Patent GB-1377684-A (GB)
Title: Data-transmission filter
IPC: H04L25/04
Filed: 1972-04-06
Abstract: 1377684 Active filters; digital data transmission; adaptive equalizers TELECOMMUNICATIONS RADIOELECTRIQUES ET TELEPHONIQUES 6 April 1972 [8 April 1971] 15912/72 Headings H3U H4P and H4R A digital filter, Fig. 2c, having as its input 20 samples of an analogue signal whose spectrum is restricted to the frequency band below a frequency f m  which is half the sampling frequency, includes 2<SP>n</SP>-1 elementary half-bandpass filter cells 13 ... 18 which are grouped in n cascade stages, the pth stage including 2<SP>n-p</SP> cells. The input series of samples at frequency 2f m  is split, at 21, into 2<SP>n-1</SP> interlaced series of samples at a frequency 2f m /2<SP>n-1</SP> which are applied to respective cells 13 ... 16 of the first stage and the outputs from these cells are combined in pairs at 22, 23 to provide 2<SP>n-2</SP> series of samples at a frequency 2f m /2<SP>n-2</SP> which are applied to respective cells 17 ... 18 of the next stage. This process is repeated for each successive stage, the single cell 19 of the last stage providing an output 25 of samples at frequency 2f m . Each cell is provided with means for reversing the sign of alternate input and output samples and with means for inhibiting its filter function, the means for the cells of each stage being connected to respective common terminals S, I, to which binary signals are applied for controlling the passband in width and position in steps of f m /2<SP>n</SP>. Filters according to the invention may be employed in a digital data transmission system, in which the transmitter provides a pre-transmission setting-up signal for a receiver equalizer, as well as pilot tones for adjustment of the equalizer during transmission. The receiver also includes means for synchronizing it to the transmitter. The basic filter cell, Fig. 8, comprises two halfband digital filter units, 27, 28 each of which includes six delay registers arranged in cascade through adders and three circuits M 1 , M 2 , M 3 , M 1 <SP>1</SP>, M 2 <SP>1</SP>, M 3 <SP>1</SP> for multiplying the input samples by coefficients stored in registers R 1 , R 2 , R 1 <SP>1</SP>, R 2 <SP>1</SP>. The series of samples entering the cell are split, by means not shown, into two series of odd and even samples which are applied to terminals 29, 30 respectively. The outputs from the two units are then recombined in OR gate 33. The assembly of Fig. 8 will operate as a highpass, lowpass or allpass filter depending on the values of the logical signals applied to the inhibiting input I and the input S which controls the inversion of alternate samples. The transmitter, Fig. 10.-Data input signals at 35 are applied to code converters 49, 50 which convert them into a bipolar code of the second order before applying them to a quadrature modulator 51. The output from the modulator is then fed to unit 52 where it is combined with full- or half-speed pilot signals from generators 53 or 55. The combined signal is then fed to a switch 56 which can selectively apply this signal, or the pretransmission setting up signal from source 57, to a carrier modulator 61, via filters 58, 59 and digital/analogue converter 60. The receiver, Fig. 15, comprises a demodulator 103 and an analogue/digital converter 107 which feeds an output on line 109 to the equalizer 110 and a synchronizing output on line 108 to adjust the local oscillator. The signal on line 108 is fed over a first path including filter 128 to a counter 121 which also receives a signal from filter 117, programmed to receive the pilot signals, the resulting output controlling the frequency of oscillator 120, and over a second path which includes filter 124 and control circuit 125 which controls a variable divider 126. The data signal is extracted from terminal 111 of equalizer 110 and, after having the pilot signal removed in circuit 113, is fed to demodulator 114. Automatic equalization, Fig. 18.-With the switch 132 in the position shown, the pretransmission setting up signal, comprising a spectrum of equally spaced tones of equal amplitude, is applied from input 130 to filter 117 which is sequentially programmed to respond to these tones, and the output from the filter provides control signals for adjusting respective stages of the coarse equalizer section 135 (detailed in Fig. 19a, not shown). Switch 132 is then moved to the second position in which fine equalizer section 136 (detailed in Fig. 22, not shown) is adjusted as a result of a comparison between the input and output signals of the equalizer. For this adjustment, further spectral lines are added from generator 137. When transmission commences, switch 132 is moved to the third position and fine equalizer 136 is continuously adjusted in response to the pilot tones, filter 117 being suitable reprogrammed. An alarm circuit 145, 146 is provided to give an indication if equalization is not achieved.