Various waveform extraction methods in measuring instruments

Almost all measuring instruments that require waveform display face a problem: the number of sampling points in the waveform segment to be displayed is not equal to the number of pixels in the screen display area. In this case, how to draw the waveform into the display area? This article will introduce you to several solutions to this problem.
A situation: The number of sampling points in the waveform segment is greater than the number of pixels in the screen display area. In different situations, different extraction schemes are used.
1. Equal interval extraction


Figure 1 Equal interval extraction


This is actually a problem of how to compress a large number of waveforms to a specific number of points. For this problem, we can naturally think of using equal interval waveform extraction. At a fixed ratio of the number of sample points of the waveform segment to the number of screen points, sample points are sampled at equal intervals, and the sample points extracted are displayed on the screen. The advantage of this scheme is that it is simple to implement and can reflect the approximate outline of the waveform, and is suitable for lower frequency signals. The disadvantage is that for signals with too high frequency, the peak value will be filtered out and cannot reflect the peak value of the signal.
2. Peak extraction


Figure 2 Peak extraction


Peak extraction is to divide the original sample fragments of the waveform into several groups, as shown in the figure, into 5 groups, each group compares the value and the value as the extraction point, and maintains the order of the two points. This extraction method is aimed at high-frequency signals. The advantage is to find the peak value, but it does not guarantee that the time interval between two adjacent points is equal.
3. Mean value extraction


Figure 3 Mean value extraction


Mean value extraction also divides the original sampled waveform into several sections, and calculates the average value for each section as the extraction point of the section. This scheme can be considered as a compromise between the first two schemes. By averaging, it not only ensures that the time interval between adjacent points is equal, but also makes the extracted waveform consistent with the original waveform as much as possible; however, there are also problems with too high frequencies. The peak value of the signal will be filtered out and cannot reflect the peak value of the signal.
Let's look at the second case: the number of sampling points in the waveform segment is less than the number of pixels in the screen display area.
In this case, it is necessary to fill in imaginary sampling points between the sampling points to solve it.
1. Direct filling


Figure 4 Direct filling


At the position where the imaginary sampling point needs to be filled, directly copy the previous real sampling point. This scheme is simple to implement, but the disadvantage is that the waveform will be stepped and unreal.
2. Linear filling


Figure 5 Linear filling


Linear filling first connects adjacent sampling points, and then finds the filled point from the corresponding position of this adjacent connecting line. This kind of scheme will not produce the steps in the scheme, but it will still artificially produce nonlinear phenomena.
3. Sine fill


Figure 6 Sine Fill


Sine filling is also called sine interpolation, which is used to restore the signal during high-speed sampling. The specific implementation requires filtering means. The realization method of this scheme is more complicated than the first two, but it avoids the non-linear phenomenon better.