CA1089018A - Digital tone generator for use with radio transmitters and the like - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Many radio continuous tone controlled squelch systems use a digitally generated tone. A digital adder is provided for the digital tone generator to introduce a phase shift to the tone to indicate that a radio transmission is ending.

Description

90~1~

My invention relates to a digital tone generator for a radio transmitter, and particularly to a digital adder for changing the phase of the digitally generated tone. `
In some frequency modulation radio communication systems, a tone of audio frequency (below that usually reproduced by a radio receiver) is transmitted to activate only a selected receiver or receivers, and thus provide privacy and prevent undesired messages from being reproduced by other receivers during the transmissionO This arrangement ; 10 is sometimes referred to as a continuous tone controlled squelch system (hereafter CTCSS) or a channQl guard system.
If the transmitted carrier is cut off when the transmission ends (by unkeying the microphone3, then a burst or tail of noise will be heard at the activated receiver until the receiver noise squelch circuit takes over and mutes the receiver. In order that this tail of noise be blocked or ;
eliminated, some frequency modulation communication systems l u~e an arrangement that continues transmission of the carrier I for a short period (such as 175 milliseconds) after the microphone is unkeyed. During this period while the carrier ,: ~ ~ , ~, ..
~ continues to be transmitted, the phase of the tone is shifted.
f! Upon receipt of the tone with a phase shift, the activated receiver triggers a circuit to mute itself. During the time ~ n,aeded for the receiver to mute itself, the presence of the I carrier pre~ents noise from being heard. Thereafter the ~, receiver remains muted until a carrier with the proper tone is received again. Since digital circuits are being used increasingly because of their small size and versatility, . ;.,.
~ digit-l tone generators are being used so that a tone of any 1~ 3Q ~ ~ desired frequency can be easily selected. Thus, there is a need for an arrangement to shift the phase of a digitally ~ generated tone.
sf ~ O 1 ~ 45-MR-105 Accordingly, a primary object of my invention is to provide a new and improved arrangement for shifting the phase of a digitally generated tone transmitted by a radio transmitter.
Another object of my invention is to provide a digital adder that responds to keying of a radio transmitter ~ -and changes the phase of a digitally generated tone trans-mitted by the transmitter.
Briefly, these and other objects are achieved in accordance with my invention by a digital adder inserted between a digital counter and a digital tone function gener-ator. The digital adder has a control input that responds to the operation or keying of a radio transmitter microphone.
When the microphone is keyed, the digital counter signals are applied to the function generator in a first digital relation. When the microphone is unkeyed, the digital counter signals are applied to the function generator in a second digital relation to cause the function generator to shift its output a selected digital amount representing the 20 ~ desired angular phase shift. The function generator maintains the shifted condition and produces the digital outputs for ;~
producing the phase shifted tone as long as the microphone remains unkeyed. When the microphone is keyed again, the digital signals are applied to the function generator in the 1 first digital relation again to provide the original condition.
The su~ject matte~ which I regard as my invention is particularly pointed out and distinctly claimed in the claims. The structure and operation of my invention, together ~;
~ with further objects and advantages, may be better under- ~
1 : : , . .
3Q Ctood from the following description given in connection ~

with thè accompanying drawing, in which: -:
i ~ FIGU~E 1 shows a block diagram of a digital tone

2 -.. ... ~ ., ., . . ; , . . .... .. . .. .... . . . . . . .

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generator having a digital adder in accordance with my invention;
FIGURE 2 shows a wave form representing the digital signals produced by a microphone when keyed and unkeyed;
FIGURE 3 shows a logic circuit diagram of a phase shift adder in accordance with my invention as used with a particular type of tone function generator; and FIGURE 4 shows wave forms representing the operation of my phase shift adder.
Two-way radio communication systems frequently use the continuous tone controlled squelch system (CTCSS) to ~ -provide privacyl and to eliminate activating undesired receivers -~
during a transmission. In CTCSS, one of 37 standard tones between 71.9 and 250.3 hertz is transmitted along with other ;
information. This tone activates one or more receivers -having ilter equipment responsive to the selected tone.
Thus, only those receivers having a filter responsive to the transmitted tone will be activated, so that privacy is j . :
;20~ increased~and unnecessary transmissions are not heard by other receivers. Because oE the relatively low tone freq- -uencies (which are typically not reproduced by the receiver audio system), the filtering equipment responsive to those ~requenaies has a relatively long time constan~. That is, once the filter produces an output responsive to the frequency, :
: ~ . . .
it continues~ to produce thalt output for a considerable time after the tone has~actually stopped. This output keeps the reaeiver activated, so that even though the carrier trans-; misslon is stapped, the reaéiver remains unmuted. If the 3a ~ CTCSS operates~ln the~ frequency modulation bands, noise will be heard until the receiver ~noise squelch circuit takes over and mutes the receiver. ~This~short time period of noise is ~931~
sometimes referred to as a squelch tail. When a person must listen to transmissions for extended periods of time, these squelch tails can be bothersome. In order to eliminate the squelch tails, the CTCSS may include an arrangement at the transmitter which is responsive to the end of the transmission, ~ -such as by release of a microphone button, to maintain the . ~ .
carrier for a short period of time and continue transmitting the channel guard tone in a different phase relation. When a receiver receives the tone in a different phase relation, its filter circuit is arranged to immediately start squelching :
or muting the receiver. By the time the carrier is actually :~
stopped, the receiver is muted, so that loss of the carrier will cause the receiver noise squelch circuit to operate and `.-continue muting the receiver.
As mentioned previously there are presently 37 . :
; standard tone frequencies which are used in CTCSS. Since `.
it may be desirable for a radio transmitter to transmit any one of those selected tones in order to activated any selected radio receiver in the CTCSS communication system, the tone : 20 generators have used digital techniques for producing the :.
selected tone. These digital techniques permit a fixed .-.
I frequency clock oscillator to have its output divided in :~
i such a manner that any of the desired tone frequencies can . .:
~be produced. However, I have found that while such techniques '::' :'..`
1 are.good in order to.produce any one of the 37 selected tones, it does present a problem where a phase shift must be ;.
;i introduced in order to eliminate squelch tails or noise at ~ .
~` th.e end of a transmi.ssi.on. Accordingly, I have provided a ~
new and improved phase shift circuit for use in radio trans- :
mitters which have a tone generator using digital techniques. .
~;~` In FIGURE 1:, clock pulses at an appropriate .
recurring rate ~determined by the desired tone frequency) are ~: :
~ serially applied to a typical and known 4 bit binary counter 10. ~!r .i .~ ' !
~ 4 ~

~89V~B 4 5--MR--10 5 This counter 10 takes the serial clock input pulses and divides or counts these pulses and produces them at four parallel outputs indicated Al, A2, A4, and A8, where A1 provides the least signiicant bits or pulses (which occur at the highest rate) and A8 provides the most significant bits or pulses (which occur at the lowest rate that is one eighth the Al rate). These outputs have 16 different combinations which are repeated once for every 16 clock pulses. Previously these four parallel outputs would be applied to four parallel inputs of a function generator for producing a sine wave. Such a function generator could be a Walsh function generator 12 which is known in the art. One description of such a generator is given in a paper entitled ;;
"Walsh Function Generator for a Million Different Functions"
by Fredrick J. Lebert, given at the Walsh Function Proceedings held at the Naval Research Laboratory in Washington, D.C. on March 31, 1970. As known in the art, Walsh functions are described as Sal and Cal, and a typical generator such as the generator 12 produces the functions Sal 7, Sal 5, Sal 3, and Sal l. These functions are applied to a digital to analog .
converter 13 which, as known, weights the functions, combines the weighted functions, and filters them to produce a sine wave whose frequency is determined by the clock pulse rate.
.
In accordance with my invention, I provide a 4 bit phase shift~adder 11 between the counter 10 and the ~ generator 12 to introduce,`in digital fashion, signals that ¦ provide the desired phase shift in order to eliminate squelch tails. The phase shift adder 11 is a known device that is sometimes referred to as a 4 bit binary full adder. The adder

3~ 11 adds, in binary fashion, the two bits A and B at each of j its four dual inputs 1, 2, 4, and 8, and produces the binary sum at its four outputs ~ 2, ~4, and ~8. The bits Al, , ~ : ~

~5-MR-105 .8 ::

A2, A4, and A8 are derived from the counter 10. The bits Bl, B2, B4, and B8 are derived from respective switches Sl, S2, S4, and S8. The switches can be electronic or solid state switches ~ . .
as well as mechanical switches or wired connections. Each of .~ .

these switches is represented as a single pole, double throw :~.
- . . .
switch that can be connected to a lower contact that is .:
grounded (which I assume is a logic 0) for no bit to be added, ~
or that can be connected to an upper contact which receives ;.~
the microphone signal. If the microphone is unkeyed or off, .
It produces (by any suitak,le means) a logic 1. If the ~-:
microphone is-keyed or on, it produces (.by any suitable means) '~
a logic 0. FIGURE 2 shows the microphone signals. Initially, I have assumed that the microphone is unkeyed and a logic l .::~
is applied to the upper switch contacts. When, at the time . .
Tl, the microphone is keyed and the carrier is turned on, a :;
logic 0 is applied to the upper switch contacts. When the microphone is unkeyed at the time T2, a logic l is applied .~. `
to upper switch contacts. Then, at some later time T3 :
:: (usually about 175 milliseconds~ the carrier is turned off.
This 175 milli.second time during which the carrier is trans- -mltted wlth the phase shifted tone is sufficient to enable th.e receiver to mute itsel~ until the receiver noise squelch.
circuit responds to the loss.of carrier and continues muting the receiver. :`
One common phase shift used to indicate the end of a transmission is I35~ degrees. For the 16 different output comkinati.ons ok,tained rom th.e counter 10, this phase shift can be obtained either by adding ~ x 16 or 6 units; or .:
by adding ~363a69135 ) x 16 or~ 10 units. Six units can be 30: : added by connecting switches S2 and S4 to the upper contacts.
;~ ~ Ten units can be added~by connecting switches S2 and S8 to : the upper contacts. In either case, the e~fect is to add ,

4 5--MR--1 0 5 6 or 10 units (each representing 22.5 degrees phase shift) when the microphone is keyed, and not to add a shift when the ;
microphone is unkeyed. FIGURE 3 shows one circuit diagram which can be used for my phase shift adder 11, for the Walsh .
function generator 12, and for the converter 13. In this adder 11, I have elected to add ten units (B2, B8) as it .
simpli~ies the circuits for carrying. And, I have assumed that the phase shift of 135 degrees is not to be changed.
Hence, the adder ll of FI:GURE 3 has ~een simplified. The Al lQ output from the counter 10 is passed directly through the.
adder ll to ~ecome the 1 output of the adder 11. The A2 input is applied to one input of an EXCLUSIVE OR gate EOR l.
As known in the art, an EXCLUSIVE OR gate produces a logic 1 when one of its two inputs is a logic l and the other of its ;.:
two inputs is a logic 0. An EXCLUSIVE OR gate produces a logic 0 when both of its ~oth inputs are a logic l or a logic 0. The A2 input is also applied to one input of an AND gate AG l for carrying to the next more significant .:
adder. As known in the art, an AND gate produces a logic 0 if either of its inputs is a lcgic 0, and produces a logic 1 :
only when both of its i.nputs are a logic 1. The output of the AND gate AG 1 is applied to one input of an EXCLUSIVE
gate EOR 2 and to one input of another AND gate AG 2 for .i .
carrying. Th.e A4 input is applied to the EXCLUSIVE OR gate I ~ EOR 2 and the AND gate AG 2. The output o the AND gate AG 2 ¦ is applied to an EXCLUSIVE OR gate EOR 3 along with the A8 ~;
input~ And the output of the gate EOR 3 is applied to one input of an EXCLUSIVE OR gate E.OR 4. The digital microphone signals are applied to the other inputs of the gates EOR 1, .
30 ~ ~; AG L for B2, and~EOR 4 or B8. These microphone signals ~: ~ causes th.è pulses produced:at the outputs ~ 2,~4, and :. :
8~to ~e changed or shi~ted by an amount which, as explained ~ :
,:
,~. .
7 - ` . :

~ ~ 89 n ~ ~ 45-MR-105 a~ove, results in a phase shi.ft of 225 degrees in the decoded wave.
The ~ 2, ~ 4, and ~ 8 outputs are applied .. ;-to the function generator 12. Specifically, the output ~ 1 is applied to the inputs of two EXCLUSIVE OR gates EOR 5, EOR
7. The output ~ 2 is applied to the other input of the gate ~ .
EOR 7 and one input of an EXCLUSIVE OR gate EOR ~. The . :.
output ~ 4 is applied to one input of an EXCLUSIVE OR gate EOR 6. The output 8 is applied to the other input of the .
gate EOR 6, one input of an EXCLUSIVE OR gate EOR 9, and a .~
logic inverter INV 1. The output of the gate EOR 6 is ~. ; .
applied to the gates EOR 5~ EOR 8. And the output of the :. .
gate EOR 7 is applied to the gate EOR 9. .~
The outputs of the Walsh function generator 12 are .-~.
indicated as Sal 7, Sal 5, Sal 3, and.an inverted output -Sal 1. These outputs are applied to respective weighted . ;:.:
. .
: resistors R7, R5, R3, Rl in the converter 13. The weighted .:~.
: , .
outputs rom these our resistors are applied to an amplifier .

and filter 15 which produces a sine wave that is applied as ~20 ~ tone to the transmitter. In one em~odiment of my invention `~

: that was constructed, the resistors had the following values:
, . . .
Rl : 30,100 ohms R3 71~500 ohms ..

: R5 365,000 ohms , ! R7 150,000 ohms .~.

If the highest resistor, namely R5 of 365,000 ohms, is given ~ , , an arbitrary weighted binary output of 1.0, then the weighted outputs o the other are aprpoximately:

Rl ~ : 12.1 30 ~ ~ : R3 5.1 `! : : ~~ :.
~ R7 ~ 2.4 1 : , ; ~ :.: .

., , . .... ,.. ~ , .. , . . . ........ ... ,.. ............. . .. ,, .. .... ... ,. ~ . .... . . . .. . . , ., . ~ .. .... ...

31Q~39~

With this arrangement, the following table represents the operation of my phase shift adder 11:

Sequence MIC. ~alsh Fl Inction ~neratc r. ~otal From Counter Status -SAL 1 SAL 3 SAL 5 SAL 7 Weighted (12.1)(5.1) (1.0) (2.4)Binary Output _ .
1 KEYED-~ 0 1 1 1 8.5 , 2 0 1 0 0 5.1 '~
3 0 0 0 1 2.4 4 0 0 1 0 1.0 0 0 1 0 1.0 6 0 0 0 1 2.4 7 0 1 ~ 0 5.1 8 0 1 1 1 8.5 9 1 0 0 0 12.1 1 0 1 1 15.5 11 1 1 1 0 18.2 12 1 1 0 1 19.5 13 1 1 0 1 19.5 , 14 1 1 1 0 18.2 ~ ~ 1 0 1 1 15.5 `
16 1 0 0 0 12.1 , 1 UNKEYED-] 1 1 1 Q 18.2 2 1 1 0 1 19.5 3 1 1 0 1 19.5 1 1 1 0 18.2 1 0 1 1 15.5 , 6 1 0 0 0 12.1 , ~' 7 0 1 1 1 8.5 8 0 1 0 0 5.1 9 0 0 0 1 2.4 0 0 1 ~ l.Q
:: 11 . O O 1 O 1.0 ' , 12 0 0 0 1 2.4 13 Q ' 1 5.1 14 0 1 1 1 8.5 165 _ ~ 1 0 0 0 12.1 `'' The first 16 lines Oe the,table represent the bit numbers from the counter 10 when the microphone is keyed and . . .
;, produced a logic 0. The outputs of the Walsh function ., ~ ..
- 40 generator had the logic conditions shown, and these conditions ~ had the total weighted binary output indicated in the last ' ,: - :
column. SimLlarly, therext 16 lines show the operation '~
when the microphone was unkeyed and produced a logic 1 signal. '~
In FIGURE 4a, I have plotted a wave form illustrating ~
` '' ,: .

..

~ 45-MR-105 ~8~

the operation of the microphone being on or keyed, and in FIGURE 4b I have plotted another wave form showing the output when the microphone is off or unkeyed. Both of these wave forms are plotted along a common time axis or with corresponding sequences from Table 1. The wave forms shown in FIGURE 4 represent the Walsh function generator output, with the weighted conditions indicated in Table 1. These wave forms can be easily filtered to produce a relatively ;
accurate sine wave. FIGURE 4, as well as Table 1, shows the conditions of how a phase shift is introduced. For example, if the microphone is on or keyed when bit count 4 occurs, the weighted binary output is 1Ø If at this time the microphone is turned off or unkeyed, the weighted output shifts to a binary weighted output of 18.2. FIGURES 4a and 4b and Table 1 show that a phase shift of ten units of 22.5 degrees each is introduced between the keyed and unkeyed conditions for any counter sequence. The total effect is a phase shift of 225 degrees. This shift is provided by my ;
phase shift adder 11, which is compatible with digital signals for producing tones, and which serves to introduce a phase shift to those digital signals. The phase shift of ten units is introduced since the digital microphone signal is applied ; to the input B2 and the input B8 for the adder 11. Other phase shifts are possible, depending upon the angular phase '~ shift desired. Por a known adder 11 of FIGURE 1, if a 180 :
degree phase shift were desired, the digital microphone signal would be applied only to the input B8 which has a binary weight . . .
of eight units of 22.5 degrees each. If a 90 degree phase ~ shift were desired, then the digital microphone signal would ;~ 30 ~e applied only to the input B4 which has a binary weight of ;~:
four units of 22.5 degrees each. Thus, for a 16 pulse sequence, angular variations oE one pulse out of 16 or 22.5 degrees can . , .
-- 10 -- ' :' ,': "
: :: .
.': ~ ' , ":' ""

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be obtained and, of course, the pulse rate may be any - -suitable value.
It will thus be seen that I have provided a new and improved arrangement having a phase shift adder connected between a binary output, such as a counter or shift register, and a function generator for producing tones from this binary output. While I have shown only one embodiment, persons skilled in the art will appreciate that modifica~ions may be made. For example, other types of function generators can a be utilized in place of the particular Walsh function generator --shown in FIGURE 3~ The adder 11 may have switchable B inputs, or may be wired for a fixed number of added B inputs. And the logic (i.e. a logic 1 or a logic 0) may be reversed for the microphone signals, since the phase shift itself (and not the advanced or retarded phase shift) is the effect desired to indicate a transmission is ending. Other bit sequences and weights can be used. Therefore, itis to be understood that modifications may be made without departing from the spirit of the invention or from the scope of the claims.

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Claims (6)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. For use in a radio communication system in which a tone of selected frequency is transmitted for activating at least one selected radio receiver and in which the phase of said tone is shifted to indicate the end of a transmission, an improved digital tone generator comprising:
a) a counter having an input for a clock signal and a plurality of digital signal outputs;
b) a function generator having a plurality of digital inputs and an output for generating a tone in response to digital signals applied to said function generator inputs;
c) and a digital adder connected between said counter outputs and said function generator inputs for applying digital signals from said counter to said function generator, said digital adder having means for adding selected digital signals to said function generator inputs in response to the end of a transmission, thereby changing the phase of said tone at said function generator output to indicate to said one selected radio receiver that the transmission is ended.

2. The improved tone generator of claim 1, wherein said digital adder adds selected digital signals to at least two of said function generator inputs.

3. The improved tone generator of claim 1, wherein said function generator is a Walsh function generator.

4. An improved tone generator circuit for use with a radio transmitter which transmits a tone of selected frequency during operation to activate a predetermined radio receiver, and which transmits said tone at a different phase for at least a predetermined time following the end of a transmission to eliminate noise in said radio receiver, said tone generator comprising:

a) a source of recurring clock signals;
b) a counter having an input connected to said source of clock signals and having a plurality of parallel digital outputs for producing pulses of predetermined repetition frequencies in response to said clock signals;
c) a function generator having a plurality of parallel digital inputs and a single output for producing a substantially sinusoidal output signal in response to digital pulses applied to said function generator parallel inputs;
d) and a digital adder circuit connecting said parallel outputs of said counter to said parallel inputs of said function generator for supplying pulses from said counter to said function generator, said digital adder circuit having an input for connection to said transmitter and responsive to the end of a transmission thereof to cause selective pulses to be applied to said function generator inputs and thereby change the phase of said substantially sinusoidal output signal during said predetermined time.

5. The improved tone generator circuit of claim 4, wherein said digital adder circuit causes said function generator to shift the phase of said output signal by approximately 135 degrees.

6. The improved tone generator circuit of claim 5, wherein said function generator is a Walsh function generator.