Adaptive removal of disturbance in TDMA acoustic peripheral...

Details Adaptive removal of disturbance in TDMA acoustic peripheral... Adaptive removal of disturbance in TDMA acoustic peripheral...

1997-12-16

2001-07-31

Marcelo, Melvin (Department: 2663)

Multiplex communications

Communication over free space

Combining or distributing information via frequency channels

C375S132000, C375S285000

Reexamination Certificate

active

06269093

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to methods and apparatus for removing Time Division Multiple Access (TDMA) induced disturbances in an audio signal.
BACKGROUND OF THE INVENTION
A Time Division Multiple Access (TDMA) system transmits and receives data in time slots. Well-known types of wireless TDMA systems are the European digital cellular telephone system known as the Global System for Mobile communications (GSM) and the U.S. digital cellular telephone system (e.g., IS-136). For GSM the use of TDMA implies that all of the transmit energy of the signal is concentrated in ⅛ of the frame time. The transmitted time slot is often called a TX burst.
Acoustic peripheral devices induce harmonics of these TX bursts. The result is an undesirable audible humming sound that emanates from the loudspeaker. The magnitude of the hum depends on a number of factors, such as the electrical construction of a hybrid in the peripheral device, the distance of the TDMA transmitter from the peripheral device, the mechanical construction of both devices, the strength of the received signal in the GSM transceiver that in turn determines the strength of the transmitted signal, the phase of the call (e.g., the call set-up phase and the call connected phase may have different TX levels), and certain GSM features (e.g., Discontinuous Transmission (DTX) and frequency hopping).
When the transmitter and the peripheral device can be mechanically constructed in a predetermined manner, the audible humming can be eliminated by preventing the induction of TX energy into the audio band circuits. However this is not always possible or desirable. For example, mobile station (e.g., cellular telephone) peripheral devices are typically sold as separate units that connect to a mobile station through a standard interface. As such, it is difficult or impossible to control the layout and relative positions of the transmitter and the audio band circuits.
The frequency of the induced TX signal corresponds to the frame time, wherein one time slot (and the TX burst) is transmitted per frame from the mobile station. For the GSM case one frame is 4.615 ms in duration, which equals a frequency of 216.667 Hz (i.e., about 217 Hz). The shape of the induced 217 Hz noise pulses is dependent on the mechanical construction, as well as the distance to the audio device, the orientation of the device, the construction of the coils in the speaker and microphone, etc., and can vary significantly with changes in these mechanical parameters. The nature of the undesirable disturbance in phase, shape and amplitude cannot thus be determined a priori.
Furthermore, because of the multi-frame structure in GSM and the multiplexing of different logical channels into a dedicated channel, not every frame will contain a TX burst. By example, Slow Associated Control Channels (SACCH) are inserted every 13th frame and IDLE frames are inserted every 26th frame (for a full rate traffic channel), resulting in drop outs of the disturbance every 120 ms. For half rate frames, every 26th frame contains the SACCH, but only every second frame contains a TX burst for traffic data. As such, further variability of the audio hum can occur.
Another factor that influences the cyclo-stationary disturbance arises from the Discontinuous Transmission (DTX) mode of operation. The DTX mode is used in order to reduce overall interference of multiple users in the system by transmitting speech data only when the user is talking. Furthermore, in GSM there exists a so-called hangover period of four speech frames, meaning that the DTX function does not mute the transmission immediately when no speech is detected (typically by a Voice Activity Detector (VAD)). In this case so-called Comfort Noise Update (CNU) frames are transmitted, as long as there is no speech, at the rate of 1 CNU frame every 480 ms. The DTX mode of operation is controlled by the base station in such a manner that it informs the mobile station to either use or not use DTX, or leaves the decision to the mobile station.
FIG. 1
illustrates one example of a disturbance
9
that is induced into the audio signal of a peripheral device
2
that is connected to a GSM Fixed Wireless terminal
1
via a 2-wire connection comprised of an output loudspeaker line
10
and input microphone line
11
. The hybrid
3
of the peripheral device
2
changes the connection from 4-wire to 2-wire, and a hybrid
4
of the terminal
1
connects the signal back to a 4-wire line. The GSM disturbance is induced inside the peripheral device
2
and generates a disturbance in both loudspeaker signal
10
a
and microphone signal
11
a
. The interfered microphone signal
11
a
is converted into a digital signal by an A/D-converter
5
. The digital signal is then coded by a Digital Signal Processor (DSP)
6
and fed to a Radio Frequency (RF) unit
7
of the terminal
1
, which sends TX bursts
8
spaced 4.615 ms apart to the network. The DSP
6
is used to perform channel coding and modulation, as well as speech coding for the input microphone signal
11
a
. For the signal coming from the network opposite processing is done in order to generate an analog loudspeaker signal.
In the case illustrated in
FIG. 1
the GSM disturbance
10
a
that is detectable in the output of the loudspeaker
10
(an audible hum of 217 Hz) cannot be measured directly. As a result, it is difficult to remove the unknown disturbance signal. Another disturbance occurs in the direction of the microphone line
11
a
. As a result the audible humming can also be detected in the earphone of the answering phone. However, this disturbance can be directly measured and suppressed.
OBJECTS AND ADVANTAGE OF THE INVENTION
It is a first object and advantage of this invention to provide a method and apparatus for reducing or eliminating an undesirable audible disturbance that results from burst mode operation of a transmitter.
It is a further object and advantage of this object to reduce or eliminate an undesirable audible disturbance that is coupled into a Fixed Wireless terminal through an attached peripheral devise (e.g., an ordinary telephone device), the disturbance resulting from a burst mode operation of a transmitter.
It is also another object and advantage of this invention to reduce or eliminate an undesirable audible disturbance that is coupled into a Fixed Wireless terminal through an attached peripheral devise (e.g., an ordinary telephone device), the disturbance resulting from burst mode operation of a transmitter, while also taking into account irregularities that may occur in the transmitted bursts due to operation with a multiframe structure and discontinuous transmission.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the objects and advantages of the invention are realized by methods and apparatus in accordance with embodiments of this invention for suppressing transmitter burst mode disturbances in an audio band. The method and apparatus extracts a TDMA burst interference signal portion from a disturbed signal and processes the disturbed signal to generate a compensation signal. Logic is provided to recognize the multi-frame structure and the DTX mechanics of GSM, or any burst type (TDMA) air interface standard of interest, such as one defined by IS-136.
In accordance of this invention, a method for removing a disturbance signal from a framed input signal that is induced by a transmitter that transmits bursts of energy includes the steps of: (a) detecting those signal frames wherein the input signal contains signal energy comprised substantially of only the disturbance signal; (b) characterizing the disturbance signal in the detected signal frames to obtain a disturbance signal correction; and (c) compensating the input signal, only for those signal frames where a transmission burst exists, using the obtained disturbance signal correction so as to remove the disturbance signal from those signal frames that contain voice, and also from certain signal frames that do not contain voice.
The step of detecting the disturbance

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