Tired of noise? Noise that
isn't the signal you want to copy? White noise, line noise, hiss? And off
frequency stuff you don't want to hear? Do you want to modernize your old rig
without spending thousands of dollars? The new SCAF-1 filter from Idiom Press
makes your radio listener friendly, whether you are an SSB or CW operator.
Modern IF filtering design is the best ever. But the engineer still has to
design an audio output that will accommodate the widest signal the radio is
designed to copy. Typically, this is 6 kHz. for wide band AM or for FM. The
audio passband must be capable of passing such signals through. The designers
have accomplished this beautifully - and as a result, when using narrower band
modes, off frequency QRM can be passed through with brilliant fidelity. Even if
you don't want to hear it.
And then there is white noise - noise created in the IF and the audio stages
that can be loud enough to drive the operator up the wall, especially when you
are trying to pull in weak signals. White noise is very fatiguing for any
operator tuning the bands.
For the CW operator, these problems are particularly galling. When everything
you want to copy is below 800 Hertz, why should you suffer QRM and receiver
artifacts up to 6 kHz or more off frequency ? Even key clicks from the station
you are working are annoying, not to mention QRN. And line noise will drive you
to watching daytime TV.
Another problem is the headphones you use. Virtually all headphones available
are designed for high fidelity performance - flat to 20,000 Hertz or more - and
thus willing partners in passing though wide bandwidths with all the excess
baggage. If it's there they deliver it all to your ears, with superb fidelity.
Several of the latest high-end radios actually deal with these problems, by
tailoring the audio passband to match the communications bandwidth. Sadly, the
vast majority of modern rigs and all older radios do not.
The new SCAF-1 filter from Idiom Press makes your radio listener friendly,
whether you are an SSB or CW operator. Using a modern but little known
technology called SCAF filtering, the SCAF-1 is an active audio low pass filter
offering user control of the filter cut-off frequency, yielding a stunning 96dB
per octave roll-off of signals above the cut off frequency, and no white noise.
It is the perfect partner for your crystal or mechanical filters. And, unlike
most audio filters, there is absolutely NO ringing.
How do you use the SCAF-1?
There are two controls - the in/out switch, and the low-pass cut off tuning
control. By turning the single front knob, you set the audio low pass filter
cut off frequency anywhere you want it from 450 Hertz to 3.5 kHz . And that's
it. Every signal or noise above the frequency you set is rolled off at 96 dB
per octave. And it is done at audio frequencies - no white noise!
What
does "roll off" mean?
It means that if you set the cut-off frequency for say 450 Hertz, any signal
coming through to the input of the filter is 96 dB down at 900 Hertz. Set it
for 1000 Hertz, signals at 2000 Hertz are down 96 dB.
Here is a response curve. Note that the
measurement equipment used to make the curve had a calibration range accuracy
of 60 dB so the curves do not go to their floor.
Why doesn't this filter use DSP?
DSP can offer brilliant results in signal processing, but good ones require
significant computing power, and are expensive. They can be complex to adjust,
and may require retuning any time the received signal changes frequency. Also,
they can offer real problems on CW with processing delays that destroy the CW
monitoring function. Unlike most DSP audio filters offered, the SCAF-1 filter
passes signals through in real time, for perfect CW monitoring and QSK, and
with no recovery pause on SSB when going from transmit to receive.
Once you start using SCAF-1 you will never want your rig to be without it.
SCAF-1 does for your audio stages what crystal or mechanical filters do for
your IF stages. SCAF-1 has two output jacks, (not switched) and will drive
headphones or a speaker.