H.264, or MPEG-4 Part Ten (AVC, Advanced Video Coding), is the latest generation of video compression standards jointly launched by the International Telecommunication Standardization Department ITU-T and the International Organization for Standardization ISO/IEC in 2003. At present, the H.264 standard is widely used in wired/wireless video remote monitoring, network interactive media, digital TV and video conferencing, etc.
Chinese name H.264 + alias MPEG-4 Part 10 Standard time for quality video compression in 2003
table of Contents
1 Basic introduction
2 Technical highlights
3 performance comparison
Basic introductionedit
H.264 is a new digital video developed by the joint video team (JVT: joint video team) of VCEG (Video Coding Experts Group) of ITU-T and MPEG (Moving Picture Coding Experts Group) of ISO/IEC
Video server
Video server
Coding standard, it is both ITU-T H.264 and ISO/IEC MPEG-4 Part 10. The solicitation of drafts started in January 1998. The first draft was completed in September 1999. The test model TML-8 was developed in May 2001. The FCD board of H.264 was passed at the 5th meeting of JVT in June 2002. . Officially released in March 2003. Like the previous standard, H.264 is also a hybrid coding mode of DPCM plus transform coding. However, it adopts a simple design of "return to basics", without many options, and obtains much better compression performance than H.263++; strengthens the adaptability to various channels, 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; its basic system is open, and no copyright is required for use. Technically, there are many highlights in the H.264 standard, such as unified VLC symbol coding, high-precision, multi-mode displacement estimation, integer transformation based on 4×4 blocks, and layered coding syntax. These measures make H.264 algorithm have very high coding efficiency, under the same reconstructed image quality, it can save about 50% of the code rate than H.263. H.264's code stream structure has strong network adaptability, increases error recovery capabilities, and can well adapt to IP and wireless network applications.
Technical highlightsedit
Layered design
The H.264 algorithm can be conceptually divided into two layers: the video coding layer (VCL: Video Coding Layer) is responsible for efficient video content representation, and the network abstraction layer (NAL: Network Abstraction Layer) is responsible for the appropriate way required by the network Pack and transmit data. A packet-based interface is defined between VCL and NAL, and packaging and corresponding signaling are part of NAL. In this way, the tasks of high coding efficiency and network friendliness are completed by VCL and NAL respectively. The VCL layer includes block-based motion compensation hybrid coding and some new features. Like the previous video coding standards, H.264 does not include functions such as pre-processing and post-processing in the draft, which can increase the flexibility of the standard. NAL is responsible for encapsulating data using the segment format of the underlying network, including framing, signaling of logical channels, utilization of timing information, or sequence end signals. For example, NAL supports video transmission formats on circuit-switched channels, and supports video transmission formats on the Internet using RTP/UDP/IP. NAL includes its own header information, segment structure information, and actual load information, that is, the upper layer VCL data. (If data segmentation technology is used, the data may consist of several parts).
High-precision, multi-mode motion estimation
H.264 supports motion vectors with 1/4 or 1/8 pixel precision. At 1/4 pixel accuracy, a 6-tap filter can be used to reduce high-frequency noise. For motion vectors with 1/8 pixel accuracy, a more complex 8-tap filter can be used. When performing motion estimation, the encoder can also choose "enhanced" interpolation filters to improve the effect of prediction. In the motion prediction of H.264, a macro block (MB) can be divided into different sub-blocks as shown in Figure 2, forming block sizes of 7 different modes. This multi-mode flexible and detailed division is more suitable for the shape of the actual moving objects in the image, which greatly improves the accuracy of motion estimation. In this way, 1, 2, 4, 8 or 16 motion vectors can be included in each macro block. In H.264, the encoder is allowed to use more than one previous frame for motion estimation, which is the so-called multi-frame reference technology. For example, if 2 or 3 frames are just coded reference frames, the encoder will select a better prediction frame for each target macroblock, and indicate for each macroblock which frame is used for prediction.
Integer transformation
H.264 is similar to the previous standard, using block-based transform coding for the residual, but the transform is an integer operation rather than a real number operation, and its process is basically similar to DCT. The advantage of this method is that the same precision transformation and inverse transformation are allowed in the encoder and the decoder, and it is convenient to use simple fixed-point operations. In other words, there is no "inverse transformation error". The unit of transformation is 4×4 blocks, instead of 8×8 blocks commonly used in the past. As the size of the transform block is reduced, the division of the moving object is more accurate, so that not only the transformation calculation amount is smaller, but the convergence error at the edge of the moving object is also greatly reduced. In order to make the small-size block transformation method not produce the grayscale difference between the blocks in the larger smooth area in the image, the DC coefficient of 16 4×4 blocks of the intra-frame macroblock brightness data (each small block One, a total of 16) performs the second 4×4 block transformation, and performs 2×2 block transformation on the DC coefficients of 4 4×4 blocks of chrominance data (one for each small block, 4 in total).
In order to improve the rate control ability of H.264, the change of the quantization step size is controlled at about 12.5%, instead of a constant increase. The normalization of the transform coefficient amplitude is processed in the inverse quantization process to reduce the computational complexity. In order to emphasize the fidelity of color, a smaller quantization step size is adopted for the chrominance coefficient.
Unified VLC
There are two methods of entropy coding in H.264, one is to use unified VLC (UVLC: Universal VLC) for all symbols to be coded, and the other is to use content-adaptive binary arithmetic coding (CABAC: Context-Adaptive Binary Arithmetic Coding). CABAC is optional, and its coding performance is slightly better than UVLC, but the computational complexity is also higher. UVLC uses a code word set of unlimited length, and the design structure is very regular, and different objects can be coded with the same code table. This method can easily generate a codeword, and the decoder can easily identify the prefix of the codeword, and UVLC can quickly obtain resynchronization when a bit error occurs.
Intra prediction
In the previous H.26x series and MPEG-x series standards, inter-frame prediction methods are used. In H.264, intra-frame prediction is available when encoding Intra images. For each 4×4 block (except for the special treatment of the edge block), each pixel can be predicted with the different weighted sum of the 17 closest previously encoded pixels (some weights can be 0), that is, this pixel 17 pixels in the upper left corner of the block. Obviously, this kind of intra-frame prediction is not in time, but a predictive coding algorithm performed in the spatial domain, which can remove the spatial redundancy between adjacent blocks and achieve more effective compression.
As shown in Figure 4, a, b, ..., p in the 4×4 square are 16 pixels to be predicted, and A, B, ..., P are pixels that have been coded. For example, the value of point m can be predicted by the formula (J+2K+L+2)/4, or by the formula (A+B+C+D+I+J+K+L)/8, etc. . According to the selected prediction reference points, there are 9 different modes for brightness, but only 1 mode for intra prediction of chroma.
For IP and wireless environments
The H.264 draft contains tools for error elimination to facilitate the transmission of compressed video in an environment with frequent errors and packet loss, such as the robustness of transmission in mobile channels or IP channels. In order to resist transmission errors, time synchronization in the H.264 video stream can be accomplished by using intra-frame image refresh, and spatial synchronization is supported by slice structured coding. At the same time, in order to facilitate resynchronization after a bit error, a certain resynchronization point is also provided in the video data of an image. In addition, intra-frame macroblock refresh and multiple reference macroblocks allow the encoder to consider not only coding efficiency, but also the characteristics of the transmission channel when determining the macroblock mode.
In addition to using the change of the quantization step size to adapt to the channel code rate, in H.264, the data segmentation method is often used to deal with the change of the channel code rate. Generally speaking, the concept of data segmentation is to generate video data with different priorities in the encoder to support the quality of service QoS in the network. For example, the syntax-based data partitioning method is adopted to divide the data of each frame into several parts according to its importance, which allows the less important information to be discarded when the buffer overflows. A similar temporal data partitioning method can also be adopted, which is accomplished by using multiple reference frames in P and B frames.
In the application of wireless communication, we can support large bit rate changes of the wireless channel by changing the quantization precision or space/time resolution of each frame. However, in the case of multicast, it is impossible to require the encoder to respond to varying bit rates. Therefore, unlike the FGS (Fine Granular Scalability) method used in MPEG-4 (with lower efficiency), H.264 uses stream switching SP frames instead of hierarchical coding.
Performance comparisonedit
TML-8 is a test for H.264. The PSNR provided by the test results has clearly shown that compared to the performance of MPEG-4 (ASP: Advanced Simple Profile) and H.263++ (HLP: High Latency Profile), the results of H.264 have obvious advantages.
The PSNR of H.264 is obviously better than that of MPEG-4 (ASP) and H.263++ (HLP). In the comparison test of 6 speeds, the PSNR of H.264 is 2dB higher than MPEG-4 (ASP) on average. It is 3dB higher than H.263 (HLP) on average. The 6 test rates and their related conditions are: 32 kbit/s rate, 10f/s frame rate and QCIF format; 64 kbit/s rate, 15f/s frame rate and QCIF format; 128kbit/s rate, 15f/s Frame rate and CIF format; 256kbit/s rate, 15f/s frame rate and QCIF format; 512 kbit/s rate, 30f/s frame rate and CIF format; 1024 kbit/s rate, 30f/s frame rate and CIF format.
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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?
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1.How far you want to cover in diameter ?
2.How tall of you tower ?
3.Where are you from ?
And we will give you more professional advice.
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.
Our solution has FM Radio Station / Analog TV Station / Digital TV Station / Audio Video Studio Equipment / Studio Transmitter Link / Transmitter Telemetry System / Hotel TV System / IPTV Live Broadcasting / Streaming Live Broadcast / Video Conference / CATV Broadcasting system.
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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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