On Tue, 11 May 2004 22:37:13 -0500, John Rader wrote:
>grounding the shell of the 4 pin mic connector solved this.
This is yet another example of what we in the pro audio world call "the pin 1
problem." That is, to be effective, the shield of a cable must be connected
DIRECTLY to the shielding enclosure (chassis), NOT to the interior circuitry or
the
circuit board. When it goes to the circuit board first and then to the chassis,
there
will be a voltage drop across the inductance of that connection due to RF
current
flow on the shield. That voltage drop will be added to the signal at random
points in
the circuit depending on circuit board layout.
The pin 1 problem is a MAJOR cause of RFI interference at audio interfaces, and
it
is most certainly the cause of the one you have just fixed.
Note that "the pin 1 problem" is an equipment design error! The radios that
connect
the cable shield to any point other than their chassis are built WRONG! The
fact that
MANY radios are designed this way does not change that fact -- the laws of
physics
are not the result of a popular vote!
BTW, it's called "the pin 1 problem" because pin 1 is the designated shield
contact
of the XLR connector, and it that contact that must be connected to the chassis
within equipment.
Jim Brown K9YC
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