Multilevel fixed-length block codes which have a dcconstraint property are discussed, and the upper bounds of the transmission rates of such codes are described under the limitations of the running digital sums (RDS) of coded signal streams. Then stateindependent decodable codes whose transmission rates are equal to the upper bounds are presented for small values of word length. In these codes, four code words or less correspond to a source symbol. As an example, a 5-level code of length 4 whose RDS's do not exceed 4, whose transimission rate is equal to 2 bits per symbol, and from which good performance for bit timing, gain adjustment of repeaters, and word synchronization can be expected, is also described.

Various image coding schemes have been studied for digital transmission of videophone signals. The Hadamard transform, which is now studied for the transmission of pictures such as those from satellites, has been considered too complicated for public use, though the characteristics such as the ratio of bit-rate reduction are more desirable than those of differential pulse-code modulation (DPCM). We have found a very simple scheme of the transform, where digitized videophone signals are transformed to Hadamard components all digitally just byndigital adders and some shift registers for 2nth-order transform. For example, three adders are necessary for eighth-order transform. It is extendable to two-dimensional transform with ease. We have made an experimental model running in real time. Experiments and theoretical calculation have shown that 3 bits/sample are required for good picture quality in the case of two-dimensional(4 \times 2)th transform and 0.5 bits more for one-dimensional eighthorder transform.