Discussion on the non-compression scheme of analog high-definition and high-definition cameras

When the "HD storm" of megapixel network cameras swept the video surveillance market, analog cameras, which have a dominant position in the video surveillance market, also began to improve their image quality. High-definition analog cameras using non-compression solutions have also become the target of choice when upgrading the image quality of monitoring systems. At the same time, while front-end megapixel network camera suppliers continue to develop their product lines to the back-end, suppliers entering the front-end from the back-end have also attracted the attention of the security industry.

Achieving high definition requires higher technology

At present, the definition of high-definition only appears in the radio and television industry. According to relevant standards, the general display ratio is 16:9, and it can at least resolve 1080i (resolution 1920×1080) digital signals, or the number of scanning lines in both vertical and horizontal directions must reach 720P digital signals. Only products that meet or exceed the above standards can be classified as high-definition products.

For a single HD camera, its high resolution and HD resolution depends on whether the camera meets the SMPTE (Society of Motion Picture and Television Engineers) standard, that is, whether it reaches one of the three resolutions of 720P (1280×720, progressive scan), 1080i (1920×1080, interlaced scan), and 1080P (1920×1080, progressive scan), and has a 16:9 display mode and full frame rate. Otherwise, even if it is a so-called megapixel camera, it may not be HD.

Million-level high-definition surveillance systems are no longer just about buying a solution like standard-definition and analog systems in the past. Due to the particularity of million-level high-definition and the massive entry of back-end IT manufacturers, the threshold for million-level high-definition surveillance is much higher than that of traditional surveillance. It requires certain core technologies in high-definition digital image acquisition and processing technology, image encoding and decoding, back-end transmission management, and software platform construction, and requires stronger comprehensive strength, especially a large amount of R&D power.

Overview of HD Camera Solutions

The popular high-definition camera solutions on the market today can be roughly divided into two types: compressed solutions and non-compressed solutions.

The compression scheme refers to the video signal collected by the sensor (CCD/CMOS), which is pre-processed and post-processed to improve the picture quality in real time with low latency, and then transmitted to the network using various encoding and decoding standards (such as H.264 and MPEG4). Due to the large amount of traffic and the very fast rate, the data must be compressed. For example, transmitting 1920×1080 pixels at 30fps requires a data rate of 1.5Gbps in the uncompressed case. Add multiple channels, and the required rate is even higher.

The non-compression solution refers to the original signals from the lens, CCD, etc., which are transmitted directly without compression after encoding, eliminating the video compression step in the middle. It has high real-time performance and is known as zero delay.

Which solution to adopt is also the most controversial issue in the current high-definition video front-end equipment. In fact, the two solutions have their own advantages, and there is no question of which one will replace the other. Each solution has its own application occasions.

Transmission technology for non-compression schemes

The transmission of non-compressed solutions is mainly through four high-definition interfaces: HD-SDI, YPbPr, HDMI, and DVI. How to choose among these four interfaces has become a topic of concern.

HD-SDI interface

The SDI standard is defined by SMPTE and is widely used in broadcast and video production. The SDI standard describes how to transmit uncompressed serial digital video data between editing devices over video coaxial cables. There are two SDI standards: SD-SDI and HD-SDI, which are based on different data rates. The basic electrical specifications of these two standards are the same. The main difference is that HD-SDI has a higher data rate, 1.485 Gbps, while the SD-SDI data rate is between 143 Mbps and 540 Mbps, with 270 Mbps being the most widely used rate.

YPbPr Interface

YPbPr: YPbPr is also called color difference component interface, which adopts the EIA-770.2a standard of the American Electronics Industry Association. There is also an interface called YCbCr interface. The difference between the two is that the former is interlaced color difference output, and the latter is progressive color difference output. The color difference output decomposes the chrominance signal C transmitted by S-Video into color difference Cr and Cb, thus avoiding the process of mixing and decoding two color differences and separating them again, and maintaining the maximum bandwidth of the chrominance channel. And Y is the brightness signal. The YPbPr interface is not a digital interface, but is still defined as an analog interface. The YPbPr interface can use coaxial cable, BNC head, or ordinary lotus head as the interface standard.

HDMI interface

HDMI stands for "High Definition Multimedia". The HDMI interface can provide up to 5Gbps data transmission bandwidth, and can transmit uncompressed audio signals and high-resolution video signals. At the same time, there is no need to perform digital/analog or analog/digital conversion before signal transmission, which can ensure the highest quality audio and video signal transmission.

DVI interface

DVI stands for Digital Visual Interface. It is an interface standard launched by DDWG (Digital Display Working Group) in 1999 by Silicon, Image, Intel, Compaq, IBM, HP, NEC, Fujitsu and other companies. DVI-D interface can only receive digital signals. There are only 24 pins in 3 rows and 8 columns on the interface, and one pin in the upper right corner is empty, which is not compatible with analog signals.

How should we choose among these four interfaces? Table 1 is a simple comparison and analysis of the above four interfaces. We find that the HD-SDI interface is currently the better high-definition interface, and the HD-SDI interface is also the current standard high-definition interface in the broadcasting and television industry.

· HD-SDI is more often used for front-end equipment to transmission equipment, or front-end equipment to center/control equipment, due to its longer transmission distance and higher transmission rate. For example, the connection between HD-SDI camera and HD-SDI optical transceiver/matrix;

· In situations where the transmission distance is not very far, such as in a control center room, and the transmission rate is very high, HDMI and DVI can meet the requirements. For example, the connection between a matrix supporting HDMI/DVI and a monitor/large-screen splicing wall;

Display terminals generally have a YPbPr interface. Although the YPbPr transmission rate is low, it is still used in some occasions where the transmission rate is not high because its transmission line is relatively cheap and the transmission distance is long.

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Advantages of HD-SDI

HD-SDI is a broadcast-grade high-definition digital input and output standard that has been successfully applied in the field of radio and television. The HD-SDI standard stipulates the use of coaxial cable and BNC connector as the interface standard. The effective distance is 100 meters. Its greatest advantages are:

Maturity of interface technology. HD-SDI is a very mature interface technology that supports high-speed transmission of video signals with almost zero delay. However, ordinary IP cameras have a large delay due to the need for data encapsulation and encoding.

The physical form of the HD-SDI transmission protocol is the BNC interface, which is a very important feature for monitoring projects. That is to say, when we convert the existing traditional analog framework system into a high-definition monitoring system, we do not need to rewire, but only need to replace the imaging part, which will save huge time and labor costs for the project;

Single-channel HD-SDI can transmit lossless 1080p high-definition digital video signals up to 1485Mbps;

It supports multi-channel parallel transmission, up to six channels in parallel. At this time, the transmission bandwidth of HD-SDI will reach 8.9Gbps ​​data transmission flow.

Non-compressed solution architecture

Charge-coupled device (CCD) cameras use FPGAs for signal synthesis and color processing and are connected to CCD drivers. The CCD driver in turn drives the CCD, mechanical shutter control, and trigger control. The analog-to-digital converter converts the input video signal into a digital format and then stores it in an off-chip memory. When the transmission of the entire frame of data is completed, the FPGA synthesizes the data in the memory and sends it to the network using the HD-SDI standard. The processing time required from trigger to HD-SDI output is 1 second or less. The FPGA can also control the memory and ADC (analog-to-digital converter).

Technical Difficulties of Non-Compression Solutions

The core of the non-compression solution system is FPGA, which is mainly responsible for two aspects: CCD drive algorithm implementation and protocol conversion. The technical difficulties of the entire solution are also concentrated on these two points.

The implementation of the CCD drive algorithm is our common 2A algorithm (automatic exposure and automatic white balance). If the front end is connected to a lens module, the 3A algorithm is implemented (with an additional automatic focus compared to the 2A algorithm). At present, the core of the compression solution architecture is DSP, because there are many high-definition solutions to choose from, such as: TI, HiSilicon, Maxim, Shengmai, Atmel, FreeScale, etc. When these manufacturers promote their own solutions, they generally integrate 2A/3A algorithms into the solutions. There are also companies that specialize in providing 2A/3A algorithms, such as Taiwan's Oppro. Therefore, when implementing a high-definition compression solution, development engineers can bypass the complex 2A/3A algorithm and directly apply it to the high-definition solution, reducing R&D investment and reducing project implementation time.

Compared with compression solutions, except for a few traditional companies with technical accumulation in this area, the application of FPGA-based 2A/3A algorithms on the market is not very widespread, and there are not many options available. If companies invest in the research and development of 2A/3A algorithms themselves, it will be a relatively large investment in terms of cost and time. Therefore, the biggest difficulty in the application of non-compression solutions lies in the implementation of CCD drive algorithms. Protocol conversion is less difficult than the implementation of CCD drive algorithms. There are many new optical terminals, matrices and other transmission, conversion and control devices based on HD-SDI on the market, but there are very few cameras based on HD-SDI. This has a lot to do with the low threshold for HD-SDI protocol implementation and the high threshold for CCD drive algorithms. Altera and Xilinx have both launched solutions based on HD-SDI. With the technical assistance of these two manufacturers, HD-SDI can be implemented at a lower cost and in a shorter time.

Conclusion

High-definition cameras based on the HD-SDI standard interface can truly achieve 1080p high-definition video data output and achieve real-time image acquisition at 25 frames per second. Emerging digital movies have higher definition images, higher resolutions, and richer colors (such as SMPTE 428-1K PGYQR 4096×2160 4∶4∶4/X'Y'Z'/12-bit@24P ). The SMPTE N26 Technical Committee is developing the 435M series of standards: a serial data interface with a clock frequency of 10.692 Gb/s, whose physical media is an optical cable that complies with the IEC 60793-2 standard and an optical connector that complies with the IEC 61754-20 standard. This 10 Gb/s-level interface can multiplex up to 8 HD-SDI data streams together, and can also map existing 1.5 Gb/s and 3 Gb/s data structures to the 10 Gb/s interface. Therefore, high-definition cameras based on the HD-SDI standard interface will have broad application prospects.