BUX Bitz, Bytez, & Baudz
"Home of the RASCAL"
HAMs "Helping All Mankind" Buck Rogers K4ABT
BUX Bitz, Bytez, & Baudz
"Home of the RASCAL"HAMs "Helping All Mankind" Buck Rogers K4ABT
A Handbook for PSK31 Users complete with transceiver to PC sound card interface diagrams. © 1999-2004 By Buck Rogers K4ABT
IT’S A “RAVE:”
To speak of it any other way, or in any manner other than; It’s a rave, would
be understating its presence and impact on the world of hobby communications.
PSK31 is sweeping through the SWL and Amateur Radio ranks like a brushfire.
Not since the early days of FM repeaters has any mode inspired so much
interest in HAM radio and short wave radio monitoring than PSK31.
If you have a short-wave receiver that supports single-sideband reception, and
a PC (486/100 MHz or faster) running Windows 95, 98, or XP, equipped with a
Creative Labs™ compatible SoundBlaster (SB16 or later) sound card, then you
have the ingredients to receive PSK31.
Figure 1
At figure one, I’ve drawn a very simple and basic interface diagram that makes
receiving PSK31 easy for anyone with the two basic items I’ve listed above.
You’ll need the FREE software which I’ll talk about later in the text, but
first, I need to explain what PSK31 is.
What is PSK31?
As the name implies, PSK (phase shift keying) modulates the phase of a
carrier, and the number ''31'' references the actual bandwidth (31 Hz)
occupied by the PSK31 signal.
The software that implements PSK31 with a Windows PC and soundcard is a
program written and developed by Peter Martinez G3PLX. Two of the most
significant features that make this the ideal mode for digital communications
is the extremely narrow band width, and the fact that it is highly immune to
noise and QRM.
Up front, phase modulation has more advantages than (morse code), CW. CW uses
amplitude (On/Off) keying. In a noisy or distorted propagation environment,
the amplitude of a signal will shift and vary much more than the phase of a
signal. When compared to CW, PSK31 is a much more resourceful, and robust
operating mode.
The baud rate used by PSK31 is 31.25 baud. This is fast enough to handle most
operators manual typing capabilities, a speed of about 50 words per minute. It
is intended as a means of keyboard to keyboard communication between two or
more operators (oh happy day, we’re talking to each other again), using a very
small amount of frequency spectrum.
By comparing the small bandwidth of PSK31 and measuring its gain against a CW
filter of 500 Hz; 10 * log (500/31) dB = 12 dB, quickly reveals that a CW
transmitter must put out 15 to 18 times more power than a PSK31 transmitter,
just to achieve the same signal to noise ratio at the receiving station. This
is the reason the PSK31 operating mode has gained so much popularity in such a
very short period.
The end result is that a PSK31 station can operate with reduced power and
smaller antenna installations. As an example; My 20 meter, 14.073 MHz,
antenna, is located in the attic of my garage (about 14 feet above ground),
and my DX70H runs around 35 watts output.
Now comes the real tease, just for the DX hounds among us. In the last three
months, I’ve work over 75 countries, and each morning I spot a new one that I
have not yet worked. It gets better! For some reason… I think it’s because
PSK31 is so much fun…, I’m finding more DX stations on PSK31 than I’ve ever
encountered on SSB and other modes combined. At any given moment, day or
night, it appears that 20 meters is always open.
Twenty meters is not the only band that has PSK31 activity, but it is a good
place to start when you first begin operating PSK31.
AND, IT’S ALMOST FREE:
When the prospective PSK31 user discovers PSK31 for the first time, a look of
both extreme pleasure, and amazed disbelief appears on their face. WOW, we’ve
just discovered a mode that is so much fun, yet, it’s almost FREE! After a few
days of operating PSK31, the internet becomes obscured in the mist of all the
fun we’re having “doing” PSK31.
Yes, there are a large number of PSK31 programs that support the PC, LINUX,
and the MAC. We’ll also cover them later in this article.
KUDOS ARE DUE THE AUTHORS OF PSK:
Before I get into interfacing our HF transceivers and the sound cards, I would
be remiss if I didn’t give credit to the author(s) of PSK.
Here, in his words is the background and the PSK31 philosophy, of and by Peter
Martinez G3PLX.
PSK31 is the result of my belief that the present batch of "data" modes have
left a gap in amateur radio operating, the gap that was previously filled by
AMTOR or even traditional RTTY, in which two or more operators chat to each
other on an open channel. Modes such as packet radio, Pactor, and others, are
highly complex, are unsuited to multi-way conversations, and in particular,
the long block lengths introduce an unacceptable delay in the processing of
text such that even normal conversation is unpleasant and quick-break
question/answer sessions are impossible.
The move to automated unattended message forwarding has left a gap in the
person-to-person communication field, and PSK31 is an attempt to remedy this
situation with a simple but efficient code structure coupled with the
narrowest possible bandwidth, and with only enough error-correction to match
typical typing-error rates, and with no time-consuming synchronization,
changeover, and ARQ processes.
The 31 baud BPSK modulation system used in PSK31 was introduced by SP9VRC in
his SLOWBPSK program.
Instead of the traditional frequency-shift keying, the information is
transmitted by patterns of polarity-reversals (sometimes called 180-degree
phase shifts).
This process can be thought of as equivalent to sending information by
swapping-over the two wires to the antenna, although, of course, the keying is
more usually done back in the audio input into the transceiver. A
well-designed PSK system will give better results than the conventional FSK
systems that amateurs have been using for years, and is potentially capable of
operation in much narrower bandwidths than FSK. The 31 baud data rate was
chosen so that the system will handle hand-sent typed text easily.
There is a problem with PSK keying which doesn't show up with FSK, and that is
the effect of key-clicks.
We can get away with hard FSK keying at moderate baud rates without generating
too much splatter, but polarity reversals are equivalent to simultaneous
switching-off of one transmitter and switching-on of another one in
anti-phase: the result being key-clicks that are TWICE AS BAD as on-off
keying, all other things being equal.
So if we use computer logic to key a BPSK modulator such as an exclusive-or
gate, at 31 baud, the emission would be extremely broad. In fact it would be
about 3 times the baudrate wide at 10dB down, 5 times at 14dB down, 7 times at
17dB down, and so on (the squarewave Fourier series in fact).
The solution is to filter the output, or to shape the envelope amplitude of
each bit which amounts to the same thing. In PSK31, a cosine shape is used. To
see what this does to the waveform and the spectrum, consider transmitting a
sequence of continuous polarity-reversals at 31 baud. With cosine shaping, the
envelope ends up looking like full-wave rectified 31Hz AC. This not only looks
like a two-tone test signal, it IS a two-tone test signal, and the spectrum
consists of two pure tones at +/-15Hz from the center, and no splatter. Like
the two-tone and unlike FSK, however, if we pass this through a transmitter,
we get inter-modulation products if it is not linear, so we DO need to be
careful not to overdrive the audio.
However, even the worst linear will give third-order products of 25dB at
+/-47Hz (3 times the baud rate wide) and fifth-order products of 35dB at
+/-78Hz (5 times the baud rate wide), a considerable improvement over the
hard-keying case. If we infinitely overdrive the linear, we are back to the
same levels as the hard-keyed system.
There is a similar line of reasoning on the receive side. The equivalent to
"hard-keying" on the receive side is a BPSK receiver which opens a gate at the
start of a bit, collects and stores all the received signal and noise during
the bit, and then "snaps" the gate shut at the end. This process gives rise to
the receive-side equivalent of key-clicks, namely side-lobes on the receiver
pass band. So, although this "integrate-and-dump" method is 100% efficient in
the task of sorting out signal from noise, it will only reject signals by 10dB
at 3 times the baud rate wide and so on, the same spurious rejection figures
that we got as spurious emission figures for the transmit side.
The PSK31 receiver overcomes this by filtering the receive signal, or by what
amounts to the same thing, shaping the envelope of the received bit. The shape
is more complex than the cosine shape used in the transmitter: if we used a
cosine in the receiver we end up with some signal from one received bit
"spreading" into the next bit, an inevitable result of cascading two filters
which are each already "spread" by one bit.
The more complex shape in the receiver overcomes this by shaping 4 bits at a
time and compensating for this inter-symbol interference, but the end result
is a pass band that is at least 64dB down at +/-31 Hz and beyond, and doesn't
introduce any inter-symbol interference when receiving a cosine-shaped
transmission.
Contact Information:
Peter Martinez G3PLX
High Blakebank Farm
Underbarrow
Kendal
Cumbria LA8 8BN
United Kingdom
My thanks and with credits to Peter for permission to reprint his PSK31
philosophy here.
THE QUICK BROWN “VOX”
For this discussion, we will NOT consider using the VOX (voice operated relay)
circuits in our transceiver to control PSK31 changeover from transmit to
receive and vice versa. Especially since most of us have a spare or extra
serial comport(s) that we can use to do the same thing, and without having the
transceiver hiccup a transmitted signal every time the sound card burps. As
you will soon see, there is an easier way to handle the push-to-talk (PTT)
function in PSK31.
ATTENUATOR; YES OR NO:
Yes, we should consider the use of some level of attenuation either into the
sound card or out of the sound card into the transceiver. I think the easiest
way to approach this is to understand first that modern day transceivers are
designed for inputs into the transceiver for SSB, FM, AFSK, RTTY, and in some
cases, SSTV. We should maintain the levels similar to those we normally use
when using a microphone.
Since many transceivers that we will be using are beyond ten (10) years since
their purchase, some of us will be using the microphone port for our PSK31
input. If this is the case, and you plan to drive the microphone with your
sound card (LINE OUT), then the level from the sound card should be comparable
to the output level of your microphone.
But I’ve already said that haven’t I? YES, I have, but I want to stress a
point. In this case, we will need to employ padding of about 40 DB. I refer to
these attenuators as “pads.” This term comes from a time when we referred to
it as “padding the signal down.” For the new HAM, padding down of a signal can
reference either audio or radio frequencies (AF / RF).
Figure 2
To keep it simple, I’ve drawn an attenuator “pad” at figure two. Again, it is
a device that is simple to build since it employs only two resistors. Unless
your sound card is a slam-dunk’n, speaker popper, you should be able to get by
with quarter watt sized resistors.
While many sound blaster™ compatible sound cards have the capability to be set
for high or low level outputs, they are almost always providing outputs which
are much to high for the input level to the microphone port of our
transceiver.
If you cannot use the sound card “Control Panel (settings)” in Windows (or
your WINPSK, or other software) to decrease the output level of your sound
card to meet the level requirements of your microphone input, then you are
about to construct the resistive attenuator shown at figure two to reduce the
LINE OUT level to your transceiver MIC port. The idea is to keep your signal
at a low enough level to have a clean PSK31 signal on the air.
With PSK31, the first thing I do is set my transceiver for normal single
side-band (SSB) operation. I turn off any speech processing and after I have
the microphone level properly set for PSK31, I don’t change this setting
again.
All future levels are set into and out of the sound card using the volume and
wav controls built into the sound card “control panel” software. In DigiPan
II, these controls are easily accessed from the task bar at the top of the
screen. Simply click on “configure” and use the “Transmit Drive” (Photo B),
command to properly drive the transceiver so that no ALC action is observed.
This is how I avoid causing phase distortion or overdriving the inputs of my
transceiver.
While we’re discussing sound card I/O levels, in case of feedback (usually "motorboating"),
which may be caused by the common ground of the soundcard input and output,
“MUTE” the “line in” ON THE "VOLUME CONTROL" panel under Windows or in WINPSK
or digiPan configure/transmit/check ‘Mute’. This change will stop the sound in
your PC speakers, but it may also stop the motor-boating of your PSK31
transmit signal.
Let me say it again, in another way, so the message is well understood. The
Microphone and Line inputs on the Volume Control panel, controls the SOUND to
the speakers (not the spectral display) and muting these inputs will render no
sound (receive) from the computer speakers, but the transceiver speaker can
still be used to audibly monitor the PSK31 signal if you so desire.
If you are using the external speaker jack for receive audio to the sound
card, then a 3.5 MM plug and two-jack splitter may be required to feed both an
external speaker and the sound card. This is because the internal transceiver
speaker will be muted by the closed-circuit (opened) jack.
LET’S GET BUSY:
In figures 3, 4, 5, 6, and 7, I’ve drawn several full blown interface drawings
and diagrams for specific makes and model transceivers. Please notice that in
most of the diagrams, I’ve use transformer coupling for both transmit and
receive audio paths/signals. In addition, I’ve employed ferrite devices to
further isolate and prevent any RF from reaching the transceiver input or the
sound card LINE IN or LINE OUT ports.
If you are taking the constant level output of audio from your transceiver and
driving the sound card then you can rest assured you need to be very critical
of levels into the sound card. Read your transceiver manual as it should tell
what the levels are at the various inputs and output jacks.
READY, SET, PSK31:
The following is a list of frequencies (in kHz) where you may find more PSK31
activity are:
18100.150
21080.150
24920.150
28120.150
Because I am most familiar with WINPSK 2.12 PSK31 software, I will use it as a
reference during my explanation for setting up the transceiver and sound card
levels.
First we’ll set the soundcard output level to approximately one third to
half-scale (click the speaker symbol on the Windows taskbar). Connect a dummy
load to the transceiver antenna output and click on the software ‘TX’ menu
button.
Figure 4
Next, let’s make sure we have our
transceiver set for USB (Upper side-band) operation, and the AGC set for Slow
(recovery). We’re now set to begin our on-the-air (dummy-load) setup.
Using your favorite watt meter and dummy load connected to the output of your
transceiver, click on the PSK software ‘TX’ menu button and adjust the Speaker
volume slider on the Windows taskbar tray for an output power reading of about
one third to one half the power you would normally see when operating CW. DO
NOT operate your transceiver at more than half its maximum power level rating!
Once you have the power level set, type a few characters and watch the power
meter. As you type, the text you are sending should cause the meter pointer to
have a slight jitter. If you have the LINE OUT/MIC IN level set properly, you
should see the power move slightly upwards. At no time should you see the
power level go above 50% or your transceiver’s rated power output. If it does
go higher, simply turn the Transmit Drive level down, using the “SETTINGS,
/transmit Level Icon in WINPSK 2.12.
SOFTWARE, SOFTWARE, EVERYWHERE:
There’s plenty of software to help you get on the air with PSK31. You can
choose the one you like best and of course some of your decisions will be
based on the particular operating system for your computer.
Be sure you read the PSK31 software user’s manual or the “help” files that
relate to the software you plan to use.
In the next few paragraphs, I’ll try to name a few software packages for PSK31
operation. Before I go there, I’d like to point out that so much of this
software is FREE, however, the software writer and author cannot live by bread
alone; He/she must have peanut-butter, and peanut-butter cost money! Which
ever software you finally choose, remember the author of that software package
dedicated his or her time to making your hobby more fun. If you like the
software, send the author a small donation as a sign of your appreciation for
their contribution to making your hobby fun and exciting. By sending a small
contribution to the software author, you are also encouraging them to extend
additional effort to add more features and updates for your future enjoyment.
WINPSK uses digital signal processing (DSP)
to do all these features and then decode the resulting PSK31 sounds into
characters on the receive screen. The display shown at photo A, is how a
typical PSK software spectrum (panoramic waterfall) window appears.
A bright yellow bar with the diamond-shaped cursor in it is a PSK31 station.
To the left and right of the station select cursor, are other PSK31 stations.
Some appear weaker and others will display brighter, which indicates stronger
signals. Although some signals in the display appear weak, they will still
print perfect copy.
The most recent receive signals are displayed at the top of the PSK software
waterfall display. To the right of the cursor, is another PSK31 station that
is almost too weak to copy.
To tune and receive any of these stations, it is only necessary to point to
the station signal with the mouse and click! The diamond-shaped cursor will
jump immediately to the center of the station signal and the text from the
station will begin to appear on screen in the receive window.
During an idle moment, you can read the IMD of the selected station on the
lower feature bar in most PSK software programs. The IMD reading of the
station displayed in photo “A” is minus 23.5. A good signal and ideal IMD is
between minus (-)22 and -32.
Other sections of the software screen are devoted to buttons to control the
functions of program, and select keys for the activation of a large number
macros. The “text” button on the task bar enables the operator to choose any
number of text files to send, including the familiar “BRAG.TXT” that all us
ole RTTY ops are familiar with.
Just a word to the wise; In order for the program to provide a panoramic
display with point-and-click tuning of stations, it is necessary for our
transceiver’s to provide both panoramic reception and transmission. The
receive band pass should be capable of receiving a large number of PSK31
stations at one time. So far, all the transceivers I’ve operated or
interfaced, have this capability.
Figure 6
© 1998-2004 G. E. "Buck" Rogers Sr. E-mail BucK4ABT
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