APPARATUS FOR PROVIDING TONE SIGNALS IN TERMINALS FOR A DIGITAL TELECOMMUNNICATION SYSTEM

Abstract: An apparatus for generating tone signals in the terminal equipment of a digital telecommunication system, the form of the signals being determined by digital condition information obtained from the exchange, e.g. a ringing tone signal or busy tone. The apparatus includes a memory (4) with a number of memory fields, the binary content of which represents discrete values in a tone frequency signal between two zero passages. The counter positions in a counter (5) are each associated with a memory field such that the periodical activation of the positions after each other provides a read-out to a D/A converter of the memory field associated with the respective position. A control circuit (9, 10) modulates the signal obtained from the periodical read-out in time with a condition signal determined by the condition information obtained from the exchange. The control circuit is activated when the counter is set to zero such that the beginning and end of the modulation coincide with the zero passages of the tone frequency signal.

Description: APPARATUS FOR PROVIDING TONE SIGNALS IN TERMINALS FOR A DIGITAL TELECOMMUNICATION SYSTEM. TECHNICAL FIELD The invention relates to an apparatus for generating tone signals in the terminal equipment of a digital telecommunication system, these signals being dependent on digital condition information obtained from an exchange. The terminals in a digital telecommunication system include, inter alia, a digital section attending to the reception and transmission of the digital signals on the line and an analogue section which, with guidance from received digital information, generates tone frequency signals or receives tone frequency signals for conversion to digital signals. BACKGROUND ART In a digital telecommunication system there are different possibilities for providing tone information such as busy tone, register tone etc in a terminal. One possibility is to actuate a tone generator in the terminal with the output signal from a decoder which obtains tone information-defining digital code signals from the exchange. Another solution is to utilize the digital speech connection for sending samples of tone frequency signals from the exchange, which are then converted in the terminal to analogue tone information. The first of the mentioned known solutions results in relatively expensive equip¬ ment, because it must be realized with the aid of discrete components. The other solution necessitates utilizing the speech band for the signal transmission, which should be avoided for enabling the generation of tone information to an optional terminal even when the channel is occupied by a call. DISCLOSURE OF INVENTION The solution according to the invention implies that the tone information signal is generated in the terminal by forming digital patterns, which are controlled with the aid of digital control signals obtained from the exchange. The apparatus can be produced in the form of an integrated circuit, and different tone information may be generated simultaneously over a bus structure for the different terminals. The invention is characterized as disclosed in the claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in detail below with the aid of an embodiment and with reference to the accompanying drawing, where Figure 1 is a block diagram of an apparatus in accordance with the invention, and Figures 2a-2d are diagrams over the signal sequence. BEST MODE FOR CARRYING OUT THE INVENTION In Figure 1, digital terminal equipment 1 includes a digital section 2 connected to the exchange via a digital line, and an analogue section 3 generating or receiving acoustic signals. The apparatus according to the invention, generating tone signals digitally, is included in the digital section of the equipment, and of this section only a codec 14 and a signal decoder 15 are schematically indicated. Both these devices are well known and are included in the terminal equipment according to planned standard. The codec 14 converts digital signals to analogue and vice versa, and the signal decoder 15 converts received binary information to signals in the terminal. Discrete values of a tone signal curve are written in digital form into a memory 4, there being 19 values according to the embodiment. These values are periodically read out with the aid of a counter 5 at a rate corresponding to the desired tone frequency. 421 Hz have been selected according to the embodiment, since the sampling frequency of 8 KHz, which is standard in the telecommunication equipment, may be utilized for stepping forward the counter 5 in this case. In this way, the 19 sample values in the memory 4 may namely be read out 421 times per second. The counter 5 is adapted such that it is returned to the zero step after 19 steps. This is achieved with the aid of two AND circuits 6 and 7, the inputs of which are connected to the positions 1, 2 and 5 in the counter, so that they provide a resetting signal when it reaches the value 19, i.e. 10011. A parallel-series converter 8 is connected to the memory 4 for feeding the sample values written into the memory in series form to one input of an AND circuit 9. The other input of this circuit obtains a signal from a D flipflop 10, which receives on one input a continuous binary signal from the deconder 15 corresponding to desired tone information, e.g. zeros for 0,25s and ones for 0,75s, while on its other input b it receives the resetting signal of the counter 5. The function of the D flipflop results in that if the signal at the input a has the value 1 this value occurs on the output only when the resetting signal of the counter 5 is fed to the input b. The gate 9 is thus opened, and the signal of the parallel-series converter 8 is fed to the digital-analogue converter in the codec 14. The one- signal on the output of the D flipflop is retained for the moment, but when the signal at the a input is altered to zero, the D flipflop output will also be zero at the next resetting signal, and the gate 9 is blocked. Figure 2a illustrates the resetting signal of the counter 5 which occurs at about 2,5 ms intervals, Figure 2b illustrates the digital tone information signal fed from the decoder 15 to the input of the D flipflop, and Figure 2c the signal obtained on the D flipflop output. Figure 2d illustrates the digital values obtained on the output of the gate 9 to an enlarged time scale. As will be seen, the tone signal can only be started and broken off for zero passages, thus avoiding disturbing clicking noises which would occur if a sinus tone was started at amplitudes differing from the zero value indicated by K in Figure 2d. The invention is of course not limited to the values according to the embodi¬ ment, and optional stepping speeds and numbers of samples per period may be utilized. vi __.

Claims: CLAIM Apparatus for generating tone signals in the terminal equipment of a digital telecommunication system, the form of the signals being determined by digital condition information (ringing tone signal, busy tone and the like) obtained from the exchange, characterized in that the apparatus includes a memory (4) with a plurality of memory fields, the binary contents of which represent discrete values in a tone frequency signal between two zero passages, a counter (5) which is periodically set to zero and in which the counter positions are each associated with a memory field, so that the periodical activation of the positions after each other provide a read-out to a D/A converter of the memory field associated with the respective position and a control circuit (9, 10) modulating the signal obtained from the periodical read-out in time with a condition signal determined by the condition in¬ formation obtained from the exchange, the control circuit (9, 10) being adapted for activation when the counter is set to zero such that the beginning and end of the modulation coincide with the zero passages of the tone frequency signal.

Inventors: SJOEBERG PER-OLOF, SJOEBERG, PER-OLOF
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1983
Main classification: H04M19
IPC codes: H04M19, H04Q1