I will not enter into a discussion of
typographical terminology here, attempting to explain the finer differences and
relationships between characters, character sets, symbols, glyphs, alphabets,
fonts, types, typefaces, etc. Suffice it to say that the term "font", as the
word literally suggests, originally refers to metal printing types that are cast
in a foundry, and are of a single style and size. So, the ubiquitous
"Arial" is not a font but a typeface, that is: a font family; but
"Arial 14 points
bold" is a font. However, with the advent of computer-based
word-processing, the word font has assumed the meaning of typeface. In this
section, I will use the incorrect "new speak" and use the wordscharacter and font in general.
Over the years (ca. 1929 - 1960), several official Hellschreiber fonts have been designed. They all have in common that they were intended to maximize legibility (even with poor signal quality),
preclude misinterpretation (mistaking one character for an other - even with poor signal quality), and minimize required signaling bandwidth. For this reason, Hell fonts are "upper case" only (capital letters), and some characters have an awkward shape - on purpose! In addition, Hell fonts are mono-spaced fonts: the bitmap of all characters of a particular font has the same width. Unlike a proportional font, in which for instance the letter "i" is narrower than the letter "w". Not so in Hell fonts.
Of course today's PC software implementations of the Hellschreiber can use any font set, and the Hell-system as such has no limitations in this respect. Obviously modern "PC" fonts were not designed with legibility and bandwidth limitations in mind - which is why they should not be used for Hellschreiber communication. Not abiding by the minimum pixel duration rule unnecessarily increases
the occupied bandwidth
of the transmitted signal (easily as much as 4 times!)
The Hell font is designed by rasterizing scripted characters. The resulting matrix can then be transmitted column-by-column (or row-by-row) as a pixel sequence:
Rasterization
of the figure "3"
Serialization of the rasterized figure "3"
The characters of all Hell fonts consist of a matrix of dots or "pixels" (picture elements). That is, they are simple bitmaps. Hence they are "bit-map fonts" (a.k.a. "raster fonts")avant la lettre, as opposed to stroke fonts and outline fonts (a.k.a. vector fonts).
The Hell fonts are presented below, using the following characteristics:
dot-matrix dimensions: the number of rows and the number of columns of the character bitmap. In all the documentation of the Hell company and of Siemens-Halske, the number of columns is referred to as the number of lines (with a few exceptions).
scandirection: the pixels of the Hell font are transmitted sequentially, by "scanning" each column (see the "How it works" page). First the left-most column is scanned, then the next column to its right, etc. Scanning can be done by in a top-down manner, or bottom-up. Yes, there are other ways to do this (see further below).
inter-character space: in Hell fonts, the space between characters is created by leaving the first and/or last column(s) of the matrix blank (white). In most cases, this is the first and the last column, but some Hell fonts use the three last columns or the first two plus the last column. Note that Hellschreiber printers (other than synchronized "start-stop" models) use a two-turn spindle that prints two identical parallel lines of text. For these systems, the top and bottom row of the font are also left blank. This is done , so as to avoid that the top of the lower text line, touches the bottom of the upper text line.
start-pulse: some Hellschreiber models use a start-stop system. Their font conveniently "hides" a start-pulse in the normally blank first column: no need to expand the dimensions of the font matrix. This start-pulse only uses part of the first column. There is no need for a stop-pulse: upon reception of a start-pulse, the drive shaft of the Hell-printer makes a fixed number of revolutions and then stops. Note that the column that contains the start-pulse, is not printed. This reduces the space between printed characters by the width of one column.
character transmission rate: for hand-sent transmissions, the standard Hellschreiber rate is 2.5 characters/sec (150 chars/min); machine-sent transmission rates are 5 cps (300 cpm), 5.5 cps (330 cpm), and 6.1 cps (396 cpm).
pixel rate ("Punkfrequenz"): to limit the required signaling bandwidth, each Hell font has a specific minimum pixel duration. Hence, there is a minimum duration of a pixel cycle (1 black pixel, followed by 1 white pixel, or vice versa). The minimum is applied to both black and white pixels, and not only within columns, but also at the transition from column to the next. Combined with the number of characters that are sent per second, this minimum pixel cycle duration determines the maximum pixel rate. In turn, the latter rate drives the required signaling bandwidth (not to be confused with the "actual" or "occupied" signal bandwidth, that may be much larger if improper modulation is used). The required bandwidth is typically three times the pixel rate (so as to include at least the third harmonic), though extensive experiments of the Hell company and German government institutions determined that no more than 1.6 times the pixel rate is required, and a minimum of 1.2 times the pixel rate (bandwidth limiting on the transmitter side, not filtering at the receiver).
Before delving into the fonts, I would
like to point out one consequence of the scanning direction. At the heart of
all Hellschreiber printers, there is a spinning helix. In all
Hellschreiber printers, this helix spins in a clockwise (CW) direction (such
that the spindle does not push against the movement of the paper tape). In
all Hellschreiber printers, the paper tape passes this spindle from right to
left. See the figure below. The printer spindle must "scan" in the same
direction as the Hellschreiber-sender: top-down or bottom-up. Otherwise, the
text is flipped vertically: printed upside-down and mirror-image.
Opposite scan
direction
-
upside-down
& mirror-image
Spindles for top-down scanning must have a
regular "right handed" thread (a screw with a right-handed thread is tightened
by clockwise rotation). Conversely, bottom-up spindles must have a "left handed"
("British") thread. The combination of scan-direction and paper-direction causes
"top-down" fonts to lean to the left when printed. Conversely, bottom-up
fonts lean to the right when printed. Note that this has nothing to do
with slanting of the printed text lines, that occurs when the motor in the
Hellschreiber-sender and printer run at different speeds.
Top-down fonts are left-leaning when
printed-
bottom-up fonts are right-leaning
Finally, it should be noted that
Hellschreiber printers (other than start-stop models) are extremely simple
devices, and are dumb.They print all received signals
- whether pixels, or noise pulses that are sufficiently strong and long. The
Hell-printer spindle simply scans columns with a fixed speed. The printer is not
designed for any particular font. Any font that is sent with the same column
duration, will - in principle - be printed correctly. This is independent of the
number of columns in the font matrix (which may be between 1 and infinite), and
the number of pixels per column (provided the pixels are long enough with
respect to the engage & disengage time of the printer's electromagnet).
Below, the Hellschreiber fonts are not
presented in a strict chronological order, but by the size of the font matrix.
On the "Sounds from Hell" page, you
can hear what some of these fonts sound like during transmission via tone
pulses.
14-LINE FONT
Rudolf Hell's
"first generation" printers (1929) were electro-chemical: they printed on
chemically impregnated paper tape. The paper had to be moist, so as to be
conductive to electrical current. Where current passed through the moist
yellowish paper, the color of the chemical compound changed to Prussian blue.
This generation of Hell-printers did not use a
printing helix. Instead, it had 14 styluses, placed into a column across the
paper tape, touching the tape. Current could be sequentially applied to each
individual stylus ( = scan). The current circuit is closed via the moist paper
tape and a metal roller underneath the tape. The photo below shows a print-out
that was generated with this type of Hell-printer.
Sample of an
electro-chemical Hellschreiber print-out with a 14-row font
(source: figure 1
in ref. 1)
Obviously, with a 14-pixel column,
the resulting font consisted of 14 rows. The available documentation does
not indicate the number of font-columns. From inspection of the photo it appears
that the font had 9 pixel-columns, and 2 blank columns for spacing between
characters. However, the photo only covers half of the alphabet, and repeated
characters do not have the same length (e.g., the last two letters "E" in the
second word).
The second generation Hellschreibers
printers were not electro-chemical, but electro-mechanical. The
printers used a spinning helix (spindle) to obtain a scanning movement across
the paper tape. At this time, Rudolf Hell had already licensed his
"Hellschreiber"
patents to the Siemens-Halske company, who became the Hellschreiber
manufacturer. Initially (1931), these new printers were
carbon tape
printers: they used a thin carbon-ribbon between the paper tape and the
spindle. In 1933 the
carbon-ribbon approach was abandoned in favor of a simple felt ink-roller that
rests on the spindle. A 12-line font was used with these printers, and starting
in 1933/34 also with the first "Presse Hell" printer model (T
empf 12).
12x12 font - carbon-ribbon Hell-printer
(source: figure 3
in ref. 1)
There are some inconsistencies in the available
literature, regarding the dimensions of this font:
12 columns, 12 rows: with the
first 9 columns used for pixels, and the last 3 columns used for spacing
(Rudolf Hell, p. 3 & 5 in ref. 1; ref. 4)
12 columns, 12 rows, with 9
columns used for pixels, but the first 2 columns and the last column
used for spacing (ref. 2, 3)
12 columns, 13 rows.
According to ref. 4, the shortest line segment within a column was 1/78
of the total number of possible pixels, This implies that the font had
13 rows (12x13=156). It would also make this the first application of
the so-called "2-pixel rule": within a column, and between successive
columns, black or white line segments are at least two font-pixels in
length (156/2=78).
12x12
rasterization
12x13 rasterization
(source: Figure 7 in ref. 1)
(source: Figure 3 in ref. 4)
12-line
font ("12-Linien-Schrift") - left-leaning
(source: Figure 92 in ref. 2, Figure 2 in ref. 3)
According to ref. 4, this font was used for manual
transmission at a rate of 2.5 characters/sec (150 cpm), and
machine-transmission (punch tape) with 5 chars/sec (300 cpm).
character transmission rate:
5 characters/sec (300 chars/min)
pixel rate: 5x(144/2)=360 Hz; telegraphy speed: 5x144=720 baud; shortest pulse: 1000/720=1.39 msec. Note that this required a very fast electromagnet in the printer!
Ref. 2:"Siemens-Hell-Schreiber",
Fritz Schiweck,pp. 149-166
"Fernmeldetechnik",
Band 9 of “Lehrbücher der Feinwerktechnik“,
1st ed., 1942, 526 pp., C. F. Winter'sche Verlagsbuchhandlung
Ref. 3: "Stand
der Siemens-Hell-Fernschreibtechnik",
Rudolf Zimmerman, Siemens & Halske A.G. - Wernerwerk, Technische
Mitteilungen des Fernmeldewerks, Abteilung
für Telegrafengerät, SH 7997. 0,5. 1043. TT1. M/1401,
May 1940, 10 pp. (courtesy Siemens
Corporate Archives, München)
Ref. 4: "Der
Siemens-Hell-Schreiber",
Alexander B. Damjanovic, Zeitschrift für Fernmeldetechnik, Werk- und
Gerätebau, Siemens & Halske A.G., Wernerwerk, Jg. 17, Nr. 12, 1936, 7
pp., SH 6654, 1. 37. 0,5 T.
10-LINE FONT (10x11)
1935 was de first year of production
of the famous Hell Feldfernschreiber, a Hellschreiber sender/printer
for military field operations. This "Feld-Hell" is referred to as "T typ 58"
(the type designator of the manufacturer, Siemens-Halske), and by its
drawing number: "Typensbildschreiber" Tbs 24a. That
same year, the Hell company started the (limited) manufacturing an other
version of the "Feld-Hell":
Tbs 24b. It
has a font that is quite different from Tbs 24a Feld-Hell. The latter
has a 7x14 font, described further below.
The Tbs 24b uses a 10x11 font.
This appears to be a transition between the 12x12 font and the 7x14 font
(the latter font applies the "2-pixel rule", and can be considered as a "7x7
font with 1-pixel resolution").
The font of Hellschreiber Tbs 24b
(some characters look strange or incorrect (e.g. "Y"), but
the depicted font is that of the actual character drum;
as the number of columns is even, characters such as T and I are not
centered on the pixel mosaic.)
Spindle of the
Tbs 24b Spindle of the
Tbs 24a
"Feld Hell"
The characteristics of this font are as follows:
dot-matrix dimensions: 10 columns, 11 rows
scandirection: top-down
inter-character space: 2 columns (first and last)
start-pulse: no
character transmission rate:
2.5 characters/sec (150 cpm)
The
12x12 font at 5 chars/sec has a
required signaling bandwidth
of about
2150 Hz
(12x12x5x2,
assuming F1 modulation).
This had at least two disadvantages, primarily during transmission via radio
(cf. p. 5 in ref. 1):
it did not permit the use of
narrow filters in the receivers (to suppress interference from other
transmitters, or certain effects of signal propagation
echoes during longwave (LF) and
very-longwave (VLF) communication caused unacceptable distortion of
printed characters.
Also, not all "land line" telephone
systems could accommodate that bandwidth in the 1920s/30s.
The only solution was to reduce the telegraphy speed. This
could be done in two ways: reduce the character transmission rate
(characters per second), and/or reduce the number of pixels in the font. The
character transmission rate could really not be reduced: 2.5 chars/sec
corresponds to a decent manual typing speed, and the 5 chars/sec machine-sent
rate had to compete with other types of teleprinter systems. Hence, the number
of pixels in the font matrix had to be reduced. Hell settled on a 7-line font.
The 7-line font was first used in 1935
with the military Hell Feldfernschreiber ("Feld-Hell", T typ 58, Tbs 24a).
It has a transmission rate of 2.5 characters/sec. Several years later
(1939/40) it was also used with the second generation "Presse Hell" printers
(T
empf 14), at 5 chars/sec. With this font change, the scan direction was
also reversed: from top-down to bottom-up.
The initial 7-line font was a
7x7 matrix:
Quasi-7x7
rasterization (source: figure 8 in
ref. 2)
However, it was realized that the
resolution could be doubled to that of a 7x14 matrix - without
doubling the signaling bandwidth! This is cleverly done by applying the
"2-pixel rule" to the 7x14 font. This is equivalent to shifting pixels in
the 7x7 font by half a pixel. The minimum pixel duration is not affected!
So,
the final 7-line font design settled on a 7x14 matrix: 7 columns
of 14 pixels. The first and the last column are used for
character spacing. The top and bottom rows are also not used (exceptions are
the "tails" of the characters Q, 3, 6, 9 ). As it turned out, this font
actually also improved legibility compared to the 12-line font (ref. 5, 7)!
Sample print-out
of the Hell Feldfernschreiber
(source: figure 9 in ref. 5)
Comparison of
12-line font (top) and
7-line font (bottom)
(source: figure 9 in ref. 1)
7-line font
("7-Linien-Schrift") - right-leaning when printed
(source: Figure 92 in ref. 2, Figure 2 in ref. 3)
Compare the 1930s Hell-font to a "modern" PC font
on an LCD screen. OK, OK, it is the close-up of a small-size true-type font,
so vector based, rather than bitmap or raster. But still, the idea goes back
at least 80 years.
The
7x14 Hellschreiber font
(also: Fig 1 in
ref. 6)
The character set of the Feld-Hell
I have used
FontStruct™ to
capture the above Feld-Hell character set as a "TrueType" font that can be
used in regular Windows®
and MacOS® programs (e.g., Word®,
PowerPoint®). Note that only the 41
characters above are defined; use the # key for the pause character.
Lower-case A-Z map to upper-case.
dimension of printed characters:
the hub of the printer spindle is 2x6=12 mm, the paper tape moves
at nearly 47 cm/min (≈1½ ft/min), 150 characters are sent per minute
(2.5 cps),
the first & last column of the font are blank for
character-spacing, the two top & two bottom rows are blank for
line-spacing (with some exceptions). Hence, the printed
characters are
(10/14)x6= 4.3 mm high and (5/7)x470/150=2.3 mm
wide.
Ref. 2:"Siemens-Hell-Schreiber",
Fritz Schiweck,pp. 149-166
"Fernmeldetechnik",
Band 9 of “Lehrbücher der Feinwerktechnik“,
1st ed., 1942, 526 pp., C. F. Winter'sche Verlagsbuchhandlung
Ref. 3: "Stand
der Siemens-Hell-Fernschreibtechnik",
Rudolf Zimmerman, Siemens & Halske A.G. - Wernerwerk, Technische
Mitteilungen des Fernmeldewerks, Abteilung
für Telegrafengerät, SH 7997. 0,5. 1043. TT1. M/1401,
May 1940, 10 pp. (courtesy Siemens
Corporate Archives, München)
Ref. 6:
"Der
SH-Feldschreiber", Fernmeldetechnik, Siemens
& Halske A.G., Wernerwerk, Berlin-Siemensstadt, 1940, 14 pp., SH. 7535a,
1.1.40 TT1. N/1069 [note: not the same as SH 7535 from
1939, 11 pp.]
Ref. 25: "Der
Hell-Schreiber”, pp. 15-19 in “Einführung in die
Nachrichtenübertragungstechnik”, Volker Aschoff, Springer-Verlag, 1968.
147 pp.
7-LINE
FONT (7x14 WITH START-PULSE)
Ca. 1952, Siemens-Halske introduced a
Hellschreiber with a start-stop system: the T typ 72 "GL" ("Gleichlauf",
i.e., synchronized). It used the same 7x14 font as the Feld-Hell and the
T empf 14 "Presse Hell", with some minor stylistic modifications (e.g.,
to the K, Q, and ?), and several additional characters (the
punctuation marks . , : ' ) (
as well as the=). The font does not include the
Feld-Hell's pause-character.
Start-stop operation was enabled by a
start-pulse hidden in the first column. It
has a duration of 8 pixels (out of the 14 pixel
column), but this column is not printed (obviously, the
start-pulse must be detected before the printer mechanism can be enabled).
As a consequence, the spacing between printed characters is reduced by 50% (from two columns to
one).
Start pulse "hidden" in the first column of the font in
"start-stop" Hellschreibers
Printout of the "GL" character set
(Source: ref. 1)
Print-out of the
"GL" character set with my own machine (ruler scale is in cm)
The "GL" basically uses
the same typeface as the Feldfernschreiber, without the pause-character, but
expanded with . , ' = ( ) : and some modifications to the E, K,
Q, =
and ?.
The font of the Hell 72 "GL"
(start pulse in first column)
Timing of the
7x14 Hellschreiber start-stop font at
6.13 characters/sec
The characteristics of this font are
as follows:
dot-matrix dimensions: 7 columns, 14 rows
scandirection: bottom up
inter-character space: 2 columns (first and last)
start-pulse: yes (pixels 5-12 in the first column; 13.33 msec)
character transmission rate:
6.13 characters/sec (367 chars/min).
This is similar to the the 6 or 6.6 cps of RTTY
"telex". The traditional US speed is 45.45 Bd
/ 6 cps / 60 wpm, whereas the traditional European speed is 50 Bd, 6.6
cps / 66 wpm.
As with this system the sender and
receiver/printer are synchronized, there is no need to print two identical lines
of text. Hence, the thread of the printer
spindle only has a single turn (vs. two in, e.g., the
Feld-Hell), and the paper tape is narrower (9.5 mm vs 15 mm).
Ref. 7: "Siemens-Hell-Schreiber
„GL““ T typ 72 c – Beschreibung",
St Bs 1211/2, October 1955, 31 pp. + schematics , Siemens & Halske AG,
Wernerwerk für Telegrafen- und Signaltechnik, T Werb 2880 R GN, 956.0,2
(courtesy Heinz Blumberg, DC4GL) [52 MB].
7-LINE FONT (7x14 WITH
START-PULSE) - ATF
In the early 1950s,
a Hellschreiber was developed in the German (not-so) Democratic Republic ("East
Germany"). It went into service with the
Kasernierte Volks Polizei (KVP - Barracked
People's Police). In 1956 the KVP was absorbed into newly formed army of East
Germany; the Nationale Volks Armee (NVA - National People's Army),
where these machines continued to be used. This Hellschreiber was directly based
on Feld-Hellschreiber technology, and early production units actually contained
components abandoned by the Hell company in Berlin at the end of World War 2. It
is referred to as an Abtastfernschreiber
(ATF), so as to avoid using the Hell name (and make the patent infringements
too obvious ). The term "Abtastfernschreiber" was
actually not new: Hellschreibers were refered to as "Abtast-Telegrafen" at least
as early as 1940 (ref. 8).
The ATF basically uses the same 7x14 font
as the Feld-Hell and the Hell "GL". It has a slightly longer start-pulse than
the Hell "GL". With 4 characters/sec, the transmission speed is between the
Feld-Hell and the Hell "GL".
7x14
font of the ATF-Hellschreiber (9-pixel start-pulse
in the first column)
A print-out made
with an ATF Helschreiber
The characteristics of this font are as follows:
dot-matrix dimensions: 7 columns, 14 rows
scandirection: bottom up
inter-character space: 1 column (last)
start-pulse: yes (pixels 3-11 in the first column; 22.96 msec)
character transmission rate:
4 characters/sec (240 cpm)
Ref. 8: "Abtast-Telegrafen"
[incl. Presse-Hell, Feld-Hell, 7-tone], chapter IV in
“Taschenbuch für Fernmeldetechniker", H.W.
Goetsch, Oldenbourg Verlag, 1940, pp. 411-427 of 787
7-LINE
FONT (7x10)
During the mid 1930s,
the French company LMT developed a7-tone
teleprinter system. Like the Hellschreiber system,
it is based on transmitting a pixel stream that are printed in real-time,
without encoding.
Like the old 14-line and 12-line Hellschreibers with
electro-chemical and carbon-ribbon printers, the LMT system simultaneously scans
all rows of the font. A separate tone frequency is used for each row. These
days, this would be referred to as "Concurrent Multi-Tone" (C/MT) Hell.
The LMT system uses a font that has 7 rows
of 10 pixels (max). Black pixels are actually represented by absence of
the tone. Each transmitted character is preceded by a start-pulse that is not
considered part of the font. This start-pulse is generated by briefly
suppressing the tones of the odd-numbered rows of the font (1, 3, 5, 7).
Some examples of
the LMT 7-line font
(source: Figure 1
in ref. 5; numbers to the right of each character
refer to the elemental rows shown below)
For a standard 46 character alphabet
(letters, numbers, punctuation marks), the font consisted of 39 different
elemental row-patterns of 10 pixels each:
The 39 elemental
line patterns
(source: Figure 2 in ref. 5)
Of these 39 patterns, several are the
combination of two other such patters. E.g., pattern 16 is the combination
of pattern 2 and 7. This allowed the number of required elemental line
patterns to be reduced to 23. Hence only these 23 were "programmed" into
memory, in the form of 23 continuously turning notched disks. A rather
complicated electro-mechanical buffer/combiner/sequencer was required to
generate the character patterns (cf.
figure 3 in ref.
5).
Ref. 4: "Der
7-Frequenzen-Schreiber" [7-tone printer],
pp. 166-167 in “Fernmeldetechnik“,
Band 9 of “Lehrbücher der Feinwerktechnik“,
Fritz Schiweck,1st ed., 1942, 526 pp.,
C. F. Winter'sche Verlagsbuchandlung
Ref. 5: "Der
7-Frequenz-Funkschreiber der Les Laboratoires L.M.T.", L. Devaux, F.
Smets, Elektrisches Nachrichtenwesen (German edition of "Electrical
Communication" of International Standard Electric Corp.), Volume 17, Nr.
1, December 1938, pp. 22-34
During the late 1950s, the
worlds first fully electronic
teleprinter was developed for the West-German military (the
Bundeswehr) and commercial customers: the Hell-80. It can
operate in two modes: asynchronous (start-stop) via keyboard or punch-tape,
and synchronous (like the Feld-Hell) via the keyboard. It uses a 7x9 font.
The start-pulse is only used in the start-stop mode.
7x9 font of the Hell-80 (the 5-pixel start-pulse in the first
column is only used in start-stop mode)
inter-character space: 1 column (in start-stop=asynchronous mode), or two columns (first and last, in synchronous mode)
start-pulse: yes (pixels 3-7 in the first column; 15.76 msec). Tests with several Hell-80 machines have shown that a start-pulse is already detected after 4.4 - 4.8 msec. That is, after about 1.5 out of the 5 transmitted start-pulse pixels.
character transmission rate:
5 characters/sec (300 cpm)
During the 1970s, the Swedish company FACIT developed an
"alphanumeric
strip
printer". Basically a digital remote
dot-matrix printer, per the Hell system.The
maximum printing speed is 15 characters per second. The
system uses a 5x7 font, with 64 defined characters. Each column takes 6.9
msec to print (1 msec per pixel). Including dwell time between columns, a
character takes 45.7 msec.
(source: ref. 9)
The 5x7 FACIT font (source: ref. 10)
The characteristics of this font are as follows:
dot-matrix dimensions: 5 columns, 7 rows
scandirection: bottom-up
inter-character space: 3 columns
start-pulse: yes, part of the digital transmission (not considered part of the font)
HELL FONT MEMORIES -
MECHANICAL, MAGNETIC, OPTICAL
In order to send a Hell-character, we need
a character generator. It retrieves the pixel-map that corresponds to the
selected character, and sends it out as a pixel stream. Characters are typically
selected manually via a keyboard or machine-read from a punch tape. The
pixel-maps are retrieved from some form of character memory or font memory
("Hell Schriftart-Speicher").
Before the advent of electronic memory chips in the early 1970s (ROM, RAM,
FLASH, bubble, etc.), other forms of "memory" had to be used. The following
"read-only" memory technologies were used in (or conceived for) Hellschreibers
senders:
Electro-mechanical:
stack of cam wheels (notched
disks) with relay contacts, "Nockenscheibensatz
+ Zeichenkontakte")
character drum with slip-contacts
("Kontaktwalze")
Through the 1950s, most of the
Hellschreiber character generators ("Sender", "Geber") used a stack of
notched disks as character memory. This was used in both keyboard and punch-tape
reader senders. The stack has a separate disk ("Nockenscheibe") for each
individual character. Each disk has its own set of contacts. The size and
position of the notches on the circumference of the disk corresponds to the
pixel pattern of the character (including a start-pulse, if part of the font).
The pixel sequence is obtained by arranging the font columns in a tail-to-head
manner (bottom of column 2 after top of column 1, etc.). All disk-contacts are
connected in parallel. When a contacts is closed by a passing notch, the sender
outputs a tone pulse or closes the keying contact of transmitter.
A notched disk with
its contacts
Hellschreiber
keyboard-sender with a stack of notched disks
The stack spins continuously. The contact
of a character-disk is only engaged (mechanically), upon selection of the
associated character via the keyboard or punch tape. This is done to avoid
excessive wear on the notches and the contacts. The pixel stream must start at
the first pixel of the first column, and "scan" from there. For a transmission
speed of 5 characters/sec, the stacks spins at 300 rpm (150 rpm for 2.5
chars/sec). Obviously it is physically impossible to time this correctly when
typing characters on a keyboard. This is why the keyboard has a lock-out
mechanism that is driven by a separate single-notch cam wheel at the end of the
stack. The keys of the keyboard can only be depressed during a short period just
before the first pixel of the first column. The selected character-contact
emains engaged for one revolution of the stack. the contact is disengaged at the
end of the revolution, end the keyboard is enabled again for the next character,
etc.
Principle
diagram of the "GL" character-drum and keyboard mechanism (source: Figure 3 in
ref. 11; note the keyboard-enable cam on the far right)
In an other implementation, the
start-pulse is also implemented as a separate 1-noth disk.
Notched disks for
the letter "E" and for the start-pulse (source: ref.
12)
Note that the idea of using a stack of
notched disks combined with a "fingerboard" was already proposed for Morse
telegraphy in 1894 (ref. 22). There is no evidence of a working model ever
having been constructed. There is one disk per character, and the Morse code of
the character is repeated several times along the circumference of the disk. The
disk is rotated over the length of a single character - with the non-constant
speed of pushing the associated key of the "fingerboard".
Concept of a
"fingerboard" for Morse telegraphy (source: ref.
22)
A stack of notched disks with
pre-programmed Morse code messages is also at the heart of the "Omnigraph"
telegraphy training device of the early 1900s (ref. 23, 24). Each disk
captured five sets of 6-7 Morse characters. Depending on the model, 1, 5, or
15 disks could be stacked. It was advertised in the "Popular Mechanics"
magazine throughout the 1910s and 1920s, as well as in the Sears-Roebuck
catalog.
An "Omnigraph"
Morse code practice device - with a stack of notched disks (original
photo: M.
Schultz; used with permission)
Ref. 22: "Finger
board telegraph key", Elmer E. Mullinix, United States Patent
Office, Patent No. 530857, filed 31 July 1894, issued 18 December
1894, 5 pp.
Ref. 23:
"Instrument for the teaching and practice of telegraphy", Charles E.
Chinnock, United States Patent Office,
Patent No.
736936, filed 27 April 1901, issued 25 August 1903, 4 pp., and
Patent No. 773374, filed
20 January 1902, issued 25 October 1904, 4 pp.
There is a
slight variation on the above "stack of notched disk" method. It is used in the
Hell Feldfernschreiber (both model
Tbs 24a(made by Siemsn-Halske) and
Tbs 24b
(made by the Hell company)) and the post-war Abtastfermschreiber
(ATF, made by RTF). Here,
the stack of notched disks is retained, but the space between the notches is
filled in with a hard insulating material ("Isoliermasse"). The surface is
turned to its final
diameter on a lathe. The
stack is now a smooth cylinder, referred to as the "character drum"
("Kontaktwalze", "Geberwalze").
The "black" pixels are captured in the form of conductive metal patches that are
embedded into the surface of the drum. The "white" pixels are simply formed by
the dark, non-conductive material of the drum. The notched disk are made of
metal. As they are tightly stacked, they are electrically interconnected. Hence,
all pixel-patches are also interconnected. The final disk of the stack
forms a continuous "common" track at one end of the drum.
Per ref. 14 (below), the metal patches
are made of nickel.
Several Feld-Hell owners have observed that the slip-contacts cause wear
marks on the drum-contacts that reveal brass underneath. This implies nickel
plated patches, rather than
notched disks that are entirely made of a nickel-steel alloy. A standard V2A
nickel-chrome-steel alloy was used for the tips of slip-contacts. This is a
very hard alloy, highly corrosion-resistant, and the surface can be polished
to the permanent mirror finish that we see on the character drum. Ref 15,
16. The V2A alloys contain 15-40% chrome, 4-20% nickel, and have a low
carbon content (less than 1%). It was patented by C. Pasel of Essen/Germany
in 1912 (ref. 15), and was covered during the early 1920s by related patents
of Benno Strauss (of the renowned Krupp steel company, also located in
Essen).
Instead of a relay-contact, each disk
has a slip-contact ("Schleifkontakt"). Again, a contact is only engaged for
one revolution of the drum, and the keyboard is only briefly enabled just
before the first pixel of the first column (1-notch cam attached to the end
of the drum). All slip-contacts are also interconnected. To close the
circuit, there is a carbon brush that rides on the continuous common track
of the drum.
Cross-section of the
character-drum- with a slip-contact
(notched disk shown in
black, insulating material in grey)
Details of the key locking/enabling and slip
contact (dis)engaging mechanism
(source: figure 11 in ref. 13)
The
photo below shows a number of the slip-contacts and associated springs. The
actual contacts are attached with two copper rivets.
Close-up of the slip-contacts of the Hell
Feldfernschreiber
- one contact in the engaged position
(character drum removed)
The
circumference of the character-drum of the Hell Feldfernschreiber Tbs 24a
The tracks of the character-drum divided into individual "pixels"
There are 41 characters of 7 x 14 bits
each. Hence, the Feld-Hell's character drum is actually an
electro-mechanical 4 kilobit Non-Volatile Read-Only Memory!
The binary version of the character-drum
(7 columns of 14 pixels per character
A text file with the above binary
pixel-pattern is available
here.
Here is a
2:44 minute video clip that I made of the spinning character drum of the
Feld-Hell machine:
(use player controls, or start your own
playerhere (MOV-format), here
(.WMV) or here (.MP4))
The concept of using a drum on which characters are
encoded as a sequence of conductive and non-conductive patches, dates back
to the early 1900s. The "Natrometer" is a variation on the Omnigraph Morse
code training device that is shown further above.
It has a drum that consists of a stack of smooth metal
(aluminium?) rings. Each
ring
captures the "dits and dahs" of several dozen Morse code characters. The
rings have a dark, non-conductive coating, except for
each "dit" and "dah".
The photo below shows a Nanometer with nine tracks and associated
slip-contacts.
"Natrometer" - drum
diameter appr. 12 cm
(source: July 2011 eBay on-line auction
260818624176)
Ref. 13: "Der
Feldfernschreiber", document D 758/1 of the Oberkommando des Heeres,
Heereswaffenamt, Amtsgruppe für Entwicklung und Prüfung, Berlin, 1 April
1941
Ref. 14: bottom of page 154 "Siemens-Hell-Schreibers",
pp. 149-166
in “Fernmeldetechnik“,
Band 9 of “Lehrbücher der Feinwerktechnik“,
Fritz Schiweck,1st ed., 1942, 526 pp.,
C. F. Winter'sche Verlagsbuchandlung
Ref. 15:
pp. 28-30 in "The History of Stainless Steel", Harold M. Cobb, ASM
International, 2010, 360 pp., ISBN: 1615030107 / 978-1615030101
The 10x11 font
of the Hell Feldfernschreiber model Tbs 24bis also implemented as a character drum:
The circumference of the character-drum of Hellschreiber model Tbs 24b
The 7x14 font of
the Abtastfermschreiber (ATF),
is also implemented as a character drum. The start-pulse is clearly visible at
the top of the photo below. Also note that, unlike the Tbs 24a and Tbs 24b
above, pixel patches of adjacent characters are interconnected at the surface of
the drum. Hence, it is unclear if the drum indeed consists of a stack of notched
disks.
The circumference of the
character drum of the ATF Hellschreiber
(photo courtesy Heinz,
DC4GL)
The
character drums of the Feld-Hell (bottom) and
the ATF Hellschreiber (top) side by side
MAGNETIC-CORE MEMORY
The 7x9 font of the Hell-80 machine is
stored inamagnetic core memory - a technology dating back to the early 1950s.
It uses pairs of small ceramic magnetic rings
(the
"cores", "Ringkerne"). Several
wires are treaded through each of the cores, to provide read/write access.
Addressing signals are provided by the decoder-card shown further below.
1949 Magnetic core memory (source: public domain, GNU Free
Documentation License)
Magnetic cores, used
in a character generator memory
(source: Figure 2
in Rudolf Hell's patents 1086738 (Germany, ref. 17) and 3255313 (US, ref. 18))
The Hell-80
"Bildpunktregisterkarte" - the pixel-memory card
(the black cylindrical
components are transistors)
Close-up of the
decoder card
- magnetic-cores shown in the center
(photo courtesy Remmelt-Jan, PAØRJW)
Rudolf Hell also patented another form of
magnetic font memory. The sender device resembles a conventional typewriter, but
is combined with magnetic tape recorder technology. Here, each "type" (i.e., the
"hammer" head of a type-bar of the keyboard) has magnetic strips on it, instead
of a mirror-image character glyph.
Typewriter
Hell-sender with magnetic types
(source: figure 1 in
ref. 19a and 19b)
Type-bar and type of
a regular typewriter
(original image: P. Krok; Creative
Commons license)
Type-bar with
magnetic Hell type
(source: figure 3 in ref. 19a and
19b)
The magnetic strip on the type represents
the pixel sequence of the character to be transmitted. When pressing a key of
the keyboard, the magnetic type touches a continuously spinning disk (or a loop
of magnetic tape). The disk is made of magnetizable material. The magnetic pulse
sequence is
somehow transferred from the type to the disk. This is similar to the
write/record-head of a magnetic tape recorder. The pulse sequence is then be
picked up by a read-head, and the pulses are used to key a transmitter or
tone-oscillator. An erase-head wipes the disk clean, to prepare it for the next
character. There are no indications that a working prototype was ever made...
Of course, sequences of black and white pixels can also
be captured optically. One way to do this is with a transparent disk (e.g.,
made of glass). A
photographic process could be used to implemented
the pixels as dark spots or line segments. Conversely, the disk could have a
dark surface, and engraving or a photochemical process used to remove this
surface for pixels. The pixels could be arranged as a ring near the edge of
continuously spinning disk, as shown below.
A focused source of light would illuminate the pixel track, and a photocell
on the opposite side of the disk would detect the presence or absence of
light, according to the pixels passing by the light source.
It would require some special
mechanism to use the optical disk memory with "random access" via a keyboard
or punch-tape reader. However, it is very well suited for applications where
a long, fixed pixel sequence is repeated continuously. This was actually
used during World War 2, in the German "Bernhard/Bernhardine"
radio navigation system for aircraft. This system used an enormous antenna
array that rotated continuously (2x per minute). The momentary azimuth of
the antenna ("pointing direction" relative to magnetic north) was
continuously transmitted in Hellschreiber format, in the form of a compass
rose. The pixel sequence of the entire compass rose (("Gradskala"), with
10-degree values and degree tick-marks) was implemented as an optical disk.
The (large) disk was mounted onto the shaft of the rotating antenna. A
Hellschreiber printer in the aircraft would print the compass rose onto
paper tape. A second Hellschreiber printer would simultaneously print a
bar-graph of the signal strength of a second antenna, pointing in the same
direction as the primary antenna. However, this second antenna had a sharp
null in the direction of the primary antenna, and transmitted a continuous
signal. The sharp minimum of the bar-graph printout pointed precisely at the
value of the antenna azimuth on which the aircraft was located.
Optical disk with
360 degree compass rose pixel sequence (figure 2 in ref. 20)
Signal
strength bar graph, azimuth data, and station identifier as printed with the
Hellschreiber.
The strip
indicates that the receiver is at a bearing of 239 degrees from ground station
"R". (source: ref. 22)
Re-created
signal strength bar graph, azimuth data, with station identifier "M"
The letter "M" in the partial compass rose
shown above, is the identifier of the transmitting station. There was a number
of these navigation stations in Germany, France, Denmark, The Netherlands,
Poland, Czechoslovakia, and Austria (not all operational). Each had its own
identifier letter. The Hellschreiber pixel pattern of the identifier was
implemented on a separate disk.
Two stacked
optical disks (d, e), light sources (f, g; only 2 of 4 shown) and photocells (h,
k)
(source:
Figure 4 of ref. 21)
These optical disks had a single, circular
recorded track, and turned at 2 rpm (the antenna had a diameter of about 35
meters / 120 ft).
An other optical disk system for "on-off
keyed" signals, is the Morse-code practice device shown below. It uses two-sided
cardboard disks, with a diameter similar to that of conventional LP gramophone
records. The disks shown (per standard LDv
704) are from the
Luftnachrichtenschule (Luftwaffe Signal Corps School), whereas the record payer
from the early 1940s is a Kriegsmarine (Navy) device. The player
collimates light into a tiny square
spot of light (≈1 mm2). Reflections off the disk are
detected by a photo cell. Replay speed can be varied (30/60/75/150 chars/min =
6/12/15/30 WPM), and the audio tone can be varied independently. I have put a
red/green 3D photo of this training equipment
here.
Click
on image for full size, and here
for the entire equipment
Optically encoded
disk of a MÜG.Mar.
(Morse-Übungsgerät
Kriegsmarine - Navy Morse-Code Practice Device)
This optical disk concept was expanded
decades later (in the 1970s, by Sony and Philips), in the form of the Laserdisc
and Compact Disc (CD). Here, information bits are implemented as small
reflective indentations ("pits") on the surface of the disk, and a laser is used
as a light source. The single track spirals from the edge of the disc to the
center - as on a gramophone record.
Ref. 20:
"Verfahren
zur Richtungsbestimmung" [Method for direction finding [optical
disks, quadruple antenna], Reichspatentamt Nr. 757528, A. Lohmann,
(Telefunken G.m.b.H.); filed: 17-July-1936,
awarded 17-July-1952
Ref.
22:pp.
96-102in “Die deutschen Funk-,
Navigations-, und Funk-Funkführungsverfahren bis 1945“, Fritz
Trenkle, Motor Buch Verlag, ISBN 3879436150,1st ed., 1979,
208 pp.
