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 "self talk-off" 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 '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.TM. 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 "voice-protected" 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 "voice-protected" 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 '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) "non-voice protected" DTMF receiver 80 which receives input from hybrid network 60; (b) integrator 90 which receives input from "non-voice protected" 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 "non-voice protected" 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 "voice-protected" DTMF receiver 70 receives DTMF signals which are transmitted to controller 40 from the user as commands. Later, when "non-voice protected" 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 '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 "non-voice protected" 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 "voice protected" DTMF receiver 70 is slower and more selective than "non-voice protected" DTMF receiver 80, in the apparatus disclosed in FIG. 2, "non-voice protected" 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 '872 patent, which is intended to overcome this latter difficulty by utilizing a separate "non-voice protected" 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 "non-voice protected" DTMF receiver 110 which receives audio output from speech decoder 50; (b) inverter 130 which receives input from "non-voice protected" 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 "non-voice protected" 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 "non-voice protected" 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 "non-voice protected" 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 "non-voice protected" DTMF receiver 80 detects a DTMF signal and "non-voice protected" 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 "non-voice protected" 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.