[Orion] AGC Threshold

[Sinisa Hristov]

Richard Detweiler wrote:

> When the noise drops down as the agc threshold is increased,what is
> happening, is the noise is dropping below the point at which the AGC will
> try to amplify.

When AGC Threshold is increased, gain is
immediately reduced in the same proportion.
No automatic action here, and AGC never "tries"
to do anything else apart from maintaining
constant volume.

There is only one AGC Threshold,
and, for any given threshold value,
signals above it are attenuated
to maintain constant volume.

No attempt is made to amplify signals
below the threshold, an no attempt is made
to improve S/N ratio or anything else,
apart from maintaining constant volume.


> This is a Good thing,  because hopefully the weak signal just above the
> noise is not below that point and it will then amplify only the signal of
> interst,

This reminds me of a myth about Orion's AGC
being fundamentally different from usual
implementations, i.e. being able to reduce
noise and amplify desired signals.

Unfortunately, there is no such a miracle.

At instants when both signal and noise are present,
signal to noise ratio does NOT depend at all on AGC
Threshold setting. And AGC action tends to reduce,
NOT increase the apparent signal to noise ratio,
as perceived with intermittent signal.

Orion's AGC action does not bring anything fundamentally new,
although the level of user control is greater than usual.
The user may or may not achive optimum settings,
depending on skill and willingness to play with controls.

By setting the AGC Threshold just above the
signal + noise level, AGC is essentially
turned off, with two benefits:

  a) intermodulation between the signal and the noise
     through AGC action (acting as undesired amplitude modulation)
     is avoided;

  b) our aural system doesn't get confused by fast and
     frequent changes of gain.

The sole remaining AGC function is that of
protecting our ears from unexpected
strong signals. Not a miracle, but useful.


73,

Sinisa  YT1NT, VE3EA