearly all, even with the most
inexpensive headphones. But many have trouble localizing sounds
directly in front or in back, and for some the sounds seem to
occur inside their skull (just as with listening to stereo on
headphones) rather than outside. Better matching of HRTFs (Head
Related Transfer Functions) can correct some of these problems,
and with recent advances in digital signal processing there may
soon be a solution. It would involve a processor similar to the
Dolby Headphone circuit - which provides a virtual 5.1 surround
field on ordinary headphones, but allowing for the proper EQ
and phasing to map the binaural sounds seamlessly in a
360-degree sphere around each listener."
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