Framing signal generator for electrical recorders

A facsimile recorder for marking an electrosensitive web with a remote scanner signal including a periodically interrupted framing signal and a video signal comprises a scanning electrode and a cooperative electrode for marking the signals on the web during successive scans across the web. Scanning of the web is controlled by a framing circuit which compares the remote scanner framing signal with a recorder generated signal and uses the asynchronism of the two signals to control the scanning electrode until it frames the recorded scanner signal on the web. The recorder pulse generator comprises means sensing the voltage impressed through the web by the scanner signal. When the scan completes its crossing of the web the voltage drop across the scanning electrodes increases producing a recorder pulse which is delayed and applied to the framing circuit.

BACKGROUND OF THE INVENTION 
In graphic recording of electrical signals, for example facsimile signals 
electrically marked during line by line scanning across a moving web, it 
is not only necessary that the applied facsimile signals received from 
independent equipment occur at the same rate as the line by line scanning 
of the web, it is also necessary that sucessive lines of received signals 
be in phase with the line scan of the web. That is, the start of each line 
of facsimile signals must be brought into coincidence with the start of 
line recording scan of the web so that the recorded information is framed 
on the web. The process of bringing the scanning of the web into phase 
with the incoming signals is termed framing. 
In facsimile recorders scanning of electrosensitive paper or like web is 
effected by scan means moved or commutated by a driver. For example, in 
electrolytic recorders a motor drives a helical electrode curved around a 
drum or a number of styli carried on an endless belt. It is customary to 
derive a phasing pulse from the scan means, which pulse is called a 
recorder pulse or start of line pulse or signal to distinguish it from the 
framing pulse or signal which constitutes a part of the incoming facsimile 
signal. One form of facsimile framing signal is a series of substantially 
continuous sinusoidal modulated lines interrupted or altered for a short 
interval between each line. This framing signal or tone is transmitted for 
seconds before the transmission of graphic facsimile information, and a 
framing pulse can be derived from the interval between each line of tone 
for comparison with the recorder pulse or framing signal. 
Typically the recorder pulse or framing signal has been derived from the 
recorder scan means by electromechanical or electromagnetic means. For 
example, a scanning helix drum or a stylus on a scanning belt can generate 
a pulse just before each scan of a recording web by momentarily closing a 
switch mechanically or magnetically. Mechanical switches closed millions 
of times are themselves notoriously subject to failure, and may 
undesirably affect the recording characteristics of the stylus. Magnetic 
reed switches are relatively free from wear but are difficult to adjust 
for precise closure at a selected position of the scan means. And since 
the preferred metals for stylii, e.g. stainless steel, are weakly-or 
non-magnetic, they cannot be used with reed switches. 
Accordingly, the objects of the present invention are to provide a way of 
generating a graphic recorder framing signal, which avoids use of 
electromechanical or electromagnetic devices and is free of their 
adjustment and maintenance problems, and which utilizes the available 
interrupted framing tone of facsimile signals. 
STATEMENT OF INVENTION 
According to the invention, in elecrical recording apparatus including scan 
means for marking through an electrosensitive web with graphic electric 
signals including a periodically altered framing signal, the apparatus 
including framing means for comparing a start of scan signal with the 
framing signal, a start of scan signal generator comprises, circuit means 
adapted to be coupled between the scan means and the framing means, the 
circuit means including means for sensing alteration of the framing signal 
voltage drop through the web between scans, and switching means responsive 
to alteration of the voltage drop to produce a start of scan signal for 
comparison with the framing signal so that asynchronism of the framing 
signal and start signal causes the framing means to bring the two signals 
into phase when the framing signals are framed on the web.

DESCRIPTION 
The facsimile recorder shown in FIG. 1 receives from a remote or 
independent source 1 a facsimile signal which, as shown in voltage versus 
time waveform 2A of FIG. 2, includes an initial framing signal F of one 
half second duration of modulated framing tone Ft interrupted 
approximately 5% (sometimes a higher percent) of each half second which is 
also the duration of each line of video or graphic information signals V 
which follow about fifteen seconds of framing signals. The framing signals 
F and the video signals V are applied through a marking signal amplifier 2 
to a recorder 3 comprising a scanning electrode E and a cooperative blade 
electrode B between which a web of electrosensitive recording paper P is 
fed by a motor M as shown, for example in U.S. Pat. No. 3,369,250, 
incorporated by reference herein. As shown at 2B the envelope Fe of the 
framing signal F is rectified in a frame pulse detector 5 wherein a 
framing pulse Fp (waveform 2C) is derived at the beginning of each line of 
framing tone and applied to a framing circuit 6 such as that shown in U.S. 
Pat. No. 2,686,832 which is incorporated herein by reference. The recorder 
pulse generator 4 of the present invention enclosed in dot-dash lines also 
applies a recorder pulse Rp to the framing circuit 6 which adjusts the 
cycling of a mechanism 7 driving the scanning electrode E by applying an 
error or correction signal Es to the scan mechanism 7 until the electrode 
E scans the paper P in phase with the incoming facsimile framing signals 
F. 
The electrode E is shown schematically as a stylus moved by the scan 
mechanism, e.g. a motor and belt, between positions E* and E**. In leaving 
and entering these positions the stylus, which may be one of several on 
the belt, moves off the paper or out of recording relation to the paper 
web P for a short interval between each scan across the web between 
positions E* and E**. A similar interval between scans, called the off 
time, occurs with a facsimile helix electrode curved less than 360.degree. 
around a scanning drain, or with a commutated series of stationary 
recording electrodes such as shown in U.S. Pat. No. 3,827,056. The present 
invention makes use of the above described recording off time between 
successive scans of the recording paper to generate the recorder pulse R 
which is compared with the framing pulse Fp for framing the framing 
signals F and the video signals V on the paper web P. 
The framing signal tone Ft and later the video signals V are applied from 
the marking signal amplifier 2 to terminals F,V one of which is grounded 
and connected to the terminal Tb of the blade electrode B of the recorder 
3. The other terminal is connected through a protective resistor R3 to the 
terminal Te of the scanning electrode E. The recording circuit is 
completed by the resistance Pr of the electrosensitive paper P which is 
shown in dashed line between the electrode terminals Te and Tb. 
The recorder pulse generator circuit 4 is connected between the signal 
terminals F,V and the electrode terminals Te and Tb. A wheatstone bridge 
is formed by two fixed resistors R1 and R2 on one side, and on the other 
side by the protective resistor R3 and the paper resistance Pr. The bridge 
is crossed by the inductive coil K connected in series with a capacitor C1 
between the junction J of resistors R1 and R2 and the junction J of 
resistors R1 and R2 and the junction at the scan electrode terminal Te of 
resistor R3 and the paper resistance Pr. The coil K is associated with and 
electromagnetically closes a reed switch S when the voltage between the 
junctions J and Te crosses a threshold of unbalance sufficient to energize 
the coil K. A capacitor C1 in series with the inductance of the coil K 
passes only abruptly positive going changes in the marking voltage RI. 
Negative going changes are shunted by a switching diode D1 or integrated 
by an averaging capacitor C2. 
The values of resistor R1, R2 and R3 are selected so as to maintain the 
bridge balanced and the switch S open when the paper resistance Pr is 
present between the recording electrodes E and B. When the framing tone 
signal Ft is applied through the paper while the scanning electrode is in 
contact with the paper the marking voltage drop through the paper is 
relatively low owing to the electroconductivity of the paper. As shown in 
waveforms 2D and 2E of FIG. 2, when the stylus scanning electrode E is 
crossing the paper web P and is in electrical contact with the paper, the 
voltage drop due to the paper resistance Pr is, for example, 75 volts. But 
when the stylus crosses off the paper, leaving an infinite or very high 
resistance between the electrode terminals Te and Tb, the voltage drop 
across the terminals rises to the maximum applied by the marking signal 
amplifier, for example 150 volts. The excursion from 75 volts to 150 volts 
between the time when the scanning electrode E crosses beyond one edge Pe 
of the paper P and begins another crossing at the opposite paper edge Pe 
(waveforms 2D and 2E of FIG. 2). 
In diagram 2D the recording paper is shown out of phase by lateral 
displacement with respect to the remote scanner signals 2A, 2B and 2C, for 
the purpose of illustrating the lack of phasing of the scanning electrode 
E with the incoming facsimile signals F,V. The subesequent lateral 
shifting of the paper P in diagram 2H represents a timewise adjustment of 
the starting position of the scanning electrode E into phase with the 
incoming facsimile signals, not a physical displacement of the paper. As 
shown in diagram 2D the recorder's scanning electrode E randomly starts to 
scan at position E* out of phase with the incoming framing pulse Fp. 
Consequently the framing tone Ft is recorded from one extreme edge Pe of 
the recording paper until the periodic interruption of the one occurs. The 
framing tone is marked as a solid line called the (unframed) framing bar 
UFb which continues after the midscan interruption to the other extreme 
edge Pe of the paper. At this point the scanning electrode leaves the 
paper and encounters an abrupt change in voltage drop with respect to the 
blade electrode B and terminal Te as shown in waveform 2E of FIG. 2. 
The abrupt change in voltage at terminal Te unbalances the bridge causing a 
momentary flow of current through the coil K until the series capacitor C1 
is charged. The inductance of the coil and reactance of the capacitor C1 
introduce a time delay between the initial rise of the marking voltage 2E 
and the delayed recorder pulse Rd generated by the closing of the reed 
switch S and appearing at the recorder pulse output terminals Rp. The 
value of capacitor C1 is selected such that the delay time (TD in waveform 
2F) is approximately one half the off paper time OT. This delay TD 
provides margins at the right and left recordings of the framing and video 
signals F,V shown on the paper P in diagram 2H. 
The delayed recorder pulse Rd of waveform 2F is compared with the framing 
pulse Fp of waveform 2C in the framing circuit 6. The error signal Es 
(FIGS. 1 and 2E) resulting from asynchronism of the compared pulses 
adjusts the speed and phase of the scan drive mechanism 7 in a known way 
to bring the delayed recorder pulse Rd into coincidence with the framing 
pulse Fp. 
Thus the recorder pulse generator 4 of the present invention eliminates 
electromechanical and electromagnetic recorder pulse devices and makes 
possible the completely electronic generation of the recorder pulse needed 
for framing. 
It should be understood that the present disclosure is for the purpose of 
illustration only and that this invention includes all modifications and 
equivalents which fall within the scope of the appended claims.