With this survey, an attempt is made to describe the great majority of all known methods of digital transmultiplexing (i.e., conversion) of time-division-multiplex (TDM) to frequency-division-multiplex (FDM) signals, and vice versa. To this end, the individual transmultiplexer approaches are classified into four categories according to the underlying algorithm: Bandpass filter bank, low-pass filter bank, Weaver structure method, and multistage modulation method. Finally, the overall performance of the various transmultiplexer approaches are compared with each other by means of different criteria [1], such as stability under looped conditions, absolute value of the group delay, computational and control complexity, modularity, potential of intelligible crosstalk, absence of an additional analog frequency conversion, and the impact of out-of-band signaling For a more profound understanding of the individual digital trsnsmultiplexer approaches, the main chapter is preceded by an introductory discussion on analog and digital generation of single-sideband signals. In this context, the associated problems of sample rate alteration and multi-rate filtering arising from digital signal processing are dealt with.

A comprehensive survey of digital transmultiplexing methods

The design of a transmultiplexer capable of performing the bilateral conversion between one 1544 kbit/s digital signal (which represents 24 PCM coded voice channels) and two analog group signals (each one containing 12 voice channels in the 60-108 kHz band) is investigated. It is shown that an FIR filter bank required as part of such a transmultiplexer can be realized efficiently by cascading a discrete cosine transform processor and a weighting network. Fast convolution algorithms are derived for evaluating the cosine transform. A method of using the symmetry conditions to reduce the computation rate in the weighting network and an elegant hardware configuration for implementing it are also discussed.

Design of a 24-channel transmultiplexer

A new method for digital implementation of SSB-FDM modulation and demodulation is presented, which utilizes the FFT algorithm to reduce the multiplication rate. The hardware realization of the presented method is shown to be the simplest; it consists of only an FFT processor and a set of complex bandpass filters which operate at the same rate as the input baseband sequences; no signal conversions are required prior to FFT processing. Simple and practical approaches are presented for design and implementation of the EFT processor and the set of complex bandpass filters. Design examples, which are made for implementation of a TDM-FDM translator at the supergroup level, show that the proposed method can attain the minimum multiplication rate among methods previously reported.

An Improved Method for Digital SSB-FDM Modulation and Demodulation

A FDM/TDM transmultiplexer uses sampling rate multiplication to increase the sampling rate for time division multiplexed (TDM) to frequency division multiplexed (FDM) conversion and decrease the sampling rate for FDM to TDM conversion. The rate multiplication filters are realized digitally in order to exploit the computational advantage of Fast Fourier Transform (FFT) algorithm, and channel filtering is implemented by a single time-shared sixth-order elliptic digital recursive filter. A novel FFT processor and recursive filter are disclosed which may be used in the system.

FDM/TDM Transmultiplexer

A generalized scheme for an all-digital time-division multiplex (TDM) to frequency-division multiplex (FDM) translator is discussed. It is shown that existing systems for this translation, e.g., those in which a fast Fourier transform (FFT) processor is used, can be derived from this generalized scheme as special cases. Several new Implementations can also be derived from this scheme when different processors are used. A specific example using a Hadamard processor is discussed.

A generalized scheme for an all-digital time-division multiplex to frequency-division multiplex translator

A new algorithm is proposed for the digital implementation of the M -channel SSB-frequency-division multiplexing (FDM) modulator and demodulator, which is computationally attractive and inherently insensitive to parameter quantization.

P. Rayner

Papers

A Digital Block-Processor for SSB-FDM Modulation and Demodulation