Converting a video signal into a two-level signal

In a method usable in a facsimile scanner for converting a video signal obtained by scanning an original point by point and line by line into a two-level signal, wherein first and second companion signals are generated to derive a threshold signal from said companion signals, which follows the video signal dynamically, and wherein said video signal and said threshold signal are compared to obtain said two-level signal, the method comprising the steps of generating said first and second companion signals:

• charging first capacitor means, the voltage of which corresponds to said first companion signal, to the actual level of the video signal under the control of first switching means when said video signal rises until the maximum level of the video signal is reached, whereby the first companion signal substantially follows a rise in said video signal;
• interrupting the charge of the first capacitor means by said first switching means under the control of the video signal at the maximum level of the video signal, whereby the first companion signal remains at a constant level in the first instance when said video signal declines;
• generating a first distance value from said video signal by generator means;
• determining continuously actual first difference values between the first companion signal and a first reference signal by first differential means;
• discharging the first capacitor means under the control of the first switching means and the first differential means when the actual first difference value is equal to said first distance value, whereby the first companion signal comes close to the decrising video signal;
• interrupting the discharge of said first capacitor means under the control of the first switching means and the first differential mean when the actual first difference value is equal to said first distance value again, whereby the first companion signal remains at a constant level in the first instance;
• continuing the charge of the first capacitor means under the control of the first switching means when the first companion signal substantially coincides with the rising video signal, whereby the first companion signal follows the rising video signal again;
• discharging second capacitor means, the voltage of which corresponds to said second companion signal, to the actual level of the video signal under the control of second switching means when said video signal decrises until the minimum level of the video signal is reached, whereby the second companion signal substantially follows a decrise in said video signal;
• interrupting the discharge of said second capacitor means by said second switching means under the control of the video signal at the minimum level of the video signal, whereby the second companion signal remains at a constant level in the first instance when said video signal rises;
• generating a second distance value from said video signal by said generator means, whereby said first and second distance values are proportional to the mean brightness values of the background of the original to be scanned;
• determining continuously actual second difference values between the second companion signal and a second reference signal by second differential means;
• charging the second capacitor means under the control of the second switching means and the second differential means when the actual second difference value is equal to said second distance value, whereby the second companion signal comes close to the rising video signal;
• interrupting the charge of said second capacitor means under the control of the second switching means and the second differential means when the actual second difference value is equal to said second distance value again, whereby the second companion signal remains at a constant level in the first instance;
• continuing the discharge of the second capacitor means under the control of the second switching means when the second companion signal substantially coincides with the decrising video signal, whereby the second companion signal follows the decrising video signal again.