Toll Quality Research Articles

Time-domain coding of (near) toll quality speech at rates below 16 KB/S

Time-domain coding of (near) toll quality speech at rates below 16 KB/S

A Methodology for the Evaluation of Real-Time Speech Digitization

Real-time speech digitizing technologies underlie such modern communications products as voice store and forward systems and digital PBX's. Among the human factors design issues associated with this technology, three of particular importance can be identified: i) speaker identifiability, ii) acceptability of speech quality, and iii) speech intelligibility. An experimental method for addressing issues of identifiability and intelligibility was developed and used to compare a commercial speech digitizing device with a standard toll quality telephone channel. It was found that the identifiability and acceptability of the telephone was slightly superior to the digitized speech. Additionally, results on an MRT showed intelligibility scores somewhat below optimal.

Quality limitations in residual-excited LP speech coding

In this paper, we report on a study of two factors which determine the RELP speech coder overall transmission rate and output quality, namely, the transmitted residual baseband bandwidth and the baseband encoding technique. The first subjective test revealed a linear relationship between uncoded baseband bandwidth and coder quality referenced to the log PCM standard. The optimum baseband encoding method out of a subset (PCM, APCM, ADM, PPADPCM, ATC) was determined by both an objective (SEGSNR) measure and a subjective (preference) measure derived from the results of a second subjective test. The ADPCM based RELP coder maximized both measures and was used thereafter in a final subjective test which sought to establish a relationship between transmission rate and quality. Test results showed a quasi-linear dependance of quality and transmission rate and also showed that toll quality (7-bit log PCM) cannot be presently achieved with RELP coding.

Technique for frequency division/multiplication of speech signals

As a follow-up to two previous studies, we examine the feasibility of dividing the bandwidth of a speech signal while keeping the time dimension intact. We utilize the real and imaginary parts of the short-time Fourier transform to describe contiguous bandpass segments of the signal spectrum. We implement a frequency division that maintains unchanged the Hilbert envelope of the waveform for each bandpass segment. We show that the resulting signal can be usefully confined to one-half the original bandwidth. Transmission can be effected either by an analog channel of one-half bandwidth, or by digitally encoding at one-half the conventional Nyquist rate. Reanalysis at the receiver and complementary multiplication of the spectrum parameters reconstruct the signal to the original bandwidth. Analysis, transmission, reanalysis, and reconstruction processes are not transparent. They provide an output of less than typical toll quality, but of reasonably good communications quality. A potential use for the method might be the simultaneous transmission of voice and data over a single voice circuit. We also show that the division and multiplication technique can be used to modify the time dimension of the speech signal (i.e., compress or expand the time scale) while keeping the frequency dimension intact.

A segmented μ-255 law PCM voice encoder utilizing NMOS technology

A technique for pulse code modulation (PCM) encoding according to the 15-segment approximation to the /spl mu/-255 nonlinear encoding law is described. The technique makes possible the realization of a one-channel encoder together with the associated sample/hold function as a single NMOS integrated circuit. The requirements imposed on the individual components for satisfactory system performance are investigated, and it is shown that a performance can be achieved which is comparable with that required in toll quality PCM telephone transmission. Experimental results from a partially integrated prototype are presented.

Highly Efficient Voice Modulation For Low C/N0Communications Channels with Hard Limiting Repeaters

A highly efficient subcarrier modulation technique, suppressed clock pulse duration modulation (SCPDM) has been developed that outperforms narrow-band frequency modulation in very noisy channels. Developed primarily for hard limiting satellite repeaters, the technique utilizes a phase-shift key (PSK) carrier and phase-lock demodulators. The SCPDM technique is shown to be extremely useful and cost effective where the overall objective is to obtain maximum traffic rather than toll quality voice communication. Design considerations are discussed and test data from a typical hardware implementation are presented.

Development of Sydney, Nova Scotia-St. John's, Newfoundland Canadian National Telegraphs Microwave System

A 650-mile radio-relay system composed of 22 "lineof-sight" paths was constructed to supplement the Canadian National Communications between Sydney, Nova Scotia, and St. John's, Newfoundland. Relay repeater points were chosen by means of all available contoured maps and on-site surveys of the various points, bearing in mind radio transmission characteristics and also accessibility for maintenance. All the radio paths were field-tested to confirm their transmission capabilities. The overwater hop between Cape North and Red Rocks, which is 69 miles long, required careful consideration, and special arrangements were made to ensure that this hop would provide a satisfactory performance. Standard Telephones and Cables Ltd.,were requested to provide this system using their latest equipment. This is 5-watt equipment working on 4000 Mc and able to carry the equivalent of 600 toll quality message circuits or one video program. To handle the present Canadian National requirements, three channels were installed in an eastward direction and two in a westward direction. The system is divided into three sections with back-to-back terminals at Corner Brook and Gander for general communications and television branching circuits. Each section has a separate comprehensive supervisory control facility for providing dual information and controls at both section terminals.

Microwave facilities with built-in reliability

THIS PAPER describes a short-haul microwave radio system now providing toll quality telephone message service between San Diego (elevation 320 feet), Calif., and the Laguna Mountain recreation area (elevation 6100 feet), Calif. The microwave portion of the system employs Motorola frequency diversity equipment operating in the 5,925 to 6,425 mc (megacycle) common carrier allocation. The microwave system is presently multiplexed with 28 channels of Lenkurt 45 BX carrier telephone channelizing equipment.

Microwave relay system between Saint John and Halifax

A microwave relay system employing time-division multiplex (TDM) has been constructed to provide toll telephone circuits for the Canadian New Brunswick Telephone Company and the Maritime Telegraph and Telephone Company. This system links Saint John in New Brunswick and Halifax in Nova Scotia via five intermediate unattended repeaters and yields 46 four-wire circuits of toll quality that provide out-of-province connections to Montreal, Toronto, Boston, and New York. This system has been in partial use since December 1952, and the complete system has been in continuous service since December 1954. A description of the radio relay link is given primarily from the systemplanning aspects. Performance figures are outlined.

A Simplified 48-Channel Carrier Telephone System

This paper describes a 48-channel single side band carrier system which provides toll quality telephone transmission bands with exceptional simplicity and dependability of equipment. Through the use of a novel frequency translation pattern, speech bands covering the range from 200 cycles to 3,500 cycles are secured at 4-kc channel separation without resort to crystal filters. In this system, improved translating modulator techniques and other advanced features reduce the number of vacum tubes to a minimum while achieving exceptionally high performance standards, stability, and ease of maintenance.