Design and implementation of video subtitle module based on MAX7456

O Introduction

To meet the needs of the CCTV (closed-circuit television) security industry to generate text and images and superimpose them on video images, MAXIM has launched the MAX7456 video subtitle module. This device is a single-channel, monochrome on-screen display (OSD) generator with built-in nonvolatile memory (EEPROM). The device can generate user-defined OSD and insert it into the video signal. All functions are required, and it is compatible with NTSC and PAL video standards. This article discusses the implementation process of a monochrome OSD system based on MAX7456 with examples.

1 Internal structure of MAX7456

The MAX7456 integrates all the functions and EEPROM required to generate user-defined OSD and insert it into the composite video signal. The device has built-in input clamps, synchronization generators, video timing generators, OSD insertion MUX, non-volatile (EEPROM) character memory, display memory, OSD generator, crystal oscillator, SPI compatible interface that can read/write OSD data, and video driver. The internal structure of the MAX7456 chip is shown in Figure 1.

The MAX7456 provides functions such as blinking, color inversion, and background control. At the same time, the MAX7456 can accept signals commonly used in security industry video standards such as standard NTSC or PALCVBS. Therefore, the device can meet the needs of most security markets.

2 OSD module system structure

The single-channel OSD generator using MAX7456 will greatly reduce the complexity and cost of the OSD system. The system only needs an ordinary microcontroller chip and corresponding software to achieve flexible and changeable subtitle display requirements. Figure 2 shows the application system block diagram of MAX7456. In the figure, the PC can download the font information generated by the software to the microcontroller (MCU) through RS232 (serial port), and the font information is then stored in the EEPROM of MAX7456 by the microcontroller (MCU) through the SPI (Serial Peripheral Interface Bus) interface. After the analog video signal output by the camera is processed by the OSD module, the required image with subtitles can be obtained. The software running in the microcontroller can control the display content of the subtitles in real time through the SPI interface.

3 Hardware structure of OSD module

As the core component of the OSD system, MAX7456 needs the cooperation of corresponding peripheral devices to play its role. Figure 3 shows the hardware structure block diagram of the OSD module. Among them, MAX7456 is responsible for adding subtitles to the input analog video signal; MAXQ2000, as a low-voltage micro-power single-chip computer , can provide corresponding control signals for MAX7456 through GPIO analog SPI interface timing, and is responsible for communicating with the PC through the serial port to obtain user-defined character sets and configuration information; MAX3002 is a bidirectional level conversion chip, which can convert the 2.5V system signal of MAX7456 into a 5V system signal that can be used by MAX7456; MAX3311 is a serial port chip matching MAX02000, responsible for providing serial port signals for communicating with PC; MAX8881 can convert the 5V power supply into the 2.5V power supply required by the single-chip computer and buffer chip.

4 OSD module software structure

The software that works with the OSD module consists of two parts: one part is the firmware running in the microcontroller, which includes the SPI interface driver, memory configuration program, register configuration program, serial port transaction communication, etc.; the other part is the user interface running on the PC, which can provide users with simple OSD settings and memory upgrade functions.

4.1 MAX7456 Control Routine

The MAX7456's SPI interface supports up to a 10 MHz interface clock (SCLK). When writing to a register, pulling low enables the serial interface to read data from SDIN on the rising edge of SCLK; when going high, the data is latched into the input register. If SD-IN goes high during a transfer, the procedure is terminated (i.e., data is not written to the register). After going low, the device waits for the first byte to be read from SD-IN to determine the type of data transfer being performed. Figure 4 shows the timing of writing data.

When reading registers, pull low. The address will be clocked into SDIN on the rising edge of SCLK. Then, the data can be output from SDOUT on the falling edge of SCLK. Figure 5 shows the timing of reading data from the device.

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The following is the specific operation method code:

4.2 PC User Interface

The software on the PC side can provide users with common OSD configuration information, such as video format selection, subtitle brightness, flip mode, flashing, subtitle file selection, etc. Figure 6 shows its user interface diagram.

In this way, through the software, users can also edit subtitle display content, customize character sets, and set OSD background color, etc.

4.3 Compile and develop environment

IAR EmbeddedWorkbench 2.10A running on Windows platform is one of the integrated development tools specially used for MAXO series microcontrollers, and has been used by most MAXQ series chip development. However, after creating a new project under IAR, you also need to set the compilation options: General Option/Target/Device and select MAX200X.

5 Conclusion

MAX7546 is a single-channel, monochrome on-screen display (OSD) generator. Using it to develop an OSD system can not only greatly save circuit board space and design time, but also provide higher performance at a lower cost. This article introduces the basic composition of the OSD system, and explains the technical points of using MAX7456 to build an analog TV subtitle on-screen display system from both hardware and software aspects, hoping to provide some guidance for OSD system developers.