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    Introduction to H.264

     

    H.264/MPEG-4 AVC (H.264) is the latest and most promising video compression standard since the release of the MPEG-2 video compression standard in 1995. H.264 is the latest international video coding standard jointly developed by the joint development group of ITU-T and ISO/IEC. Through this standard, the compression efficiency under the same image quality has been increased by more than 2 times compared with the previous standard. Therefore, H.264 is generally regarded as the most influential industry standard.

       One, the development history of H.264

    H.264 was called H.26L when it was proposed by the Video Coding Experts Group of the ITU in 1997, and was called MPEG4 Part10 (MPEG4 AVC) or H.264 (JVT) after ITU and ISO collaborated on research. .

       High-level technical background of H.264

       The main goal of the H.264 standard is to provide better image quality under the same bandwidth than other existing video coding standards.

       And, compared with the previous international standards such as H.263 and MPEG-4, H.264 has the biggest advantages in the following four aspects:

      1. Each video frame is separated into blocks composed of pixels, so the encoding process of the video frame can reach the block level.

      2. The spatial redundancy method is used to perform spatial prediction, conversion, optimization and entropy coding (variable length coding) on ​​some original blocks of the video frame.

      3. The temporary storage method is adopted for different blocks of consecutive frames, so that only the changed parts of the consecutive frames need to be coded. The algorithm uses motion prediction and motion compensation to complete. For some specific blocks, a search is performed on one or more frames that have been coded to determine the motion vector of the block, and thus the main block is predicted in the subsequent coding and decoding.

      4. The residual space redundancy technology is adopted to encode the residual blocks in the video frame. For example: for the difference between the source block and the corresponding prediction block, conversion, optimization and entropy coding are used again.

      H.264 features and advanced advantages

    H.264 is a new generation of digital video compression format after MPEG4 jointly proposed by the International Organization for Standardization (ISO) and the International Telecommunication Union (ITU). It not only retains the advantages and essence of previous compression technologies, but also has incomparable other compression technologies. Many advantages.

      1. Low Bit Rate: Compared with compression technologies such as MPEG2 and MPEG4 ASP, under the same image quality, the amount of data compressed using H.264 technology is only 1/8 of MPEG2 and 1/3 of MPEG4. Obviously, the adoption of H.264 compression technology will greatly save users' download time and data traffic charges.

      2. High-quality images: H.264 can provide continuous and smooth high-quality images (DVD quality).

      3. Strong fault tolerance: H.264 provides necessary tools to solve errors such as packet loss that are prone to occur in an unstable network environment.

      4. Strong network adaptability: H.264 provides a Network Adaptation Layer, which enables H.264 files to be easily transmitted on different networks (such as the Internet, CDMA, GPRS, WCDMA, CDMA2000, etc.).

       2. H.264 standard overview

      H.264, like the previous standard, is also a hybrid coding mode of DPCM plus transform coding. However, it adopts a concise design of "back to basics" and does not require many options to obtain much better compression performance than H.263++; it strengthens the adaptability to various channels and adopts a "network-friendly" structure and syntax. Conducive to the processing of errors and packet loss; a wide range of application targets to meet the needs of different speeds, different resolutions and different transmission (storage) occasions.

      Technically, it concentrates the advantages of previous standards and absorbs the experience accumulated in standard formulation. Compared with H.263 v2 (H.263+) or MPEG-4 simple profile (Simple Profile), H.264 can save up to 50 at most code rates when using the best encoder similar to the above encoding method % Bit rate. H.264 can continue to provide high video quality at all bit rates. H.264 can work in low-latency mode to adapt to real-time communication applications (such as video conferencing), and it can also work well in applications without delay restrictions, such as video storage and server-based video streaming applications . H.264 provides tools for handling packet loss in packet transmission networks, and tools for handling bit errors in error-prone wireless networks.

    At the system level, H.264 proposes a new concept, which is a conceptual division between the Video Coding Layer (VCL) and the Network Abstraction Layer (NAL), the former being the core of the video content The expression of compressed content, the latter is the expression delivered through a specific type of network, this structure facilitates the packaging of information and better priority control of information. The system coding block diagram of H.264 is shown as in Fig. 1.

    Figure 1 H.264 system block diagram

       Three, the key technology of H.264 standard

          1. Intraframe prediction coding

       Intra-frame coding is used to reduce the spatial redundancy of the image. In order to improve the efficiency of H.264 intra-frame coding, the spatial correlation of adjacent macroblocks is fully utilized in a given frame, and adjacent macroblocks usually contain similar attributes. Therefore, when encoding a given macroblock, first predict based on the surrounding macroblocks (typically based on the macroblock in the upper left corner, because this macroblock has been encoded), and then calculate the difference between the predicted value and the actual value The value is encoded, so that compared to directly encoding the frame, the bit rate can be greatly reduced.

       H.264 provides 6 modes for 4×4 pixel macroblock prediction, including 1 DC prediction and 5 directional prediction, as shown in Figure 2. In the figure, a total of 9 pixels from A to I of the adjacent block have been coded and can be used for prediction. If we choose mode 4, then the 4 pixels a, b, c, and d are predicted to be equal to E Values, e, f, g, and h4 pixels are predicted to be equal to F. For flat areas in the image that contain little spatial information, H.264 also supports 16×16 intra-frame coding.

     

    Figure 2 Intra coding mode

      2. Interframe predictive coding

       Inter-frame predictive coding uses temporal redundancy in consecutive frames for motion estimation and compensation. H.264 motion compensation supports most of the key features in the previous video coding standards, and flexibly adds more functions. In addition to supporting P frames and B frames, H.264 also supports a new inter-stream transmission Frame-SP frame. After the code stream contains SP frames, it can quickly switch between code streams with similar content but with different bit rates, and supports random access and fast playback modes at the same time.

       H.264 motion estimation has the following four characteristics.

      (1) Segmentation of macroblocks of different sizes and shapes

       The motion compensation of each 16×16 pixel macroblock can adopt different sizes and shapes. H.264 supports 7 modes, as shown in Figure 4. The motion compensation of the small block mode improves the performance of the motion detailed information processing, reduces the block effect, and improves the image quality.

      (2) High-precision sub-pixel motion compensation

       In H.263, half-pixel precision motion estimation is used, while in H.264, 1/4 or 1/8 pixel precision motion estimation can be used. When the same accuracy is required, the residual error after H.264 using 1/4 or 1/8 pixel accuracy motion estimation is smaller than the residual error after H.263 using half pixel accuracy motion estimation. In this way, under the same accuracy, H.264 requires a smaller bit rate in inter-frame coding.

      (3) Multi-frame prediction

       H.264 provides an optional multi-frame prediction function. During inter-frame encoding, 5 different reference frames can be selected, which provides better error correction performance, which can improve the video image quality. This feature is mainly used in the following situations: periodic motion, translational motion, and changing the camera lens back and forth between two different scenes.

      (4) Deblocking filter

       H.264 defines an adaptive filter to remove block effects, which can handle horizontal and vertical block edges in the prediction loop, greatly reducing block effects.

      3. Integer transformation

    In terms of transformation, H.264 uses a transformation similar to DCT based on 4×4 pixel blocks, but uses an integer-based spatial transformation. There is no inverse transformation. There is an error problem due to the trade-off. The transformation matrix is ​​such as Shown in Figure 5. Compared with floating-point operations, integer DCT transform will cause some extra errors, but because the quantization after DCT transform also has quantization error, compared with it, the influence of quantization error caused by integer DCT transform is not big. In addition, the integer DCT transform also has the advantages of reducing the amount of calculation and complexity, which is conducive to transplantation to fixed-point DSP.

      4. Quantify

    There are 32 different quantization steps in H.264, which is very similar to the 31 quantization steps in H.263, but in H.264, the step size is progressive at a compound rate of 12.5%, and Not a fixed constant.
    In H.264, there are also two ways to read transform coefficients: Zigzag scanning and double scanning. In most cases, a simple zigzag scan is used; dual scan is only used in a block with a smaller quantization level, which helps to improve coding efficiency.

       5. Entropy coding

       The last step of video coding processing is entropy coding. Two different entropy coding methods are used in H.264: Universal Variable Length Coding (UVLC) and Text-based Adaptive Binary Arithmetic Coding (CABAC).
    In standards such as H.263, different VLC code tables are used according to the type of data to be coded, such as transform coefficients and motion vectors. The UVLC code table in H.264 provides a simple method, no matter what type of data the symbol represents, the uniform variable word length code table is used. The advantage is simplicity; the disadvantage is that a single code table is derived from the probability statistical distribution model, without considering the correlation between code symbols, and the effect is not very good at medium and high code rates.
    Therefore, the optional CABAC method is also provided in H.264. Arithmetic coding enables the use of probability models of all syntactic elements (transform coefficients, motion vectors) on both encoding and decoding. In order to improve the efficiency of arithmetic coding, through the process of content modeling, the basic probability model can adapt to the statistical characteristics that change with the video frame. Content modeling provides conditional probability estimation of coded symbols. Using a suitable content model, the correlation between symbols can be removed by selecting the corresponding probability model of the coded symbols adjacent to the currently coded symbol. Different syntactic elements are usually kept Different models.

       Fourth, the application of H.264 in video conferencing

       At present, most video conferencing systems adopt H.261 or H.263 video coding standards, and the emergence of H.264 enables H.264 to reduce the bit rate by 50% compared with H.263 at the same rate. In other words, even if users only use 384kbit/s bandwidth, they can enjoy high-quality video services up to 768kbit/s under H.263. H.264 not only helps to save huge expenses, but also improves the efficiency of resource use, and at the same time enables commercial-quality video conferencing services to have more potential customers.

       At present, there are already a few video conferencing products of manufacturers that support the H.264 protocol, and the manufacturers are committed to popularizing the new industry standard of H.264. As other video conferencing solution vendors follow their example one after another, we will be able to fully experience the advantages of H.264 video services.

     

    How far(long) the transmitter cover?

    The transmission range depends on many factors. The true distance is based on the antenna installing height , antenna gain, using environment like building and other obstructions , sensitivity of the receiver, antenna of the receiver . Installing antenna more high and using in the countryside , the distance will much more far.

    EXAMPLE 5W FM Transmitter use in the city and hometown:

    I have a USA customer use 5W fm transmitter with GP antenna in his hometown ,and he test it with a car, it cover 10km(6.21mile).

    I test the 5W fm transmitter with GP antenna in my hometown ,it cover about 2km(1.24mile).

    I test the 5W fm transmitter with GP antenna in Guangzhou city ,it cover about only 300meter(984ft).

    Below are the approximate range of different power FM Transmitters. ( The range is diameter )

    0.1W ~ 5W FM Transmitter :100M ~1KM

    5W ~15W FM Ttransmitter : 1KM ~ 3KM

    15W ~ 80W FM Transmitter : 3KM ~10KM

    80W ~500W FM Transmitter : 10KM ~30KM

    500W ~1000W FM Transmitter : 30KM ~ 50KM

    1KW ~ 2KW FM Transmitter : 50KM ~100KM

    2KW ~5KW FM Transmitter : 100KM ~150KM

    5KW ~10KW FM Transmitter : 150KM ~200KM

    How to contact us for the transmitter?

    Call me +8615915959450 OR
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    1.How far you want to cover in diameter ?
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    About Us

    FMUSER.ORG is a system integration company focusing on RF wireless transmission / studio video audio equipment / streaming and data processing .We are providing everything from advice and consultancy through rack integration to installation, commissioning and training.
     
    We offer FM Transmitter, Analog TV Transmitter, Digital TV transmitter, VHF UHF Transmitter, Antennas, Coaxial Cable Connectors, STL, On Air Processing, Broadcast Products for the Studio, RF Signal Monitoring, RDS Encoders, Audio Processors and Remote Site Control Units, IPTV Products, Video / Audio Encoder / Decoder, designed to meet the needs of both large international broadcast networks and small private stations alike.
     
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    We have customers of public and commercial broadcasters, telecom operators and regulation authorities , and we also offer solution and products to many hundreds of smaller, local and community broadcasters .
     
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