Chapter
22 Contents
Chapter 24
FROM THE
ORIGINATOR'S STANDPOINT
Communication
fails unless our message gets across and is understood. Weak signals and
poor conditions during
transmission
(static, interference, fading) all contribute to partial failure to get
through. In all of these conditions,
telegraphic
communication is vastly superior to voice because almost all its energy
is effectively concentrated within a very
narrow band.
Yet it pays a cost for this by taking more time to communicate the
same words. In addition, it too can suffer
partial loss
due to transmission conditions as well as from just plain accidental misunderstanding.
How can we reduce these
losses to
a minimum? Let's focus on the originator's use of the words themselves
(by "words" we include the use of abbreviations and Q-signals).
FEEDBACK
AND REDUNDANCY
We rarely
think much about how we speak when we are conversing. When we speak face
to face we can generally tell whether we are being understood or not by
feedback through the reactions and responses of the listener. But
when our communication is remote, by voice over a wire or the radio, the
visual clues to the hearer's understanding are missing. When the
telegraph code is the link, auditory clues (tone of voice of a comment
or reply, "uh-huh", "yeah", etc.) are also missing. Relatively awkward
break-in is the only possible direct feedback while transmitting in code,
and it is an ambiguous interruption, until the receiving operator explains
his problem.
It is when
we speak, whether face to face or by remote means, that most of us tend
to use more words than the bare minimum
necessary
to be understood: this is called redundancy. The degree of redundancy
varies from person to person and from
situation
to situation. Redundancy increases the context from which the
hearer to may understand.
When we write we generally are much more careful of how we say things that are important than when we speak. We give more thought to the choice of words and the way we write them: we becomes more circumspect and precise in order to minimize the reader's possible misunderstanding of what we mean. Since we have no feedback at all, we generally tend to use more words than the minimum necessary in order to make up for that lack.
In telegraphic
communication the tendency, largely because of the time required to transmit,
is to eliminate every word which
does not seem
to be absolutely necessary. We abbreviate in various ways-- generally
down to bare bones: the minimum
required to
express the thought. First we leave out words, and then we tend to
abbreviate what is left as much as we think we
dare to omit
and still have it understandable. (This is especially true when paying
on a per-word basis for transmission.)
What we have
been saying is this: redundancy helps to insure adequate
and more accurate communication. That is, we normally use more words
and expressions than the bare minimum required to get our meaning across.
Time, however, is a factor working against telegraphic communication.
It is not as rapid as speech in terms of words per unit time. In
order to balance the time factor against the intelligibility factor, the
originator of a telegraphic message generally weighs more carefully exactly
what words to use and how to put them together. If he is wise he
will also consider the effect of possible mistakes or distortion during
sending and receiving which might produce ambiguity.
REPEATING
AND COUNTING WORDS
What can we
amateurs do to minimize misunderstanding or complete failure of our communications?
One of the commonest things is simply to repeat each word or words, or
the whole message. We may repeat only the most critical words or
numbers two or three times. (Numbers are almost impossible to correct
because there is no significant context to help out.)
Another form
of repetition is to ask the receiving station to repeat the message back
to the sender word by word. This nearly
assures perfection.
But this, like repeating each word as it is sent, requires at least twice
the original time on the air.
Counting the
words in a transmission has long been a common commercial practice, but
is not generally used except for
message type
traffic. It does not assure complete accuracy (exact words and spelling).
USING REDUNDANCY
INTELLIGENTLY
We can often
prevent misunderstanding by adding a word or two to a short communication.
For example, to confirm a scheduled QSO later in the day, to say "CUL this
afternoon," or "CUL in pm" instead of just "CUL" helps insure that the
other operator knows that you mean today, and that you are not cancelling
it (as he might assume otherwise due to some interference, etc.).
When conditions are rapidly deteriorating this may be our only hope to
get across before further communication becomes impossible.
A little forethought
along these lines on the originator's part may help avoid unfortunate misunderstandings.
Especially when
we simply
must get through, and conditions are very poor, we should choose our words
and expressions carefully.
AT THE RECEIVING
END
Here we ask
"Will I be able to copy (or read) it?" and if I can't, "What is the problem?"
-- "What can be done to improve
the quality
of this material I am receiving," or "What can be done to make
sense out of this somewhat garbled transmission
which is all
I have?" -- "What is the nature of the problem?"
During the
communication, speed of transmission is an important factor, one directly
controlled by the sender. Both too fast
and too slow
sending can cause trouble in receiving -- here the receiving operator must
tell the sender to slow down or speed up to meet the receiver's needs.
Quite naturally, speed of transmission must set be within the receiving
operator's capability.
It may
be that the weighting of the dits is too light and I'm missing some of
them. If so, can the sender make them a bit
longer (heavier)?
Maybe the sharpness of the pulses has been rounded off too much to remove
"clicks" and the signals sound
mushy.
At higher speeds, perhaps the dits are too heavy and confusing the ear.
These are things which the sender may be
able to modify
on the spot, but he must be told.
In Chapter
14 "The Ear" we have discussed some of the things which can be done
to help, especially the use of filters. Here
we look at
the filter requirements for an audio filter. We want a filter which
will separate the desired signal and still keep
it intelligible.
At this point we are not concerned with any of the radio frequencies of
the signal as it passes through the
receiver,
but only with the audio beat signal which is output.
That audio signal consists of
The minimum
basic telegraphic element is the "dit, an "on" signal lasting a given length
of time in seconds. For example,
a 10 baud
rate of signaling means that there are ten basic telegraphic elements per
second (or 5 cps or Hertz), and each
element lasts
1/10 of a second, the reciprocal of the baud rate. Obviously, to perceive
a dit or a dah requires silence both
before and
after it. The minimum element of silence (space) is also equal to
one dit. One dit followed by one element of
space constitutes
a square wave two telegraphic elements long and may be called one "cycle,"
by analogy with a cycle of
sinusoidal
wave. (This is expressed symbolically in Chapter
28 by "10".) A continuous series of dits would then for a given
length of
time have twice as many bauds as cycles per second. A sequence of
25 such dits and spaces (10101010..., 50 elements) in one second would
thus correspond to a frequency of 25 Hertz, 50 bauds.
It is in this sense that we compare these two frequencies (audio frequency
and telegraphic keying frequency).
For a filter
the two predominant factors for intelligibility are passband width and
center frequency of the beat note. (The
actual shape
of the filter's frequency-amplitude response curve is also of importance
but for other reasons: see Chapter 24 and
engineering
manuals.)
There must
be enough audio cycles to fill in the keying pulse shape of the smallest
code element, the dit, in such a way that
all code elements
begin and end clearly and are therefore properly timed. That means
that the audio center frequency
(pitch of
the beat note) must be high enough to preserve the square wave shape closely.
A mathematical (Fourier) analysis
shows that
the center audio frequency needs to be about 7 times the telegraphic cycle
rate to give the best shape of telegraphic
pulses.
A square wave
frequency related to words-per-minute, and the duration of one telegraphic
unit can be worked out for English
using the
data in Chapter 28 as follows:-
For standard
English text, there are 49.38 elements per word. This is only 1% less than
the standard 50 elements used as
today's standard
word, so we shall use the 50 element standard here.
If this 50
element word is, for example, assumed to be sent in one second, it will
be at the rate of 50 bauds, or 25 Hertz,
(cps
square wave equivalent). For this example there will then be 60 words
in one minute -- 60 wpm, a high speed. Using this to convert wpm
to bauds we multiply (wpm) by 60/50, that is by 1.2. Since the duration
of one basic telegraphic element is the reciprocal of the baud rate, in
this case it will be 1/50 second.
Now to determine
the minimum audio frequency needed to fill in the telegraphic square wave
shape well and give really high
quality audio
code signals, the following factors must be taken into account:
For our 60
wpm example above, this means an audio frequency of 50 x 7 = 350
Hertz for best quality of code pulses. Thus it
can be seen
that, except for extremely high speed transmissions, there will be
no problem, since the typical values of beat frequency are in the 400 -
1000 Hz. range.
The minimum
bandwidth will be concerned with signal stability and intelligibility limits.
If the bandwidth is too narrow the
signal may
drift out and be hard to find again. If it is too wide the risk of
random noise and interfering signals increases.
The rise-fall
time of a filter to square wave input should not exceed about half a dit
length. Working through the arithmetic
for 6 dB down
shows that the minimum bandwidth for Standard English should not be less
than about 1.33 x (wpm). This is
well below
the bandwidth needed for signal stability, so there is no problem here
for normal CW use.
Finally, if
your copy doesn't seem to make good sense, and there is no way to verify
it, see the end of Chapter 8 "Copying" for
suggestions.
Signal required for CW with 5% character errors is 20 dB below that of double-sideband a.m. A good operator with CW at 15 wpm in presence of thermal noise, a signal to noise ratio (in one kHz bandwidth) of -1 dB is required for 10% character errors and +1 dB for 1% character errors. This latter is 22 dB below double sideband order-wire quality. However, 17 dB below double-sideband a.m. for CW was chosen to account for differences between operators.
Thus:
CW
needs at 0 dB
compare with
SSB needs at +14 dB
(room for improvement)
DSB needs at +17 dB
(5 dB Difference in operators!)
Reference:
Power relationships and operator factor: (QST Fe 1967 p 46, US Army Rept):