http://www.arrl.org/news/features/2008/03/11/10007?nc=1
10 Tips for the PSK31 Digital Mode
By Frank Ravenswood, K2NCC
[EMAIL PROTECTED]
March 11, 2008
There's more to going over the waterfall than just having a good barrel.
An example of a Waterfall display. [photo by Frank Ravenswood, K2NCC]
Use the center of your waterfall (see definitions of italicized terms below)
display. Testing will show that your transmit (TX) and receive (RX) will be
strongest there. Don’t blindly use a 1000 Hz tone or strictly follow the VFO
“set it and forget it” concept. You can easily lose 20% or more of your power
on each edge of your passband. Passband centering of the signal will give the
best results of both RX and TX.
There's no need to have the waterfall streaking bright red. Set your rig's
volume to a low level (less than 25% of maximum) and adjust your waterfall and
sound card levels for good contrast. Do not overdrive your sound card! Get the
background noise and the transmit trace well defined and separate. Keep in mind
that how your waterfall looks does not impact decoding, but it is harder to
work it if you can’t see it.
Use UPPER CASE characters sparingly. Lower case text in PSK31 varicode
transmits fewer bits of data. You'll increase transmit speed and improve the
likelihood of proper decoding on the other end by using lower case text as much
as possible. (For example, the difference between a lowercase “e” (11) and an
uppercase “E” (1110111) is three times more bits.)
Enable your RF attenuation and increase the volume. This can help keep a strong
signal from wiping out the weaker ones. Attenuation will probably be around 20
dB, but by dropping the noise level, the signal readability may improve. AGC
(Automatic Gain Control) does nothing for a weak signal; it only levels the
louder ones.
Use your digital mode’s software, or a program like Spectrogram, to see what
your noise level is with the radio off. This will give you an idea of how
“clean” your sound card is. Typically, onboard (built-in) sound hardware (as
found in most “mainstream” computers like Dell or HP) does not have a
signal-to-noise ratio as good as an inexpensive (less than $50) separate sound
card. When purchasing a sound card, look for one with a signal-to-noise ratio
over 100 dBA. [A-weighted decibels; a scale of sound “loudness’ used in
acoustics that simulates the response of human hearing. — Ed.] [Many onboard
sound interfaces, while not optimum for high fidelity stereo sound, perform
well for many amateur applications. For a technical review of various sound
cards see J. Taylor, K1RFD, “Computer Sound Cards for Amateur Radio,” QST, May
2007, pp 63 — 70. — Ed.]
Consider dual monitors (most modern video cards have two jacks). This allows
you to have the waterfall or spectrum display on one screen and your logger,
text window, etc on the other. It makes a huge difference in speed and
ease-of-use when you don't have to swap between screens or use smaller windows
for your contact.
When transmitting, keep your ALC reading as close to zero as possible. This
will help keep your signal clean and your IMD at an acceptable level (-20 dB or
better is ideal). Your power output will drop, but there's no need to overload
the transmit level. PSK31 uses an 80% duty cycle. Even with a full duty cycle
rig, it still needs to dissipate heat! Besides, 20 W more makes little
difference. Output of around 50 W is enough to work the world, and your fellow
operators will appreciate the courtesy. Also be sure your voice processor is
not enabled when using digital modes.
Ask for an RSQ (readability, strength, quality) report. When in a contact, send
just a tone and ask for your IMD and a report on how your trace looks. This
will give you a better idea of what adjustments may be needed.
There are hundreds of digital modes. To get started or to learn more about the
most common ones, refer to the list of digital mode references at the end of
this article.
Bonus Tip:
Try 30 meters PSK31. It’s a robust band, offering the best of 20 meters and 40
meters. Only digital modes and CW are permitted and contesting is not allowed.
Be sure to operate within your privileges. PSK31 can typically be found around
10.140 MHz.
Definitions of Terms:
AGC (automatic gain control): The ability to reduce signal strength on-the-fly,
giving you more level audio reception on stronger stations.
ALC (automatic level control): A voltage adjustment or reading, indicating your
transmit signal levels. ALC is designed to control voice and carrier signal
levels, not digital modes. Typically for digital modes, if the ALC meter moves,
then the microphone gain is too high.
Signal-to-noise ratio (S/N): A comparison of the signal levels to the relative
noise level. Ideally, a perfect signal would have no noise, but realistically,
you’ll want an S/N ratio well within the tolerances of the mode you’re using.
PSK31 tolerates about a 10 dB S/N ratio.
dB: a logarithmic unit of relative measurement that expresses the ratio of two
levels.
Digital mode: Any method for conveying digital information using radio signals.
Much like a computer modem, a digital feed is converted to analog, sent across
a transmission medium, then reconverted back to a readable signal at the
receiving station.
Duty cycle: The total time during a transmission period that the transmitter is
delivering power to the antenna. When your radio is transmitting, there’s an
on/off process that takes place. Transmitting at a 100% duty cycle indicates
that you are using 100% of your radio’s power, 100% of the time. Better radios
will allow this, while others will eventually fail under the pressure of such a
load.
IMD (intermodulation distortion): The ratio, in dB, used to determine the
quality of your transmission. Unwanted modulation products or signals will
reduce IMD levels. More power does not mean better copy!
Overdrive: Turning the volume of your radio up so high that you risk damage to
the sound card, or cause signal “splatter.” Similar to maintaining your ALC
levels.
Passband: The range of frequencies that your transceiver can receive when on a
single frequency; typically around 3000 Hz wide.
PSK (Phase Shift Keying): A form of modulation that shifts the phase of the
transmit signal in order to carry more information. PSK31 is a digital mode,
created in the 1990s by Peter Martinez, G3PLX, which has a 31.25 Hz bandwidth
on your waterfall display.
RF (radio frequency) attenuation: A suppression of signals received. You’ll
often see a noise level reduction, with a minor sacrifice to the desired signal
reception. Check your radio’s manual on how to adjust it.
RSQ (Readability, Strength, Quality): Much like the familiar “RST” reports,
using a 599-type reporting scheme. Instead of “Tone” (Morse code), use
“Quality.” 95%+ readable, with a very strong waterfall trace and a clean (no
splatter) signal would warrant a 599 report.
Sound card: A piece of hardware in your computer that acts as an
analog-to-digital or digital-to-analog converter of audio frequency signals. A
microphone input is often included.
VFO (variable frequency oscillator): An oscillator whose frequency is
controlled by varying the value of either the capacitance or inductance of its
tuned circuit. In analog transceivers the main tuning dial is used to adjust
the VFO’s tuned circuit capacitance or inductance and thereby the transceiver’s
operating frequency.
Varicode: A streamlined coding system that uses letter codes that are shorter
than those of the ASCII or Baudot coding systems
Waterfall: A visual display of radio signals (and other sounds) found on the
tuned frequency.
For More Information:
S. Ford, WB8IMY, The ARRL HF Digital Handbook, 4th edition. Available from your
local ARRL dealer, or from the ARRL bookstore: www.arrl.org/shop
R. Prosch, DF3LZ, Technical Handbook for Radio Monitoring I
S. Brown, HB9DRV, Digital Master 780: Spectrogram and other software.
Frank Ravenswood, K2NCC, earned his Technician license in June 2005 at SeaPac
Hamfest, Seaside, Oregon. His previous radio experience was in the Army
(05D/98H, EW/SIGINT identifier). After leaving the Army he became a computer
repair technician.
His HF base station is a barefoot 1982 Kenwood TS-830S into a Cobra UltraLite
JR. Recently, he has been operating mostly 20 meter PSK31 using the SignaLink
interface and hosting the W7OTV streaming audio Internet repeater.
As you might tell from his vanity call sign, he’s a fan of Star Trek and likes
to play chess online. Feel free to e-mail him for an online chess game or to
schedule an HF attempt to Oregon.
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