Data Compression

1. Storage Space

Storage space refers to the amount of memory required to store multimedia data such as images, audio, and video. Multimedia files are usually very large in size, which creates challenges in storage and transmission.

Data compression is used to:

• Reduce file size
• Save storage space
• Improve transmission speed

2. Coding Requirements

Coding requirements define the essential features that a compression technique must satisfy to be effective.

Important requirements include:

• Efficiency: High compression ratio with minimal data loss
• Speed: Fast encoding and decoding
• Accuracy: Maintains acceptable quality after compression
• Error Resilience: Ability to handle transmission errors
• Low Complexity: Should not require excessive computational power

3. Source, Entropy, and Hybrid Coding

a. Source Coding

Source coding removes redundancy in data to reduce size without losing important information.

Examples:

• Huffman Coding
• Run-Length Encoding (RLE)

b. Entropy Coding

Entropy coding is a type of lossless compression that assigns shorter codes to frequently occurring data and longer codes to less frequent data.

• Based on probability of symbols
• Improves compression efficiency

c. Hybrid Coding

Hybrid coding combines multiple compression techniques (both lossy and lossless) to achieve better performance.

• Used in image and video compression
• Example: Combination of DCT and entropy coding in JPEG

4. Lossy Sequential DCT-Based Mode

Lossy Sequential DCT-based mode is a compression technique used mainly in image processing.

• Uses Discrete Cosine Transform (DCT)
• Converts image data from spatial domain to frequency domain
• Removes less important visual information

Steps involved:

1. Divide image into blocks (usually 8×8)
2. Apply DCT transformation
3. Quantize coefficients (lossy step)
4. Encode data

This method reduces file size significantly but causes some loss of quality.

5. Expanded Lossy DCT-Based Mode

Expanded lossy DCT-based mode is an advanced version of the basic DCT method.

• Provides better compression efficiency
• Supports progressive image transmission
• Allows partial image display before full loading

It is commonly used in modern multimedia systems for improved performance.

6. JPEG (Joint Photographic Experts Group)

JPEG is a widely used image compression standard.

Features:

• Uses lossy compression
• Based on DCT technique
• Reduces file size significantly
• Suitable for photographs

Advantages:

• High compression ratio
• Good visual quality

Disadvantages:

• Loss of image quality after compression

7. MPEG (Moving Picture Experts Group)

MPEG is a standard for compressing video and audio data.

Features:

• Uses both lossy and lossless techniques
• Compresses sequences of images (video frames)
• Removes temporal and spatial redundancy

Types:

• MPEG-1 (basic video)
• MPEG-2 (DVD, TV broadcasting)
• MPEG-4 (internet streaming, modern video)

Conclusion

Data compression is essential in multimedia systems to efficiently store and transmit large amounts of data. Techniques like DCT, JPEG, and MPEG help reduce file size while maintaining acceptable quality, making multimedia applications practical and efficient.