Line magnifier
A line magnifier or line tweezer is an electronic diagnostic device for the targeted evaluation of analog video signals of old standards and is connected between the video signal source and the trigger input of an oscilloscope with the purpose of projecting a certain line of the image signal onto the oscilloscope or another evaluation device.
properties
Line magnifiers are built into other devices in an adapted form as an assembly or built as a stand-alone device for workshop and laboratory purposes. They consist of analog and digital circuits, require a DC voltage supply or have a built-in power supply . As connections, they have a video signal input, possibly also an output - to avoid reflections and the associated impairment of the image signal - and an output for the trigger signal. In addition, there is a setting option or an input for the desired line number (s), optionally for various video signal types and formats, selection and adaptation options or automatic functions for adapting to the respective signal format with regard to the number of lines and differentiation between double fields and full images. If the evaluation of other image formats with non-integrated or already separate synchronization is planned, inputs for H- and V-Sync can be available. If they are used, the line magnifier does not need the image signal.
Layout and function
A looped-through video signal or an input usually terminated with 75 Ω is used as the input . This amplifies the video signal and sends it to the detectors for H and V sync. The detector for H-Sync detects the horizontal synchronization of the image signal. This is the line synchronization and ensures that the lines of the image are projected exactly on top of each other. The detector for V-Sync recognizes the vertical synchronization and thus marks the beginning of the image. These two digital signals are switched to a synchronous counter: The V-Sync to the zero setter ( Reset ) and the H-Sync to the counter input (Count). This has the effect that the counter is set to zero at the beginning of each picture and the lines of the picture are counted at their preceding synchronization pulse. Common video signals are transmitted in fields . In the first field are the odd lines and in the second the even lines (→ interlaced ). The difference for the line magnifier is that the V-Sync detector has to be rearranged by recognizing or ignoring the second field. In one case, the two adjacent lines of the fields are evaluated, in the other case the counter continues to count and thus distinguishes the fields. Even with a standard BAS signal, which is the basis of all analog television standards , a 10-bit counter is required in order to be able to record all lines (2 9 <number of lines <2 10 → 512 <576 <1024). The downstream comparator compares the counter reading with the set line number as a binary expression for logical equivalence . The output signal of the comparator is connected to the output of the line magnifier. If the trigger input of the oscilloscope is connected to it and adjusted to the signal, the beam of the oscilloscope starts at precisely this line and shows the signal voltage curve of this line of the image signal on the oscilloscope when the probe is connected to the video signal. Positions of the line can be determined with a downstream precise timing element or a (digital) circuit with appropriate properties.
To simplify the digital part of the circuit, it is possible to load the counter with the preset line number instead of zero and let it count backwards. If it reaches zero, the output is set. Instead of the comparator, a NOR gate with 10 inputs is sufficient . The 10 XNOR gates are omitted . It is no longer possible to evaluate several lines based on their number. It is even easier to evaluate the next line by only passing the carry bit of the counter to the trigger.
application
The evaluation of certain lines is required for (selection): In the laboratory, production and workshop:
- for focusing an image or CCD sensor of a camera (all image areas and corners)
- for diagnosing image signal interference and troubleshooting various devices
To evaluate content in image signals:
- in the teletext decoder , for capturing the text data from the television picture
- was used for decoding channel video data
- when reading back data saved with video recorders such as "ArVid"
To process existing image signals:
As a precise optical sensor:
- for using the camera as an optical sensor for evaluating the image in automation and mechanical engineering,
- for using a camera as a line sensor ,
- for using a camera as a vertically adjustable line sensor by entering the line number.
See also
Web links
- Video Line Selector assembly instructions and documentation from April 12, 2002 at elm-chan.org
- PAL Line-Selector at controller-designs.de
Individual evidence
- ↑ Line magnifier as a project of the Aalen University of Applied Sciences ( Memento from February 9, 2014 in the Internet Archive )
- ↑ Funkschau magazine 84-09 / 80 - trigger attachment for oscilloscopes; Line tweezers for video service