William Chunhung Yip

Efficient codebook search for CELP vocoders

A new way of determining correlation coefficients for stochastic codebook vectors for CELP coding of speech takes advantage of the sparsely populated nature of stochastic codebook vectors. N valued input signals (e.g., convolution vectors) to be correlated with N valued codebook vectors are fed to an N by N multiplexer or other selection means and the signal values either passed to an accumulator or not according to the state of N select inputs or other identification means determined from a memory store (e.g., an EPROM) whose entries correspond to the non-zero values of the codebook vectors. The accumulator output is the correlation of the codebook vector with the input signal. A sequencer steps through the entire codebook to provide correlation values for each vectors. The results are used to determine the optimum stochastic codebook vector for replicating the particular speech frame being analyzed.

Partitioned echo canceller utilizing decimation echo location

In practice, digital echo cancellers require design trade-offs between the effective range, rate of convergence, and system resource (sizing and timing) requirements. While echo cancelling over an extended time range is often desirable, for a standard LMS approach it comes at the expense of increased system resource requirements and decreased convergence rates. In this paper, the authors investigate a Decimation Echo Location (DEL) technique which provides a novel approach for identifying the high energy regions of the echo impulse response model over an extended range. Accurate echo location provided by the DEL process enables a subsequent partitioning of an echo canceller. The Partitioned Echo Canceller (PEG) structure was developed to operate only on the identified regions of interest. In this manner, the echo canceller can be effective over an extended range without sacrificing system resource or performance requirements. DEL/PEC simulations have readily yielded a factor of five increase in algorithm efficiency compared to a conventional LMS approach, with additional refinements promising even greater gains. For a system with a predetermined modeling range requirement, DEL/PEC provides a solution with significantly reduced resource requirements. Similarly, for an application with a fixed system resource allotment, this technique greatly extends the effective modeling range capability.

Reduced codebook search arrangement for CELP vocoders

A new way of CELP coding speech simplifies the recursive loop used to poll code adaptive book vectors by reducing the number of autocorrelation operations that must be performed with the K vectors of the codebook each having N entries. Autocorrelation is initially performed for only a small number P<<N autocorrelation coefficients in each codebook vector and the values found are used to scan through all the codebook vectors looking for those S vectors (S<K) which give the best match to the input speech. The autocorrelation function for the S vectors is then recalculated for R entries (P<R≦N) in the codebook vectors to determine which of the S codebook vectors and associated gain gives the best match to the input speech.

Advanced Digital Television System

This paper will give an architectural overview of an advanced digital televisicn system which utilizes the latest VISI technology, fast digital signal proessing ard frame buffer. This digital design aproach will lead to a cost-effective chip set that will produce improved quality, and higher definition TV picture for either the NTSC or PAL system.