1、 Video transmission principle
Video is to use the principle of human eye vision retention, through playing a series of pictures, make the human eye feel of movement. The video is transmitted only, and the amount of video is very large, which is unacceptable to the existing network and storage. In order to make video easy to transmit and store, people find that video has a lot of repeated information. If the duplicate information is removed from the transmission end and recovered at the receiving end, the file of video data is greatly reduced, so H.264 video compression standard is available.
The original image data in the video will be compressed in H.264 encoding format, and the audio sampling data will be compressed in AAC coding format. After coding and compressing, video content is really conducive to storage and transmission. However, when watching the playback, the decoding process is also needed accordingly. Therefore, it is obvious that a kind of convention can be understood by both encoder and decoder is required between encoding and decoding. In terms of video image coding and decoding, this Convention is simple:
The encoder encodes multiple images and produces a GOP (group of pictures) in a segment. When playing, the decoder reads a section of GOP for decoding, then reads the picture and then renders the display. GOP (group of pictures) is a series of continuous pictures, which consists of one I frame and several B / P frames. It is the basic unit of video image encoder and decoder access. Its arrangement sequence will be repeated until the end of image. I frame is an internal coding frame (also known as keyframe), P frame is forward prediction frame (forward reference frame), and frame B is bidirectional interpolation frame (bidirectional reference frame). In short, frame I is a complete picture, while P and B record changes relative to I frame. Without I frames, P and B frames cannot be decoded.
In H.264 compression standard, I frame, P frame and B frame are used to represent the transmitted video picture.
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2、 Frame I, P, frame B, GOP
Frame I
Frame I: that is, intra coded picture. Frame I represents key frame, and you can understand that the picture of this frame is completely preserved; decoding only needs the data of the frame to complete (because it contains the complete picture). Also known as internal picture, I frame is usually the first frame of each GOP (a video compression technology used by MPEG). After moderate compression, it can be used as a random access reference point and can be regarded as an image. In MPEG coding, some video frame sequences are compressed into I frames; some are compressed into P frames; and some are compressed into B frames. I frame method is an in frame compression method, also known as "key frame" compression method. I frame method is a compression technology based on DCT (discrete cosine transform), which is similar to JPEG compression algorithm. The compression ratio of 1/6 can be achieved by using I frame compression without obvious compression trace.
[I frame features]
1. it is a full frame compression encoding frame. It encodes and transmits the whole frame image information by JPEG compression;
2. the complete image can be reconstructed only by using the data of I frame during decoding;
3. frame I describes the background of the image and the details of the moving body;
4. I frame is not generated by referring to other pictures;
5. I frame is the reference frame of P frame and B frame (its quality directly affects the quality of each frame in the same group);
6. I frame is the basic frame (the first frame) of frame group GOP, and only one I frame is in a group;
7. Motion vector is not required for frame I;
8. the information of I frame is relatively large.
[I frame coding process]
(1) The intra prediction is carried out to determine the intra prediction mode.
(2) The pixel value subtracts the predicted value to get the residual.
(3) The residual is transformed and quantified.
(4) Variable length coding and arithmetic coding.
(5) The image is reconstructed and filtered, and the image is used as reference frame of other frames.
For example, in video conference system, the image sent by the terminal to MCU (or MCU to terminal) does not send a complete picture to the remote end at a time, but only the part that changes after sending the picture based on the previous picture. If the network condition is not good, the terminal will receive the remote end or send to the remote picture with packet loss and the image will be blocked. In this case, if there is no frame mechanism to send a new complete image to the local (or local re send a new complete image to the remote), the flower screen of the output image of the terminal will be displayed The phenomenon of Caton will become more and more serious, which will cause the meeting to fail to be carried out normally.
In the process of video screen playing, if I frame is lost, the P frame behind will also appear as soon as it can not be solved, and the phenomenon of black screen will appear; if P frame is lost, the video screen will show the phenomenon of flower screen and mosaic.
In video conference system, I frame will only occur within the bandwidth limit of the meeting, and will not exceed the meeting bandwidth and take effect. I frame mechanism not only exists in MCU, but also in TV wall server and video server. It is to solve the problem of packet loss in the case of bad network conditions, such as image screen and button, which will affect the normal progress of the meeting.
P frame
P frame: that is, predictive coded picture. P frame represents the difference between this frame and the previous key frame (or P frame). When decoding, the difference defined in this frame needs to be superimposed on the previous cached picture to generate the final picture. (that is, difference frame, P frame has no complete picture data, only data different from the previous frame)
[prediction and reconstruction of P frame]
P frame is a frame of reference I, in which the prediction value and motion vector of "a point" of P frame are found in the I frame, and the prediction difference and motion vector are transmitted together. The predicted value of "point" of P frame is found from the I frame according to the motion vector at the receiver and the difference is added to get the sample value of "point" of P frame, so that the complete P frame can be obtained.
[P frame features]
1. P frame is the encoding frame with 1-2 frames behind I frame;
2. P frame adopts motion compensation method to transmit the difference and motion vector (prediction error) between P frame and I frame;
3. the prediction value and prediction error in I frame must be summed up in decoding before the complete P frame image can be reconstructed;
4. P frame belongs to forward prediction inter frame coding. It only refers to the I frame or P frame closest to it in front;
5. P frame can be the reference frame of P frame after it, or it can be the reference frame of B frame before and after it;
6. because P frame is a reference frame, it may cause the spread of decoding error;
7. because of the difference transmission, the compression of P frame is relatively high.
Frame B
Frame B: that is, bidirectionally predicted picture. Frame B is a two-way difference frame, that is, the difference between the frame and the front and rear frames is recorded in B frame. In other words, to decode B frame, not only the previous cache picture but also the decoded picture is needed. The final picture is obtained by the superposition of the front and rear pictures and the data of the frame. B frame compression rate is high, but CPU will be more tired when decoding.
[prediction and reconstruction of B frame]
Frame B is based on the front I or P frame and the P frame at the back as the reference frame. The prediction value and two motion vectors of "a point" of frame B are found, and the prediction difference and motion vector are taken to transmit. The receiver "finds out (calculates)" predicted value in two reference frames according to motion vector and sums up with difference, and obtains the sample value of "certain point" of frame B, thus obtaining complete B frame. Two way prediction coding between frames by motion prediction
[B frame features]
1. Frame B is predicted by the front I or P frame and the later P frame;
2. The B frame transmits the prediction error and motion vector between it and the I frame or P frame and the P frame at the back;
3. B frame is a bidirectional prediction coding frame;
4. The compression ratio of frame B is the highest, because it only reflects the change of the main body of the motion between C reference frames, and the prediction is more accurate;
5. B frame is not a reference frame, and it will not cause the spread of decoding error
[why B frame is needed]
From the above, we know that the decoding algorithm of I and P is relatively simple and the resource occupation is relatively small. I just need to complete it by myself. P. it only needs the decoder to cache the previous picture. When encountering P, it is better to use the previously cached picture. If the video stream only has I and P, the decoder can read and decode while reading and linear advance. It is very comfortable for us to move forward I don't like it. So why do you want to introduce B frame?
Many movies on the network adopt B frame, because the difference between front and rear frames recorded by B frame can save more space than P frame. However, the file is small, and the decoder is in trouble. In decoding, not only the picture cached before, but also the next I or P picture (that is, pre read and decode) is used. Moreover, frame B can not be simply lost, Because B frame actually contains picture information, if it is simply lost and repeated with the previous picture, it will cause the picture card (in fact, it is lost). In order to save space, the movies on the network often use quite a lot of B frames. The more B frames are used, it will cause more trouble for players who do not support B frame, and the more the picture will be stuck.
GOP (sequence) and IDR
In h264, the image is organized in sequence, and one sequence is a data stream after image coding.
The first image in a sequence is called an IDR image (refresh the image immediately), and the IDR image is an I frame image. H. 264 introduces IDR image to resynchronize decoding. When decoder decodes IDR image, it will immediately empty the reference frame queue, output or discard all decoded data, re search parameter set and start a new sequence. In this way, if there is a major error in the previous sequence, you can get the opportunity to resynchronize here. Images after IDR images will never be decoded using data from previous IDR images.
A sequence is a series of data streams generated after the image coding with little content difference. When the motion changes less, a sequence can be very long, because less movement changes represent the content change of the image picture is very small, so you can make an I frame, and then always P frame and B frame. When the motion changes a lot, one sequence may be shorter, for example, it contains an I frame and 3 or 4 P frames.
In video coding sequence, GOP is group of picture, which refers to the distance between two I frames, and reference refers to the distance between two P frames. A group of pictures is formed between the two I frames, namely GOP (group of picture).
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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?
Call me +8618078869184 OR
Email me [email protected]
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.
We are using advanced technology products for all the systems, because we know the high reliability and high performance are so important for the system and solution . At the same time we also have to make sure our products system with a very reasonable price.
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 .
FMUSER.ORG has been exporting more than 15 years and have clients all over the world. With 13 years experience in this field ,we have a professional team to solve customer's all kinds of problems. We dedicated in supplying the extremely reasonable pricing of professional products & services. Contact email : [email protected]
Our Factory
We have modernization of the factory . You are welcome to visit our factory when you come to China.
At present , there are already 1095 customers around the world visited our Guangzhou Tianhe office . If you come to China , you are welcome to visit us .
At Fair
This is our participation in 2012 Global Sources Hong Kong Electronics Fair . Customers from all over the world finally have a chance to get together.
Where is Fmuser ?
You can search this numbers " 23.127460034623816,113.33224654197693 " in google map , then you can find our fmuser office .
FMUSER Guangzhou office is in Tianhe District which is the center of the Canton . Very near to the Canton Fair , guangzhou railway station, xiaobei road and dashatou , only need 10 minutes if take TAXI . Welcome friends around the world to visit and negotiate .
Contact: Sky Blue
Cellphone: +8618078869184
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Wechat: +8618078869184
E-mail: [email protected]
QQ: 727926717
Skype: sky198710021
Address: No.305 Room HuiLan Building No.273 Huanpu Road Guangzhou China Zip:510620
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English: We accept all payments , such as PayPal, Credit Card, Western Union, Alipay, Money Bookers, T/T, LC, DP, DA, OA, Payoneer, If you have any question , please contact me [email protected] or WhatsApp +8618078869184
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Pay by Western Union to me :
First name/Given name: Yingfeng
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Full name: Yingfeng Zhang
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T/T . Pay by T/T (wire transfer/ Telegraphic Transfer/ Bank Transfer)
First BANK INFORMATION (COMPANY ACCOUNT):
SWIFT BIC: BKCHHKHHXXX
Bank name: BANK OF CHINA (HONG KONG) LIMITED, HONG KONG
Bank Address: BANK OF CHINA TOWER, 1 GARDEN ROAD, CENTRAL, HONG KONG
BANK CODE: 012
Account Name : FMUSER INTERNATIONAL GROUP LIMITED
Account NO. : 012-676-2-007855-0
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Second BANK INFORMATION (COMPANY ACCOUNT):
Beneficiary: Fmuser International Group Inc
Account Number: 44050158090900000337
Beneficiary's Bank: China Construction Bank Guangdong Branch
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Address: NO.553 Tianhe Road, Guangzhou, Guangdong,Tianhe District, China
**Note: When you transfer money to our bank account, please DO NOT write anything in the remark area, otherwise we won't be able to receive the payment due to government policy on international trade business.
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* It will be sent in 1-2 working days when payment clear.
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* If the packages is below 2kg,we will be shipped via post airmail, it will take about 15-25days to your hand.
If the package is more than 2kg,we will ship via EMS , DHL , UPS, Fedex fast express delivery,it will take about 7~15days to your hand.
If the package more than 100kg , we will send via DHL or air freight. It will take about 3~7days to your hand.
All the packages are form China guangzhou.
* Package will be sent as a "gift" and declear as less as possible,buyer don't need to pay for "TAX".
* After ship, we will send you an E-mail and give you the tracking number.
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For Warranty .
Contact US--->>Return the item to us--->>Receive and send another replace .
Name: Liu xiaoxia
Address: 305Fang HuiLanGe HuangPuDaDaoXi 273Hao TianHeQu Guangzhou China.
ZIP:510620
Phone: +8618078869184
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