DATA COMMUNICATUION 5TH SEM CSE

1. UNIT -5
Introduction to Image and Video
CompressionImage Compression

3. 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.

4. DIGTIZNG VIDEO
A 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).

5. Audio Compression
Predictive 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.

6. Video Compression
Image Compression: JPEG

7. Image Compression: JPEG
Case 1: a block of uniform gray

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

9. Case 3: A Block That Changes Gradually

10. 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.

11. 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.

12. Compression

13. Video Compression: MPEG