GB2238690A

GB2238690A - Filters for analogue-digital video signal conversion

Info

Publication number: GB2238690A
Application number: GB9019422A
Authority: GB
Inventors: Josef Sochor
Original assignee: BTS Broadcast Television Systems GmbH
Current assignee: Philips GmbH
Priority date: 1989-09-09
Filing date: 1990-09-06
Publication date: 1991-06-05
Anticipated expiration: 2010-09-06
Also published as: GB9019422D0; DE3930085A1; GB2238690B; DE3930085C2

Abstract

In a system with a digital transmission path between an A/D converter (4) and a D/A converter (6), respective analogue low pass filters (2) and (7) whose orders differ by 1 are provided adjacent the converters for pre- and post-filtering to prevent aliassing. The standardised frequency response of one of the two low pass filters is multiplied by a factor so that the minima in the echo attenuation response of each of the filters corresponds to the maxima in the echo attenuation response of the other. The effect of the non-ideal filter characteristics (amplitude ripple in the pass band), which accumulates if the signal is repetitively converted between analogue and digital forms, is thus reduced. The digital path (5) may include signal processing or recording. <IMAGE>

Description

METHOD FOR THE DIGITAL TRANSMISSION OF A SIGNAL This invention relates to

a method for the digital transmission of a signal.

The conversion of an analog signal into a digital signal by a sampling (scanning) process can be considered as a multiplication of the analog signal by the sampling signal. There is a multiplication of the analog spectrum of the carriers, which occur with a multiple of the frequency of the sampling signal. If the frequency bandwidth of the analog signal is not limited to half the frequency of the sampling signal, there are disturbing overlaps in the frequency spectrum, known as alias interference. Thus, pre- filtering of the analog signal with a low-pass filter is p rescribed prior to A/D conversion. The frequency bandwidth (f 9) of the low pass filter depends on the sampling frequency (f Following digital transmission, of which the digital transmission path-.may include means for digital signal processing and/or magnetic recording and playback, the transmitted and D/A converted signal is reconstructed by post-filtering with a further low pass filter. In addition to the sin(x)/x-function of the D/A converter, here again the sampling signal frequency is decisive for the cut-off frequency of the further low pass filter.

In theory a pass band with a linear amplitude characteristic from 0 Hz to the cut-off frequency (f 9) is sufficient for the amplitude frequency response of the two low pass filters. The low pass filters should completely block any signal outside the pass band. However, in practice it is not possible to produce such sharp cut-off low pass filters at an acceptable cost.

The US journal "SMPTE Journal", October, 1981, pp. 949 to 955 discloses a method for improving the subjective image quality of digital television signals, in which several filters with amplitude fequency responses decreases with different characteristics to the cut-off frequency are inserted in the transmission path. The different filter characteristics do reduce interference in the case of brightness jumps in television signals, but the filter costs are still high. In addition, this known method has the disadvantage that in a series connection of several A/D and D/A converters the frequency response errors in the pass band of the filters accumulate. This particularly applies to simple low pass filters with a low order, for example Chebishev and Butterworth filters, whose passband changes very steeply into the stop band, but which j 1 have significant amplitude ripple in the pass band.

The object of the present invention is to provide a method for the digital transmission of a signal permitting the use of simple filters with amplitude ripple in the pass band.

According to the present invention there is provided a.method for the digital transmission of a signal, in which on the transmission side an analog signal is pre- filtered with a first low pass filter and is then converted with an A/D converter into a digital signal, and in which on the reception side the digital signal is converted back into an analog signal with a D/A converter and is then post-filtered with a second low pass filter, wherein the first and second low pass filters have respective orders which differ by one.

The invention has the advantage that it is possible to use simpler and therefore less expensive low pass filters. Therefore it is possible to obviate the all-pass filters for smoothing the group delay which are normally connected in series with the low pass filters.

An embodiment of the invention will now be described with reference to the accompanying drawing, wherein:

Figure 1 is a block circuit diagram of a digital signal transmission means, Figure 2 shows the standardised frequency response of a low pass filter for pre-filtering, Figure 3 shows the standardised frequency response of a low pass filter for post-filtering, and Figure 4 shows the standardised frequency response of low pass filters for pre- and post-filtering resulting from the embodiment of the invention.

Referring to the drawing, in the block circuit diagram of Figure 1, an analog signal is applied at a terminal 1 to a low pass filter 2 for limiting the frequency band. The cut-off frequency f 9 of the low pass filter 2 is such that the Shannon scanning (sampling) law for spectra is respected. In the present case the signal bandwidth determined by the cut-off frequency f 9 of the low pass filter 2 and the sampling frequency f S of a sampling signal applied to a terminal 3 have the relationship 2.f 9 > = fs. The sampling signal applied at the terminal 3 is used for controlling an A/D converter 4, which samples the low pass-filtered analog signal and quantizes and encodes the same.

In the present embodiment it is assumed that the z il 1 signal which is available in digital form at the output of the A/D converter 4 is a digital video signal which is supplied to a transmission path 5 which includes the magnetic recording and playback channel of a digital video recorder. The digital signal received on the playback side is converted back into an analog signal with a D/A converter 6 and is transmitted to a low pass filter 7 for postfiltering. In the usual way the low pass filter 7 is adapted to the transmission function of the D/A converter 6 and consequently has a sin(x)/x-configuration of the amplitude frequency response. A reconstructed analog signal is available at the output 8 of the low pass filter 7 and from the frequency and time standpoints corresponds to the input side, analog signal, if use is made of ideal low pass filters with a rectangular transition between the pass and stop bands and linear amplitude and phase response in the pass band.

However, as stated hereinbefore, such ideal filters cannot be produced at acceptable cost. Even sophisticated complex low pass filters have ripple in the pass band which, ib the case of a multiple copying of the video signals with a digital video recorder, can lead to considerable interference in the reproduced television picture. Particularly when editing video signals, in which, for example, 20 generations of an original video signal are used, there is an accumulation of errors or faults whose cause is the ripple in the amplitude frequency response of the low pass filters used for preand post-filtering.

Thus, according to the embodiment of the invention, the low pass filters 2 and 7 for pre- and postfiltering are designed in such a way that their orders (degrees) differ by 1. For example, if the low pass filter 2 has an order of.n = 4, then the low pass filter 7 is designed so that its order is either n = 3 or n = 5. In addition, the standardised frequency of one of the low pass filters is multiplied by a suitable factor k, so that the "hills" in the amplitude frequency response of one filter correspond to the "valleys" in the amplitude frequency response of the other filter. The procedure is such that the echo attenuation (ripple) of the two filters is made substantially the same.

The standardised frequency responses given in Figures 2, 3 and 4 serve to illustrate the foregoing, it being assumed that a Chebishev low pass filter is chosen for the pre-filtering, having an order n = 4 and an echo attenuation of 13 dB, in accordance with the book by G. Pfitzenmaier "Tabellenbuch Tiefp9sse", Siemens AG, Berlin, 1971. The low pass filter 7 is also constituted by a A Chebishev filter with a 13 dB echo attenuation, but with an order of 5. In Figures 2, 3 and 4 the amplitude attenuation a b is plotted on the ordinate and the standardised frequency 0 on the abscissa. Figure 2.shows the form of the ripple in the pass band of the low pass filter 2 provided for prefiltering, and Figure 3 the form of the ripple in the pass band of the low pass filter 7 provided for post-filtering. By multiplying with the factor k = 1.14, the scaling of the standardised frequency response D for the low pass filter 7 provided for post-filtering is extended in such a way that the minima in the ripple characteristic of each filter corresponds to the maxima in the ripple character- istic of the other filter, resulting in a smooth transmission characteristic at the end of the transmission system. The resulting echo attenuation is only a few percent of the individual echo attenuations. The insertion loss associated with this compensation can be absorbed by a linear amplification in the transmission path.

Claims

1. A method for the digital transmission of a signal, in which on the transmission side an analog signal is pre-filtered with a first low pass filter and is then converted with an A/D converter into a digital signal, and in which on the reception side the digital signal is converted back into an analog signal with a D/A converter and is then postfiltered with a second low pass filter, wherein the first and second low pass filters have respective -10 orders which differ-by one.

2. A method according to claim 1, wherein the standardised frequency response of one of the filters is multiplied b a factor such that the minima in the echo attenuation response of each of the two filters correspond to the maxima in the echo attenuation response of the other filter.

3. A method according to claim 1 or 2, wherein the low pass filters with different orders are used in association with a tape recorder for the digital recording and playback of video signals.

4. A method for the digital transmission of a signal, substantially as described herein with reference to the accompanying drawing.

Published 1991 at The Paterit Office. State House. 66/71 High Holborn' LOndor' WC I R 437P. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Foint, Cwnifelinfach. CrossKeys. Newport- NP I 7HZ. Printed by Multiplex techniques lid, St Mary Cray, Kent.

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