dl-fldigi/fldigi_doxygen/user_src_docs/audio-adjust.txt

/**
\page audio_adjust_page RX/TX Audio Adjustment

\tableofcontents

\section rx_audio Receive audio

Setting the correct hardware, operating system, and fldigi received audio
levels is not difficult, but it is the one setup procedure most often done
incorrectly.  The most commonly used sound card devices contain either a
16 or 24 bit analog to digital (a/d) converter.  A 16 bit a/d can provide
approximately 90 db of signal conversion.  For the 16 bit converter, if the 
peak audio signal that the a/d can handle is +/- 1 volt then the minimum 
discernable signal (1 bit) will be +/- 30.5 microvolts.  If more than a +/- 1 volt
signal is applied to the a/d input then either one of two things may occur,
(1) the audio is clipped, or (2) the audio is wrapped, large positive signals
wrap to large negative signals and vice versa.

The objective in adjusting the Rx audio is to use the full dynamic range of the
a/d without incurring overdrive.  fldigi provides a number of display controls to
assist in setting up the Rx audio.  There are two controls that ONLY adjust the
visual appearance of the waterfall and DO NOT effect the a/d or the signal decoders.
These controls are below and to the left of the waterfall, "Upper signal level (dB)", and
"Signal range (dB)".

<center>
\image html win10-fldigi-wf-adjust.png "Waterfall level/range controls"
\image latex win10-fldigi-wf-adjust.png "Waterfall level/range controls" width=1.5in
</center>

Change the waterfall display to the "Scope" view.  Do this by pressing the "WF" button twice if
it is currently displaying the waterfall.  You can also right click once on that button.  That
button acts as a rotary and is left/right click sensitive.  The display should show what looks 
like an oscilloscope view of the received audio.  This is the entire audio signal and not just 
the signal that is currently decoded.

<center>
\image html win10-fldigi-signal.png "Scope view"
\image latex win10-fldigi-signal.png "Scope view" width=1.5in
</center>

If your sound card a/d (also referred to as codec) is functioning correctly there
should be no signal offset and the signal will be centered vertically about the y-axis.  Set your
transceiver to a portion of the band with digital signals, 14.070 MHz is a good spot.  Now adjust
the transceiver audio output and the operating system mixer controls so that the peak signals do
not exceed the upper/lower gray lines on the display.  The diamond indicator to the left of the 
fldigi AFC button will be colored as follows:
<ul>
<li>BLACK - no signal, or insufficient Rx audio</li>
<li>GREEN - signals are in the correct range</li>
<li>YELLOW - signals are exceed 75% of maximum, but are less than 90% of maximum</li>
<li>RED - signals exceed 90% of maximum a/d capability - WARNING WILL ROBINSON!</li>
</ul>
You can then proceed to adjust the waterfall/fft appearance when you have the audio Rx level adjusted correctly.

The three signal views should be similar to this with the level/range controls set to 0 and 60 respectively.  The 
waterfall palette used is "default.pal", and the received signal is about 20 dB above the noise floor.
<center>
<table>
  <tr>
    <td>
      \image html win10-fldigi-wf.png "Waterfall"
      \image latex win10-fldigi-wf.png "Waterfal" width=1.5in
    </td>
    <td>
      \image html win10-fldigi-fft.png "FFT"
      \image latex win10-fldigi-fft.png "FFT" width=1.5in
    </td>
    <td>
      \image html win10-fldigi-signal.png "Scope"
      \image latex win10-fldigi-signal.png "Scope" width=1.5in
    </td>
  </tr>
</table>
</center>
\section rx_win10 Windows Audio Properties
A combination of the Windows operating system and certain audio codecs needs to be accommodated.  The codecs
used by devices like the SignaLink-USB, the TS590S, and recent Icom transceivers are identified by the
Windows operating system as "microphone" devices.  The Windows mixer controls for that device
are then set to insert about 30 dB of gain in the signal path.  To compensate most users simply lower the
transceiver audio output.  The result is very poor decoding.  The correct procedure is to force the Windows
mixer controls to remove that 30 dB of gain.  This procedure is for Windows 10, but is similar on Windows 7,
and Windows 8.

Right click on the task bar speaker icon and select the "Recording devices" menu item.  This will open the 
Sound devices dialog:
<center>
\image html win10-rec-properties.png "Sound Devices"
\image latex win10-rec-properties.png "Sound Devices" width=3.5in
</center>
This is the dialog on my Windows 10 test computer with a SignaLink-USB connected to a USB hub.  The 
SignaLink-USB shows up as a <b>Microphone</b> USB Audio CODEC.  Right click on the device entry and
select Properties.  This opens up the Microphone Properties dialog:
<center>
\image html win10-mic-properties.png "Microphone Properties"
\image latex win10-mic-properties.png "Microphone Properties" width=3.5in
</center>
Select the "Levels" tab and set the level to as close to 0 dB as is available:
<center>
\image html win10-mic-levels.png "Microphone Level"
\image latex win10-mic-levels.png "Microphone Level" width=3.5in
</center>
Then select the "Advanced" tab and change the type of converter.
<center>
\image html win10-mic-advanced.png "Microphone Advanced Tab"
\image latex win10-mic-advanced.png "Microphone Advanced Tab" width=3.5in
</center>
Apply and save the changes.  The last step is
usually needed to insure that the settings are not lost when the operating system is closed and restarted.


\section tx_audio Transmit audio

Too often you see an overdriven signals on the digital sub-bands;
multiple audio sidebands on PSK, splatter from overdriven MFSK and
RTTY. There is absolutely no reason for a transceiver driven
by fldigi to exhibit this type of performance. You can set up
your computer / transceiver for good solid performance without excessive
drive.
<br>

The "TUNE" button generates a continuous single frequency audio signal
at the exact frequency to which the waterfall cursor has been set. The peak
amplitude of this signal is the peak amplitude of every modem signal
generated by fldigi. None will exceed this value, even the simultaneous
multi-tone modes like Throb. Every modern SSB transmitter uses some automatic
level control ALC for preventing overdrive for SSB voice. A little overdrive
on a voice channel can be tolerated to a degree. In fact, that is what
an analog RF compressor does, overdrive and then subsequent filtering.
But you absolutely cannot tolerate that with the digital modes. Here is the
way to set up your transceiver for a clean signal. I recommend starting out
with a dummy load, but an "off hour" for a band might work just as well if
you do not have a dummy load.
<br>

<ul>
	<li>For Windows users</li>
	<ul>
		<li>Set your sound card output level to the minimum on the Windows
			mixer</li>
	</ul>
	<li>For Linux users</li>
	<ul>
		<li>Set your PCM level to about 80%</li>
		<li>Set your Transmit Level control for minimum output level.</li>
	</ul>
	<li>Set the fldigi transmit attenuator to -3 dB (control to the left of the AFC button)</li>
	<li>Set the waterfall cursor to 1000 Hz</li>
	<li>Enable the "Tune" mode in fldigi ... you do have CAT or PTT set up ...right?</li>
	<li>Make sure your transceiver's speech compression control is OFF</li>
	<li>Slowly bring up the Mixer audio out until your rig's ALC just starts
		to function (a light blinking or a meter showing this condition).</li>
	<li>Reduce the Mixer audio output until the ALC is disabled.</li>
	<li>You are now transmitting at maximum output power without distortion.</li>
	<li>You can use the fldigi transmit attenuator to make small adjustments 
	in the output power to compensate for variations in the SSB transmit filter.</li>
</ul>

You can use any level below this and be assured that your output signal
will be clean. All digital signals that fldigi generates will be limited to
this peak-to-peak voltage. You should always use the minimum power necessary
to maintain good comms, remember that even if you are clean at 100 W you
signal will be so strong among the QRP signals that it will overpower the
AGC on many receivers that are working another digital station within the
same SSB bandwidth that you are on. You will appreciate this the first
time that you are working a weak PSK DX station and someone blasts through
and captures your AGC.
<br>

You should try the the above adjustments at different audio
frequencies. Transceivers that achieve the SSB filtering with crystal or
mechanical filters will have a considerable amount of variation across the
passband of the filter. This will show up as a varying amount of ALC that is
dependent on the audio frequency. Once you are comfortable with the process
you can very quickly repeat the "Tune" and set the power for the frequency
to which the waterfall is set.  The fldigi transmit attenuator control should
have sufficient range to compensate for transmit SSB filter variation.  You
do not want to transmit (or receive) near the cutoff frequencies of the SSB
filter.  The transceiver filter will introduce both phase and amplitude 
distortion over the signal bandwidth.
<br>

\ref tune_page "Return to Top of Page"
<br>
\ref main_page "Return to Main Page"


*/