Abstract:
A modem or other telephone device ( 102, 502 ) is configured to identify (or receive notification of) the companding law used by a destination modem or telephone device. The telephone device ( 102, 502 ) can then, using stored conversion tables, predict the inverse mapping from the converted encoding law. Having done so, the telephone device ( 102, 502 ) can identify distorted portions of the decoded signal and modify the original signal before it is sent to the encoding or encoding-converter unit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to telecommunications systems and, particularly, to an improved method for converting signals between A-law and μ-law format. 
     It is conventional in telecommunication systems to digitize voice signals according to a predetermined encoding law. Currently, there are two international standards for encoding pulse code modulated (PCM) signals. In the United States and Japan, μ-law encoding is used; in Europe and the rest of the world, A-law encoding is used. Both standards are promulgated by the International Telecommunications Union (ITU) Telecommunication Standardization Sector in the ITU-T Recommendation G.711, which is hereby incorporated by reference in its entirety as if fully set forth herein. 
     When a call is placed between countries using different PCM encoding schemes, the network must provide conversion through the use of a device that converts μ-law to A-law and A-law to μ-law. Typically, the device maintains a map or look-up table to perform the conversion. Such maps are defined at Tables 3 and 4 of the Recommendation G.711. The nature of the encoding, however, is such that distortion may nevertheless result when the signals are decoded. 
     For example, FIG. 1A illustrates an exemplary digitized voice wave form  1   a  that is to be sent from a starting location that uses μ-law PCM encoding. When μ-law converted, the digitized voice wave form  1   a  may take on values (represented by the waveform  1   b  of FIG. 1B) that are slightly higher than in the original wave form. At the destination location that uses A-law PCM encoding scheme, the μ-law converted wave form is then converted to A-law format. The A-law converted wave form may take on values (as shown in the waveform  1   c  of FIG. 1C) that are even higher than in either the μ-law converted wave form or the original wave form. The A-law converted wave form  1   c  is then A-law decoded, and these cumulative changes in the wave form values due to the μ-law and A-law conversion can ultimately lead to distortion. Similarly to the above example except in reverse, there can be distortion when a signal wave form is received when sent from a starting location that uses A-law coding to a destination location that uses μ-law coding. This type of signal distortion is only an annoyance when dealing with voice signals, but the distortion when dealing with data signals can impair the receiving modem&#39;s ability to demodulate accurately. 
     Thus, there is a need for improved μ-law to A-law conversion and A-law to μ-law conversion, both for data and voice communications. 
     SUMMARY OF THE INVENTION 
     These and other disadvantages in the prior art are overcome in large part by a system and method according to the present invention. A modem or other telephony device is configured to identify (or receive notification of) the companding law used by a destination modem or telephony device. The telephony device can then, using stored conversion tables, predict the inverse mapping from the converted encoding law. Having done so, the telephony device can identify distorted portions of the decoded signal and modify the original signal before it is sent to the encoding or encoding-converter unit. 
     A telephony-over-LAN (ToL) client according to an embodiment of the invention includes a digital signal processor (DSP) and a memory for storing A-law and μ-law maps. The ToL client receives an identification from its gatekeeper of the companding law used by the destination. The ToL client then optimizes the signal for that law. Thus, the ToL client may generate a different, optimized amplitude digital signal if the destination uses a different companding law than the ToL client&#39;s location does. 
     A modem according to an embodiment of the invention similarly is configured to identify the destination device&#39;s encoding law. The modem may then adjust its output analog signal (either directly or by adjusting the input digital signal) to “trick” the central office or PBX into choosing a different signal level (different than the signal level resulting normally without use of the invention) when it converts. 
     These and other embodiments will be better understood from the following detailed description in conjunction with the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C are diagrams illustrating the conversion of an exemplary wave form after μ-law and A-law coding; 
     FIG. 2 is a diagram illustrating an H.323 system according to an embodiment of the invention; 
     FIG. 3 is a block diagram illustrating an H.323 terminal according to an embodiment of the invention; 
     FIG. 4 is a block diagram of an exemplary codec and coding resource unit for the H.323 system of FIG. 2; 
     FIG. 5 is a flowchart illustrating operation of the embodiment of FIG. 2; 
     FIG. 6 is a block diagram of an exemplary modem system according to an embodiment of the invention; 
     FIG. 7 is a more detailed block diagram of an exemplary modem according to an embodiment of the invention; and 
     FIG. 8 is a flowchart illustrating operation of an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and with particular attention to FIG. 2, a diagram illustrating a telecommunications system  100  according to an embodiment of the present invention is shown. The telecommunications system  100  is a telephony-over-LAN (ToL) system, of the type set forth in the ITU Recommendation H.323. The telecommunications system  100  thus includes a local area network (LAN) or packet network  101 . Coupled to the LAN  101  may be a variety of H.323 terminals  102   a,    102   b,  a multipoint control unit (MCU)  104 , an H.323 gateway  106 , an H.323 gatekeeper  108 , a LAN server  112  and a plurality of other devices such as personal computers (not shown). 
     The H.323 terminals  102   a,    102   b  are in compliance with the H.323 standard. Thus, the H.323 terminals  102   a,    102   b  support H.245 for negotiation of channel usage, Q.931 for call signaling and call setup, registration admission status (RAS), and RTP/RTCP for sequencing audio and video packets. The H.323 terminals  102   a,    102   b  may further implement audio and video codecs, T.120 data conferencing protocols and MCU capabilities. H.323 terminals  102   a,    102   b  may be telephony-enabled computers or digital H.323 telephones. 
     The H.323 gateway  106  generally provides a translation function between H.323 conferencing endpoints and other terminal types, and performs call setup and clearing on both the LAN side and switched circuit network side. 
     The H.323 gatekeeper  108  performs address translation from LAN aliases for terminals and gateways to IP or IPX addresses (as defined in the RAS specification) as well as bandwidth management (also specified within the RAS specification). The H.323 gatekeeper  108  may further be used for call routing. 
     According to a specific embodiment of the present invention, the gatekeeper  108  may also be configured to identify the coding scheme of the called destination and forward this information to the H.323 terminal  102 . The gatekeeper  108  may then communicate the required coding information to the H.323 terminals using RAS messaging. The H.323 terminal  102  then analyzes the incoming audio data to determine if an adjustment is necessary. In other specific embodiments, as will be discussed in greater detail below, the H.323 terminals  102   a,    102   b  themselves identify (from the country code) the encoding scheme of the destination. The H.323 terminals  102   a,    102   b  further include coding resources units  111   a,    111   b  in accordance with various embodiments. The coding resources units  111   a,    111   b  are configured to identify (or be informed of by the gatekeeper  108 ) the coding scheme used at a called destination. The coding resources units  111   a,    111   b  then access μ-law and A-law maps to determine whether the converted and decoded signal at the called destination would be relatively free of distortion. If not, then the incoming digitized voice signal received from the audio I/O of the terminal at the calling destination is adjusted prior to transmission from the terminal, so that the ultimately decoded signal at the called destination is a closer approximation to the incoming digitized voice signal. 
     A logical diagram of an H.323 interface  102  to LAN  101  is shown in FIG. 3, according to an embodiment of the present invention. Packet network interface  13  couples the H.323 device to LAN  101 . H.323 terminals/devices and equipment carry real-time voice, video and/or data. It should be noted that H.323 is an umbrella recommendation that sets standards for multimedia communications, including telephony-over-LAN communications. The network can include packet-switched Transmission Control Protocol/Internet Protocol (TCP/IP) and Internet Packet Exchange (IPX) over Ethernet, Fast Ethernet and Token Ring networks. 
     The H.323 terminal  102  includes a video input/output (I/O) interface  10 , an audio I/O interface  12 , a user application interface  19  (e.g., data interface for connecting to data equipment  21 ), and a system control user interface (SCUI)  20 . Terminal  102  also includes an H.225 layer  24 , a video coder/decoder (codec)  15 , an audio codec  14 , and a control layer  11  that includes Q.931 protocol functionality  16 , RAS protocol functionality  17  and H.245 protocol functionality  18 . 
     As seen in FIG. 3, video I/O interface  15  connects to the video codec  15  such as an H.261 codec for encoding and decoding video signals. Coupled between video I/O interface  10  and H.225 layer  24 , video codec  15  translates encoded video signals to H.225 protocol signals. Although the H.261 codec can be the video codec used for an H.323 terminal, other video codecs, such as H.263 codecs and others, may also be used for encoding and decoding video. 
     As shown, audio  110  interface  12  connects to the audio codec  14 , such as a G.711 codec, for encoding and decoding audio signals. Coupled to audio I/O interface  12 , audio codec  14  is coupled to H.225 layer  24  and translates audio signals to H.225 protocol signals. Although the G.711 codec is the mandatory audio codec for an H.323 terminal, other audio codecs, such as G.728, G.729, G.723.1, G.722, MPEG1 audio, etc. may also be used for encoding and decoding speech. G.723.1 is a preferred codec because of its reasonably low bit rate, which enables preservation of link bandwidth, particularly in slower speed network connections. 
     As will be discussed in greater detail below, audio codec  14  includes the coding resources unit  111  includes a memory for storing μ-law to A-law and A-law to μ-law conversion maps in accordance with the present invention. When a signal is received from audio I/O  12 , the coding resources unit  111  determines whether a coding conversion is required; if so, the coding resources unit  111  adjusts the input signal so that the encoded signal sent from terminal  102  that is decoded at the destination location is optimized to be closer to the original input signal from audio I/O  12 . 
     SCUI  20  provides signaling and flow control for proper operation of the H.323 terminal. In particular, all non-audio and non-video control signaling is handled by SCUI  20 . Coupled to SCUI  20  are H.245 layer  18 , Q.931 layer  16  and RAS layer  17 , which each couple to H.225 layer  24 . Thus, SCUI  20  interfaces to the H.245 standard which is the media control protocol that allows capability exchange, channel negotiation, switching of media modes and other miscellaneous commands and indications for multimedia communications. SCUI  20  also interfaces to the Q.931 protocol which defines the setup, teardown, and control of H.323 communication sessions. SCUI  20  further interfaces to the Registration, Admission, Status (RAS) protocol that defines how H.323 entities can access H.323 gatekeepers to perform among other things address translation, thereby allowing H.323 endpoints to locate other H.323 endpoints via an H.323 gatekeeper. The H.225 standard layer  24 , which is derived from the Q.931 standard, is the protocol for establishing connection between two or more H.323 terminals and also formats the transmitted video, audio, data and control streams into messages for output to the network interface  13  (e.g., transport over IP network  101 ). The H.225 layer  24  also retrieves the received video, audio, data and control streams from messages that have been input from network interface  13 . User application interface  19 , which may be a T.120 protocol interface as well as other types of protocol interfaces, also couples to H.225 layer  24 . 
     Thus, an H.323 network may be configured to include several different devices. For example, the network may include a terminal for enabling users connected to a LAN to speak, a terminal for enabling a caller resident on the LAN to call a second user through the public switched network and/or a terminal for enabling the adapter to communicate through a wireless trunk, using a wireless telephone. The terminal may also implement supplementary services according to the H.450 protocol specification. 
     Turning now to FIG. 4, a block diagram of an exemplary coding resources unit  111  with a codec of the H.323 terminal is shown. In particular, the coding resource unit  111  includes an analog-to-digital converter  304  coupled to receive and digitize audio input, and a digital-to-analog converter  306  coupled to receive digitized audio input and output analog audio output. The digitized audio input may be provided to a buffer  305  and then to a digital signal processor (DSP)  302 . The DSP  302  in turn is coupled to a memory  308  which stores μ-law and A-law conversion maps. The DSP  302  is configured to convert the digitized data stream into a μ-law or A-law encoded data stream, depending on the coding scheme of the country in which the H.323 terminal is located. 
     According to the present invention, when a call is being made (or received) the DSP  302  may identify the companding or coding scheme at the location of the remote party from call set-up information. In the case of making a call, the DSP  302  may identify the country code and is able to proceed on that basis. In the case of a received call, the DSP  302  receives a signal (e.g., RAS) from the gatekeeper  108  identifying the remote location of the caller as using μ-law or A-law coding in the standard fashion. 
     After determining if the coding scheme of the remote location is different from the scheme used in the location of the H.323 terminal, the DSP  302  then analyzes the digitized audio input data by determining what the decoded signal will look like. For example, assuming the H.323 terminal is located at a μ-law site, the DSP  302  of the H.323 terminal may accept the digitized input data stream, then determine what A-law input is required to achieve that same digitized audio data stream (e.g., by accessing the appropriate inverse conversion from the memory) as A-law output at the remote location. The A-law input is the output of the μ-law coding; the DSP  302  will then determine, again by accessing the memory, what μ-law input values are most likely to give those μ-law output values. If the original digitized input data stream values do not correspond to the determined μ-law encoder input values, then DSP  302  will adjust the incoming digitized data stream to conform to the determined μ-law encoder input values. Therefore, the μ-law output values of the determined encoded adjusted data stream will be transmitted to the remote location where it should be converted to the other coding scheme (A-law) in the usual manner and A-law decoded into audio output without the typical signal distortion conventionally experienced without use of the present invention. That is, the audio output at the remote location should be a non-distorted reproduction of the input audio of the H.323 terminal. 
     If the remote location is determined to use the same PCM coding scheme as the location of the H.323 terminal, then DSP  302  is configured to merely convert the digitized data stream into this PCM coding scheme and then to send the encoded data stream via interface  310  for transmission to the remote location. 
     Therefore, the present invention provides the capability intelligent adjustments of the encoded digitized data stream transmitted to a remote location depending on whether the coding schemes of the communicating locations differ, so that the signal distortion due to coding conversions is significantly reduced. 
     This process is illustrated in flowchart form in FIG.  5 . In a step  402 , a call is connected and the local and remote companding schemes are identified. More particularly, in the case of a call being made, the country code may used to identify that an alternative companding scheme is in use (for example, a table of country codes may be maintained in memory). If a call is being received, the gatekeeper  108  identifies the source and provides information to the H.323 terminal of the remote companding scheme. In a step  404 , the memory is accessed and the input digital stream is analyzed to determine what input to an A-law decoder will result in the desired output (assuming the local site is μ-law). In a step  406 , this A-law input is compared with the μ-law memory map to determine what μ-law input will achieve the desired output. In a step  408 , the DSP  302  will adjust the incoming digital stream such that the desired output is arrived at (i.e., to match the determined μ-law input). It is noted that, while illustrated as being embodied in a ToL system, any digital telephony system may make use of the scheme of the present invention. Thus, the figures are exemplary only. 
     An alternative embodiment of the invention is shown in the telecommunications system  500  of FIG.  6 . As shown, a local terminal  500 , such as a computer, is coupled to a local analog modem  502  via a transmit line  550  and receive line  551 . Such transmit and receive lines  550 ,  551  may be part of an RS-232 interface cable typically employed to couple a computer to a modem. The local analog modem  502  is coupled via a line  510  to the public switched telephone network (PSTN)  526 . The line  510  may be a standard twisted pair telephone line which transmits an analog signal. A remote terminal  506 , which may also be a computer, is coupled to a remote analog modem  504  via transmit line  561  and receive line  560 . The remote analog modem  504  is coupled via line  520  to the PSTN  526 . Line  520 , like line  510 , may also be a standard telephone line. As indicated, the local terminal  500  and local analog modem  502  are arranged to transfer data, information, and other signals between a remote terminal  506  having a remote analog modem  504  over the PSTN  526 . Data, including information and command signals, are transferred between the terminals and their respective analog modems over the various respective transmit and receive lines  550 ,  551 ,  560 ,  561 . Information from the terminal  500  is modulated and otherwise processed by the local analog modem  502  to form an analog modem signal transmitted to the PSTN  526  over line  510 . The analog modem signal is then transferred to the remote analog modem  506  by the PSTN as an analog signal over the line  520 . 
     Within the PSTN, the analog modem signal may be converted to a digital signal (for example, at an exchange or a Central Office (not shown)) for transmission within the network and reconverted to an analog modem signal for transmission over the analog lines  510  and  520  at another exchange or CO. In particular, the digital signal may be a μ-law companded signal (in the United States or Japan) and may be converted in the PSTN  526  to A-law for overseas transmission. Then, the receiving CO or exchange decodes the A-law signal and reconverts it into an analog signal. The remote analog modem  506  demodulates the received analog modem signal and transmits the demodulated data to the remote terminal over line  560 . Information from the remote analog modem  506  may also be transmitted to the local analog modem  502  using the same method operating in the opposite or reverse direction. 
     The analog modems  502 ,  506  according to the present invention identify the remote destination&#39;s companding scheme to determine whether μ-law to A-law (or A-law to μ-law) conversion is necessary. If so, the analog modems  502 ,  506  adjust the amplitude of their output analog signals to optimize reconversion. 
     FIG. 7 illustrates an analog modem  502  in greater detail, according to an embodiment of the invention. As shown in FIG. 7, an analog modem  502  is coupled to a terminal  500 , such as a computer, and transmits and receives data over the PSTN  526  via an analog interface circuit (AIC) or digital access arrangement (DAA)  514 . A controller  604  is coupled to the terminal  500 . The controller  604  may be a microcontroller and is coupled via a data bus to a data pump  608 , which may be a general-purpose digital signal processor programmed as a data pump. A memory  509  is further coupled to the controller  604  and stores a map of μ-law to A-law conversions (and A-law to μ-law conversion)), as will be explained in greater detail below. The data pump  608  receives data, command signals and other information from the controller  604 , and then data pump generates a sampled data signal. The data pump  608  is also coupled to a codec  610  which receives the sampled data signal from the data pump  608  and which generates a modulated analog signal from the sampled data signal. The modulated analog signal is then transmitted to the analog interface circuit  514  and out to the PSTN  526 . The analog interface circuit  514  provides a variety of functions, such as power level setting, impedance matching, and may include hybrid circuitry to transfer information from two sets of twisted pair transmission lines to one pair of transmission lines. 
     According to the present invention, the data pump or DSP  608  further determines a conversion of the sampled data signal according to a first encoding law (e.g., μ-law) and, responsive to an indication of whether the destination or remote modem functions within a second encoding law (e.g., A-law), will determine what input to the second encoding scheme will produce an output most closely corresponding to the sampled digital data stream. The sampled digital data stream is then adjusted so that the resulting output of the μ-law encoder will most closely match the closest matching input to the A-law converter. The adjusted digital data stream is then provided to the codec  610  for digital to analog conversion and modulation for transmission along the PSTN. The controller  604  may receive the information concerning the encoding scheme used at the destination in any of several ways. In the case of an incoming call, this information may be derived from standard caller identification information. For example, the terminal  500  receives the caller identification information and may thereafter send a command to the controller  504  indicating that an alternate encoding scheme is used at the remote location. In the case of an outgoing call, this information could be derived from the country code that is provided prior to the local phone number by the terminal  500 . 
     This process is illustrated in greater detail in FIG. 8, which shows a flowchart of the operation of the modem  502  of FIG. 7 (For simplicity, it will be assumed that the modem  502  is located in a μ-law site). In particular, in a step  702 , the modem  502  receives digital data from the terminal  500 . As noted above, the terminal  500  may be a computer such as a personal computer. In a step  704 , the controller  504  provides the data to the data pump  508 , which samples the data. In a step  706 , the data pump  508  determines the A-law value that would give the sampled data as an output. That is, the DSP  508  determines what A-law encoding will, if decoded, result in the desired sampled data stream. The data pump  508  does so, for example, by accessing the look-up table in memory  509 . In a step  708 , the data pump  508  determines what μ-law value will give the corresponding output (i.e., μ-law output). In other words, assuming the A-law value determined in step  706  is equivalent to a μ-law value at the modem  502 , the DSP  508  determines what μ-law input would result in that A-law value. In a step  710 , the data pump  508  will adjust the data such that the new data corresponds to the determined μ-law value. In a step  712 , that stream is sent to the codec  510 . Finally, in a step  714 , the analog data is sent out the PSTN. It is noted that, while described above in terms of adjusting a digital signal, the amplitude, for example, of the analog signal may similarly be adjusted, in a known manner.