From: "1GDW" <>
>
>
> The WL-183 /minidisk project is moving forward but I have a question
> about using more than one wire to carry the microphone signal. I
> have a piece of four conductor shielded Canare cable. I am
> thinking of using the shield as ground and then double wire the
> other microphone leads (if one line were to break I would have a
> backup feed). Would this possibly reduce the signal strength being
> transmitted? Or would the signal just take the line of least
> resistance. I have other cable that I can use in place of this idea.
> Jerry White Muscatine, IA
You should use the four conductors, but not for the reason you think.
When the cable develops a broken conductor it should be replaced. With
Canare star quad cable that's a very long way down the road if you are
at all nice to your cables. The biggest error people make with mic cable
is winding them too tightly for storage. Even this the Canare is very
resistant to damage.
Canare star quad is the commercial results of experiments done by the
BBC to minimize pickup of electrical interference by mic cables in TV
studio work. Where there are lots of high powered cables all over the place=
.
Yes, connect ground to the braided shield. This would connect to chassis
ground in your recorder. The signal is carried on the internal lines.
Blue wires carry the hot signal, white wires the cold signal. Use both
wires in each case when making up a mono cable.
The four conductors are arranged like a twisted pair, except there are 4
conductors instead of two. The two of the same color are on the opposite
sides of the twisted bundle. This effectively makes the center of
conduction go straight down the center of the cable for both pairs.
Since they are virtually in the same location they pick up
Electromagnetic interference exactly the same, thus canceling it. The
star quad arrangement is stated as providing a 20 dB drop in pickup of
this kind of noise vs the standard two conductor with shield mic cable.
In addition, the use of two cable for each signal reduces the reactive
inductance of the cable. The effect of this is less loss of higher
frequencies compared to single conductor systems. The inductance acts as
a resistor that increases it's resistance with frequency, so the lower
it is the better. (Aside for those experimenting with frequencies higher
than 20 kHz, it's extremely important that you use the exact same cable
with mics when making comparisons, and star quad is going to help you
the most)
Because star quad uses finer wire strands than regular mic cable, it's
phase shift is less too.
The advantages of star quad are in much better signal transmission. If
you break one of the conductors it then becomes unterminated and it's
inductance goes very high, a absolute worst case for signal
transmission. Thus the rule, a break in one conductor of star quad and
you replace or repair it. Repair is only possible at the ends.
For nature recording the level of sources of electromagnetic
interference is low as you are far away from any other conductors
normally. However, the other advantages of star quad are still there.
The longer the cable run you are using the greater the advantage.
I also use Canare star quad as a stereo cable. I carry the signal hot of
the two channels on the two blue wires and the signal cold on the white.
In the arrangement I have the two channels are somewhat separated
physically as they are on opposite sides of the star quad. I've had no
crosstalk problems at all. This negates the star quad advantages,
effectively it's two twisted pair, but I don't tend to do really long
runs, and as I'm nature recording don't need the ultimate in noise
resistance. In fact most of the time I'm using less than 5' of cable as
I'm hand holding. Even my high tripod recording is normally done with a
25' cable. I have compared this arrangement with 5 pin XLR's and a
single star quad cable to using a dual star quad snake with 3 pin XLR's.
The dual cable is very large by comparison, and I cannot tell any
difference listening. Using a single cable for the stereo has large
practical advantages for field recording over using two cables. Less to
snag on brush, less tangled cable to deal with.
Here's a fairly good discussion of mic cable:
http://www.procosound.com/whitepapers.htm
Walt
________________________________________________________________________
________________________________________________________________________
|