Method and apparatus for muting switching noise while switching between CODECS

Abstract: A communication apparatus having a plurality of audio CODEC (5) and communication method thereof. When a coding mode for transmitting audio information is set, the audio CODEC (5) corresponding to the coding mode is selected and switched. When the coding mode of the audio information is instructed from the terminal on the transmission side, the audio CODEC on the reception-side terminal is switched according to the instruction. When these audio CODEC are switched, generation of audio noise can be suppressed by setting the mute on the audio information.

Description: The present invention relates to communication apparatuses capable of coding and transmitting audio information and/or receiving and decoding coded audio information, and communication methods thereof. Recently, communication services by ISDN (Integral Service Digital Network) have been put to practical use and audiovisual teleservices using such digital network, an audiovisual telephone apparatus and videoconference, have been attracting attention. Services, protocol, and frame structure of multimedia services for this audiovisual teleservices are defined in CCITT (The International Telegraph and Telephone Consultative Committee) Recommendations H.320, H.242, H. 221, H.261. Recommendation H.221 defines coding allocation of a Bit rate Allocation Signal (BAS) which is used for exchange of frame structures and has channel capabilities from a 64 to 1920 kbit/s channel in audiovisual teleservices and mode designation. Recommendation H.242 defines a protocol such as capability exchange and communication mode switching between audiovisual terminals using BAS. Recommendation H.320 defines system aspects of overall audiovisual services. In the above recommendations, a method for performing multimedia communications, such as image, sound, and data communications, between the terminals by procedures such as terminal capability exchange sequence using the BAS in-channel and mode switching sequence by mode designation, after establishment of end-to-end physical connection and synchronism in-channel has been specified. Information transfer speed in each medium in the multimedia communication is determined by designating an audio coding mode on audio information. The information transfer speed of data information such as written on a document is determined by designating a transmission speed of the data information. Furthermore, transfer speed of image information is determined as the remaining bandwidth after obtained the transfer speed of the audio information and that of the data information are subtracted from the information transfer speed of all communication paths. Accordingly, the information on the transfer speed when the transmission mode of a transmitting party is informed to the terminal of a receiving party in the BAS command needs to have only an audio coding mode and data transfer speed. As for the transfer speed of the image information, it is automatically set in the terminal of the receiving party. Recommendation H.242 defines communication capability of audio information so as to enable to transmit/receive the audio information in accordance with (A-law) and (µ-law) of Recommendation G. 711. Designation of the audio mode which transmits/receives the audio information is performed within the audio communication capability when the terminal of a transmitting side transmits the BAS command to the terminal of the receiving party. The terminal which has received this BAS command must set the reception mode to the mode designated by the BAS command. However, the transmission mode and reception mode need not always be the same. It can be respectively set. In the conventional multimedia communication apparatus, in the audio mode, when the communication mode which transmits the audio information is switched, noise is generated since the audio CODEC is switched. When the reception mode which receives the audio information is switched, there is a case where the audio information of the reception mode before the switching remains in the network. In this case, the coding mode of the audio information differs from the decoding mode, resulting in audio noise at the terminal of the reception side. Furthermore, when the communication mode is initialized, at the start of communication, the synchronizing procedure and audio CODEC are switched and timing for synchronism is staggered, thus resulting in generation of noise. EP-A-0361384 discloses a variable rate coder. A signal is input in parallel to a plurality of ADPCM encoders having different quantising scales. The encoder is selected for each frame of data so as to maintain a preset quality of regenerated signal with the lowest bit rate. A selection code is added as a header to each frame of data transmitted identifying the encoder used and therefore the decoder required at the receiving side. In the light of the above drawbacks, it is an object of the present invention to provide a communication method and apparatus with reduced noise generation when a communication mode of audio information is switched or initialisation of the communication mode at the in-channel is set. In a first aspect, the invention provides a transmission apparatus for transmitting audio information using a plurality of different coding modes, the apparatus comprising: a plurality of coding means for coding audio information for transmission, using different respective coding modes; selection means for selecting one of said plurality of coding modes for transmission; switching means for switching between said coding means in accordance with the selection by said selection means; characterised by further comprising: mute setting means for muting the transmitted audio information while said switching means switches between said coding means. By muting the transmitted audio sound while switching the transmission audio CODEC, embodiments of the invention do not output noise while switching the transmission coding mode. The mute may be set on the transmission sound for a predetermined period of time after transmission of mode information to the receiving terminal. In a second aspect, the invention further provides a receiving apparatus for receiving audio information transmitted using a plurality of different coding modes, the apparatus comprising: a plurality of decoding means for decoding and outputting received audio information for different respective coding modes; switching means for switching the decoding means so as to select one of the plurality of decoding means in response to a mode instruction received from a transmitting terminal; characterised by further comprising: mute setting means for muting the output of the apparatus during switching by said switching means. A received sound may be muted for a predetermined period of time after reception of the mode instruction from the transmitting terminal. It is noted that EP-A-0039428 describes an error protection technique for digitised sound transmission, in which the most significant bits of each audio sample are protected by a parity bit. In the event of serious corruption, the decoded sound is muted by the decoder until the quality of the received data recovers. Other aspects of the invention are set out more particularly in the appended claims. Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Fig. 1 is a block diagram of a multimedia communication apparatus according to an embodiment of the present invention; Fig. 2 is a block diagram showing the internal construction of the audio CODEC in the communication apparatus of the present embodiment; Fig. 3 is a block diagram showing the internal construction of the system controller in the communication apparatus of the present embodiment; Fig. 4 is a flowchart to explain the operation of the communication apparatus which switches an audio transmission mode, and is a first embodiment of this present; Fig. 5 is a flowchart to explain the operation of the communication apparatus which switches the audio transmission mode, and is a second embodiment of this present; Fig. 6 is a flowchart to explain the operation of the communication apparatus which switches an audio reception mode, and is a third embodiment of this present; Fig. 7 is a flowchart to explain the operation of the communication apparatus which switches the audio reception mode, and is a fourth embodiment of this present; Fig. 8 is a flowchart to explain the operation of the communication apparatus at the start of communication, and is a fifth embodiment of this present; and Fig. 9 is a flowchart to explain the operation of the communication apparatus at the start of communication, and is a sixth embodiment of this present. DESCRIPTION OF EXEMPLARY EMBODIMENTS Embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. Fig. 1 is a block diagram of a multimedia communication apparatus of the present embodiment. This communication apparatus can transmit/receive audio information and image information in addition to normal data. In Fig. 1, numeral 1 is a handset for inputting/outputting sound, numeral 2 is a microphone for inputting sound, and numeral 3 is a speaker. Numeral 4 is an audio interface (I/F) comprising: a function to switch the connection between an audio CODEC 5 and the handset 1, microphone 2, and speaker 3 respectively; an on-/off-hook detection function for detecting whether the handset 1 is on hooked or off hooked, an echo cancel function for canceling echo when the microphone 2 and speaker 3 are used; and a generation function for generating dial tone, call signal tone, busy tone, and incoming call tone. Numeral 5 is an audio coder/decoder (audio CODEC) having the function for performing A/D conversion on transmission audio information and coding the information and the function for decoding the received audio information and performing D/A conversion in accordance with the audio signal coding/decoding algorithm provided by the instruction from the system control unit 13 (which is described later). The audio signal coding/decoding algorithm includes 64 kbit/s PCM (A-law), 64 kbit/s PCM (µ-law), 7 kHz AUDIO (SB-ADPCM), 32 kbit/s ADPCM, 16 kbit/s (e.g. APC-AB) 8 kbit/s. Numeral 6 is a camera which is used to input a portrait, for example. Numeral 7 is a display for displaying the input image from the camera 6, an image received from the terminal of the other (transmitting) party, and operational screen. Numeral 8 is a video interface having the function for switching the connection with the camera 6 and display 7, the function for switching a display on the display 7 in the input image, received image, and operational screen, and the function for synthesizing image signals to display a split screen on the display 7, by the instruction of the system controller 13. Numeral 9 is an image coder/decoder (video CODEC) which has functions for performing A/D conversion on the transmission image information and coding the information, and for performing D/A conversion on the reception image information and decoding the information in accordance with CCITT Recommendation H.261. Numeral 10 is a data terminal for data communication and numeral 11 is a data interface for routing the transmission data from the data terminal 10 and system controller 13 (to be described later) to a multi-separator 14, and for routing the reception data from the network 16 to the data terminal 10 or system controller 13. Numeral 12 is an operation panel such as a keyboard or touch panel used for inputting control information. The system controller 13 comprising CPU, ROM, RAM, timer T1, and auxiliary storage generates an operation/display screen corresponding to the state of control all over the main apparatus, input control information, and occupied line, and executes application programs. The timer T1 times a predetermined time set by the CPU. When the predetermined time has passed, the timer T1 notifies the CPU. Numeral 14 is a multiplexor/demultiplexor which multiplexes an audio signal from the audio CODEC 5, image signal from the video CODEC 9, data from the data interface 11, and command from the system controller 13 in a unit of transmission frame in accordance with CCITT Recommendation H.221. The multiplexor/demultiplexor 14 separates the received frame into each medium which is a basic unit for the construction, and informs of this. Numeral 15 is a network interface which controls an ISDN 16 in accordance with the ISDN user network interface. Fig. 2 is a block diagram showing the construction of the audio CODEC 5 shown in Fig. 1. In Fig. 2, numeral 5a is a transmission mute set/release which sets or releases the mute on the transmitting sound based on the instruction from the system controller 13. When the transmission mute set/release is set, transmitting the sound is inhibited. Numeral 5b is a reception mute set/release which sets or releases the mute on the reception sound based on the instruction from the controller 13. When the reception mute set/release is set, outputting the sound is inhibited. Numeral 5c is a CODEC switcher for respectively switching various types of audio CODEC for transmission and reception based on the instruction from the system controller 13. Numeral 5d is a 64 kbit/s PCM A-law CODEC, numeral 5e is a 64 kbit/s PCM µ-law CODEC, numeral 5f is a 16 kbit/s (e.g. APC-AB) CODEC, and 5g is a 64 kbit/s / 56 kbit/s / 48 kbit/s SB-ADPCM CODEC. Fig. 3 is a block diagram showing the functional construction of the software of the system controller 13. In Fig. 3, numeral 13a is a main control unit for controlling each device comprising the present apparatus and performing a communication between devices. Numeral 13b is a D-channel control unit for performing a D-channel control such as call set-up and call disconnection. Numeral 13c is an in-channel control unit for performing synchronization of in-channel, capability exchange, and audio communication mode switching. The operation of the multimedia communication apparatus of the present invention is described along with the flowcharts of Figs. 4 - 9. First, referring to Fig. 4, an example of the operation of the transmission-side communication apparatus where the audio transmission mode switching of the first embodiment is described. The control program which executes this processing is included in the in-channel control unit 13c. In Fig. 4, when the information to switch the audio transmission mode from (µ-law) to (A-law) is received from the operation panel 12 at step S401, a control signal 21 is outputted to audio CODEC 5 and a transmission mute set/release 5a is turned on at step S402. The process proceeds to step S403 where (µ-law → A-law) BAS command to switch the audio transmission mode is transmitted to the terminal of the other party via the network interface 15. Then, at step S404, the control signal 21 is outputted and the transmission audio CODEC is switched from the µ-law CODEC 5e to the A-law CODEC 5d by the CODEC switcher 5c. The process proceeds to step S405 where the transmission mute set/release 5a of the audio CODEC 5 is turned off by the control signal 21 and the processing ends. Next, referring to Fig. 5, another example of the operation of the transmission-side communication terminal where the audio transmission mode switching is described. This is a modification of the processing shown in Fig. 4 and the second embodiment. At step S501, when the instruction to switch the audio transmission mode from (µ-law) to (A-law) is inputted from the operation panel 12, which is similar to step S401, the timer T1 is started at step 502. The process proceeds to step S503, similar to step S402 of Fig. 4, the transmission mute set/release 5a is turned on. At step S504, the (µ-law → A-law) BAS command to switch the audio transmission mode of the terminal of the other party is transmitted from the in-channel control unit 13c. Then, the process proceeds to step S505 where it is determined if the timer T1 which is started at step S502 has expired. When the timer T1 expires, similar to step S404 of Fig. 4, the process proceeds to step S506 where the transmission CODEC is switched from the (µ-law) to (A-law) by the CODEC switcher 5c. Then, the process proceeds to step S507, similar to step 405 of Fig. 4, where the transmission mute set/release 5a is turned off by the control signal 21, and the processing ends. The transmission of audio information is inhibited after the BAS command is transmitted until the audio reception mode is switched in the reception terminal. Therefore, generation of noise is suppressed at the reception terminal. Referring to Fig. 6, another example of the operation of the communication apparatus where an audio reception mode switching is described as a third embodiment. The program which executes this processing is included in the in-channel control unit 13c. At step S601, the (µ-law → A-law) BAS command to switch the audio reception mode is received from the transmission terminal of the other party via the network interface 15. This BAS command is transmitted from the transmission-side terminal in accordance with the flowchart of Fig. 4 or Fig. 5. Then, the process proceeds to step S602 where the reception mute set/release 5b of the audio CODEC 5 is turned on by the control signal 21. The process proceeds to step S603 where the reception audio CODEC is switched from the µ-law CODEC 5e to the A-law CODEC 5d by the CODEC switcher 5c. The process then proceeds to step S604 where the fact that the audio reception mode is switched from (µ-law) to (A-law) is indicated to the operation panel 12. Accordingly, a lamp (not shown) indicating the reception mode is lit on the operation panel 12. Then, the process proceeds to step S605 where the reception mute set/release 5b is turned off by the control signal 21 and the processing ends. Next, referring to Fig. 7, another example of the operation of the communication terminal where the audio reception mode switching is described. This is a modification of the processing shown in Fig. 6 and a fourth embodiment. At step S701, similar to step S601 of Fig. 6, when the (µ-law → A-law) BAS command to switch the audio reception mode is received from the terminal of the other party, the process proceeds to step S702 where the timer T1 is started. Then, the process proceeds to step S703, similar to step S602 of Fig. 6, where the reception mute set/release 5b is turned on. The process then proceeds to step S704, similar to step S603, where the audio reception CODEC is switched from (µ-law) to (A-law) by the CODEC switcher 5c. At step S705, similar to step S604, the fact that the audio reception mode is switched from (µ-law) to (A-law) is indicated to the operation panel 12. The process proceeds to step S706 where it is determined if the timer T1 which is started at step S702 has expired. When the time has expired, similar to step S605 of Fig. 6, the process proceeds to step S707 where the reception mute set/release 5b is turned off in the same way as that of step 605, and the processing ends. The audio information which has been transmitted before the audio transmission mode is switched at the transmission terminal is not outputted from the reception terminal. Referring to Fig. 8, the control operation of the audio CODEC according to the program in the main control unit 13a at the start of the communication, which is a fifth embodiment of this embodiment, is described below. First, at step S801, the information such as a call set-up has been completed is received from the program of the D-channel control unit 13b. At step S802, the transmission mute set/release 5a and the reception mute set/release 5b are both turned on by outputting the control signal 21 to the audio CODEC 5, and the audio information to be transmitted or received is put in the state of mute. The process proceeds to step S803 where a request signal for synchronizing with the in-channel is transmitted to the program of the in-channel control unit 13c. The process proceeds to step S804 where it is determined if this information is received from the in-channel control unit 13c. If received, the process proceeds to step S805 where the transmission and reception mute set/release 5a, 5b are both turned off by the control signal 21. After the state of mute is released, the processing ends. Referring to Fig. 9, an example of the control operation of the audio CODEC according to the program of the main control unit 13a at the start of communication is described. This is a modification of Fig. 8 and a sixth embodiment of this invention. At step S901, similar to step S801, when the information such as a call set-up has been completed is received from the D-channel control unit 13b, the process proceeds to step S902 where the control signal 21 is outputted to the audio CODEC 5 and the transmission and reception mute set/release 5a, 5b are both turned on. The process proceeds to step S903 where the timer T1 is started for timing out the predetermined time. At step S904, similar to step S803, a request signal for synchronizing with the in-channel is transmitted to the program of the in-channel control unit 13c. The process proceeds to step S905, similar to step S804, where it is determined if the information is received from the in-channel control unit 13c. If received, the process proceeds to step S906 where it is determined if the timer T1 which is started at step S903 has expired. If the predetermined time has passed and the timer T1 has expired, the process proceeds to step S907 where the transmission and reception mute set/release 5a, 5b are both turned off. After the state of mute is released, the processing ends. The audio noise can be suppressed when the transmission audio CODEC and the reception audio CODEC are switched in a case of initializing of the audio communication mode, even if the coding method is not corresponding to the decoding method, for the difference between the timing of switching of the transmission audio CODEC and the timing of switching of the reception audio CODEC. The time set in the timer T1 in Figs. 5, 7, and 9 can be arbitrary. As described above, according to the communication apparatus of the first embodiment, the audio information when the transmission audio CODEC is switched can be prevented from being outputted to the network. Furthermore, according to the communication apparatus of the second embodiment, the audio information which has been received before switching can be prevented from being inputted to the reception audio CODEC after the switching, even if the timing for switching the reception audio CODEC is staggered in the reception-side terminal. Accordingly, generation of noise which is caused by the fact that coding mode differs from the decoding mode can be suppressed. Furthermore, according to the communication apparatus of the third embodiment, the sound at switching of the reception audio CODEC can be prevented from being outputted. Still further, according to the communication apparatus of the fourth embodiment, the audio information between the time when the transmission audio CODEC is switched and when the reception audio CODEC is switched can be prevented from being outputted to the network. Still further, according to the communication apparatus of the fifth embodiment, the audio information when the transmission audio CODEC and reception audio CODEC at setting the audio communication mode initialization can be prevented from being outputted. Still further, according to the communication apparatus of the sixth embodiment, the data when the transmission audio CODEC and reception audio CODEC are switching at setting the audio communication mode initialization and the audio information when the audio coding and decoding are performed by a different audio CODEC caused by a timing gap between the transmission and reception can be prevented from being outputted. As many apparently widely different embodiments of the present invention can be made without departing from the scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims: A transmission apparatus for transmitting audio information using a plurality of different coding modes, the apparatus comprising: a plurality of coding means (5d, 5e, 5f, 5g) for coding audio information for transmission, using different respective coding modes; selection means (12, 13) for selecting one of said plurality of coding modes for transmission; switching means (5c) for switching between said coding means (5d, 5e, 5f, 5g) in accordance with the selection by said selection means (12, 13); characterised by further comprising: mute setting means (5a, 13) for muting the transmitted audio information while said switching means (5c) switches between said coding means (5d, 5e, 5f, 5g). The apparatus according to claim 1, further comprising: means (403) for transmitting mode information to a receiving terminal specifying the coding mode selected by said selection means (12, 13); the apparatus being arranged such that the transmitted audio information is muted (503 to 507) for a predetermined time duration after transmission (504) of said mode information, prior to switching (506) to the selected mode by said switching means (5c). A receiving apparatus for receiving audio information transmitted using a plurality of different coding modes, the apparatus comprising: a plurality of decoding means (5d, 5e, 5f, 5g) for decoding and outputting received audio information for different respective coding modes; switching means (5c) for switching the decoding means (5d, 5e, 5f, 5g) so as to select one of the plurality of decoding means (5d, 5e, 5f, 5g) in response to a mode instruction received from a transmitting terminal; characterised by further comprising mute setting means (5b, 13) for muting the output of the apparatus during switching by said switching means (5c). The apparatus according to claim 3, the apparatus being arranged such that the output is muted for a predetermined time duration (702 to 707) after said mode instruction is received from a transmitting terminal (701). A communication apparatus for transmitting and receiving audio information using a plurality of different coding modes, the apparatus comprising: a plurality of coding means (5d, 5e, 5f, 5g) for coding audio information for transmission, using different respective coding modes; initialisation means (13a, 13c) for initialising a transmission/reception mode; first switching means (5c) for switching between said coding means (5d, 5e, 5f, 5g) in response to a selection of the transmission mode by said initialisation means (13a, 13c); a plurality of decoding means (5d, 5e, 5f, 5g) for decoding and outputting received audio information for different respective coding modes; second switching means (5c) for switching between said decoding means (5d, 5e, 5f, 5g) so as to select one of the plurality of decoding means (5d, 5e, 5f, 5g) in response to an instruction of the coding mode by said initialisation means (13a, 13c); and mute setting means (5a, 5b, 13) for muting the audio information during switching by either the first switching means (5c) or the second switching means (5c). The apparatus according to claim 5, the apparatus being arranged such that the output is muted for a predetermined time duration (902 to 907). An apparatus according to claim 5 or 6, wherein said initialisation means provides in-channel synchronisation and capability exchange, in addition to audio communication mode switching in accordance with CCITT Recommendations H.221 and H.242. A transmission method in a communication apparatus having a plurality of coding modes (5d, 5e, 5f, 5g) for coding audio information for transmission using respective different coding modes, the method comprising the steps of: selecting one of said plurality of coding modes for the transmission of audio information; switching between said coding means (5d, 5e, 5f, 5g) in response to the selection of coding mode for the transmission of audio information; characterised by the step of: muting the transmitted audio information while said coding means (5d, 5e, 5f, 5g) is switched. The method according to claim 8, wherein said switching step switches the coding means (5d, 5e, 5f, 5g) when a predetermined period of time has passed after information of the selected coding mode has been transmitted to a receiving terminal. A receiving method in a communication apparatus having a plurality of decoding means (5d, 5e, 5f, 5g) for decoding audio information transmitted using a plurality of different coding modes, the method comprising the steps of: switching between said plurality of decoding means (5d, 5e, 5f, 5g) in response to an instruction received from a transmitting terminal; characterised by the step of: muting the output of the apparatus while said decoding means (5d, 5e, 5f, 5g) is switched. The method according to claim 10, wherein said muting step mutes the output of the apparatus for a predetermined period of time after said instruction from a transmitting terminal is received. A communication method in a communication apparatus having a plurality of audio CODECs (5d, 5e, 5f, 5g) for coding or decoding audio information using a plurality of different coding modes, the method comprising the steps of: initialising a transmission/reception mode; selecting one of said plurality of coding means for the transmission of audio information; switching between said audio CODECs (5d, 5e, 5f, 5g) in response to the selection of coding mode for the transmission of audio information; switching between said plurality of audio CODECs (5d, 5e, 5f, 5g) in order to decode audio information received from a transmitting terminal; and muting said audio information while switching the audio CODEC (5d, 5e, 5f, 5g) for coding or decoding audio information. A method according to claim 12, wherein said muting is performed for a predetermined time duration. A method according to claim 12 or 13, wherein said initialising step comprises in-channel synchronisation and capability exchange, in addition to audio communication mode switching in accordance with CCITT Recommendations H.221 and H.242.

Inventors: HABA YOSHITO, HABA, YOSHITO, Haba, Yoshito, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H04B14
IPC codes: H04M11, H03G3, H04B14, H04M1