RE: Volume control in filter window.

["Leif Åsbrink" <SMTP:LEIF.ASBRINK@xxxxxxxxxxxxxxxx>]

Hi Rein and All,

>     When I loaded and started de software the very first time I saw signal
>     in all three windows but no sound! I wrongly assumed all kinds of 
>     problems basically around the back end of the sound card not getting
>     signal to send to the output ( speaker ) Though the 
> instructions mention
>     this volume control later in the text, I overlooked this at the start
>     or what have you.
>     Also, the red bar is not visible when I start-up the software. 
When you start for the first time, the volume control is set at about 30%
which means that a signal that is about 40dB above the noise floor will
give a good loudspeaker output.
  
>     I am having trouble to understand exactly what this control 
> is doing or
>     what its function is and have not found anything about it.
This is just a volume control like "RF volume" on an analog radio.


>     My guess is that it acts as a variable level switch to output
>     sound. It sounds to be discreet and not continuous.
It is perfectly continous, but it has a step size forced onto it
by the screen size. If you make the baseband window higher, the
steps are finer.


>     With no signal and the bar all the way down the output is 
> totally blocked
>     no sound or noise is coming from the speaker. Have not been able to 
>     check yet how the transition from no sound to sound is 
> happening, most 
>     likely as a function of the desired signal level and the 
> volume control 
>     setting or, one effecting the other. 
>     It appears that around the 20 scale divisions point the sound 
> channel is
>     opening up and I can hear clean weak audio depending on the 
> position of 
>     the bfo signal though quite weak.
>     Again with no signal going in, when the setting of this control is 
>     increased however, the audio output is increasing up to the 
> point that it
>     starts to ring or even start to oscillate.
Linrad still has only the weak CW mode. Among other things this means 
that there is no AGC. The AF volume control is on your loudspeakers
or in the mixer program, the RF volume is the red bar.

Bring the red bar to 100%, then adjust the AF volume for the maximum
audio you want for a saturating signal. Then turn the red bar down
for a suitable signal level. Exactly the same procedure as you would
do on an analog receiver in which you switch off the AGC and use
AF and RF volumes this way for weak signal work.

>     There is then most likely a 
>     higher noise level present that is causing the ringing. 
I have problems with the concept of "ringing"

A filter with a bandwidth of X Hz will have an output that
lasts 1/X seconds minimum. A short pulse will be lengthened.
It is a consequence of the very nature of a filter.

Poorly designed filters may have a pulse response that lasts
for a much longer time than the minimum time.

>     It appears to me that this control does more than just volume control
>     It looks like as if it is some level detector to enable the output.
>     Or perhaps that the filtered signal needs to have a certain amplitude
>     to overcome some set level before audio is generated.
You may have enabled the audio expander. It may give this impression
although there is no threshold. When the dynamic range is expanded, weak
signals become even weaker. If you selected 8 bit output weak signals 
will just disappear. Try the F1 help function to find out what all 
the controls are.

When using Linrad, the amplitude indicator, the little white square at the
left side of the red bar (amplitude control) should be somewhere mid-range.
If the signal saturates, the square becomes red and placed at the top.
If the signal is too low, the box becomes green. Do not expect to
hear anything from the computer loudspeaker when the indicator is
near the level where it goes green. You need good headphones to 
really use the dynamic range of your brains for weak signal reception. 
Typical computer loudspeakers will not come anywhere near.

>     With signal, I find it relative hard to set the audio freq without 
>     any trace of ringing, with increasing of the volume setting 
> the ringing 
>     level is increasing quite a lot as well as the desired signal. 
>     Perhaps the same old problem!  
You may record the raw data on the hard disk. Press S to start saving.
Then stop after about one minute and press G to make a screen dump.
Send the files to me or upload them somewhere if the size is above 
1 megabyte. Then pack all the par_* files and send them to me.

I will listen to the signal that you point out with a reference to the 
screen dump and send a .wav file of the output with what I think
is optimum processing. 

You will most probably think that the output has far too much "ringing".
That is because I am used to work with filters that are close to 
optimum for the signal. Theory says that "a matched filter will have
a pulse response that is similar to the desired signal". This will
of course implicate that noise will sound very similar to a CW
signal. If your brains are not used to it, you will get confused
by it - and then the audio expander helps a little. The Linrad filters
have close to minimum length of the pulse response - you have the freedom
to select the curvature where the flat region starts to fall off so
you can make your own compromise

If you do not like narrow filters, just open the bandwidth and Linrad 
will sound exactly as or better than any other radio with the same 
bandwidth and without AGC.

73

Leif / SM5BSZ



LINRADDARNIL