Hi,
I am writing this note because I am frustrated when running PSK31 on
the SDR Console. It is ironic as the SDR console has the ability to
be the best PSK31 ever developed as changes can be made in
software. By relating my experiences I hope Flex-Radio and
associated developers can achieve this goal.
Now don't get me wrong. The SDR console is a good PSK31 transceiver
for the casual operator. It's where you need to do contesting or
work DX that other transceivers are better. The areas that can be
improved are in the way filters are managed, the delay in the time
between the SDR Console and the third party software initiate
processing, and in the available displays and how they present
information. These items can be vastly improved in software.
I have been operating PSK31 since 1999 using the latest software and
my own interfaces and enjoy DX and contesting. My current setup
consists of the SDR console operated at 35 watts on an AMD 2.1 Ghz
computer, a linear amp running 400 watts occasionally used, and MixW
software. Alternatively, I use a K2/100 and OMNI-VI. I am using the
SDR v1.6.0, WA6AHL Apr 23 release. I am using the FireBox sound card
sampling at 96000. The VAC and virtual com ports work well except
the CAT fails after a period of time(known bug). The first work
around I need is to reset CAT when it fails. I even found that click
tuning fails after a period of time but I have not fully investigated
this problem or reported it. I set the power control at 100 watts
and control input/output using the Setup/Audio/VAC/Gain controls.
First let me address filter management. I have been using DigU on
PSK31. If you use the variable filters, I use Var1 set at 100->3000,
the DIGU SSTV click tuning will center the frequency at 1200 Hz which
is fine, However if you need the narrow the band pass to copy a weak
signal, you have problems. The SDR console is superb on weak signal
detection as you can manage AGC, but MixW has fixed AGC and it
reduces the gain if there is a strong signal anywhere in the filter
band pass. So you need a narrow filter to get the strong signal out
of the band pass in order to copy a weak signal. I have Var 2 set at
1100 -> 1300 Hz. However, the click tuning will not center the
frequency at 1200 Hz any more. The second work around I need is not
to use click tuning when I use the narrow filter. So I tried the
fixed filters. The click tuning centers the signal at 1200 Hz when
the 2.6 or 2.1 KHz filter are selected. No problem so far. But if
you need a more narrow filter, the click tuning no longer will center
the frequency in the center of the filter, so you are have the same
problem as before. As a fix, I recommend that when using the DigU/L
modes, that when using the narrow filters, either Var or fixed that
the console software do something similar as is done for CW. Center
the filter around the signal. 1200 Hz is better 600 Hz (for example)
for the digital modes. If you use 600 Hz, there is a good chance
that you will transmit audio harmonics: 1200, 1800, 2400 Hz, so you
need to use the transmit filter. I have two profiles defined,
Digital: 400 -> 2500 Hz and Digital Narrow: 400 ->1000 Hz. You need
to go through two layers of menus to select it,
Setup/Transmit/profiles. For awhile Jeff had the TX filter on the
front of his console, but moved it on the last release. If you have
selected the narrow filter, the next time you transmit you need to be
sure you are below the cutoff frequency. Just centering the filter
around the signal would allow the operator to use a frequency and
filter he wanted and solve this problem. This centering of the
frequency should apply to both the fixed filters and the variable
filters. You can only define two variable filters; it is too
inconvenient (read hard) to define the filters using the sliders on
the fly. I have copied PSK 31 using the 25 Hz filter, and copy is
superb using the 100 Hz filter.
There is the issue of processing delay. I use the waterfall on both
the SDR console and MixW. Presumably there is a negligible delay
between a station starting to transmit and the signal appearing on
the waterfall display. When it does, there is a possibility it is a
new CQ, and during a contest, the CQs will be short. There is a
noticeable time delay between the time the signal first appears on
the Console waterfall and the MixW waterfall (or the MixW spectrum
for that matter). So you have to wait until it appears on the MixW
display, then click it to lock in and start copying. By this time,
the station has stopped calling CQ, and you miss him. Similar to the
receive time delay, there is a measurable time delay between pressing
the MixW TX button and the SDR console starring to transmit. If you
are working a contest or DX, you need to be at least as fast as
everyone else in order to get the contact. I have minimized the
delays as much as I can by using the smallest buffer selectable on
VAC and the smallest buffer useable on the FireBox. Using the
smaller buffers on the FireBox affects the processing, and the
smaller ones are not useable. So what we need is some attention
given by the developers to minimizing the time from the receipt of a
signal and the ultimate processing of the signal by a third party
software. I am not recommending building PSK31 or any digital
mode into the SDR console. Developers have devoted a career (since
1999) building digital mode software, and few have got it right.
There is also the issue of color resolution on the SDR
waterfall. MixW has 9 colors for the various signal levels on the
waterfall; the SDR console has three. I use 6. You need the better
color resolution to see the received signal sidebands and weaker
signals. A bad signal will have higher sidebands which are
observable on the MixW waterfall, but are not observable on the SDR
console. On MixW, the color levels are not adjustable; they are at a
fixed level, probably 10 dB apart. You could do the same thing on
the SDR console but have the lower level selectable. I recommend six
levels 10 dB apart with the lower level adjustable.
The transmit display could be improved. On the waterfall now, it
just is completely consumed with signal. What is needed is a
replication of the received signal on the transmitted display to the
extent that the levels of the sidebands can be observed. This is an
indirect measurement of IMD; a direct measurement of IMD and can
should be made for best results, . The heat level, a measurement of
dissipation of power in the power amplifier can be added depending on
the feasibility within the existing hardware envelop. The efficiency
of the power amplifier is less transmitting basically the two tone
signal of PSK31 than it is for CW or RTTY, transmitting a single tone
signal. The drop in efficiency from 80 to 60 percent, for example,
would increase the power dissipation in the power amplifier from 20
to 40%, or watts, if 100% were 100 watts. I happened to have burned
out the power amplifier in a transceiver, not knowing this. This can
easily happen on the SDR PA.
All of these items are offered in the spirit of improvements to the
basic SDR console to make it undoubtedly the finest digital mode
transceiver ever. Some others may have some other ideas of what are
basically enhancements to the SDR Console, and should submit them to
get them included.
73, Chas w1cg
- [Flexradio] Observations on PSK31 and other dig... Charles Greene
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