Coding of sound information, Coding of video information...

Encoding of sound information

Sound is a continuous signal. When binary coding of an analog audio signal, the continuous signal is sampled, i.e. is replaced by a series of individual samples with a given periodicity. The quality of the binary coding depends on two parameters: the number of discrete signal levels being recognized and the number of samples per second. The sampling frequency is determined by the sampling rate. The digitized audio signal corresponds to the original analog signal in the event that the sampling frequency is not less than twice the highest harmonic frequency of this original signal. A person hears sounds in the range from 20 Hz to 20 kHz, so the maximum sampling frequency should be at least 40 kHz.

Simultaneously with the sampling, quantization of amplitude samples is performed - measurement of the instantaneous values ​​of the amplitude and their conversion into a digital code. The accuracy of the measurement depends on the number of bits of the codeword. With a code word length of 8 bits, the number of amplitude gradations is 256, with 16 bits - 65 536. Fig. 1.2 shows the process of digitization and quantization of an analog signal by 3-digit numbers.

For stereo sound, sampling and quantization are performed separately and independently for the left and right channels. To record and play sound in computers, use -

Time sampling and quantization by analog signal level

Fig. 1.2. Time sampling and quantization by analog signal level

There are sound cards that provide 8- or 16-bit samples.

Audio quality in discrete form may be poor (broadcast quality) at 8 bits and 5.5 kHz and high enough (audio CD quality) at 16 bits and 44 kHz. The volume of an audio file with a sound duration of 1 s with good sound quality will be V = 16 bits • 44 000 = 88 KB. To reduce the amount of storage of audio information, compression (compression) methods are used, reducing the volume without degrading the quality to 20% of the original.

When synthesizing the sound of various musical instruments, synthesizers are used, using methods such as the method of frequency modulation (FM synthesis) and wave tables ( WT -synthesis).

Video encoding

Video information is formed as a result of the organization of a streaming video sequence of "moving images". Digitization of the video fragment is associated with the problems of providing very high data rate and data rate. The problem of increasing the speed of exchange is solved by developing high-speed data storage devices. To reduce the amount of data contained in the video stream (up to 9 MB/s), compression of the data stream is usually used to write information to a computer. The file size of a compressed discrete still image depends on four parameters: image area, resolution, the number of bits required to represent the pixel, and the compression ratio. In the video, the number of still images forming it is added to this. The choice of the compression ratio is a compromise between the system capacity and the quality of the image to be restored. The higher the compression ratio, the lower the image quality. Therefore, the choice of these parameters is justified by technical and economic analysis and compression algorithm.

There are many technologies for image compression/recovery. The most popular is offered by the Joint Photographic Experts Group (JPEG) and allows to reduce the size of the graphic file by 10-12 times. For video compression, MPEG technology is used

(Motion Picture Expert Group). The MPEG algorithm converts the image η the stream of compressed data, given that a person seeing a moving object focuses on it, and the fixed background perceives to a lesser degree. This allows you to highlight changing and frozen fragments in the frame: the actor moves, and the decoration does not change, which saves on the size of the information, the main picture is digitized once, and then only capture and transmit only the changes. Video format MPEG-1, created in the late 1980's. and used in Video-CD, gave way to higher-quality MPEG-2, and the new MPEG-4 standard, developed by Microsoft in 1999, and its modification of DivX allowed to place a video film of good quality on a regular CD.

Multimedia information - a combination of text, audio, graphic, video information, presented on the computer screen or multimedia projector. Multimedia information has huge volumes, therefore it is compressed by compression programs, and before playback is restored, it is said, "on the fly as the data stream arrives. Multimedia computer programs allow you to create parallel streams of information: text, visual and audio.

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