Abstract:
Apparatus for detecting and identifying DTMF signals in connection with a voice store and forward equipment (VSF) and for preventing talk-off, whether generated by the VSF or by a user of the VSF. In particular, the apparatus for use in connection with an interface which receives an analog signal from and applies an analog signal to a telephone line, the apparatus including: (a) detection means, responsive to an analog signal received from the interface, for detecting DTMF signals in the received analog signal and for generating DTMF indication signals in response thereto; (b) means for storing a representation of the received analog signal and at least some of the DTMF indication signals and for applying at least some of the DTMF indication signals to a decision means; and (c) means for retrieving a representation of an output analog signal and a representation of output DTMF indication signals and for applying them to the decision means; wherein the decision means is means for applying the output analog signal to the interface.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention pertains to method and apparatus for detecting and identifying Dual Tone Multifrequency (DTMF) signals and, in particular, for detecting and identifying DTMF signals in connection with voice store and forward apparatus (VSF) for preventing talk-off, whether generated by the VSF or by a user of the VSF. 
     BACKGROUND OF THE INVENTION 
     A voice store and forward apparatus (VSF) such as, for example, the ROLM PhoneMail voice store and forward apparatus produced by ROLM Systems of Santa Clara, Calif., typically receives a voice message from a user and stores the message for later transmission to other users. The later transmissions are typically provided in response to commands which are received from the other users who communicate with the VSF. It is well known in the art that a VSF typically permits users to utilize Dual Tone Multifrequency (DTMF) signals to communicate therewith. This is advantageous because a user may use a telephone which is equipped with a DTMF pad--as is well known, this includes almost all telephones that are commonly used in the United States today--to generate DTMF signals to send commands to the VSF. For example, such commands may be used to request the VSF to transmit a number of stored messages. In addition, while the user is receiving stored messages, he may generate further DTMF to provide further commands to the VSF to cause it, for example, to skip one of the stored messages or to go back and replay a stored message which was previously transmitted, and so forth. 
     Although there is a great advantage in using DTMF signals to send commands to a VSF due to the easy availability of DTMF signaling devices in the form of pads on a telephone set, there are, at the same time, certain problems which occur when DTMF signals are used to send commands to the VSF. In particular, such problems arise from the use of a hybrid network to interface with a 2-wire telephone line which carries signals to and from the VSF. The hybrid network gives rise to problems because a typical such hybrid network has a limited ability to separate transmitted and received signals. As a result, the hybrid network converts signals which are transmitted from the VSF to a user--such signals being, for example, input command prompts and stored messages--into an additive component of signals which are received from the user, which phenomenon is referred to as crosstalk. As described below, this signal conversion causes two types of problems. The first type of problem occurs whenever the user generates a DTMF signal while the VSF is transmitting signals, i.e., voice prompts or stored messages, to the user. Due to the signal conversion which is caused by the hybrid network, the DTMF signals which are generated by the user may be corrupted by the VSF output and, as a result, be unrecognizable to the portion of the VSF which receives and identifies DTMF signals. The second type of problem occurs whenever the VSF is transmitting signals, i.e., voice prompts or stored messages, while the user is silent. Due to the signal conversion which is caused by the hybrid network, the portion of the VSF which receives and identifies DTMF signals may misidentify the additive component of the VSF output as a DTMF signal which has been received from the user. This is referred to as &#34;self talk-off&#34; at the DTMF receiver of the VSF and, as a result, the VSF may interpret the additive component as a command from the user and provide an unexpected and inappropriate message to the user. 
     Many attempts have been made to cure the above-described two problems. For example, FIG. 1 shows the block diagram of FIG. 1 of U.S. Pat. No. 4,431,872 (the &#39;872 patent) which illustrates certain components of a typical VSF, i.e., VSF 30. As shown in FIG. 1, user telephone 10 is interconnected to VSF 30 through public switched network 20 and telephone 10 comprises TouchTone™ pad 15 which can generate DTMF signals. VSF 30 comprises controller 40, speech decoder 50, hybrid network 60, and receiver 70. Speech decoder 50 receives a message from controller 40, which message has been stored by controller 40 in, for example, digital form, and converts it into an audio signal which is applied as input to hybrid network 60 for transmission to the switching network 20 and from there, in turn, to telephone 10. In addition, receiver 70 receives signals which were generated by telephone 10. As shown in FIG. 1, receiver 70 is a &#34;voice-protected&#34; DTMF receiver and, as such, it decodes i.e., recognizes and identifies, DTMF signals and, in response, transmits identification and/or command codes to controller 40. The term &#34;voice-protected&#34; DTMF receiver refers to a DTMF receiver which is designed so that the detection requirements are relatively stringent so that the receiver will not be activated by voice signals. In a typical embodiment of telephone 10, DTMF signals are generated thereat with a tone amplitude of approximately -10 dBm. Further, in a typical worst case estimate, there is a tone loss through public switched network 20 of -20 dB and, as a result, DTMF signals which are generated at telephone 10 arrive at hybrid network 60 with an amplitude of approximately -30 dBm. In addition, as is well known to those of ordinary skill in the art, there is a further loss of approximately -3 dB in hybrid network 60, all of which provides that the DTMF signals arrive at DTMF receiver 70 with an amplitude of, approximately -33 dBm. Finally, there is a worst case -15 dBm crosstalk at hybrid network 60 which arises from the audio output from speech decoder 50. As one can readily appreciate from this, DTMF receiver 70 has to distinguish -33 dBm DTMF signals from a -15 dBm audio signal. 
     FIG. 2 shows the block diagram of FIG. 3 of the &#39;872 patent which addresses the first type of problem discussed above, namely, the problem that occurs whenever a user generates a DTMF signal while the VSF is transmitting signals to the user and, as a result, the DTMF signals which are generated by the user may be corrupted by the VSF output and be unrecognizable. FIG. 2 discloses apparatus which attempts to solve this problem by shutting off the output of the VSF at certain critical times. The equipment denoted by boxes 10 through 70 are the same for FIGS. 1 and 2. However, VSF 35 of FIG. 2 further comprises: (a) &#34;non-voice protected&#34; DTMF receiver 80 which receives input from hybrid network 60; (b) integrator 90 which receives input from &#34;non-voice protected&#34; DTMF receiver 80; and (c) switch 100 which receives input from integrator 90 and from speech decoder 50. As shown in FIG. 2, the output from &#34;non-voice protected&#34; DTMF receiver 80 is applied as input to integrator 90 and, in response, integrator 90 outputs a signal which is applied to switch 100 which causes it to open. This interrupts the audio path between speech decoder 50 and hybrid network 60. During this time period, crosstalk across hybrid network 60 ceases and &#34;voice-protected&#34; DTMF receiver 70 receives DTMF signals which are transmitted to controller 40 from the user as commands. Later, when &#34;non-voice protected&#34; DTMF receiver 80 has determined that the user has stopped transmitting DTMF tones, switch 100 is closed and the path between speech decoder 50 and hybrid network 60 is reestablished. 
     However, as recognized in the &#39;872 patent, the apparatus shown in FIG. 2 hereof has a problem in that audio output from speech decoder 50 may produce a signal that is detected by &#34;non-voice protected&#34; DTMF receiver 80 as a result of crosstalk through hybrid network 60. Whenever this occurs, as was described above, switch 100 will be opened and the output from VSF 35 to the user at telephone 10 will be unnecessarily interrupted. Thus, although the apparatus disclosed in FIG. 2 hereof protects against self talk-off because &#34;voice protected&#34; DTMF receiver 70 is slower and more selective than &#34;non-voice protected&#34; DTMF receiver 80, in the apparatus disclosed in FIG. 2, &#34;non-voice protected&#34; DTMF receiver 80 will cause switch 100 to open too often. This effect is noticeable to a user and results in substantially degraded system performance. 
     FIG. 3 hereof shows the apparatus disclosed in FIG. 5 of the &#39;872 patent, which is intended to overcome this latter difficulty by utilizing a separate &#34;non-voice protected&#34; DTMF receiver to monitor the output from speech decoder 50. The equipment denoted by boxes 10 through 100 are the same for FIGS. 2 and 3. However, VSF 37 of FIG. 3 further comprises: (a) second &#34;non-voice protected&#34; DTMF receiver 110 which receives audio output from speech decoder 50; (b) inverter 130 which receives input from &#34;non-voice protected&#34; DTMF receiver 110; and (c) AND logic circuit 120 which receives input from integrator 90 and from inverter 130. 
     As shown in FIG. 3, the output from &#34;non-voice protected&#34; DTMF receiver 110 is applied as input to inverter 130 and, in response, inverter 130 outputs a signal which is applied to AND logic circuit 120. If the audio output from speech decoder 50 is identified as a DTMF signal by &#34;non-voice protected&#34; DTMF receiver 110 and, at the same time, the crosstalk of that signal through hybrid network 60 is also identified as a DTMF signal by &#34;non-voice protected&#34; DTMF receiver 80, then one input to AND logic circuit 120 will be up (1) and the other input will be down (0). Thus, there will be no output from AND logic circuit 120 to operate switch 100. Further, as one can readily appreciate, in order for switch 100 to be opened, both inputs to AND logic circuit 120 must be enabled. This only occurs when &#34;non-voice protected&#34; DTMF receiver 80 detects a DTMF signal and &#34;non-voice protected&#34; DTMF receiver 110 does not detect a DTMF signal in the audio output from speech decoder 50. 
     The solution in the prior art which was discussed above with reference to FIG. 3 hereof--wherein separate &#34;non-voice protected&#34; DTMF receiver 110 is used to monitor the signal output by speech decoder 50 to detect talk-off and, thereby, to minimize false output signaling--is inadequate for several reasons. First, the prior art apparatus disclosed in FIG. 3 requires the use of three DTMF receivers and this results in increased system cost and complexity. Second, the prior art apparatus disclosed in FIG. 3 does not detect and prevent self talk-off due to crosstalk from the system output. 
     As a result of the above, there is a need in the art for method and apparatus for detecting and identifying DTMF signals in connection with a VSF and for preventing talk-off, whether generated by the VSF or by a user of the VSF. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention advantageously satisfy the above-identified need in the art and provide method and apparatus for detecting and identifying DTMF signals in connection with a voice store and forward apparatus (VSF) and for preventing talk-off, whether generated by the VSF or from a user of the VSF. A VSF generally operates in one of two modes: (a) a first mode wherein the VSF is transmitting a signal and the user is listening and (b) a second mode wherein the user is talking and the VSF is quiet and recording the user&#39;s speech. Potential corruption of DTMF signal reception by the VSF as a result of output from the VSF occurs only while the VSF is operating in the first mode whereas talk-off is a potential problem which occurs only while the VSF is operating in the second mode. As a result, specific embodiments of the present invention are directed to method and apparatus which optimize the first and second modes of a VSF separately to provide a VSF which provides optimum performance at all times. 
     In particular, one embodiment of the present invention is apparatus for use in connection with an interface which receives an analog signal from and applies an analog signal to a telephone line, the apparatus comprising: (a) detection means, responsive to an analog signal received from the interface, for detecting control signals in the received analog signal and for generating received control indication signals in response thereto; (b) means for storing a representation of the received analog signal and at least some of the received control indication signals and for applying at least some of the received control indication signals as input to a decision means; and (c) means for retrieving a representation of an output analog signal and a representation of output control indication signals and for applying them as input to the decision means; wherein the decision means is means for applying the analog signal to the interface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A complete understanding of the present invention may be gained by considering the following detailed description in conjunction with the accompanying drawing, in which: 
     FIG. 1 shows the block diagram of FIG. 1 of U.S. Pat. No. 4,431,872 (the &#39;872 patent) which illustrates certain components of a typical voice store and forward apparatus (VSF); 
     FIG. 2 shows the block diagram of FIG. 3 of the &#39;872 patent which addresses a problem that occurs whenever a user generates a DTMF signal while the VSF is transmitting signals to the user; 
     FIG. 3 shows the block diagram of FIG. 5 of the &#39;872 patent which utilizes a separate &#34;non-voice protected&#34; DTMF receiver to monitor the output from a speech decoder 50 to prevent inadvertent cutoff of VSF transmission; 
     FIG. 4 shows a block diagram of an embodiment of the present invention which solves the same need solved by the prior art embodiment of FIG. 3, however, in a manner which eliminates the need to utilize two &#34;non-voice protected&#34; DTMF receivers; 
     FIG. 5 shows a block diagram of an embodiment of the present invention which solves the same problem solved by the embodiment of FIG. 4 and, in addition, solves the problem of self talk-off; and 
     FIG. 6 shows a block diagram of an embodiment of the present invention which solves the same problem solved by the embodiment of FIG. 5 without the use of a second &#34;voice protected&#34; DTMF receiver. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 shows a block diagram of an embodiment of the present invention which solves the same need solved by the prior art embodiment of FIG. 3, however, in a manner which eliminates the need to utilize two &#34;non-voice protected&#34; DTMF receivers. As shown in FIG. 4, telephone 10 is connected through public switched network 20 to voice store and forward apparatus (VSF 38). Telephone 10 includes TouchTone™ pad 15 which can be used to generate DTMF signals. VSF 38 is comprised of controller 40, speech coder 500, multiplex 510, demultiplex 520 and speech decoder 50. Speech coder 500 is apparatus which is well known to those of ordinary skill in the art which receives an input in the form of, for example, an audio message which has been transmitted by a user from telephone 10 through hybrid network 60 and converts the input into a digital signal. The digital signal which is output from speech coder 500 is applied as input to multiplex 510. Multiplex 510 is apparatus which is well known to those of ordinary skill in the art which receives digital output from speech coder 500 and digital output from &#34;non-voice protected&#34;  DTMF receiver 80 and multiplexes these two signals into a combined digital signal. Further, the combined digital signal is applied as input to controller 40 for storage thereby in a manner which is well known to those of ordinary skill in the art. 
     As is well known to those of ordinary skill in the art, the output from &#34;non-voice protected&#34; DTMF receiver 80 is a digital signal which indicates the presence and identity of a DTMF tone. In general digital data represented by the digital output from &#34;non-voice protected&#34; DTMF receiver 80 has little impact on the storage requirements of controller 40 because voice audio data has a digital bit rate in the range of 15-30 Kbps whereas digital output from &#34;non-voice protected&#34; DTMF receiver 80 which outputs a one bit presence indicator of a DTMF tone can be sampled at a rate of about once every 10 milliseconds to yield a 100 bps data rate. 
     When VSF 38 transmits to a user, demultiplex 520 receives, as input from controller 40, digital data which has been stored in controller 40. Demultiplex 520 is apparatus which is well known to those of ordinary skill in the art, which separates the input into a digital signal which corresponds to the digital signal which was produced in response to an audio message that was received from a user previously and a digital signal that was received from &#34;non-voice protected&#34; DTMF receiver 80. Demultiplex 520 applies the digital signal relating to the audio message as input to speech decoder 50 and demultiplex 520 applies the digital signal relating to input from &#34;non-voice protected&#34; DTMF receiver 80 to inverter 130. As is well known to those of ordinary skill in the art, speech decoder 50 takes an audio message which has been stored by controller 40 in, for example, digital format and converts it into an audio signal which is presented to hybrid circuit 60 for transmission to the switching network 20 and from there, in turn, to telephone 10. During this transmission to the user, &#34;voice-protected&#34; DTMF receiver 70 receives signals transmitted to VSF 38 from telephone 10, decodes any embedded DTMF tones, and transmits the command codes to controller 40. 
     The output of &#34;non-voice protected&#34; DTMF receiver 80 passes through integrator 90 and the output from integrator 90 is applied, in turn, as input to AND logic circuit 120. The output from inverter 130 is applied as input to AND logic circuit 120 and the output from AND logic circuit 120 is applied, in turn, as input to switch 100. 
     In the second mode of operation, whenever a user at telephone 10 calls VSF 38 to input an audio message, &#34;non-voice protected&#34; DTMF receiver 80 generates a digital indication each time it &#34;detects&#34; a DTMF tone. This indication is multiplexed by multiplex 510, together with a digital representation of the audio message which is output from speech encoder 500, into a combined digital signal. Then, controller 40 stores the combined digital signal. Later, in the first mode of operation, whenever a user calls VSF 38 and requests a playback of a previously recorded message, the audio output from speech decoder 50 may produce signals that are detected as DTMF tones by &#34;non-voice protected&#34; DTMF receiver 80 as a result of crosstalk on hybrid network 60. However, at substantially the same time, the digital signal output from demultiplex 520 which is applied to inverter 90 and which relates to the original output from &#34;non-voice protected&#34; DTMF receiver 80 should also indicate that a DTMF tone is present, i.e., demultiplex 520 will send a signal to inverter 130 whenever &#34;non-voice protected&#34; DTMF 80 has &#34;originally detected&#34; a DTMF tone. As a result, in this case, both inputs of AND circuit 120 will not be enabled and switch 100 will not be opened. However, on the other hand, if the input to AND logic circuit 120 from &#34;non-voice protected&#34; DTMF 80 is &#34;up&#34; (logical 1) to indicate that the user is transmitting a DTMF tone while VSF 38 is transmitting an audio message and the other input to AND logic circuit 120 from inverter 130 is up (logical 1) because there was no DTMF tone in the original user&#39;s audio transmission to VSF 38, AND logic circuit 120 will produce an output to operate switch 100. As a result, switch 100 will open to interrupt VSF 38 output transmissions whenever a user inputs a DTMF tone, however, switch 100 will not interrupt VSF 38 output transmission if there is a spurious DTMF tone emulation in the VSF 38 output transmission. In other words, the embodiment of the inventive VSF 38 shown in FIG. 4 removes interference between user generated DTMF tones and VSF output transmission signals. 
     Although the above-described embodiment of the present invention prevents interference between VSF output transmission signals and user generated DTMF tones, the problem of self talk-off still remains. Self talk-off occurs because the portions of the VSF output transmission signals that are most likely to cause talk-off are applied to &#34;voice-protected&#34; DTMF receiver 70 by hybrid network 60. In accordance with the present invention, the problem of self talk-off can be prevented by cutting off the output transmission signal from the VSF whenever it is likely to cause &#34;voice-protected&#34; DTMF receiver 70 to talk-off. In a first embodiment which addresses this problem shown in FIG. 5, this is accomplished with the use of an additional &#34;voice protected&#34; DTMF. VSF 39 shown in FIG. 5 is the same as VSF 38 shown in FIG. 4 except for the addition of delays 600 and 610, &#34;voice protected&#34; DTMF receiver 620, and OR logic circuit 630. The following explanation will focus on the features of VSF 39 of FIG. 5 which differ from those of VSF 38 of FIG. 4. 
     The audio output from speech decoder 50 is applied as input to delay 600 and is applied as input to &#34;voice protected&#34; DTMF receiver 620. After passing through delay 600, the audio signal may, as a result of crosstalk through hybrid network 60, be applied as input to &#34;voice protected&#34; DTMF receiver 70. If &#34;voice protected&#34; DTMF receiver 70 identifies the signal as a DTMF tone, it sends a signal to controller 40 and, in response, controller 40 will cease output, believing that the DTMF tone was generated by the user. In the embodiment shown in FIG. 5, &#34;voice protected&#34; DTMF receiver 620 is used to &#34;predict&#34; when this event might occur and, in response thereto, to cut off the output from VSF 39 by causing switch 100 to open. However, in order to cause switch 100 to open at the proper time to prevent &#34;voice protected&#34; DTMF receiver 70 from operating, &#34;voice protected&#34; DTMF receiver 620 needs a &#34;head start.&#34; This &#34;head start&#34; will enable &#34;voice protected&#34; DTMF receiver 620 to be able to cause switch 100 to open in time to prevent tripping &#34;voice protected&#34; DTMF receiver 70. The required &#34;head start&#34; is provided by delay 600. Thus, when &#34;voice protected&#34; DTMF receiver 620 detects a DTMF tone, it applies an up (logical 1) to OR logic circuit 630 and OR logic circuit, in turn, applies a signal to switch 100 that causes it to open and prevent self talk-off. While this will cause the VSF output to be cut off, this will happen infrequently as compared to the apparatus shown in FIG. 2 because &#34;voice protected&#34; DTMF receiver 620 will talk-off much less frequently than &#34;non-voice protected&#34; DTMF receiver 80. The remainder of VSF 39 operates in the same manner as does the embodiment shown in FIG. 4, however, delay 610 is needed to match the delay introduced into the audio output by delay 600. As one can readily appreciate, the use of second &#34;voice protected&#34; DTMF receiver 620 can also be added to the prior art embodiment shown in FIG. 3 to perform a similar function. However, if second &#34;voice protected&#34; DTMF receiver 620 were added to the prior art embodiment shown in FIG. 3, then only one delay would be needed instead of the two delays indicated for the embodiment shown in FIG. 5, i.e., a delay would be added after the output from speech encoder 50 and before &#34;non-voice protected DTMF receiver 110. Further, if second &#34;voice protected&#34; DTMF receiver were added to the prior art embodiment shown in FIG. 3, &#34;voice protected&#34; DTMF receiver 620 should be positioned so that it would receive the audio signal output from speech decoder 50 before the delay in order that it have a &#34;head-start&#34; on &#34;voice protected&#34; DTMF receiver 70. 
     FIG. 6 shows a block diagram of an embodiment of the present invention which solves the same problem solved by the embodiment of FIG. 5 without the use of a second &#34;voice protected&#34; DTMF receiver. In particular, the following will focus on the features of VSF 33 of FIG. 6 which differ from those of VSF 39 of FIG. 5. The embodiment shown in FIG. 6 utilizes three different types of DTMF receivers: (a) &#34;voice protected&#34; DTMF receiver 710 which is optimized for talk-off; (b) &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference performance; and (c) &#34;non-voice protected&#34; DTMF receiver 80. We will discuss below in detail how such DTMF receivers are fabricated, however, at this point, we will discuss some overall features that differentiate their performance. In particular: (a) &#34;voice protected&#34; DTMF receiver 710 which is optimized for talk-off is the most selective of the three receivers in terms of determining whether or not a DTMF tone was detected and it operates the slowest of the three receivers; (b) &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference performance is designed to detect DTMF tones generated by a user in the presence of interference from, for example, crosstalk through hybrid network 60 while VSF 33 is transmitting a message to a user, is the next most selective of the three receivers, and it operates more quickly than receiver 710; and (c) &#34;non-voice protected&#34; DTMF receiver 80 is the least selective of the three receivers and it operates the fastest. 
     First consider the operation of VSF 33 of FIG. 6 when VSF 33 is receiving an audio message from a user. In this case, controller 40 sends a signal on line 730 to switch 720. In response, switch 720 is placed in an &#34;up&#34; position which provides that output from &#34;voice protected&#34; DTMF receiver 710 which is optimized for talk-off is passed through switch 720 to controller 40 to protect against the spurious detection of DTMF tones which are simulated by the user&#39;s audio input. The outputs from &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference and &#34;non-voice protected&#34; DTMF receiver 700 are combined with the digital output from speech encoder 500 in multiplex 510 in the same manner as was described above with reference to FIG. 5 and the combined digital signal is transmitted to controller 40 for storage thereby. 
     Second, consider the operation of VSF 33 of FIG. 6 when VSF 33 is transmitting an audio message to a user. In this case, controller 40 sends a signal on line 730 to switch 720. In response, switch 720 is placed in a &#34;down&#34; position which provides that output from &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference is passed through switch 720 to controller 40. A combined, stored digital signal is output from storage by controller 40 and it is applied as input to demultiplex 520 which is apparatus which understood by those of ordinary skill in the art. Specifically, demultiplex 520 is apparatus which demultiplexes the input thereto into three signals: (a) the first signal is a digitized audio signal which is applied as input to speech decoder 50; (b) the second signal represents the output from &#34;non-voice protected&#34; DTMF receiver 80 and that signal is applied as input to delay 610; and (c) the third signal represents the output from &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference and that signal is applied as input to OR logic circuit 630. In response to the first signal, speech decoder 50 provides an audio output which is applied as input to delay 600 and, after that, through switch 100 to hybrid network 60. After passing through delay 600, the audio signal may, due to crosstalk through hybrid network 60, be applied as input to &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference. If &#34;voice protected&#34; DTMF receiver 700 identifies the signal as a DTMF tone, it sends a signal to controller 40 and, in response, controller 40 will cease output, believing that the DTMF tone was generated by the user (if this happens, it is undesirable). The third signal which is output from demultiplex 520, which third signal represents the output from &#34;voice protected&#34; DTMF receiver 700 which is optimized for interference signal and which third signal was produced when the audio message was recorded, is used to &#34;predict&#34; when this false detection by receiver 700 might occur and to cut off the output signal from VSF 33 by causing switch 100 to open. However, in order to be able to cause switch 100 to open in time to prevent &#34;voice protected&#34; DTMF receiver 700 from operating, the third signal needs a &#34;head start&#34; to be able to open switch 100 in time to prevent tripping. As was described above with respect to the embodiment shown in FIG. 5, this &#34;head start&#34; is provided by delay 600. Thus, when the output from &#34;voice protected&#34; DTMF receiver 700 would detect a DTMF tone, the third signal applies an up (logical 1) signal to OR logic circuit 630 and OR logic circuit 630, in turn, applies a signal to switch 100 that causes it to open and prevent self talk-off. The remainder of the circuit operates in the same manner as does the similarly numbered apparatus of the embodiment shown in FIG. 5. As one can readily appreciate, the second &#34;voice protected&#34; DTMF receiver of FIG. 5 has been removed by storing the output of &#34;voice protected&#34; DTMF receiver 700. 
     One of ordinary skill in the art can fabricate embodiments of the present invention as disclosed in FIGS. 4-6 by utilizing an industry standard SSI203 DTMF receiver chip and by utilizing the &#34;Early Detect&#34; signal output therefrom as the &#34;non-voice protected&#34; DTMF receiver output and by utilizing the &#34;Data Valid&#34; signal output as the &#34;voice protected&#34; DTMF receiver output. However, the need to use separate &#34;voice protected&#34; and &#34;non-voice protected&#34; DTMF receivers as shown in the embodiments of FIGS. 4-6 can be eliminated in accordance with the following. The appendix hereto discloses a quadrature-phase, matched filter bank which receives an input audio signal and, in response thereto, produces measurements which can be used to implement &#34;voice protected&#34; as well as &#34;non-voice protected&#34; DTMF receivers. Specifically, the filter bank periodically outputs measurements of the DTMF component amplitudes and total signal power in the form of three numbers. The signal component amplitudes are compared to each other to determine signal twist and the total signal power is tested to determine whether the input met well known minimum requirements for DTMF tones. Further, the signal component amplitudes are compared to total signal power to estimate the signal-to-noise ratio. A counter is used to indicate the time duration of the signal. 
     Each of the above-described tests utilizes thresholds and a signal input measurement must exceed all thresholds before a DTMF tone can be reported. By varying the thresholds, different levels of receiver performance can be achieved. For example, strict thresholds result in a &#34;voice protected&#34; DTMF receiver which is optimized for talk-off performance; relaxing these thresholds slightly results in a &#34;voice protected&#34; DTMF receiver which is optimized for recognition in the presence of voice interference; and relaxing these thresholds by a large amount results in a &#34;non-voice protected&#34; DTMF receiver. 
     Further, a &#34;voice protected&#34; DTMF receiver is generally designed to trade off talk-off performance with recognition of DTMF tones in the presence of voice interference. Even when one uses the apparatus shown in FIGS. 2-5, voice interference will be present during part of the received tone. This can effectively shorten the length of the received tone, causing misrecognition. The effect of this shortening of the effective length of the tone can be reduced by making the receiver more tolerant of voice interference. Unfortunately, making the receiver more tolerant of such interference necessarily makes the receiver more likely to talk-off from user input, thus different receivers are used at different times. 
     Although we have described the present invention in terms of the different functional components in order to better be able to describe the functional components, the manner in which these functional components operate and the manner in which these functional components interact with one another should be clear to those of ordinary skill in the art. Further, one of ordinary skill in the art should readily appreciate that present day circuitry has been able to provide chips which combine several functional components together on a single chip. For example, the hybrid network and the speech digitization are combined in an industry standard &#34;combo codec.&#34; Still further, preferred embodiments of the inventive apparatus disclosed in FIGS. 4-6 comprise software which is executed on special purpose microprocessors. Specifically, the speech encoder and speech decoder comprise speech data reduction algorithms which execute on a digital signal processor (DSP) such as, for example, the TMS320C25 DSP which is commercially available from Texas Instruments. The DTMF filter bank and the DTMF detection algorithms which evaluate the output from the filter bank to provide the various receivers are implemented in software which is executed on the same DSP. The delay functions are implemented as LIFO operations in the memory of the DSP and the random logic circuits may be implemented as decision algorithms within the DSP. Finally, the data multiplexing and demultiplexing and the data transferral are performed by a general purpose microprocessor such as, for example, the Intel 80186. Each software portion of the preferred embodiments is a straightforward translation of the functions which have been described in detail above to code which anyone of ordinary skill in the art can implement. ##SPC1##