A conference call can be set up by interconnecting the speech paths in a distributing network. The quality of conference speech is dependent on the circuit conditions and the speech leakage through the hybrids which convert the two wire paths into four wire paths and vice versa. In this paper we analyze the summing algorithm and extend previously published results. We also present a single speaker algorithm that is experimentally shown to be effective.

Intraframe transform coding of pictures for the case of a nonseparable covariance model is considered. Performances of the Walsh-Hadamard, discrete cosine and Karhunen-Loeve transforms are compared based on the compaction of signal energy in the transform components, and, the degree of decorrelation of the data. The results demonstrate that the performances of the discrete cosine and Karhunen-Loéve transforms compare closely, as is the case with a separable covariance model. The corresponding performance of the Walsh-Hadamard transform is inferior.

This paper is concerned with interframe coding of monochrome pictures using 3-dimensional transforms. First, an algorithm which enables the vector processing of 3-dimensional arrays is derived. Next, this algorithm is used in an interframe transform coding experiment. Picture data (6 bits/pel) is processed in (4 \times 4 \times 4) blocks using the Walsh-Hadamard transform (WHT), and the discrete cosine transform (DCT). The corresponding reconstructed pictures (1 bit/pel) are included.

Rate-distortion functions for 2-dimensional homogeneous isotropic images are compared with the performance of five source encoders designed for such images. Both unweighted and frequency weighted mean-square error distortion measures are considered. The coders considered are a) differential pulse code modulation (DPCM) using six previous samples or picture elements (pels) in the prediction--herein called 6-pel DPCM, b) simple DPCM using single-sample prediction, c) 6-pel DPCM followed by entropy coding, d)8 \times 8discrete cosine transform coding, and e)4 \times 4Hadamard transform coding. Other transform coders were studied and found to have about the same performance as the two transform coders above. With the mean-square error distortion measure, 6-pel DPCM with entropy coding performed best. Next best was the8 \times 8discrete cosine transform coder and the 6-pel DPCM--these two had approximately the same distortion. Next were the4 \times 4Hadamard and simple DPCM, in that order. The relative performance of the coders changed slightly when the distortion measure was frequency weighted mean-square error. FromR = 1to 3 bits/pel, which was the range studied here, the performances of all the coders were separated by only about 4 dB.

A discrete cosine transform (DCT) is defined and an algorithm to compute it using the fast Fourier transform is developed. It is shown that the discrete cosine transform can be used in the area of digital processing for the purposes of pattern recognition and Wiener filtering. Its performance is compared with that of a class of orthogonal transforms and is found to compare closely to that of the Karhunen-Loève transform, which is known to be optimal. The performances of the Karhunen-Loève and discrete cosine transforms are also found to compare closely with respect to the rate-distortion criterion.