- Vocoders stands for Voice Coders.
- Synthetic sound is reproduced with artificial quality.
- Vocoders transmit signals with low bit rate, usually in the range of 1.2 to 2.4 KB.
- Model parameters are used by the receiver along with the transmitted parameters.
- Model parameters then synthesizes the approximation to the source output.
- The channel vocoders are linear predictive coders and code excited linear prediction.
- It is one of the static dictionary coding forms.
- The dictionary consists all of the letters of the source alphabet.
- These letters are followed by many pair of letters. These are known as Digrams.
- Two character input is read by digram encoder.
- The dictionary is searched by the encoder for the existence of inputs.
- If input exists, the index is encoded and transmitted.
- The lossless channel is described by a channel matrix.
- It is described with only one non-zero element in each column.
- During transmission, no source information is lost.
- A low resolution of an image is sent first.
- IT needs only few bits for the purpose of encoding.
- The image is then updated to the required fidelity.
- This is done by transmitting more information.
- The sequence encoding in the look ahead buffer is encoded in this technique.
- The encoding id done by moving the encoder to a search pointer.
- The search pointer is through until a match to the first symbol is encountered.
- This symbol is available in the look ahead buffer.
- The actual distance between the pointer and the look ahead buffer is known as offset.
1. Forward Adaptive Quantization
2. Backward Adaptive Quantization.
1. Granular error
2. Slope over load error.
- A code should be decodable.
- The code words are shorter than the letters which occur less frequently, has code word letters that occur more frequently.
- 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.
- 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.