Connection chip and technical features in video transmission technology

1, the video format is related to the clarity and truth of the signal

As we all know, the video transmission technology is very concerned about the clarity and truth of the video signal, as shown in Figure 1 for the common component video signal. Can you do this? Video signal filters, drivers, and multiplexers and cable transmissions have a variety of input/output coupling and clamping combinations, including input AC or DC coupling, output AC or DC coupling and the combination of various input clamps, etc., are the connection chips and technologies in video transmission technology, which should be said to be video formats. This technical feature will be discussed and introduced.

2. Input technology of interface in video transmission technology

2.1 AC-coupled video input method

When designing an analog video circuit, the analog video signal is generally input by AC coupling (as shown in Figure 2). This allows the receiving circuit to establish an optimum DC bias level, while the capacitor isolates the DC voltage of the drive signal on the output side of the output device. The classic way to recover DC in an AC-coupled video signal is by separating the sync portion of the entire embedded video signal and establishing a digital pulse signal.

2.2 DC-coupled video input mode (as shown in Figure 3 (a))

To accommodate single-ended input drivers with reference voltages that are ground-level, DC-coupled, as shown in Figure 3, filter/drive devices such as the FMS641 7A, FMS6418B, and FMS6419 are specifically designed for DC input coupling. For example, the video/graphic display shows the standard current mode output of the DAC. In order for a current-excited general-purpose DAC to generate an output voltage, this DAC circuit often uses a double-terminated 75Ω (AC impedance equivalent to 37.5Ω) resistor as the load. Therefore, this type of DAC output has a known ground reference DC level. Devices with this type of DAC circuit can be seamlessly connected to the video DAC output and have the following advantages: no input coupling capacitor is required; no clamp settling time; no signal amplitude reduction due to input capacitor discharge; no generation Glitch pulse; there is no input impedance limitation when using a pulsed DC recovery loop; no on-chip sync separation, charge pump circuit, and servo loop are required.

2.3 About the input level design in TV design

Figure 3(b) shows an input stage circuit for modern television. The video signal is AC coupled to the DC recovery circuit so that the input video signal can have any DC offset. The input circuit is compatible with AC coupled and DC coupled video sources.

It should be noted, however, that there has never been a generally accepted solution for input-level design in television design. In the past, a large number of TV sets used different input stages, regardless of whether the signal came from an AC or DC-coupled signal source, which may cause problems in some TV sets. For so many different types of receiving devices, universal compatibility is required. Impossible. Based on cost considerations, most of the low-end video sources that account for signal transmission equipment use DC-coupled output.

3. Output technology of interface in video transmission technology

3.1 AC coupled output and DC coupled output

Whether the output is AC-coupled depends mainly on the technical and cost factors. The AC-coupled output circuit includes a series capacitor (Figure 4a), while the DC-coupled output circuit does not have this capacitor (Figure 4b). The addition of a capacitor in the output channel will result in cost. Problems such as increased space, increased video signal distortion, etc., cause the output waveform to "tilt" up or down relative to the input waveform. Therefore, this field distortion is called "field tilt". The DC-coupled output has no field tilt, so the designer of the video output circuit must take it seriously.

Since there are shortcomings in AC coupling, why have they been used for a long time? There are two reasons. One is that in order to protect, the integrated circuit has not been widely used to drive a video output circuit with a simple NPN emitter follower. In case the output connector is shorted to ground or supply voltage, the capacitor prevents damage to the NPN transistor. Current integrated video amplifiers have a robust short-circuit protection circuit that will not be damaged in the event of a short circuit. Second, starting from the technical characteristics, the most common method of outputting video signals to a media display device is AC coupling (as shown in Figure 4(c)), which allows the receiving circuit to establish a common mode level at its own input. This level is independent of the DC level of the input video signal. A 75Ω series resistor should be placed as close as possible to the output, which helps isolate the downstream spurious interference from the output and provides the best signal conditions. The MAX4090 in Figure 4(c) is the filter driver.

One of the concerns with AC coupling is that the coupling capacitor is typically large, ie 220μF or greater. This is because the capacitance of the capacitor and the 150Ω load (the sum of the reverse termination resistor and the input termination resistor) should be much lower than the frame rate (25Hz or 30Hz). A 220μF capacitor forms a 5Hz pole frequency, which is difficult to meet performance requirements. Broadcasting equipment typically uses a 470μF or 1000μF coupling capacitor. Figure 4(d) shows the high-pass response characteristics of the AC-coupled output with 220μF and 22μF capacitors. The minimum capacitance of the ac coupling capacitor should be 200μF, which is the minimum coupling capacitor that can be used to achieve acceptable signal distortion.