Add video output to TV for mobile phones and portable devices

With mobile phones and othersUltra Portable DeviceWith the rapid development of media playback technology in, video in ultra-small size and low power devices is more and more widely used. A new feature is the ability to drive video signals from mobile phones to traditional TV sets for viewing (Fig. 1). Sending video signals to different applications is useful in many ways, including video conferencing, photo browsing, movie streaming, videophones, online games, and other applications that you would never dream of.

Semiconductor manufacturers are developing video for ultra-portable video technologyEncoderAnd integrated videoWave filter/Driver to drive 75 ohm coaxial cable directly. Encoder functions are performed after the main controller chip, including NTSC or PAL formatting, depending on whether only usecomposite videoOr add S-video to determine if integrated video-to-analog converters are integrated (DAC). Add a filter/driver after the DAC to rebuild the signal and eliminate high-frequency pseudo-signals to obtain higher-quality images. In addition, the technology provides 75 ohm coaxial cable drivers to directly drive cables connected to the TV.

Composite video output

The TV output function of a mobile device outputs a composite video signal, which is the most commonly used video signal at present and can be received by almost any TV set. In addition, portable devices such as mobile phones or portable media players need a way to convert digital video signals into analog signals and into NTSC or PAL composite video signals. This allows you to see the signal on an external TV set. In addition, the analog signal needs to be amplified and matched to a cable with a characteristic impedance of 75 ohms. This implementation is shown in Figure 2.

Composite video is expected to remain as a regular signal and remain a way to display analog video signals in the near future. This composite video signal contains all the information needed to recover the video at the receiving end, including line and field synchronization, brightness and chroma signals (Fig. 3).

Figure 1: Video signals can be transmitted from a mobile phone to a TV.

Figure 2: Video encoder and video filter/driver in portable device.

Figure 3: The composite video signal shows an excellent band.

Because the standard composite video connector is very large for portable devices, there is an improved connector called mini A/V connector that is more suitable for portable video transmission. It also transmits left and right audio signals on the same cable, thereby saving more space. Usually, one end of the connector is the mini A/V plug, and the other end is the larger RCA composite video signal and the left/right channel plug (Fig. 4).

Figure 4: Mini A/V connecting RCA cables.

Video Encoder

To build a composite video signal, a process called encoding is required. This process requires the digital signal to be formatted and converted into an analog composite video signal of NTSC or PAL format. Video encoders can be integrated into a larger digital integrated circuit or used as a stand-alone device depending on the functional specifications.

A stand-alone video encoder converts digital component video (in 8-bit parallel CCIR-601/656 or ANSI/SMPTE 125M format) output from a main system processor (i.e., a baseband chip) to a standard analog baseband TV composite video signal (NTSC or PAL standard) with modulated color subcarriers. The signal is then fed into an integrated DAC for processing and output from the device (Fig. 5).

Figure 5: NTSC / PAL video encoder.

Video Reconstruction Filtering

After digital video encoding, the signal is usually restored back to the analog domain through a DAC. This process is called rebuilding (Figure 6). In this process, a high-frequency pseudo-signal which distorts the picture is introduced. The reconstruction filter is used to filter out these spurious signals. The reconstruction performance of the filter depends on whether the high frequency pseudo-signals can be well filtered without affecting the valid signals in the pass band. These high frequency pseudo-signals cause the amplitude changes of detail components in the picture, such as highlights or fine pattern details, which affect the quality of the video signal because they move relative to the sampling clock. The results are similar to jagged problems, which cause details to distort as they move through the picture.

Figure 6: Ultra Portable Video Reconstruction Filter/Driver.

In order to achieve filter function, it is recommended to use integrated video filter/driver, such as FMS6151 of FeiMega Semiconductor Company. This integrated active filter can replace several discrete elements. Generally speaking, the filter used in video multimedia devices is a low-pass active filter. The main components of the filter are operational amplifier, capacitance, resistance and inductance. FMS6151 is a 5th order Butterworth filter. Its overall performance is very good, such as low phase error, high stability, small number of components and efficient filtering characteristics. Therefore, it is an excellent choice for filtering consumer video devices. Because of its high reliability and guaranteed parameter specifications, these integrated active filters have more stable and consistent filtering performance than discrete active and passive filters (Fig. 7).

Figure 7: The Output Reconstruction (Image Suppression) filter eliminates clock and sideband components generated by sampling and analog reconstructions.

Due to voltage swing requirements and the need for a higher ESD (electrostatic discharge) protection level, the reconstructed filters and cable drivers are usually placed outside the encoder.

Video Filter/Driver

In addition to rebuilding the filter, a video driver is needed to amplify the video signal and drive a 75 ohm coaxial cable. The amplifier needs a gain of 6dB to accommodate the dual terminal load. FMS6151 Integrated Video Filter/Driver Solution combines a rebuild filter with a low impedance video driver. The operating voltage range of the device is from 2.5V to 5.5V. Fifth-order filters provide better image quality than typical second-order and third-order passive solutions.

This filter/driver is directly driven by a DC-coupled DAC output, but it can also work when AC-coupled input is used. The input common mode range of the device is 1.2V with ground reference._PP. Its output can drive a single 75 ohm coaxial cable (150 ohm) load that is AC or DC coupled. DC coupled output does not require expensive output coupled capacitance. If the output is AC coupled, the SAG correction circuit can be used to reduce the capacitance and physical size of the AC output coupling capacitance while still generating an acceptable field skew.

SAG Correction

Traditionally, if video applications are AC-coupled, a very large output coupling capacitance (between 220F and 1000F) will be required. The SAG correction circuit provides excellent performance with a small output coupling capacitance, so large coupling capacitance is not required. The usual output circuit (a capacitance of 220F at 150 ohm load) creates a single pole (-3dB) at 5Hz. Reducing this capacitance can cause excessive phase shifts, which can tilt the video field, hindering the correct recovery of the synchronization signal.

The peak value of the SAG correction circuit in FMS6151 is small, so it can provide phase response compensation that can significantly reduce the tilt of the video field. This compensation enables the designer to reduce the large 220F output coupling capacitance. The SAG correction uses a 22F capacitor and the output coupling capacitor uses a 47F capacitor, which are much smaller than the volume and cost of capacitors required by other circuits (Fig. 8).

Figure 8: Video filter/driver with SAG correction.

Activation and shutdown

FMS6151 has shutdown function, it can stop output and reduce static current below 25nA, which reduces power consumption and prolongs battery life. This feature is particularly important for portable devices such as mobile phones, handheld game consoles and cameras that require filtering and driver capabilities. In addition, FMS6151 provides 12kV ESD protection.

Small size driver to drive large screen

In order to realize the output function of composite video TV on portable devices, many considerations are needed. There are many design challenges in the division of device functions, maintaining low power consumption and high image quality. Video encoder can easily convert digital signal to NTSC and PAL analog composite video.

Figure 9: The body of FMS6151 encapsulated in Micropak is very small.

The micropak-encapsulated FMS6151 is very small, making it ideal for cell phones with dense components and providing high-quality video for TV viewing. The device has a powerful 12kV ESD protection to ensure that mobile devices are not harmed. A 5th-order low-pass reconstruction filter can be used to smooth the output video signal to avoid unnecessary distortion. The device amplifies, drives and matches the impedance of a 75 ohm coaxial cable. Selecting the minimum configuration of direct DC coupling mode can further save the space of the printed circuit board. Because the output current of the coaxial cable driver is very low when working and less than 25nA when not in use, this technology can significantly reduce battery energy consumption.