Basic concepts of video codec

Analog technology was used in the field of audio and video in the early stage, and it has been developed into digital technology. The main advantages of digitization are: high reliability, can eliminate transmission and storage loss, and facilitate computer processing and network transmission. After digitization, audio and video processing has entered the field of computer technology. Audio and video processing is essentially the processing of computer data.

The original video data generated by image information acquisition is very large. For some applications that are directly played locally after acquisition, compression technology is not needed. But in reality, more applications involve video transmission and storage. Transmission network and storage equipment can not tolerate the huge amount of data of original video data. The original video data must be encoded and compressed before transmission and storage.

There are two kinds of signal components in all the actual program materials: abnormal, unforeseen and predictable. The abnormal component is called entropy, which is the real information in the signal. The rest is called redundancy because it is not required information. Redundancy can be spatial, for example, in large areas of an image, adjacent pixels have almost the same value. Redundancy can also be temporal, such as a similar part between continuous images. In all compression system encoders, entropy is separated from redundancy, only entropy is encoded and transmitted, and redundancy is calculated from the signals sent by encoder in decoder.

Intra frame coding is a space domain coding, which uses spatial redundancy to compress images. It processes an independent image and does not span multiple images. The spatial domain coding depends on the similarity between adjacent pixels in an image and the main spatial frequency of the pattern area.
JPEG standard is used for still image (i.e. picture), only space domain compression is used, and only intra frame coding is used.

Inter frame coding is time domain coding, which uses the temporal redundancy between a set of continuous images to compress images. If a frame image can be used by the decoder, the decoder can obtain the next frame image only by using the difference between the two frames. For example, the similarity of the moving flat frame images is large and the difference is small, while the images with intense exercise are similar and different. When a frame of complete image information is obtained, the difference value between the image and the latter frame can be used to calculate the image of the latter frame, so that the data amount can be compressed. Time domain coding relies on the similarity between consecutive images, and the current image is predicted by using the received image information as much as possible.

MPEG standard is used for moving image (i.e. video), which uses space domain coding and time domain coding, so it is used in combination with intra frame coding and inter frame coding.

A set of continuous images record the movement of the target. Motion vector is used to measure the motion degree of the target between two frames. The motion vector is composed of horizontal displacement and vertical displacement.

The motion of the target reduces the similarity between images and increases the amount of data difference. Motion compensation reduces the amount of data difference between images by running vectors.

The following figure shows the schematic diagram of motion compensation. When a target moves, its position changes but the shape color and so on remain unchanged. The encoder can reduce the image difference by using motion vector, and the decoder can move the target to the right position according to the motion vector in the image difference. If the image is ideal, there is no change in any attribute except the moving position, the difference between the two images only contains the data amount of motion vector. Obviously, motion compensation can reduce the amount of image difference data significantly.

In the three consecutive images, the target block moves vertically, and the background block does not move. We consider how to obtain the current frame image (picture n):

In the screen n-1, because the background block is blocked by the target block, there is no background block related information.

In screen n+1, the data of background block is completely included, so screen n can obtain the background block from screen n-1.
How can I get screen n? The decoder can decode the screen n-1 and the picture n+1 first. The target block data in picture n-1 can be obtained by combining the target block data in the picture n-1 with the motion vector. The background block data in picture n can be obtained by the background block data in the screen n+1. The decoding sequence of the three pictures is n-1, n+1, n. The display sequence of the three pictures is n-1, N, n+1. Picture n is obtained by the calculation (predicted) of the former drawing face n-1 and the latter drawing surface n + 1, so this method is called bidirectional prediction (or forward prediction, bidirectional reference).

Frame I: the I frame (intracoded picture, often referred to as keyframe) contains a complete image information, which belongs to the intra coding image, without motion vector, and does not need to refer to other frame images during decoding. Therefore, channel switching can be performed at I frame image without loss or decoding of image. I frame image is used to prevent the accumulation and diffusion of errors. In closed GOP, the first frame of each GOP must be I frame, and the data of the current GOP will not refer to the data of the GOP before and after.

IDR frame: the IDR frame (instant decoding refresh picture) is a special I frame. When decoder decodes to IDR frame, DPB (decoded picture buffer) will be cleared, all decoded data will be output or discarded, and then a new decoding sequence will be started. The image after IDR frame does not refer to the image before IDR frame, so IDR frame can prevent the error propagation in video stream, and the IDR frame is also a secure access point for decoder and player.

P frame: P frame (predicted encoded picture) is an inter frame coding frame, which is predicted and encoded by using the previous I frame or P frame.

B frame: b-directional predicted picture (bi-directional predicted image frame) is an inter frame coding frame, and bi-directional prediction coding is carried out by using the I frame or P frame before and / or after. B frame can not be used as reference frame.
B frame has higher compression rate, but it needs more buffer time and higher CPU occupancy. Therefore, B frame is suitable for local storage and video on demand, but not for live broadcast system with high real-time requirements.

GOP (group of pictures) is a group of continuous images, which consists of one I frame and multiple b/p frames, which is the basic unit of codec access. Two parameters m and N commonly used in GOP structure specify the distance between two anchor frames (I frame or P frame) in GOP, and N specifies the size of one GOP. For example, m=3, n=15, GOP structure is ibbpbbpbbpbbpbb
Todo: is the interval of every two anchor frames the same in GOP? Speculation: not necessarily the same. In fact, many video files are analyzed, and the rules are not consistent. This is not completely clear, and it needs to be further accumulated, analyzed and confirmed.

Closed GOP: closed GOP only needs to refer to the images in this GOP, and does not need to refer to the data of the GOP before and after. This mode determines that the display order of closed GOP always starts with I frame and ends with P frame
Todo: is closed GOP bound to end with P frame? Speculation: this definition may not be necessary. Some video files GOP are seen ending with frame B.

Open GOP: the B frame in open GOP may be decoded using some frames of its previous GOP or the latter GOP. Open GOP will only appear when the stream contains B frames.

Todo: does open GOP stipulate that it starts with frame B and ends with P frame? Speculation: this definition may not be necessary. Start with frame B? The online data are different. End with P frame? Some video files GOP are seen ending with frame B.
In open GOP, the functions of common I frame and IDR frame are different, so it is necessary to distinguish two frame types clearly. In closed GOP, there is no difference between the function of ordinary I frame and IDR frame, so it can not be distinguished.

DTS (decoding time stamp) represents the decoding time of compressed frame.
PTS (presentation time stamp) indicates the display time of the original frame after decoding the compressed frame.

DTS and PTS are the same in audio. Because B frame needs two-way prediction in video, B frame depends on the frame before and after it, so the video decoding sequence and display sequence of B frame are different, that is, DTS and PTS are different. Of course, video without B frames has the same DTS and PTS. The following figure takes an open GOP diagram as an example to illustrate the decoding sequence and display sequence of video stream

Acquisition sequence refers to the sequence of image frame acquired by the original signal collected by image sensor.
Coding sequence refers to the sequence of image frames after encoder coding. The image frames stored in the local video file stored on disk are in the same order as the encoding order.

Transmission sequence refers to the sequence of image frames in the process of the encoded stream transmission in the network.
Decoding sequence refers to the order in which the decoder decodes the image frame.

Display order refers to the order in which image frames are displayed on the display.

The order of acquisition is the same as that of display. The coding sequence, transmission sequence and decoding sequence are the same.

Taking the "b[1]" frame as an example, it is shown that "b[1]" frame decoding requires reference to "i[0]" frame and "p[3]" frame, so "p[3]" frame must be decoded first than "b[1]". This leads to the inconsistency between decoding order and display order, and the frame displayed needs to be solved first

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.

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