Abstract: Due to the advantages of high definition and real-time transmission of SDI, it was initially applied in the field of professional video broadcasting and has been increasingly adopted in the field of security in recent years. However, due to the high data transmission rate and large storage capacity of SDI, there are still certain difficulties in design and application for some security practitioners who previously used IP network high-definition monitoring solutions. In order to help readers have a more comprehensive understanding and design of SDI, this article introduces how to choose appropriate SDI signal chain devices, how to design high-performance SDI signal chains, and introduces equalizers, cable drivers, and retiming

abstract

Due to its advantages such as high definition and real-time transmission, SDI was initially applied in the field of professional video broadcasting and has been increasingly adopted in the field of security in recent years. However, due to the high data transmission rate and large storage capacity of SDI, there are still certain difficulties in design and application for some security practitioners who previously used IP network high-definition monitoring solutions. In order to help readers have a more comprehensive understanding and design of SDI, this article introduces how to choose appropriate SDI signal chain devices, how to design high-performance SDI signal chains, the basic working principles of equalizers, cable drivers, and retimers, suggestions for PCB layout and power supply design, and TI’s specific solutions in the SDI field.

1. Introduction to SDI

SDI, Serial Digital Interface, is a standard used to transmit uncompressed digital video signals such as standard definition, high-definition, 3G high-definition, etc. The most popular SDI video formats are shown in Table 1. Due to its high-definition characteristics, low latency, and the ability to reuse analog video cables already deployed, SDI is gradually being widely adopted in fields such as security and monitoring. At present, SDI related equipment on the market mainly includes SDI extenders, distributors, matrices, multi screen segmentation, codecs, SDI optical transceiver, DVR, etc.

SDI Introduction

Compared to traditional IP monitoring networks, the advantages of SDI are very obvious:

SDI has unparalleled advantages in image clarity

High definition not only means high resolution, but also requires excellent performance in ultra wide dynamic, white balance, signal-to-noise ratio, brightness, contrast, sharpness, and other aspects. Due to encoding and compression, IP network surveillance videos are far inferior to uncompressed SDI in terms of image quality, image details, and other aspects

SDI transmission has strong real-time performance

The transmission of SDI signals does not go through compression and there is no processing delay; Not passing through the IP network and not affected by network latency.

Upgrading from analog monitoring system to SDI allows for the reuse of existing wiring systems

SDI also uses coaxial 75 ohm cables and BNC interfaces, which can easily and quickly upgrade from traditional analog monitoring systems to SDI without the need to rearrange the network like IP networks. This feature has a huge advantage in upgrading analog monitoring systems, as the construction of IP networks is not allowed for many buildings.

On the other hand, SDI also has its drawbacks, such as high cost, large data storage capacity, and difficulty in designing long-distance transmission. However, as SDI is gradually widely adopted in the market, these drawbacks will gradually weaken.

2. Working principle of SDI devices and introduction to TI related products

Figure 1 is a typical application diagram of SDI input, output, and processing. TI can provide a complete set of SDI transmission solutions, including equalizer, cable driver, retimer, cross switch matrix, video clock, display driver, storage driver, and power supply.

Typical SDI applications

Figure 1 Typical SDI Applications

2.1 Equalizer

The high-frequency components of a signal are attenuated more relative to the low-frequency components of the signal after being routed through PCB or cable transmission. This phenomenon is called skin effect, which disrupts the signal integrity of high-speed signals, closes their eye patterns, and increases signal jitter. In order to compensate for the skin effect, people have invented equalizers, preamplifiers, and de emphasis devices to compensate for the unevenness of the frequency response of transmission lines. Figure 2 is a frequency response diagram of a transmission line and an equalizer. The transmission line model attenuates more at high frequencies, while the equalizer has higher gain at high frequencies. Setting the high-frequency gain of the equalizer to an appropriate value will result in a roughly flat frequency response across the entire frequency band when the transmission line and equalizer are connected in series.

Usually, pre emphasis and de emphasis devices are used at the transmitting end of high-speed digital signal transmission, while equalizers are used at the receiving end. However, in SDI links, only adaptive equalizers are used at the receiving end, and pre emphasis or de emphasis is not used at the transmitting end because SDI devices may be connected through user-defined coaxial cables of any length. Any fixed equalization or pre/de emphasis value cannot flexibly meet various cable lengths, and there is no adaptive pre emphasis and de emphasis device in the industry. In addition, SDI devices must be plug and play, and customers are not allowed to manually set appropriate balance values on the application site to obtain the best cable transmission characteristics. Therefore, only adaptive equalizer is the ideal solution, which can automatically detect signal quality and set the optimal equalization value to obtain the best transmission channel frequency response.

Frequency response of transmission lines and equalizers

Figure 2 Frequency response of transmission lines and equalizers

Figure 3 shows the effect of using the equalizer. It can be seen that the eye diagram and jitter performance of the high-speed SDI signal are significantly improved by the equalizer after passing through a section of cable or PCB wiring.

The effectiveness of using an equalizer

Figure 3. Usage effect of 3500-42M 176449-02 equalizer

TI provides a full range of equalizers that support SD, HD, and 3G SDI, as shown in Table 2.

equalizer

Table 2 Equalizer

Among them, LMH0394 is a high-performance equalizer, and its competitive analysis is shown in Figure 4, with a long transmission distance and low power consumption.

Competitive Analysis of Equalizer LMH0394

Figure 4 Competitive Analysis of Equalizer LMH0394

2.2 Retimers

The SDI retimer is used to automatically detect the type of input signal, adjust its own PLL and CDR circuits to restore and shape a low jitter clock, and then retime the received SDI signal to reduce the jitter of the SDI signal. Although equalizers can also reduce the jitter of SDI signals, they are two completely different devices and cannot be replaced by each other. The function of an equalizer is to improve eye pattern and signal jitter by increasing high-frequency gain to flatten the frequency response of the transmission channel, while a retimer suppresses and reduces cumulative noise through PLL and CDR. If the SDI transmission channel is very long or worsened by other noise and interference during the transmission process, an equalizer alone is not enough to improve the quality of the signal. In this case, connecting a re timer in series at the output end of the equalizer is an ideal solution to improve the quality of the SDI signal. Figure 5 illustrates the deblurring effect of the retimer, and it can be seen that the eye image and jitter have been significantly improved.