[1000mp] my SuperCable is done

[Kok Chen]

<w9ol@billnjudy.com> wrote:

> Shields grounded at each end.

Actually, believe it or not, this is not advisable
for low level audio signals.  Could lead to hum loops,
unless you have a lower impedance (audio frequencies)
ground strap between the two said pieces of equipment.

One end of the braid is best left open.  Or ground it
through a 100 pF cap or something to give it a good RF
ground but not 60 Hz path.

But you can check if you hear any low level hum (using
the monitor feature (from the sound card) and by recording
the audio (to the sound card) and doing a spectral analysis.
The rig-to-soundcard direction is more critical since
it could restrict the dynamic range of your demodulator.

Any hum will raise the "noise" floor of what the sound card
sees, and you lose dynamic range.  Remember that if
you have 1 volt of audio and just 10 millivolt of hum, your
dynamic range could never exceed 40 dB, etc, etc.  1 millivolt
of hum is probably OK, since that is 60 dB and garden variety
sound cards probably have no more than 50 dB of DR to
start with.  But if you want to experiment with really high
DR FSK stuff, like the ST-8000, you will need to bet the
hum below 100 uV to get any benefit from the good modems.

From their studies in the 40's, Ma Bell never believed in
shielded audio cables.  They have always used balanced twisted
pairs.  But you will need two center tapped transformers at
each end of the cable.

Would you believe my HAL DSP-4100 comes with a balanced input.
There is a center tapped transformer inside the modem.  The Ham
version, the DXP-38 did not have that option.

73 de w7ay