Hellschreiber printers (other than start-stop models) are really simple and "dumb": the printer blindly prints all received pulses, without any interpretation. Hellschreiber characters are pixel (bit) sequences with a fixed-length. Each character of the alphabet is mapped to a particular sequence, but is not encoded. This is not all that different from teletypewriters ("telex"). Here, each character of the alphabet is typically encoded as a sequence of a fixed number of bits, preceded by a start-pulse and followed by a stop-pulse. Again, all characters have the same length.
In case of the Baudot code (e.g., the CCITT-2 or ITA-2 alphabet), there are five data-bits, one start-bit, and stop pulse with the duration of 1-2 bits (typ. 1.5 bits). That is, 7.5 bits in total. A character that is sent with a telegraphy speed of 50 baud has a duration of 1000 / 50 = 20 msec per bit, and a total duration of 7.5 x 20 = 150 msec.
In the end, both Hellschreiber and teletypewriter characters are fixed-length pulse sequences. Hence, a Hellschreiber printer can print the pulse sequence of any teletypwriter - as long as the printer solenoid is fast enough. For standard 7-column Hellschreiber font, the solenoid can print pulses with a minimum duration of 8.16, 4.08, 3.7, or 3.3 msec (at standard 2½, 5, 5½, 6 and 6.1 chars/sec respectively). Printing individual 20 msec teleprinter pulses (bits) is no problem whatsoever.
A single column of the Hell-font is printed every 57.1, 28.6, 23.8 or 23.4 msec. As stated above, a 50 Bd character (with a 1.5 bit stop pulse) has a duration of 150 msec. If the spindle of a Hell-printer is slowed down enough, each printed column represents the duration of exactly one complete teleprinter character. The printed columns will be vertical, if both Hell-printer and teletypewriter are running at exactly the same speed. Note that that printer should print a single line, not two identical parallel lines. Hence, the spindle only has one thread that makes one turn..
If the speed of the Hell-printer is calibrated, it can be used to verify the speed of a teletypewriter - like a simple electro-mechanical signal analyzer! Such Hellschreibers are called "Drehzahlfehlerschreiber", i.e., an "rpm-error printer". Ref. 1-4. Note that speeds can also be checked with tuning-fork and stroboscopic disk methods.
Fig. 1: Printed tape segments from a Hellschreiber "rpm-error printer"
(source: figure 538 in ref. 1)
Fig. 2: Printed tape segments from a Hellschreiber "rpm-error printer"
(source: figure 170 in ref. 2)
As illustrated in the figure above, the angle between the printed columns and the edge of the paper tape is directly related to the rpm-error. The angle is measured with a transparent plastic template, overlaid on top of the paper tape. The practical rpm-error range that can be measured with this method is +/- 5%. For a printer with a synchronous AC-motor, the measurement accuracy depends on the accuracy of the public power grid frequency (50/60 Hz), assumed to be within +/- 0.3%. The allowed error is +/- 0.75% per CCITT standards, though typically +/- 0.5% is applied.
Fig. 3: Siemens-Halske Hellschreiber "rpm-error printer"
Fig. 4: Close-up of the (standard) printer module, with print-out of a telex signal
Fig. 5: Equipment label of the Siemens-Halske Hellschreiber ""rpm-error printer"
Fig. 6: Power supply and other components of the S-H "rpm-error printer"
Fig. 7: Synchronous motor and gear train of the S-H "rpm-error printer"
Fig. 8: Concept of a "distortion recorder", based on a Hell-printer
(source: Figure 528 in ref. 3)
This particular printer is not based on an inked spindle and a solenoid-driven printer blade. Here, a stationary blade is used, as well as special paper tape. The white paper is coated blue on top. The arrival of a tone pulse is transformed to high-voltage pulses. The voltage is applied between the blade and the spindle. The high-voltage pulses cause arcing that obliterates the blue coating of the paper. Thus, each pulse causes a white pixel to appear. Note that the pixel is associated with the leading edge of the received pulse; the size of the pixel is not related to the duration of the pulse. The paper transport speed is very low: 1.8 m/hr, compared to 28 m/hr in a Feld-Hell machine. When the printer is operated over a longer period of time, a distribution of pixels is printed, that should be (tightly) centered around the middle of the paper tape. A 175 Hz tuning fork is used to calibrate the speed of the printer motor.
- Ref. 1: "Geschwindigkeitsmessungen" [speed measurements], pp. 781-784 in "Fernmeldetechnik", Vol. 9 of "Lehrbücher der Feinwerktechnik", Fritz Schiweck, 4th ed., 1962, C. F. Winter'sche Verlagsbuchhandlung, 894 pp.,
- Ref. 2: "Drehzahlfehlerschreiber" [rpm-error printer], pp. 262 in Section X of "Telegrafentechnik", Vol. 6, Part 6 of "Der Dienst bei der Deutschen Bundespost - Leitfaden für die Ausbildung", Fritz Schiweck (ed.), R. v. Decker's Verlag, G. Schenck, 1960, 970 pp.
- Ref. 3: "Fernschreib-Verzerrungsschreiber (Bezugverzerrungsschreiber)" [teleprinter distortion-printer], pp. 768-770 in "Fernmeldetechnik", Vol. 9 of “Lehrbücher der Feinwerktechnik“, Fritz Schiweck, 4th ed., 1962, C. F. Winter'sche Verlagsbuchhandlung, 894 pp.
- Ref. 4: "Fernschreib-Verzerrungsmesser Tmse 89b", Beschreibung und Bedienungsanleitung Fs Bs und Ba 5624/30, Siemens & Halske A.G., Wernerwerke für Telegrafen- und Signaltechnik, January 1963
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