(1) Field of the Invention
The invention relates to a transmission system comprising a transmitter and a receiver, for transmitting an information signal, more specifically a video signal, in a digital format obtained by means of differential pulse code modulation (DPCM). 
(2) Description of the Prior Art
The transmitter of a transmission system generally comprises a source which produces an information signal, in the analog or the digital form, which must be transmitted to the associated receiver.  In a DPCM transmission system, this information signal is first applied to a source encoding circuit, which is here in the form of a DPCM encoder, which comprises a difference producer to which the information signal and a prediction signal are applied and produces a difference signal.  This difference signal is applied to a quantizing arrangement which produces a quantized difference signal.  The DPCM encoder further comprises a prediction circuit to the input of which the quantized difference signal is applied and at whose output the said prediction signal is produced. 
The quantized difference signal occurring at the output of the quantizing arrangement is applied to a channel coding circuit, for example an analog-to-digital converter or a code converter, which converts this quantized difference signal into a digital channel signal which will be designated DPCM signal and consists of a sequence of codewords occurring at a predetermined rate f.sub.s, alternatively designated sampling frequency.  It should be noted that the inverse quantity 1/f.sub.s will be designated sampling period and will be denoted by the symbol T.
The codewords used by the channel coding circuit are transmitted via a transmission means to the associated receiver where they are converted in a channel decoding circuit into a decoded channel signal which, in the case of non-disturbed transmission, accurately corresponds to the original quantized difference signal.  This decoded channel signal is further applied to a DPCM decoding arrangement.  The latter comprises a summing arrangement to which the decoded channel signal and a second prediction signal are applied and which produces a sum signal.  This DPCM decoding arrangement also comprises a prediction circuit, the decoded channel signal being applied to its input and the second prediction signal being produced at its output.  The prediction circuit in the transmitter is of a similar construction as the prediction circuit in the receiver in order to achieve that the sum signal accurately corresponds to the original information signal. 
In order to obtain an impression of the operation of the prediction circuit, it is customary to divide each line of the TV picture into a series of picture elements, each having a given picture value, that is to say brightness and/or color.  The prediction circuit produces a prediction value for each picture element.  More specifically, it holds that the prediction value for an actual picture element is equal to the sum of a number of picture values associated with different picture elements, each picture value being weighted with a weighting factor which is characteristic of the relevant picture element.  These weighting factors are chosen such that their mathematical sum is not more than one.  If the prediction circuit is of such an implementation that for the determination of the prediction value for an actual picture element it only takes account of the picture values of one or more picture elements belonging to the same line as the actual picture element, then a one-dimensional prediction is involved.  If, in contrast therewith, account is taken of picture values of a number of picture elements which belong to lines different than those to which the actual picture element belongs, then two-dimensional prediction is involved.  When, analogous to the foregoing, also picture values belonging to picture elements of preceding pictures are used, then three-dimensional prediction is involved. 
As will be obvious from the foregoing, a prediction circuit can be implemented in different manners.  Possible implentations are decribed in, for example, the reference 1, 2, 3, 4, 5 and 6.  From these references, it will be clear that generally a prediction circuit is in the form of a recursive time-discrete filter, in the majority of cases a recurve digital filter. 
Because of the recursive behavior of this filter, each received codeword contributes in the receiver to the formation of the picture values of a number of picture elements.  This number will be denoted signal response number hereinafter.  If now a codeword is disturbed in the transmission means, then the associated picture values of a number of picture elements will also be disturbed.  This number of visibly disturbed picture elements is equal to the signal response number. 
The signal response number is closely related to the magnitude of the mathematical sum of the weighting factors used in the prediction circuit.  If this mathematical sum is equal to unity, then the signal response number is infinitely large and, after the occurrence of a transmission error, each picture element will be further disturbed.  If the mathematical sum of the weighting factor is chosen below unity, then the signal response number decreases and consequently also the number of disturbed picture elements, but the quality of the TV picture becomes poorer.  The highest picture quality is obtained when the sum of the weighting factors is equal to unity. 
In order to obtain the situation that in a DPCM transmission system in which prediction circuits are used having weighting factors whose mathematical sum is equal to unity, a reduction of the number of disturbed picture elements is nevertheless effected after the occurrence of a transmission error without the picture quality becoming poorer, it is proposed in the reference 7, 8 and 9 to add in the transmitter an error reducing signal to the DPCM signal.  This error reduction signal is produced by an error reducing circuit to which the information signal itself, or the prediction signal is applied.  In the associated receiver, an error reducing signal is subtracted from the received sum signal, as a result of which the original DPCM signal is obtained again when no transmission errors have occurred.  This error reducing signal is generated by a local error reducing circuit to which a signal generated in the DPCM decoding arrangement is applied.  A transmission system of such a type is known as a "Hybrid DPCM transmission system". 
In actual practice this prior art transmission system has been found to come fully up to expectations, provided one-dimensional prediction is used in the prediction circuit.  If multi-dimensional prediction is used, then the influence of transmission errors has been found to be considerably greater than in the event that one-dimension prediction is used. 
In order to keep, in a DPCM transmission system, the influence of transmission errors as small as possible, even if multi-dimension prediction is used in the system, reference 10 proposes to form the prediction circuit in both the transmitter and the receiver from two or more prediction channels, which are each provided by a non-linear network followed by a recursive digital filter, the mathemetical sum of the weighting factors of which is less than unity.  The inputs of these non-linear networks are connected to the inputs of the prediction circuit.  The outputs of the recursive digital filters are connected to the inputs of an adder arrangement, the output of which is connected to the output of the prediction circuit.  The recursive digital filters are all of a similar structure and a unique system of weighting factors is associated with each filter.  This transmission system has the disadvantage that in practice it has been found that each factor must have a very high arithmetic accuracy.  The weighting factors must, for example, be so accurate that 12 to 14 bits are required for their presentation.  This means that a considerable number of components is required for the implementation of these filters.