Transmission bandwidth efficiency enhancement techniques

All communication channels transmit infonnation fonn one user to another using channel bandwidth mostly measured in bits/second. With the advent of latest technological tools, the bandwidth efficiency of all the channels may be enhanced beyond their normal transmission capability; mean more bits can be transmitted through the same channel. Techniques such as low bit rate encoding, variable bit rates, compression of source data, digital speech interpolation, discontinuous transmission and lost frame reconstruction are some of those signal processing techniques that can be exploited to enhance bandwidth efficiencies of the channels. In this chapter bandwidth efficiency techniques are detaile

Linear predictive coding of speech

Linear Predictive Coding (LPC) is a mathematical (signal processing) technique used to remove the redundancy from speech signals. To remove the redundancy from speech signal, is done in 1\vo stages, first stage is to remove short term correlations form the signal and the second is to remove long term correlations. The short term and long term redundancies are modelled by digital filters very precisely. This chapter is dedicated for LPC ofspeec

Digital multiplexing techniques

Multiplexing techniques are broadly classified as analogue and digital, digital being the latest. Time Division Multiplexing (TDM) and Statistical TOM are most commonly used digital multiplexing techniques. The TOM is a detenninistic method of multiplexing various sources in pre-determined fixed slots. Statistical TDM, (STDM) uses dynamic method of allocating time slots to only the active users. The dynamic nature of slot allocations enhances bandwidth utilization better than TDM however this property of STDM is always suitable for non-delay sensitive systems. For real-time and delay sensitive systems the TDM approach is the better optio

Low bit rate speech coding

In recent years much work has be concentrated on encoding speech. Various coding methods are developed such as Low-Delay Codebook Excited Linear Prediction (LD-CELP), Codebook Excited Linear Prediction (CELP), Pulse Residual Excited Linear Prediction (PRELP) and others. These encoders operate at very low bit rates (4.8Kbps) as well as maintained speech quality. In this chapter some ofthese encoding algorithms are explaine

Codebook excited linear predictive coding

In standard Codebook Exited Linear Coding (CELP) the encoding method uses random values for excitation vectors. The excitation is modelled by the codebook. Each time a segment of speech is encoded and an excitation vector is matched to minimise the error between original and encoded signal to maintain quality of speech. This chapter narrates the CELP encoding algorithm. The conceptual block diagram of twoO time varying filters and a Gaussian codebook shown in figures, fc encoder and decoder respectively. This simplified block diagram mainly consists ofthree blocks, Short-Term Prediction, Long-Term Prediction and a random code-book. The parameters of these predictors SIP, LIP and the codebook are optimised and estimated in many ways. If these estimated parameters are accurate, the synthesized speech \vilI sound the same as original speech. Because ofthe limitations ofthe coding build blocks the estimated filter parameters case as the estimation errors in a result speech quality suffers degradations. The standard CELP algorithm explained gradually as follows and its block diagram shown in figur

Discontinuous transmission and multiplexing

Discontinuous Transmission (DTX) is a concept used in some mobile standards, mostly in Groupe Special Mobile (GSM) systems. In DTX systems, Voice Activity Detection (VAD) and comfort noise are essential element. The usage of DTX in the systems allo\vs transmissions during activity periods and remains silent during non-activities. This chapter is dedicated for DTX and VAD, their usage in STDM design

Low bit rate speech multiplexer tools

In multiplexing many sources are combined in a common channel to maximise the bandwidth of the system. There can be many components of the STDM such as compressed speech sources at lower rates, voice activity detectors and lost frames reconstruction. This chapter briefly explains all three major components for the under lying multiplexer architectures

Pulse residual excited linear prediction

Pulse Residual Excited Linear Prediction (PRELP) is a variant ofCELP encoding algorithm. The PREP, as its name suggests, instead of codebook, use pulses in codebook. This method reduces the searching time for a matching pulse from the codebook. In this chapter, PRELP algorithm is explained along with the design ofsuch codeboo