Two new information services, VIDEOTEX and TELETEXT, are now being introduced in the United States and Canada. Both are aimed at displaying frames of information retrieved from a database on a color TV set augmented by logic and memory. The frames are mixtures of characters and graphics. Both services have a very "friendly" user interface. Distinctions between the two lie in the transmission medium used and the extent of and degree of interaction with the database. TELETEXT uses broadcast TV channels and permits retrieval from a small database. VIDEOTEX uses telephone or other data communication lines and allows extensive interaction with a conceptually unlimited database. This paper focuses on design considerations and features for the recently adopted standard, North American Presentation Level Protocol Syntax (NAPLPS). This new standard specifies the data syntax for the encoding of textual and pictorial information in both VIDEOTEX and TELETEXT services in North America. These services were originally introduced in Europe using alphamosaic formats. Characteristics of the American and Canadian environment that have motivated the new standard to include both alphamosaic and alphageometric formats are examined. The paper concludes with a look at possible additions to NAPLPS.

The system concept and technology threads which are being woven together to yield the interactive picture information systems of today have evolved over many years. The first part of this paper looks at the current systems called, generically, TELETEXT and VIDEOTEX. It then traces major threads in the evolution of these and some extended systems and presents some interesting, and, undoubtedly in some instances, some surprising historical background. The final part gives an overview of the contributed papers of this special issue and some editorial comments.

Novel implementations of a binary rate multiplier (BRM) circuit are described. These BRM's, which use the input data word to load patterns into shift registers, are capable of working at higher speed than a conventional circuit, and should be more suitable for silicon integration. Long input data words can be accommodated with a long shift register or by interconnecting several short registers.

This paper describes a companded analog-to-digital (A/D) converter for voiceband signals that is simple and potentially inexpensive. The converter uses only 18 coarsely spaced analog levels. Fine resolution is obtained by oscillating between these levels at an increased speed and averaging the result over a Nyquist interval. The companding used in the converter is effectively the same as that of μ-255 pulse-code modulation (PCM). In the encoding process a one-bit code is generated at 256 000 samples/s. This 1-bit per sample signal can be transmitted and decoded directly, or a simple digital circuit will produce a 13-bit, 8-kHz linear PCM signal that can be compressed to 8-bit companded PCM format. In this paper the basic operation of the 1-bit coder is described and its performance when connected to a 1-bit decoder is illustrated. Methods for obtaining both linear and compressed PCM are then presented, and the properties of these PCM signals with respect to noise, gain tracking, and harmonic content are described. Relative insensitivity to circuit component variations, absence of analog gates, along with the need to generate only a few analog levels, make the coder especially well suited to integrated circuit realization.

Interpolative digital-to-analog (D/A) converters produce a final output via a two-step process. First, each digital input word is used to control a circuit whose output oscillates rapidly (i.e., many times faster than new digital input values are provided) between coarsely spaced analog values (i.e., many times coarser than the resolution specified by the input word). Second, the oscillating analog signal is low-pass filtered to give the final output. The oscillation pattern is chosen to produce an average value that corresponds to the fine resolution specified by the input word and to ensure that the power of the error (the difference between the oscillating signal and the desired fine resolution output) occurs predominantly out of band. By this means, high-speed operation reduces the need for many finely spaced analog signal amplitudes, a tradeoff which is especially desirable for integrated circuit implementation. In this paper, the basic operation of interpolative D/A converters is described. Three alternative means of generating patterns are compared with respect to circuit complexity, and amount of baseband distortion introduced. The relative insensitivity of these converters to circuit value variations is emphasized. Applications of the interpolative technique to decoding digital words in both linear and piecewise linearly companded formats are given.