Radio Receiver Incorporating a Data Operated Squelch System

The present invention relates to a data processing system of the kind (hereinafter referred to as the kind set forth) in which there is a transmission of a code format having a pattern that provides a plurality of reversals 10101100 etc. from which it is desirable to be able to differentiate between valid data and invalid data. Invalid data means the absence of data, data corrupted by noise or data alien to the system.

According to the present invention there is provided a data processing system of the kind set forth wherein means are arranged to provide the times between n zero crossings as herein defined to measure said times to establish the number (R) between said zero crossings as given by the expression: ##EQU2## where I is the integer part of x, n is an integer representing the number of zero crossings and t.sub.i is the duration of the ith interval between zero crossings measured on a bit base of unity thereby to permit the use of said number (R) when compared with a predetermined tolerance range to provide an indication of the quality of said transmission.

In one construction n is between 8 and 100 and in a convenient construction n is 32 or 64.

By the term zero crossing is to be understood the time at which zero volts is crossed when going from a positive voltage to a negative voltage or vice versa.

The invention will be more fully understood from the following description given by way of example only with reference to the figures of the accompanying drawings and the following Example in which

FIG. 1 is a circuit block diagram of a pager incorporating data operated squelch of the invention and

FIG. 2 is a flow diagram of the operations performed by the present invention.

FIG. 3 is a graphical presentation of the transfer function of the expression used in the invention to illustrate a refinement of the circuitry.

Consider as one Example a transmission in which the code format has sixty four zero crossings as shown in the table below and the values for z=t.sub.i -I(t.sub.i -0.5) are:

Then taking the first three examples in the table (1) is: ##EQU3## and (2) is: ##EQU4## and (3) is: ##EQU5##

Clearly the sixty four values in the table for Z summated is 60.9 thus: ##EQU6##

The value R with a tolerance can, therefore, be used to make a decision for a satisfactory squelch that is to say a system which switches at least a part of a receiver between a quiescent and an active state depending upon the quality of the signal received.

The tolerance having been chosen to be for example 0.8 to 1.1 then R at 0.9515 is seen to fall within that tolerance and thereby indicate a signal of a predetermined quality.

The squelch may be used to obtain good sensitivity in a radio pager for example without wasting battery life.

Referring now to FIG. 1 the circuitry operates in such a way as to produce a specific transfer function of the form shown graphically in FIG. 3. The input is the time interval (t.sub.i) between adjacent transitions of the input signal. The output is numerical and is averaged over the required number (n) of transition interval samples to form the resultant (R). The decision of the squelch circuit is made by comparing this resultant (R) against some preset limits so that within this range the squelch may indicate a valid signal to the rest of the circuitry. One specific embodiment utilizing a microprocessor based system is illustrated in FIG. 1 and the flow chart for this specific embodiment is given in FIG. 2. A listing is given in the attached appendix A of the logic to be applied to the ROM memory to be interpreted by the microprocessor unit MPU. In FIG. 1 a radio pager comprises a receiver board 10 having an aerial 11 by which a signal enters a radio frequency amplifier 12 a 1st mixer 13 coupled to a 1st oscillator 14 to a 1st intermediate frequency amplifier 15; from which it passes inter alia via a 2nd mixer 16 to an audio limiter 17 connected to the input/output of the microprocessor 18 (MC14605E2) which is related to the memory of ROM (MCM65516) 19 .

The MPU will receive from the ROM a logical sequence set out in the flow sheet of FIG. 2 wherein:

101 is BTSYNC

102 is initialization clear flags load count

103 is zero port A clear?

104 is BSBEG

105 is 7 BFLAG set

106 is BSNT 1

107 is zero port A set?

108 is increment BTPRD

109 is BSNT1 0

110 is zero port A set?

111 is DMT (dead man timer) reset, increment BTPRD

112 is zero port A set

113 is DMT reset, increment BTPRD

114 is BSTIM 1

115 is LDA timer BCLR 7, BFLAG

116 is zero port A clear?

117 is increment BTPRD

119 is BSNT 0 1

119 is zero port A clear?

120 is DMT (dead man timer) reset, increment BTPRD

121 is zero port A clear?

122 is DMT reset, increment BTPRD

123 is BSTIM0

124 is LDA timer B set 7, BFLAG

125 is BSNT0

126 is BCLR 7, BFLAG

127 is BSNSE 1

128 is BSET 7 BFLAG

129 is LDA 0 1/STA/BTPRD/CLR/TEMPB

130 is BSTIME

131 is timer greater than 7A

132 is BSUB

133 is subtract 1.0 from timer

134 is add 0 to timer

135 is BSPRDX

136 is LDA/BTPRD STA/TEMPA/CLR/BTPRD

137 is subtract 06

138 is A greater than zero?

139 is COMA

140 is INCA

141 is BSPRD0

142 is A greater than 0.C.?

143 is BSPRD2

144 is LDA/TEMPA/SUB/TEMP B add BTTOT STA/BTTOT

145 is C.CLEAR

146 is INC.CARRY

147 is BSPRD4

148 is LDA05 STA/BTPRD

149 is BSPRD3

150 is DEC/BTCNT

151 is BTCNT equals zero?

152 is SUBOC

153 is CLRX/TAX

154 is add 0.C to TEMPB INC/BTPRD. TXA JMP/B5 PRD0

155 is LDA carry STA/TEMPA

156 is carry less than 2?

157 is carry equals 2?

158 is carry equals 3?

159 is BSPRD8

160 is BSETOBFLAG

161 is RTS global

162 is BSPRD7

163 is LDA/BTTOT/STA/TEMPB

164 is TEMPB greater than MXLVL

165 is BSPRD6

166 is RTS global

In FIG. 3 there is shown a graphical representation of the transfer function of the expression: ##EQU7##

Clearly if for example the first three figures of the table above 3.2; 1.6 and 1.2 are entered on the abscissa than the values for Z respectively (1.2; 0.6; 1.2) are read off along the ordinate. But as is well known a system of sequential logic requires a finite time to `think` and even where the bit width uses but 1.953 milliseconds, as shown close to the origin three tenths of this time may pass before the enabling means of the transfer function is entered. One solution is self evident viz to enter after a short time interval of 1.953.times.0.3 milliseconds and advance on the transfer function at that point, or to use parallel logic circuitry or a faster clock rate or overcome the difficulty from an analog circuit. ##SPC1##