[SFDXA] For A CLEAN SIGNAL! - FT8 Adjusting Sound Card - Bandwidth

[Bill]

***FT8 – Avoiding Harmonics and Audio Settings Updated 12/6/18 *

By Mike Black/W9MDB and Eric Spero/WA1SXK

The FT8 sound card mode is an MFSK mode with redundancy and error 
checking.  If you have been hearing about this new mode or have been 
using it, this article can aid you to set up and operate the FT8 data 
mode with good clean signals.  FT8 allows you to view and decode dozens 
of signals at once with the waterfall display.  This means your audio 
setup and bandpass settings will be of utmost importance while operating 
the FT8 sound card mode.
FT8 is a very popular sound card mode thanks to the WSJT-X team's 
efforts.  See http://physics.princeton.edu/pulsar/K1JT/wsjtx.html for 
the software and check out the references for lots of details.  A sound 
card mode is what you might call a poor-person’s SDR (since the software 
is free you can be poor to enjoy it).  All the FT8 relevant 
modulation/demodulation/bit-correction/error- checking is taking place 
in the WSJT-X software.  During this process your FT8 messages 
(exchanges) are being encoded with additional information that allows 
for error detection and bit corruption correction which the receiving 
side can use to process and validate the received message.  The WSJT-X 
software sound card modes can be used with any rig, new or old.  FT8 can 
demodulate dozens of signals (50hz wide each) in up to 5000Hz of 
bandpass including overlapping signals. As you can see, many hams can 
operate at the same time in the same bandwidth.  This approach allows 
new operating paradigms like FT8 Fox&Hound mode that DXCC teams are now 
using where they can process multiple QSOs simultaneously (see 
http://physics.princeton.edu/pulsar/k1jt/FT8_DXpedition_Mode.pdf).
One of the areas that seems to be misunderstood while operating FT8 is 
the how the radio’s bandpass settings affect the operations of this 
mode. Many of the hams who operate some of the other modes like SSB and 
CW, use the default bandpass settings in their rig which are frequently 
too narrow for FT8. Note that most rigs can set the overall bandpass and 
not separate the Rx/Tx bandpass.  Your rig’s bandpass is hopefully at 
least 200Hz-2400Hz and preferably 200Hz-3000Hz since there are quite a 
few operators who transmit above 2500 (see Figure 1 with 
200Hz-3000Hz).   You can view your bandpass in the WSJT-X waterfall by 
turning off “Flatten”, below that change the pulldown to “Current” and 
set “N Avg” to 3 or less and “Start 0 Hz” so we can see the changes more 
easily.  What you should see is something like Figure 3, where you have 
a noise level signal rolling-off at lower and upper ends of the bandpass.
You should adjust the Bins/Pixel and window width, so you can see your 
entire FT8 signal bandwidth.  Then you need to learn your bandwidth 
controls in your rig, so you can maximize both the low end and high end 
cutoff points.  Once you have your bandwidth set in your rig as wide as 
you can make it you can change the “Start 0 Hz” to “Start 200 Hz”.    
Then adjust the right side of graph until it is inside your upper 
bandwidth limit.  Then turn on “Flatten”.  Note that Flatten does not 
like having the edges of your bandpass in the waterfall and will not 
behave as well if you adjust this incorrectly.  The signal bandpass area 
between the lower and upper limits of the waterfall window is where the 
FT8 signals will be
displayed and decoded in the software.  The frequency limits of the 
waterfall determine what gets decoded.
Remember your goal for FT8 is to adjust your rig for as much TX/RX 
bandwidth as possible…you can run it up to 5kHz for WSJT-X (as much as 
WSJT-X can do right now).  Most rigs should do at least 2002400Hz.  Many 
rigs have bandwidth settings called NAR/MID/WID filters and the WID 
settings should always be used and it can usually be adjusted for the 
largest bandwidth.  If you can’t find your rig’s bandwidth settings 
after consulting with your manual feel free to contact one of the authors.
Once you have your bandwidth set correctly you are now ready to set up 
your receive signals path:
#1 Open the sound control panel on your computer, right-click your rig 
sound device, select Properties, and set the Level of your sound card 
Playback device (and Recording device) to 0dB. You can get to the sound 
control panel in Windows 10 by right-clicking the speaker icon and 
select “Open Sound Settings” and find the “Sound Control Panel” entry 
and your audio is probably called “USB Audio CODEC”.  Rightclick the 
device for your rig and select “Properties”.  Select the Level tab and 
right-click in the level slider to set the scale to dB instead of the 
default percentage.
#2 In the Advanced tab set your sample rate to 48kHz 16-bit. Also, 
ensure all enhancements or spatial sound effects are turned OFF.
#3 You are now ready to adjust the level in the WSJT-X meter to minimum 
30dB and not more than 40dB when you have your rigs VFO tuned on a quiet 
spot.  You can adjust your external sound card knobs if you are using 
one, and your rigs output level to achieve this.  Do NOT make this 
adjustment using sound controls on the computer…in most cases that is 
not necessary.   The meter in WSJT-X should look something like Figure 
4.  Again, note this is on a quiet spot on the band with no signals 
present.
Now let’s talk about your transmit signal path:
   A rig that has too much input audio will create a dirty tx signal 
with very significant harmonics that can disrupt a QSO and everybody can 
see on the waterfall…. but…here is the problem…you won’t know about it 
unless you can monitor your own transmitted signal.  Any harmonics will 
show up at multiples of your Tx offset setting in the WSJT-X waterfall 
of everybody that receives your signal.  For example, you can clearly 
see the noise bars at around 2480 in Figure 1 which is the 3rd harmonic 
of the signal at 829.  In Figure 2 you can see an extreme example with 
multiple harmonics.  Running WSJT-X’s optional rig split mode or fake it 
mode will prevent harmonics from showing up in your transmitted signal 
since harmonics will show up above 3000Hz which is normally outside your 
transmit passband.  But it’s better not to generate them in the first 
place as your signal also gets affect by the clipping.  ALC (Automatic 
Level Control) is one indicator of clipping and is an easy, roundabout 
way to know if you are clipping your signal.  Another way is to ensure 
that dB level controls in WSJT-X and your soundcard behave as they should.
To set your transmit levels use the procedure below.  This procedure is 
designed to ensure your signal is just enough to drive the rig without 
any ALC taking place.  You MUST understand what your ALC meter is 
telling you.
Things you need to know to do this procedure:
• How much power your rig can transmit for FT8.  Many rigs cannot use 
full power for this type of mode and some rigs can’t even do their rated 
power on a constant envelope signal without hitting the ALC.   • How to 
read and understand your rig’s ALC meter.  Anything above 0dB ALC is bad 
for FT8/digital modes. The question is, how does your rig show 0dB ALC?  
For most older rigs 0dB is the left-hand side of the meter or just a 
red-light indicator when ALC is in effect.  On newer SDR rigs you do 
want to see some ALC action as 0dB is not the left-hand side of the 
meter...read your manual.  The procedure below will help you confirm how 
your meter behaves. • How to set the rig output power level • How to set 
rig audio input level (RAIL) – this is the audio level that the rig sees 
from the computer or the external sound card.  We will also be adjusting 
any external sound card like a SoundBlaster or SignaLink (if you have 
one) and the RAIL together.
#1 Set your rig power level to the maximum it can do for CW mode (look 
at the rig specs – could be anywhere from 30-100% of full power).  We 
just need to ensure we have enough “room” in the rig setting so that we 
can feed a signal and not be any more than 50% of the rig’s power 
setting. #2 Set WSJT-X to full power on the Pwr slider #3 Confirm your 
sound card playback device is 0dB and 48kHz sample rate. Recording 
should be the same 0dB, 48kHz. Also ensure your sound card audio 
playback is NOT the default device.  If you make it the default than all 
your windows sounds (beeps, YouTube, etc.) will playback through your 
rig. #4 Set your RAIL to minimum and your playback/outgoing/transmit 
sound card to minimum.  #5 Set the TX offset to 1500 – we want to ensure 
the tone we generate is in this “sweet spot”. #6 Press "Tune" on WSJT-X 
-- at this point you should have no power being transmitted.  If you see 
power go check your settings again. #7 Increase RAIL and external audio 
device level SLOWLY until you get about 20W (assuming your rig is set at 
100W or use 20-50% of whatever your rig power setting is).  Whatever 
your ALC meter says at that point is probably what you want and is below 
0dB. On most rigs the ALC meter won't even move or barely move.  If your 
ALC is more than minimal at this setting, then you probably have an 
Elecraft K3 which wants 4 bars + tickling the 5th bar to operate at 
capacity.  It’s important you understand what your ALC meter is telling 
you.  You do not want any ALC on an FT8 signal (or any other digital 
signal). #7 Bring up RAIL until ALC starts increasing, then decrease 
gain back to where it was.  For the K3 and such you should be able to 
put out max power without exceeding the K3’s 4bars+5th.   #9 Sanity 
check – the Pwr slider on WSJT-X should show 50% power loss for every 
3dB down.  So if you are 100W at full scale (0dB on the Pwr slider) then 
-3db should be 50W and -6dB should be 25W.  If your power does not 
decrease then your audio is still wrong.  You could be overdriving the 
A/D in your rig which may also clip your signal.  An example is the 
FT-450D which has a 60mV input with a SignaLink that provides up to 
100mV.  If you put over 60mV into the FT-450D your signal will clip.  
The Pwr slider in WSJT-X is an indicator when this is occurring since 
dropping from 100mV to 80mV won’t matter to the rig and the -3dB Pwr 
slider setting will still show 100W. #8 Repeat #9 for the lower and 
upper limits of what you believe is your bandpass.  You should see the 
same power level behavior for your entire bandpass.  If you see the 
maximum power level reduce at any point then your bandpass or notch 
filters may be the reason…double check them.  If you run WSJT-X in 
either “Rig Split” or “Fake It” (in Settings/Radio) than your audio will 
always be 1500-2000Hz and actually allow you to work signals beyond your 
bandwidth (i.e. if you can decode them you can work them).

You should now be seeing maybe 80-100% of your rig's power level 
(depending on rig) with no ALC being tickled (or the 4bars+ as explained 
above).  That's how you want to operate FT8 to ensure a clean signal. 
You can leave your rig power level at the maximum and adjust your 
transmitted power from WSJT-X (except for the K3 where you should adjust 
on the rig only due to the K3’s power hunting feature) and your signal 
will always be clean.  Changing rig power on the rig or audio levels in 
the rig or sound card that you have already set may end up triggering 
the ALC again so be vigilant that the ALC is always where it should be.

Hope to see you on the bands with a clean FT8 signal!

https://www.dropbox.com/s/mcj4hklxxgwswsy/FT8Noise5.pdf?dl=1

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