Compression Standard

- A data compression technique, which reverts an exact copy of original file.

- Lossless Source Coding is used for compressing text files in modems.

- Lossless Source Coding is a building block for designing lossy compressors.

- Lossy compression is implemented for images, sound and video files for effective data compression.

- Many compression techniques have a lossless mode.

- The lossless source coding involves a sequence of fixed length symbols.

- Each of these symbols is easily manipulated independently.

- Information Theory is about quantification of information.

- It is used in compressing data.

- Entropy is a key measure of information.

- It is expressed in terms of average number of bits that are required to store a message.

- Entropy is used to quantify the uncertainty which is a process in predicting the random variable values.

- Lossless data compression, Lossy data compression and channel coding are the fundamental topics of information theory.

- A code that maps source symbols into a set of variable number of bits.

- A VL code compresses the sources and decompresses with zero error.

- By implementing a right coding strategy, an identically distributed source might be compressed almost close to its entropy.

- This process is in contrast to fixed length coding methods.

- Examples of variable-length codes are Huffman coding, LempelZiv code.

- File compression is a process to reduce the disk space to store that file.

- File compression enables data to be transferred quickly.

- Disk space needed on internet servers is reduced. This allows the servers to store more files / information with less disk space.

- File compression reduces the amount of time on internet to upload or download a file.

- Compression hides data so that not all computers can read the information stored.

- File compression is a mandatory preference for some of the internet servers to transfer files.

- Data symbols are encoded with coding schemes for fixed length codes.

- Every coding scheme has unique code.

- This unique encoded character ensures unambiguous.

- The encoded strings have fixed length.

- The fixed length codes are always uniquely decipherable.

- The non-binary Hoffman code elements are derived from an alphabet ’m’ is > 2 letters.

- All the symbols ‘m’ which occur least frequently will be having the same length.

- The lowest probability of the symbols ‘m’ will differ only in the last position.

- The letters that combine have code words of the same length.

- The symbols that have lowest probability will have code words with long length.

- Silence compression is used in compressing sound files.

- It is equivalent to run length coding on normal data files.

- The parameters are:

1. A threshold value. It is a parameter that specifies, below which the compression can be considered as silence.

2. A silence code followed by a single byte. It indicates the numbers of consecutive silence codes are present.

3. To specify the start of a run of silence, which is a threshold.

- Distortion theory is about trade-offs between the rate and distortion.

- It is applied for compression schemes.

- An average number of bits are utilized to represent each sample value.

- If the rate of bits is decreased it is known as increase in distortion.

- If the rate of bits to represent each value is increased it is known decrease in distortion.

- An entropy encoding algorithm.

- It uses variable length code table for encoding source symbol.

- It is used to construct a prefix code that is based on a set of symbols.

- It suboptimal. The lowest expected code word length will not be achieved.

- A variable-length entropy encoding form.

- It is used for implementing loss less data compression.

- Fewer bits are occupied when frequently used characters are represented. More bits are occupied when not-so-frequently used characters are represented.

- Sub Band Coding(SBC) is a transform coding.

- A transform code can break a signal that may result in many different ‘frequency bands’.

- Every transform code is encoded independently.

- Most of the time, it is used for compressing audio and video signals.

- It is a process where a well defined temporal support for ‘wiggles’ about X-axis.

- The inner-product of the input signal is multiplied with a set of ortho-normal basis functions.

- Then the coefficients are computed by this inner-product.

- Vector Quantization has quantizes as inputs and outputs.

- The vector quantization results in a distortion rate.

- This rate is lower than the scalar quantization.

- They are classified based on the requirements of reconstruction and compression of data.

- They are Lossy Compression and Lossless Compression.

- Companded Quantization maps the input through compressor function.

- This function expands the probability to the high level regions.

- These regions are close to the origin.

- These regions are compresses the corresponding lower probability regions that are away from the origin.

- The output of Companded Quantization is resulted by using uniform quantizer.

- An expander function is used to transform the quantized value.

- Prefix coding is known as optimum coding.

- More frequently occurred symbols have shorter code words.

- Less frequently occurred symbols have longer code words.

- The less occurred frequently symbols will have equal length.

- Optimum prefix codes enhance the efficiency of data compression.

- The source output is divided into various blocks of data.

- Every block is analyzed prior to quantization.

- As per the block analyses, the quantizer parameters are set.

- These settings are transmitted later to the receiver.

- The transmitted settings are served as side information at the receiver end.

- It is not simple to use a single model to describe the source in many applications.

- In these scenarios, a composite source model is used.

- Composite Source Model uses only one source.

- Only single source is activated at a given point of time.

- A prefix code is a code which does not require code word as a prefix to another code word.

- Huffman code is an example for Prefix Code.