This invention relates generally to digital communication systems and particularly to a delta modulated communication system that transmits step sizes.
Digital communication systems are finding increasing use for the transmission of analog signals, such as voice communication. The design of such systems involves a tradeoff between the fidelity of reproduction of the analog signal and the bandwidth required. Various techniques are used for the encoding of the analog data, some of which are very simple and others of which attempt to attain high fidelity at relatively low bandwidth.
One of the simplest systems is a pulse code modulated (PCM) communication system. In a linear PCM system, the analog signal is periodically sampled in a linear analog-to-digital (A/D) converter. The digital amplitude of the instantaneous analog signal is then transmitted as a binary pulse train. PCM communication systems suffer from several problems. If the analog signal has a large dynamic range, the number of bits required to adequately represent this range is correspondingly large and requires a large bandwidth for transmission. Furthermore, the linear A/D converters are required to operate accurately over the entire dynamic range. Such A/D converters are difficult to build and are expensive.
A logarithmic PCM system uses a logarithmic A/D converter in which the levels are quantized in a logarithmic scale. Logarithmic A/D converters with large dynamic range are inexpensive and the bandwith required for logarithmic PCM is less than for linear PCM. However, some fidelity is sacrificed by the logarithmic compression.
An alternative to standard PCM is differential logarithmic PCM, in which the amplitude of the analog signal is not itself transmitted. Instead, the transmitter, in sampling the analog signal each sampling period, compares the sampled signal against the next previously sampled signal. A logarithmic difference of these two values is then transmitted. The receiver must then integrate the differential PCM signals to obtain the instantaneous digital amplitudes. Although differential PCM is more complex than standard PCM, it offers high fidelity in the presence of low frequency noise.
A completely different approach is used in the delta modulation (DM) technique. In DM systems, just like a differential PCM system, the transmitter transmits a differential of the amplitude of the analog signal rather than the amplitude itself. But unlike the differential PCM, a DM system can transmit only one of two signals, a positive step or a negative step, each of the same magnitude. A positive step would be transmitted as a 1, a negative step as 0. In the simplest DM systems, the size of the step remains constant with the result that if the analog signal is rapidly varying, the digital transmitted signal will not accurately reproduce sudden changes exceeding the size of the step. One result of this effect is that the transmitter does not compare the present value of the analog signal with its value in the previous sampling. But instead, it compares the sampled value against the value indicated by the previous digital transmissions. Eventually, after a series of steps in the same direction following a sudden change, the indicated value again returns to the neighborhood of the sampled value. The operational effect of this lag is that delta modulation poorly transmits or distorts high frequency, high amplitude analog signals.
There have been several techniques suggested which would provide for adaptive delta modulation, i.e. the size of the step is automatically altered at both the transmitting and receiving stations depending on the immediate past history of the delta modulated transmission. For example, if three successive transmitted bits are ones, it can be surmised that the transmitter is overloading and the digital signal is lagging the analog signal. If this occurs, both transmitter and receiver will assume a step size that is double the previous step size so that larger changes in the analog signal can be accommodated. Likewise, if three zeroes in a row are received, it can be assumed that the transmitter is also overloaded with a negative going analog signal and likewise the step size is doubled. On the other hand, if a train of alternating ones and zeroes is transmitted, it can be assumed that the step size is too large for the relatively small changes occurring in the analog signal. Therefore the step size is set at half of its previous value. Techniques such as these are described by Donald L. Schilling et al. in IEEE Transactions on Communications, volume COM-26, 1978, pages 1652-1659. These techniques tend to require complicated electronics to implement the algorithms necessary for the corrections of the step size.
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