Note that at that time, the corner frequency of a filter As usual, the referenced literature does not indicate steepness of Implemented low-pass filters of the era also blocked frequencies above 200 Hz Widening the transmitter's output filter beyond 1.6 times the pixel rate, i.e., 1.6 xĦ1.25 ≈ 100 Hz at 2.5 characters/sec (Feld-Hell), or 200 Hz at 5 characters/sec The above companies furthermore recommended against Same font, but at twice the speed, hence, half the fundamental pulse duration. As the Hell-pixels are binary, the equivalent This equivalent to a pixel rate ("Punktfrequenz" ref. Hence, the shortest pixel cycle ( = 1 black + 1 white pixel) is 2 x 8.16 = 16.32 That have a duration of N x 1000 msec / (2.5 characters/sec x 7 columns x 14 lines) = N x 4.08 msec, where N The font used by the Feld-Hell machines has black and white column segments ( Of course, filtering at the receiver does not affect the bandwidth of the transmitted signal, and may worsen print quality. This can actually be verified by using a very narrow IF or AF filter at the receiver. This filtering still allows the text that is printed at the receiving Hellschreiber, This effectively suppresses the third and higher harmonics of the pixel rate, and significantly reduces the second harmonic. This resulted in the recommendation for filtering the modulation (i.e., at the transmitter) with a low-passįilter that has a corner frequency of 1.2 times the pixel rate. Hellschreiber experiments and measurements around 19 35 (ref. (central office of the German national postal authority) conducted extensive Siemens-Halske, the Cable & Wireless company of the UK, and the Reichspostzentralamt Have to reduce the occupied bandwidth that results from on/off-keying. Interference, out-of-band transmission, etc. The intended communication, is highly undesirable, as it causes adjacent channel 4: spectrum of the RF output signal of an SSB transmitter (in USB mode), modulated withĬlearly, transmitting a signal that occupies more bandwidth than needed for