Oscilloscope turns into TV in seconds - female engineer DIYs it

We usually think of oscilloscopes as being used to analyze electronic circuits. But few people realize that they can also work like a TV when a video signal is present. In this article, you will learn how to turn your oscilloscope into a TV or video monitor. You will also learn the principles behind this.

Oscilloscope video signal display. We traditionally think of oscilloscopes as boring and used only to analyze electronic circuits, but they can also be used like televisions in the 20th century. If we want to understand the method, we must understand how television works.

There are three video signals in the world: NTSC, PAL and SECAM.

NTSC is the abbreviation of National Television System Committee. The main countries that use this standard are the United States, Canada and Japan. China uses the PAL television signal. These two signals are essentially the same. Here we mainly introduce NTSC. SECAM is not suitable for application here.

First, the electronic picture tube TV scans the picture from the upper left corner of the screen, then scans line by line from left to right, and from top to bottom, until a picture is completed. This picture is a frame, and then the whole process is repeated to complete the next frame. Because the whole process is very fast, it is almost impossible to detect it with the naked eye.

There are two methods of scanning the screen: interlaced scanning and progressive scanning. The difference between them lies in the method of scanning the screen. These two methods are incompatible with each other and must be converted before they can be used together. Interlaced scanning means that each picture or frame is divided into two sub-pictures (such as the two FIELDs in the figure below). Interlaced scanning consists of two paths during the screen scanning process. First, the horizontal path that constitutes Field1 is scanned, then it returns to the starting point, and the horizontal path of Field2 is scanned, and then it returns to the starting point. The advantage of this method is that there will be no gaps between rows, and the resolution of the video can also be improved.

Line-by-line scanning has only one scanning path, scanning horizontal lines from top to bottom. See the figure below.

In 1940, NTSC introduced the black and white video standard, bundling information such as horizontal sync signal, vertical sync signal and luminance signal into one channel. In fact, when we use it later, the most important thing is to understand Luminance Levels, that is, luminance level (voltage level).

Then, in the following decades, color video signal standards were introduced. In order to be compatible with black-and-white television signals, the basic structure of color video signals is the same, except that the carrier wave carrying hue and saturation information and the standard color waveform (color burst) are added to the original black-and-white video signal. Therefore, in the visible line interval period, the signal parameters expressed by the waveform are luminance, hue, and saturation. Hue is expressed by phase angle (Phase), saturation is expressed by amplitude (Amplitude), and luminance is expressed by average level (Level).

In the NTSC signal, the carrier frequency containing color information is 3.57954525Mhz, and the amplitude of the standard color waveform (Color Burst) is 20IRE (140IRE=1V). The function of the standard color waveform is to synchronize with the oscillator inside the TV . If you want to know which phase angle corresponds to which color, you can refer to the NTSC vector diagram. The figure below illustrates the color resolution process.

Generally speaking, the CRT of an oscilloscope uses vector scanning. However, in order to display a video signal, it must use raster scanning technology. To put it in a more vivid way, for example, if you want to draw a circle, imagine drawing close and horizontal horizontal lines on a piece of paper lightly, and only darken the area where you want to show the circle. Generally speaking, the horizontal scanning frequency of NTSC signals is 15.734kHz, and the vertical scanning frequency is 59.94Hz. The horizontal refresh rate indicates how fast the electron beam moves from left to right. The vertical scanning frequency indicates how fast the electron beam moves from top to bottom. First, you need to provide the oscilloscope with sawtooth signals of these two frequencies.

Why use a sawtooth wave? Imagine that the screen is scanned from top to bottom and from left to right. Then the device or circuit that generates the signal has the ability to produce a true sawtooth waveform.

Of course, we can choose a computer sound card, a mobile phone sound card, a signal source, or build a circuit to generate the required signal. If you are lucky, you can get the required sawtooth wave signal on an old TV.

Before going to the next step, we need to check the waveform. If it is indeed a sawtooth wave, without any bends, etc., then we can go to the next step.

Next, we need to find a way to make each point in the scanning process obtain a specific brightness to form a complete picture. In addition to the vertical input and horizontal input (CH1 and CH2 channels), we can sometimes see another input terminal, which is usually marked with "Ext.Sync.", "Ext.Input" or "Z-Axis Input".

The Z-axis input is a brightness modulation input, and its function is to control the brightness of the display signal. It can be said that the X-axis is from left to right, the Y-axis is from top to bottom, and the Z-axis is forward and backward. As for its performance parameters, the oscilloscope I used is second-hand. I searched the manual online and the displayed parameters are TTL compatible (blanked at TTL high), 42Vmax, and input impedance 10 kilo-ohms.

Simply put, the higher the input voltage value, the darker the dot, and the lower the voltage value, the brighter the dot. According to my specific test, there is a significant change in brightness within 1-2V, about 2V above all dark, and 1V below all bright. The video signal must be inverted before it can be used, otherwise the white area will become black and the black area will become white.

First, connect as shown in the picture below. Required materials: oscilloscope, signal source (can be a sound card, etc.), video signal source (camera, TV signal), wire connector, etc.

The external signal source provides vertical scanning signals and horizontal scanning signals, and the Z-axis input video signal controls the brightness. If the image is found to be rolling, it means that there is a little difference between the actual scanning frequency and the standard frequency, and the scanning signal frequency needs to be readjusted. At the same time, please note that the video signal needs to be inverted and adjusted appropriately. I use an amplifier circuit to adjust the video signal here (see the circuit diagram below), of course, you can use a video signal adjuster to adjust it.

X-axis scanning waveform

Y-axis scanning waveform

The actual effect is as follows:

We can also use LM1881 to generate a stable vertical and horizontal signal source. LM1881 can generate vertical and horizontal scanning square waves that match the current input video signal. You can download the chip information of LM1881 online. We then use a simple RC circuit to change the square wave into a sawtooth wave (or a certain slope). The figure below is the circuit I use, which is divided into two parts, X-axis and Y-axis scanning signals and video signal adjustment.

The X-axis and Y-axis scanning signal part has been mentioned before, so I won't repeat it here. The diode inside uses 1N4148, and the specific values ​​of R and C are expected to be adjusted by the reader to find the appropriate values.

The video signal adjustment part is used to reverse the video signal, and at the same time can adjust the video signal amplification value and the relative value between signals, that is, the brightness and contrast of the video. There are two adjustable resistors inside, which can adjust the brightness and contrast of the video respectively. Readers can practice it by themselves, and of course they can also improve the original circuit.

LM1881 outputs square wave X-axis and Y-axis square wave signals.

In the image below, the top is the inverted video signal.

Below is the original video signal. Through the oscilloscope, we can see that the range of NTSC signal is between -1 and 1 volt. The area around -0.4v is where the synchronization signal is located. The area around 0v is where the black is located. We mainly want to obtain information above 0V.