UNIT -5Introduction to Image and Video Compression Image

Compression

DIGITIZING AUDIO According to the Nyquist theorem, if the highest frequency of the signal is f, we need to sample the signal 2f times per second.Voice is sampled at 8000 samples per second with 8 bits per sample. This results in a digital signal of 64 kbps.

Music is sampled at 44,100 samples per second with 16 bits per sample. This results in a digital signal of 705.6 kbps for monaural and 1.411 Mbps for stereo.

DIGTIZNG VIDEOA video consists of a sequence of frames. If the

frames are displayed on the screen fast enough, we get an impression of motion.

Each frame is divided into small grids, called picture elements or pixels.

For black-and-white TV, each 8-bit pixel represents one of 256 different gray levels.

For a color TV, each pixel is 24 bits, with 8 bits for each primary color (red, green, and blue).

Audio CompressionPredictive Encoding:

In predictive encoding, the differences between the samples are encoded instead of encoding all the sampled values.Perceptual Encoding: MP3:

The most common compression technique that is used to create CD-quality audio is based on the perceptual encoding technique.

Video CompressionImage Compression: JPEG

Image Compression: JPEGCase 1: a block of uniform gray

Case 2: A Block With Two Different Uniform Gray Scale Sections

Case 3: A Block That Changes Gradually

From above three cases•The transformation creates table T from table P. •The dc value is the average value (multiplied by a constant) of the pixels. •The ac values are the changes. •Lack of changes in neighboring pixels creates 0s.

Quantization•the values are quantized to reduce the number of bits needed for encoding.•divide the number by a constant and then drop the fraction.•The divisor depends on the position of the value in the T table.•JPEG is called LOSSY compression is because of quantization phase is not reversible.

Compression

Video Compression: MPEG