HK1062348B - Apparatus and method for enhanced conversion of wideband signals to narrowband signals - Google Patents

Description

Enhanced conversion of wideband signals to narrowband signals

Background

I. Field of the invention

The present invention relates to communication systems, and more particularly to enhanced conversion of wideband speech signals to narrowband speech signals.

II. background

The field of wireless communications has many applications including, for example, cordless telephones, pagers (paging), wireless local loops, Personal Digital Assistants (PDAs), internet telephony, and satellite communication systems. A particularly important application is in mobile (cellular) telephone systems for mobile users. (as used herein, the term "cellular" system encompasses both mobile (cellular) and Personal Communications Service (PCS) frequencies.) different over-the-air interfaces have been developed for such mobile telephone systems, including, for example, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). In connection therewith, different national and international standards have been established, including for example Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM) and Interim Standard95 (Interim Standard95) (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (generally referred to herein as IS-95), and proposed high data rate systems for data, and the like, are promulgated by the Telecommunications Industry Association (TIA), the International Telecommunications Union (ITU), and other well-known standards bodies.

Mobile telephone systems configured for use in accordance with the IS-95 standard use CDMA signal processing techniques to provide efficient and reliable mobile telephone systems. Exemplary mobile telephone systems configured for use in accordance with the IS-95 standard are illustrated in U.S. patent nos. 5,103,459 and 4,901,307, assigned to the assignee of the present invention (assignee) and incorporated herein by reference in their entirety. An example illustration of a system utilizing CDMA techniques is the CDMA2000 ITU-R Radio Transmission Technology (RTT) candidate proposal (referred to herein as CDMA2000) issued by the TIA. The standard given for cdma2000 IS the IS-2000 draft version and has been approved by the TIA. The cdma2000 proposal IS compatible with IS-95 in many ways. Another CDMA standard is the W-CDMA standard, which is incorporated inThird Generation partnership project "3 GPP" (3) ^rd Generation Partnership Project”3GPP”)In (3), document numbers are 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.

In a conventional landline (landline) telephone system, the frequency band of the transmission medium and the terminal is limited to 4000 Hz. Speech signals are typically transmitted in a narrow band of 300Hz to 3400Hz, outside of which control and signaling additional signals are carried. Given the physical limitations of landline telephone systems, signal propagation in mobile telephone systems is implemented with these same narrow-band frequency constraints so that calls originating from mobile subscriber units can be transmitted to landline units. However, since physical limitation requiring a narrow frequency range is not given in the mobile system, the mobile phone system can transmit signals in a wider frequency range. An exemplary standard for generating a wider frequency range signal is published in document g.722 ITU-T published in 1989, entitled "7 kHz audio coding at 64 kBits/s".

In the transmission of speech signals, the perceived quality of the sound waveform is of primary importance to users and service providers. If the wireless communication system transmits signals in a wideband frequency range of 50Hz to 7000Hz, a switching problem occurs when the wideband signal terminates in a narrowband environment that attenuates high frequency components of the wideband signal. Therefore, there is a current need in the art to be able to convert a wideband speech signal into a narrowband speech signal without degrading the sound quality.

Summary of the invention

Novel methods and apparatus for converting a wideband speech signal to a narrowband speech signal are presented. In one aspect, an apparatus for converting a wideband signal to a narrowband signal is presented, the apparatus comprising: a filter for emphasizing a mid-range portion of a frequency response of the wideband signal and de-emphasizing a high-range portion of the frequency response of the wideband signal, wherein an output of the filter is a narrowband signal of a non-flat frequency response; and a down sampler for extracting a sampling rate of the wideband signal.

In another aspect, an apparatus for converting a wideband speech signal to a narrowband speech signal comprises: a control element for deciding whether to convert the wideband speech signal into a narrowband speech signal; a switch coupled to the control element, wherein the control element triggers the switch if the control element determines that a wideband speech signal is to be converted; the bandwidth switching filter is used for receiving the broadband voice signal if the switch is triggered, wherein the bandwidth switching filter strengthens partial frequency spectrum of the broadband voice signal to generate an output signal with non-flat frequency spectrum; a down sampler for extracting the output signal of the bandwidth-switched filter.

In another aspect, an apparatus for decoding a wideband speech signal and for converting the wideband speech signal into a narrowband speech signal is presented, the apparatus comprising: a speech synthesis element for creating a synthesized wideband speech signal; and a post-processing element for enhancing the synthesized wideband speech signal, wherein the post-processing element further comprises: a post-filter element; and a bandwidth switching filter for emphasizing a middle range of the synthesized wideband speech signal frequency spectrum and de-emphasizing a high range of the synthesized wideband speech signal frequency spectrum.

In another aspect, a method for transmitting a wideband waveform in a wireless communication system is presented, the method comprising: receiving a signal carrying a base station wideband waveform, wherein the wideband waveform is for further transmission from the base station to a target destination; determining whether the target destination can process a wideband waveform; if the target destination cannot process the wideband waveform, converting the wideband waveform into a narrowband waveform with a non-flat frequency response; if the target destination can process the wideband waveform, the wideband waveform is transmitted from the base station to the target destination without converting the wideband waveform to a narrowband waveform.

In another aspect, determining whether the target destination is supported by a wideband vocoder comprises: embedding a detection code in a Pulse Code Modulated (PCM) signal, wherein the PCM signal carries a wideband waveform; and transmitting a detection code confirmation from the target destination via a second base station if the target destination detects the detection code, wherein the second base station supports communication with the target destination and the wireless communication system.

Detailed description of the drawings

Fig. 1 is a diagram of an example communication system.

Fig. 2A is a graph of a flat narrowband frequency response.

FIG. 2B is a spectrogram emphasizing a narrow-band filter at frequencies between 1000Hz and 3400 Hz.

Fig. 3A is a graph of a flat broadband frequency response.

Fig. 3B is a graph of a suitable frequency response.

Fig. 3C is a graph of another suitable frequency response.

Fig. 3D is a graph of another suitable frequency response.

Fig. 4 is a block diagram of a wideband-to-narrowband conversion device coupled with a decoder.

Fig. 5 is a block diagram of another wideband-to-narrowband conversion device coupled to a decoder.

Fig. 6 is a block diagram of a wideband decoder outputting a signal having a non-flat frequency response.

Fig. 7 is a flow chart of a method for deciding whether to convert a wideband speech signal to a narrowband speech signal.

Fig. 8 is a flow chart of another method for deciding whether to convert a wideband speech signal to a narrowband speech signal.

Detailed description of the embodiments

As illustrated in fig. 1, the wireless communication network 10 generally includes a number of mobile stations (also referred to as subscriber units or user equipment) 12a-12d, a number of base stations (also referred to as base station transceivers (BTSs) or node bs) 14a-14c, a Base Station Controller (BSC) (also referred to as radio network controller or packet control function 16), a Mobile Switching Center (MSC) or switch 24, a Packet Data Serving Node (PDSN) or internetwork function (IWF)20, a Public Switched Telephone Network (PSTN)22 (typically a telephone company), and an Internet Protocol (IP) network 18 (typically the internet). For simplicity, 4 mobile stations 12a-12d, 3 base stations 14a-14c, 1 BSC 16, 1 MSC 18, and 1 PDSN 20 are shown. Those skilled in the art will appreciate that there may be any number of mobile stations 12, base stations 14, BSCs 16, MSCs 18, and PDSNs 20.

In one embodiment, the wireless communication network 10 is a packet data services network. The mobile stations 12a-12d may be any of a number of different types of wireless communication devices such as, for example, a portable telephone, a mobile telephone that is connected to a laptop computer running IP-based, web-browser applications, a mobile telephone with an associated hands-free car kit (hand-free kits), a Personal Digital Assistant (PDA) running IP-based, web-browser applications, a wireless communication module incorporated into a portable computer, or a fixed location communication module such as may be found in a wireless local loop or meter reading system. In the most general embodiment, the mobile station may be any type of communication unit.

The mobile stations 12a-12d may be configured to implement one or more wireless packet data protocols, such as those described in the EIA/TIA/IS-707 standard. In particular embodiments, the mobile stations 12a-12d generate IP packets that are designated as the IP network 24 and encapsulate the IP packets into frames (frames) using a point-to-point protocol (PPP).

In one embodiment, the IP network 24 is connected to a PDSN 20, the PDSN 20 is coupled to a MSC 18, the MSC 18 is coupled to the BSC 16 and the PSTN 22, and the BSC 16 is coupled to the base stations 14a-14c via lines (wires) configured to carry voice and/or data packets according to any of several known protocols, including, for example, E1, T1, Asynchronous Transfer Mode (ATM), IP, frame Relay, HDSL, ADSL, or xDSL. In an alternative embodiment, the BSC 16 is directly coupled to the PDSN 20, and the MSC 18 is not coupled to the PDSN 20. In another embodiment of the present invention, the mobile stations 12a-12d communicate with the base stations 14a-14c over an RF interface that is atThird Generation partnership project 2 "3 GPP 2""physical layer standards for cdma2000 spread Spectrum System," 3GPP2 document number C.P0002-A, TIA PN-4694, published as TIA/EIA/IS-2000-2-A (draft, edit 30) (10/19/1999), which IS incorporated herein by reference in its entirety.

In typical operation of the wireless communication network 10, the base stations 14a-14c receive and demodulate sets of reverse-link signals (reverse-link signals) from different mobile stations 12a-12d engaged in telephone calls, web browsing, or other data communications. Each reverse link signal received by a given base station 14a-14c is processed in the base station 14a-14 c. Each base station 14a-14c may communicate with a number of mobile stations 12a-12d by modulating and transmitting sets of forward link signals to the mobile stations 12a-12 d. For example, as shown in fig. 1, the base station 14a communicates with first and second mobile stations 12a, 12b simultaneously, and the base station 14c communicates with third and fourth mobile stations 12c, 12d simultaneously. The resulting packet is transmitted to the BSC 16, which provides call source assignment and mobility management functionality including soft handoff coordination (authorization) for a particular mobile station 12a-12d call from one base station 14a-14c to another base station 14a-14 c. For example, the mobile station 12c communicates with two base stations 14b, 14c simultaneously. Eventually, when the mobile station 12c moves far enough away from one of the base stations 14c, the call will be soft handed off to the other base station 14 b.

If the transmission is a conventional telephone call, the BSC 16 will send the received data to the MSC 18, and the MSC 18 provides additional routing services to the connection to the PSTN 22. If the transmission is a packet-based transmission, such as that designated as the IP network 24, the MSC 18 will send the data packet to the PDSN 20, and the PDSN 20 will send the packet to the IP network 24. Alternatively, the BSC 16 will send the packets directly to the PDSN 20, and the PDSN 20 sends the packets to the IP network 24.

Typically, the conversion of an analog speech signal into a digital signal is performed by an encoder, while the conversion of a digital signal back into a speech signal is performed by a decoder. In an exemplary CDMA system, a vocoder comprising an encoding portion and a decoding portion is organized (collocated) in the mobile unit and the base station. An exemplary vocoder is described in U.S. Pat. No. 5,414,796, entitled "variable Rate vocoder", assigned to the assignee of the present invention and incorporated herein by reference. In a vocoder, the encoding portion extracts parameters relating to a model of human speech generation. The decoding section re-synthesizes the speech using the parameters received through the transmission channel. The model is often varied to accurately simulate a time-varying speech signal. Thus, during the time period in which the parameters are calculated, the speech is divided into blocks of time, or analysis frames. The parameters for each new frame are then updated. As used herein, the word "decoder" refers to any device or any portion of a device that may be used to convert a digital signal that has been received over a transmission medium. Thus, the embodiments described herein may be implemented with a vocoder for a CDMA system and a decoder for a non-CDMA system.

Acoustic speech is generally composed of low and high frequency parts. However, due to physical limitations of conventional telephone systems, the frequency band of the input speech is limited to a narrow range of 200Hz to 3400 Hz. A filter is a device that modifies the frequency spectrum of an input waveform to produce an output waveform. Such a modification is characterized by a transfer function h (f) y (f)/x (f) which relates the modified output waveform y (t) to the original input waveform x (t) in the frequency domain.

Fig. 2A illustrates a spectrum of a narrow band filter with a flat frequency response. An example of a device having this feature is a microphone. As shown, the low frequencies are emphasized too much, while the high frequencies are cut off. The input signal passing through the filter will lead to an output waveform that is uncomfortable to the human ear, and the filtered speech is silenced.

Fig. 2B illustrates the spectrum of a narrow band filter emphasizing frequencies from 1000Hz to 3400 Hz. In this example, the lower frequencies are attenuated, but the spectrum between 1000Hz and 3400Hz is enhanced. The reinforcement in this frequency range perceptually compensates for the omission of frequency components above 3400 Hz. Thus, a more "natural" and intelligible sound is experienced by the end user (end user) when hearing the filtered signal.

Due to the improvements in wireless telephony, many wireless communication systems can propagate acoustic signals over a wider range of 50Hz to 7000 Hz. Such sound signals are called broadband signals. Communications using this frequency range have been standardized in the document g.722 ITU-T entitled "7 kHz audio coding at 64 kBits/s" published in 1989. Since frequency components up to 7000Hz can be carried by a wideband system, a typical wideband decoder can be implemented with a flat frequency response. Fig. 3A is a time-wideband signal flat spectrum diagram. No emphasis is required since frequency components between 3400Hz and 7000Hz are included. The inclusion of these high frequency components produces a perceptually intelligible waveform without necessarily reinforcing the frequency range between 1000Hz and 3400 Hz.

However, problems arise when transmitting broadband signals to narrowband terminals or through narrowband systems. In the current state of the art, the frequency band of the wideband signal is limited within the constraints of the narrowband terminal/system by a simple frequency cutoff of 3400 Hz. The conversion of the wide band to the narrow band can be accomplished by passing the wide band signal through a low pass filter and down sampling the result. Thus, the spectrum of the converted broadband signal closely resembles the spectrum of fig. 2A. As described above, this flat frequency response produces a waveform that is not acceptable to human perception. Accordingly, there is a need for enhanced conversion of wideband signals to narrowband signals so that the converted narrowband signals are perceptually appropriate to end users. The embodiments described herein accomplish the conversion of a wideband signal to a narrowband signal while preserving the appropriate sound component.

Fig. 4 is a block diagram of an embodiment that may be connected with an existing wideband decoder. The embodiment is a wideband-to-narrowband conversion apparatus configured to reduce loss of signal information when a wideband signal is converted to a narrowband signal. The retention of the signal information produces a sound signal that is perceived as suitable by the end user.

The base station (not shown) receives an information bit stream for input into the wideband decoder 40. The wideband decoder 40 may be configured to follow the g.722 ITU-T waveform or any other waveform whose frequency band is not limited to 3400 Hz. Variations in the bandwidth of the waveform will not affect the scope of the present embodiment. A control element (not shown) in the base station makes a decision whether the output of the wideband decoder 40 is to be delivered to the narrowband terminal. A method and apparatus for deciding whether to convert a wideband signal to a narrowband signal is described below. If the output of wideband decoder 40 is to be sent to a narrowband terminal or narrowband system, a control element (not shown) triggers switch 42 to send the wideband decoder output to wideband-to-narrowband conversion device 44. The wideband-to-narrowband conversion device 44 comprises a Bandwidth Switching Filter (BSF) whose output is coupled to a down-sampler 48.

The bandwidth-switching filter 46 may be implemented with any filter having a frequency response characterized as having a 5dB to 10dB slope curve in the mid-range of frequencies. The optimal intermediate range is between frequencies of 1000Hz to 3400Hz, but larger or smaller ranges such as 800-3500Hz or 1100-3300Hz may be used without affecting the range of the present embodiment. Frequencies above the mid-range are attenuated for near narrow band response. Fig. 3B is a representative example of a frequency response having a desired slope. However, filters with different shape curves may also be used. For example, fig. 3C illustrates a spectrum with a straight slope (straight slope) that can also be used in the present embodiment. Fig. 3D illustrates another useful frequency response, where the spectrum contains linear segments with varying slopes. The bandwidth switching filter 46 may be implemented as a fixed filter with constant filter coefficients or an adaptive filter with updated filter coefficients. The design choice should be made in accordance with predetermined system parameters and does not affect the scope of the present embodiment.

The down sampler 48 may be implemented by any device capable of determining a new sampling sequence y (n) from the input sequence x (n) such that y (n) is x (mn), where M is a positive integer value.

In one embodiment, decimation occurs at a rate of M-2, since the wideband signal is typically sampled at 16kHz and the narrowband signal is typically sampled at 8 kHz. Since the decimation occurs after the filtering is performed by the bandwidth filter 46, an interpolator (interpolator) may be used in the narrowband target terminal to recover the decimated portion of the switching signal.

Fig. 5 is a block diagram of another wideband-to-narrowband switching device coupled with a wideband decoder. In this embodiment, the wideband-to-narrowband switching means is configured to reduce the number of calculations required to convert the wideband signal to a narrowband signal.

The base station (not shown) receives an information bit stream for input to the wideband decoder 50. The wideband decoder 50 outputs a waveform in accordance with g.722 ITU-T or any other waveform having frequency components higher than 3400Hz without affecting the scope of the present embodiment. A control element (not shown) in the base station makes a decision whether the output of the wideband decoder 50 is to be delivered to a narrowband terminal or through a narrowband system. If the output of wideband decoder 50 is to be sent to a narrowband terminal or through a narrowband system, a control element (not shown) triggers switch 52 to send the wideband decoder output to wideband-to-narrowband conversion device 54. The wideband-to-narrowband conversion device 54 comprises a down-sampler 56 whose output is coupled to a Bandwidth Switching Filter (BSF) 58.

In one embodiment, the down-sampling filter decimates samples at a rate M-2. In a typical wideband system, the signal is sampled at a rate of 16 kHz. If the sampler is operating at rate M-2, half of the samples are discarded and the bandwidth switching filter 58 is operating at the 8kHz signal. Accordingly, the wideband switching signal filter 58 of fig. 5 may be constructed with less computational complexity than the bandwidth switching filter 46 of fig. 4. However, like bandwidth-switching filter 46 of FIG. 4, bandwidth-switching filter 58 may be implemented with any filter having a frequency response characterized by a 5-10dB slope curve between mid-range frequencies.

The above embodiments have been described as additional components that can be used with existing wideband decoders. However, novel and unobvious embodiments of wideband decoders are envisioned in which the spectrum of the output signal exhibits high frequency emphasis.

Fig. 6 is a functional block diagram of a wideband decoder 60 configured to output a narrowband signal with a non-flat spectrum. The decoder 60 includes a speech synthesis element 62 and a post-processing element 64. The speech synthesis element 62 receives speech information carrying speech signals and appropriate excitation signal parameters. Many parametric examples of speech signals use Linear Predictive Coding (LPC) techniques, where the parameters of the filter model can be recreated from autocorrelation values at the decoder. Alternatively, the values of the LPC parameters may be communicated directly from the encoding source to the decoder. A more detailed explanation of different linear predictive coding techniques is described in the above-mentioned U.S. patent No. 5,414,796.

The speech synthesized from the speech synthesis element 62 is generally intelligible. However, the quality of the synthesized speech may be distorted. Therefore, a post-processing component 64 is needed to enhance the synthesized speech to produce a more "natural" effect. The post-processing element 64 includes at least one post-filter 66 and a bandwidth switching filter 68. The conventional post-filter 66 may include a combination of a pitch post-filter (pitch post filter), a formant post-filter, and a tilt compensation filter. However, because the entire wideband spectrum of the signal is processed, the conventional post-filter 66 does not guarantee the required frequency emphasis of the present embodiment. A bandwidth switching filter 68 coupled to the post-filter 66 ensures the emphasis of a particular frequency sub-group. A control element (not shown) controls whether the output of the post filter 66 is sent through the bandwidth switching filter 68.

The bandwidth-switched filter 68 may be implemented as in the above-described embodiment, wherein the spectral magnitude (spectrumagnitude) curve has a slope of at least 5dB to 10dB between the frequency range of approximately 1000Hz and 3400 Hz. The order of placement of bandwidth filter 68 and post-filter 66 may be changed without affecting the scope of the present embodiment.

Fig. 7 is a flow chart for deciding whether to implement wideband-to-narrowband signal conversion in a wideband system. At step 70, the control element located in the base station is notified of the arrival of the broadband signal transmitted from the subscriber unit. In a typical wireless communication system, notification of the arrival of any such signal transmission is transmitted during a call set-up or registration phase. In the call setup phase, information to signal the final destination address is sent to the control element. The final destination address typically corresponds to a telephone number entered by the user of the original subscriber unit or to a saved address selected by the user. An example of a call setup procedure is found in U.S. patent No. 5,844,899 entitled "method and apparatus for providing a call identifier in a distributed network system", assigned to the assignee of the present invention and incorporated herein by reference.

In step 72 the control element compares the mobile subscriber unit database used in the broadband system with the final destination address of the signaling. In a CDMA system, such as the system illustrated in fig. 1, the mobile subscriber database will be found in the mobile switching center 18. If the final destination number is found in the database, the control element proceeds to decode the wideband signal without converting to a narrowband signal at step 74. If the final destination number is not found in the database, the control element triggers a switch that sends the output of the wideband decoder to the wideband-to-narrowband conversion device at step 76, i.e. the implementation described above.

Alternatively, if the communication system supports both broadband and narrowband subscriber units and signals originating from broadband terminals, the database of mobile subscriber units may be replaced by a broadband mobile subscriber unit database and the above-described method steps may be implemented.

Alternatively, the database of the mobile subscriber unit may be replaced by all registered communication subscriber units, including mobile subscribers and landline subscribers, where the bandwidth capacity of the communication terminal is also conserved. Thus, instead of deciding on the presence of the final destination number in the database, a decision is made whether the final destination number is supported by the broadband terminal.

In another embodiment, if the broadband communication system allows multiple communication links between communication units such as teleconferencing (telecommuting), the control element may be programmed or configured to convert multiple broadband signals into multiple narrowband signals. Such a conversion would allow the system to increase the number of participants in a teleconference call.

Fig. 8 is a flow chart of another method of deciding whether to implement wideband-to-narrowband signal conversion. This embodiment is implemented by a base station wideband vocoder that converts a wideband signal to a narrowband signal if the wideband decoder is not serving the target destination.

At step 80, the base station receives and decodes the encoded signal from the remote unit. The encoded signal comprises a wideband speech signal and a signaling overhead signal. Included in the signaling overhead signal is the target destination address. At step 82, the decoded signal is transmitted to the base station controller where the wideband speech signal is converted to a multi-bit Pulse Code Modulated (PCM) output. A pseudo-random detection code is embedded in the PCM output. The embedded PCM output is transmitted to the target destination through the mobile switching center at step 84.

If the physical medium between the base station and the target destination supports wideband transport and the target destination is supported by a wideband decoder, the target destination detects the pseudo-random detection code and establishes a communication conference with the base station in step 86. Implementation details of tandem vocoder operation are described in U.S. patent No. 5,903,862 entitled "method and apparatus for detecting tandem vocoding to modify vocoder filtering," assigned to the assignee of the present invention and incorporated herein by reference. In step 87, the base station vocoder and the target destination vocoder transmit the wideband speech signal without conversion to a narrowband speech signal.

In the alternative, if the base station's wideband sound has the same configuration as the wideband vocoder at the target destination, the tandem vocoding can be set to bypass (bypass). Details of the implementation of vocoder bypass are described in U.S. patent No. 5,956,673 entitled "detection and bypass for tandem vocoding using detection codes," assigned to the assignee of the present invention and incorporated herein by reference. If the target destination wideband vocoder can be set to shunt detour, the base station can output the wideband signal without converting to a narrowband signal.

If the target destination is not serviced by the wideband decoder, the base station performs a wideband to narrowband conversion at step 88, as in the above embodiment.

Thus, a novel and improved method and apparatus for converting wideband to narrowband signals has been described. Those of skill in the art would appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, software, firmware, or combinations of both. Various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware, software, or firmware depends upon the particular application and design constraints imposed on the overall system. Skilled artisans will appreciate the interchangeability of hardware, software, and firmware under these circumstances, and how best to implement the described functionality for each particular application.

The implementation of the different illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the illustrative embodiments may be implemented or performed with a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a field programmable gate array (field programmable gate array) or other programmable logic device, discrete gate or transistor logic, or discrete hardware components. A processor executing a set of firmware instructions, any conventional programmable software module and a processor, or any combination thereof, may be designed to perform the functions of the control elements described herein. The processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. The software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An example processor is coupled to the storage medium to read information from, and write information to, the storage medium. Alternatively, the storage medium may reside in an ASIC. The ASIC may reside in a telephone or other user terminal. In the alternative, the processor and the storage medium may reside in a telephone or other user terminal. The processor may be implemented as a combination of a DSP and a microprocessor, or two microprocessors in conjunction with a DSP core, or the like. Those of skill would further appreciate that the data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description are represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Thus, various embodiments of the present invention have been shown and described. It will be apparent, however, to one of ordinary skill in the art that many changes can be made to the embodiments described herein without departing from the spirit and scope of the invention.

Claims (21)

1. An apparatus for converting a wideband speech signal to a narrowband speech signal, comprising:

a control element for deciding whether to convert the wideband speech signal into the narrowband speech signal;

a switch coupled to the control element, wherein the control element activates the switch if the control element determines that the wideband speech signal is to be converted;

a bandwidth switching filter for receiving the wideband speech signal if the switch is triggered, wherein the bandwidth switching filter emphasizes a mid-portion spectrum of the wideband speech signal to produce an output signal having a non-flat spectrum with a curve having a slope of at least 5 dB; and

a down sampler for extracting an output signal of the bandwidth-switched filter.

2. The apparatus of claim 1, wherein the intermediate portion spectrum is at a frequency between 1000Hz and 3400 Hz.

3. The apparatus of claim 1, wherein said non-flat spectrum has a curve with a slope between 5dB and 10 dB.

4. The apparatus of claim 3, wherein the curve having a slope between 5dB and 10dB is between 1000Hz and 3400 Hz.

5. The apparatus of claim 1 wherein said down sampler decimates at a rate of M-2, wherein output signal y (n) is related to input signal x (n) by the relationship y (n) -x (mn).

6. The apparatus of claim 1 wherein said bandwidth switching filter further attenuates high frequency portions of said wideband speech signal.

7. An apparatus for converting a wideband speech signal to a narrowband speech signal, comprising:

a control element for deciding whether to convert the wideband speech signal into the narrowband speech signal;

a switch coupled to the control element, wherein the control element activates the switch if the control element determines that the wideband speech signal is to be converted.

A down sampler coupled to the switch, wherein the down sampler is configured to extract the wideband speech signal if the switch is triggered; and

a bandwidth switching filter for receiving the decimated wideband speech signal, wherein the bandwidth switching filter emphasizes a mid-portion spectrum of the wideband speech signal to produce an output signal having a non-flat spectrum, and the non-flat spectrum has a curve with a slope of at least 5 dB.

8. The apparatus of claim 7, wherein the intermediate portion spectrum is at a frequency between 1000Hz and 3400 Hz.

9. The apparatus of claim 7, wherein said non-flat spectrum has a curve with a slope between 5dB and 10 dB.

10. The apparatus of claim 9, wherein the curve having a slope between 5dB and 10dB is between 1000Hz and 3400 Hz.

11. The apparatus of claim 7 wherein said down sampler decimates at a rate of M-2, wherein output signal y (n) is related to input signal x (n) by the relationship y (n) -x (mn).

12. The apparatus of claim 7 wherein said bandwidth switching filter further attenuates high frequency portions of said wideband speech signal.

13. A device for decoding a wideband speech signal and for converting the wideband speech signal to a narrowband speech signal, comprising:

a speech synthesis element for creating a synthesized wideband speech signal; and

a post-processing element for enhancing the synthesized wideband speech signal, wherein the post-processing element further comprises:

a post-filter element; and

a bandwidth switching filter for emphasizing a mid-range of a spectrum of the synthesized wideband speech signal and de-emphasizing a high-range of the spectrum of the synthesized wideband speech signal, the bandwidth switching filter having a frequency response with a slope curve of at least 5dB over the mid-range of frequencies.

14. The apparatus of claim 13, wherein the mid-range of the spectrum is between 1000Hz and 3400 Hz.

15. The apparatus of claim 13, wherein the high range of the spectrum is above 3400 Hz.

16. A method for communicating a wideband waveform sourced from a wireless communication system, comprising:

receiving, at a base station, a signal carrying a wideband waveform, wherein the wideband waveform is for further delivery from the base station to a target destination;

determining whether the target destination can process a wideband waveform;

if the target destination is unable to process the wideband waveform, converting the wideband waveform to a narrowband waveform having a non-flat frequency response with a curve having a slope of at least 5 dB; and

if the target destination can process the wideband waveform, transferring the wideband waveform from the base station to the target destination without converting the wideband waveform to a narrowband waveform.

17. The method of claim 16 wherein said determination of whether said target destination can process said wideband waveform comprises the step of determining whether said target destination is supported by a wideband vocoder.

18. The method of claim 17, wherein the determination of whether the target destination is supported by a wideband vocoder comprises:

embedding a detection code into a pulse code modulated signal, wherein the pulse code modulated signal carries the wideband waveform; and

if the detection code is detected by the target destination, transmitting an acknowledgement of the detection code from the target destination by a second base station, wherein the second base station supports communication with the target destination and a wireless communication system.

19. A method for determining whether to convert a wideband signal to a narrowband signal, comprising:

a final destination address originating from the remote unit is received,

comparing the final destination address to a number of destination addresses in an authentication database;

transmitting the broadband signal to a final destination address if the final destination address matches one of a number of destination addresses in an authentication database; and

if the final destination address does not match any of a number of destination addresses in an authentication database:

converting the wideband signal to the narrowband signal, wherein the narrowband signal has a non-flat frequency response having a curve with a slope of at least 5 dB; and

the narrowband signal is transmitted to the final destination address.

20. An apparatus for converting a wideband signal to a narrowband signal, comprising:

a filter for emphasizing a mid-range portion of the wideband signal frequency response of a wideband signal and de-emphasizing a high-range portion of the wideband signal frequency response, the filter having a frequency response with at least a 5dB slope curve over a mid-range of frequencies; and

a down sampler for extracting a sampling rate of the wideband signal.

21. An apparatus for converting a wideband signal to a narrowband signal, comprising:

means for receiving a final destination address and said broadband signal originating from a remote unit;

means for comparing the final destination address with a plurality of destination addresses in an authentication database;

means for deciding whether to deliver the wideband signal to the final destination address or to convert the wideband signal to the narrowband signal, wherein the narrowband signal has a non-flat frequency response and the non-flat frequency response has a curve with a slope of at least 5 dB; and

means for transmitting the narrowband signal to the final destination address.