Multiplex communications – Wide area network – Packet switching
Patent
1985-02-04
1987-10-13
Safourek, Benedict V.
Multiplex communications
Wide area network
Packet switching
358 36, 358 37, 358138, 358166, 364724, 364825, H04J 1500, H04N 964, H04N 712
Patent
active
047003456
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data transmission systems in which the data is in the form of analogue or digital samples.
2. Related Art
In sampled systems, transmission bandwidth can be reduced by downsampling. This, in its simplest form, involves dropping samples to reduce the number transmitted. Where more than one sample stream is to be transmitted, downsampling and then multiplexing sample streams can reduce the number of channels required. If two such streams are downsampled by dropping alternate samples and then multiplexed by interleaving samples, the resultant sample rate is the same as in one of the original streams.
SUMMARY OF THE INVENTION
The present invention provides a filter for filtering a signal in the form of analogue or digital samples, comprising a multiplier stage having a plurality of multiplier sections each with a respective output but a common input for applying a plurality of coefficients to a sample input to the stage, and a delay and summing network comprising a plurality of delay sections and alternate summing networks connected in series, the output of one of the multiplier sections being connected to the first delay section in the series with each output of the other multiplier sections being connected to a respective summing network.
BRIEF DESCRIPTION OF THE DRAWINGS
Features and advantages of the present invention will become apparent from the following description of embodiments thereof when taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a diagram showing the frequency spectrum of a sampled signal;
FIG. 2 is a diagram showing the frequency spectrum of a downsampled signal;
FIG. 3 is a diagram showing the frequency response of an ideal pre-filter;
FIG. 4 is a block diagram of a basic form of a downsampling system with pre-filtering;
FIG. 5 is a block diagram of a pre-filter used in the system shown in FIG. 4;
FIG. 6 is a block diagram of a pre-filter construction which can be used to replace the pre-filter shown in FIG. 5;
FIG. 7 is a block diagram of a pre-filter construction according to the present invention; and
FIG. 8 is a block diagram of a part of receiving apparatus for receiving a signal that has been processed by the construction shown in FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The frequency spectrum of a sampled system is outlined in FIG. 1. The spectrum has repeats at intervals which are multiples of the sampling frequency. Downsampling introduces spectrum repeats at multiples of the downsampled frequency. Downsampling to half the sampling frequency introduces repeats at half the sampling frequency as shown in FIG. 2. The repeat spectra can overlap to cause aliasing which cannot be removed by simple means.
The aliasing can however be prevented or reduced by pre-filtering prior to downsampling. The ideal prefilter has an amplitude response which is a "brick-wall" as shown in FIG. 3. The cut-off frequency is at half the downsampled frequency. The simplest practical pre-filter is a two sample average which has coefficients of 1/2:1/2. The frequency response of this pre-filter has a null at half the original sampling frequency and therefore reduces the alias but does not remove it. More complicated filters using more samples can improve the response.
Where two samples streams X and Y are to be identically filtered, downsampled and multiplexed by interleaving samples, the hardware can be implemented as shown in FIG. 4 by passing each sample stream X and Y at identical rates through respective pre-filters 10X and 10Y and thereafter through respective down sampling circuit 11X and 11Y. The downsampled streams are then multiplexed using a multiplexing circuit 12. Each pre-filter 10 is often implemented by the arrangement shown in FIG. 5 for a filter of N coefficients. The pre-filter comprises a succession of delay circuits 14, each having a delay period equal to the sample period of the data stream. The input of each delay circuit 14 is connected by a respective
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Hurley Terence R.
Morcom Richard
Huseman M.
Independent Broadcasting Authority
Safourek Benedict V.
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