JPH05300209A - Radio telephone system - Google Patents

Abstract

PURPOSE:To attain hand-free talking using a head set type handset clearly with high quality without a sense of disorder by devising the telephone set so as to prevent deterioration in speech quality even when the microphone is set in a position apart from its mouth. CONSTITUTION:A noise canceller 15 is provided in a portable radio telephone set PSS and a microphone 11 of the portable radio telephone set PSS is used to detect surrounding noise and when a headset type handset HS is used for hand-free talking, the noise canceller 15 generates a pseudo noise signal based on the surrounding noise detected by the microphone 11. Then the pseudo noise signal is subtracted from a transmission voice signal outputted from the microphone 51 of the headset type handset HS to cancel the surrounding noise component.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a handheld type radio telephone device used in a mobile radio telephone system such as a portable radio telephone system or a cordless telephone system.
In particular, the present invention relates to a wireless telephone device equipped with means for enabling hands-free communication.

[0002]

2. Description of the Related Art Cellular mobile radio telephone systems and cordless telephone systems are known as conventionally used mobile radio telephone systems.

First, a cellular type portable radio telephone system is connected to a control station CS connected to a wired telephone network NW, for example, as shown in FIG. 7, and to the control station CS via wired lines CL1 to CLn, respectively. Multiple base stations BS1
To BSn and a plurality of mobile stations PS1 to PSm. The base stations BS1 to BSn form radio zones E1 to En called cells in different areas. The mobile stations PS1 to PSm are connected to a base station in a wireless zone in which the mobile station is located via a wireless line,
From this base station to the cable telephone network N via the control station CS.
Connected to W. In this state, it becomes possible to talk with the telephone connected to the wired telephone network NW. Further, when the mobile stations PS1 to PSm move to another wireless zone during a call, the wireless line is switched to the wireless line of the destination base station by the control of the control station CS, and the call continues.

On the other hand, in the cordless telephone system, a fixed device corresponding to a base station of a portable radio telephone system is connected to, for example, a subscriber line terminal of a home or office, and a radio telephone corresponding to a mobile station is connected via a radio line. It is designed to be connected to a fixed device. This cordless telephone system is capable of communication within a small area centered around a fixed device, for example, with a radius of about 100 m, and the call is disconnected when going out of this area.

By the way, as a radio telephone used in the portable radio telephone system and the cordless telephone system, for example, a so-called handheld type of a handset type is often used. This type of wireless telephone allows you to talk by touching the wireless telephone itself to the speaker's ear and mouth like a wired telephone handset, so you can use it without feeling discomfort to the speaker. It's very convenient.

[0006]

However, the conventional wireless telephone of this type generally has only the handset communication mode as described above as its usage pattern. For this reason, it may be inconvenient when both hands cannot be used, for example, when a speaker is talking while writing and making a call while holding a luggage or an umbrella in both hands. ..

On the other hand, it has been conventionally considered to use a headset type handset as a means for realizing a hands-free call. This kind of handset
For example, it is very promising because it can be used relatively easily as if listening to a headphone stereo. But,
A headset-type handset conventionally considered for hands-free calling has a microphone placed at the speaker's mouth, such as that used by aircraft pilots. Therefore, the user is worried about the presence of the microphone and is not preferable in appearance. It is also conceivable to set the position of the microphone at a position apart from the mouth, such as the chest, instead of the speaker's mouth. However, with this configuration, it becomes difficult for the microphone to collect the transmitted voice of the speaker, and
Ambient noise such as noise is apt to be collected by the microphone, and as a result, the S / N of the transmitted voice is deteriorated and the call quality is significantly deteriorated, which has not been put into practical use.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to prevent the communication quality from deteriorating even if the position of the microphone is set away from the mouth so that the headset can be set. It is an object of the present invention to provide a radiotelephone device that can perform a hands-free call using a portable handset in a clear and high quality without feeling discomfort.

[0009]

To achieve the above object, the present invention provides a radio telephone connected to a base station via a radio communication line, and a head at least electrically connected to the radio telephone. A second microphone for collecting ambient noise in one of the wireless telephone and the handset, and outputting the ambient noise signal corresponding thereto in a wireless telephone device including a set type handset. When,
And a noise elimination circuit. With this noise removing circuit, based on the ambient noise signal output from the second microphone, the ambient sound included in the transmission voice signal output from the first microphone provided in the headset type handset The arithmetic processing is performed to remove the noise component.

The present invention also provides a third aspect for a radio telephone to collect a transmission voice and output a corresponding transmission voice signal in a state where a headset type handset is not connected.
When the headset type handset is not connected, the third microphone is also operated as a second microphone for collecting ambient noise.

Further, according to the present invention, the noise elimination circuit is composed of an impulse response estimation circuit and a subtractor, and the impulse response estimation circuit outputs the transmission voice signal output from the first microphone and the second microphone. The impulse response between the first microphone and the second microphone is estimated from the output ambient noise signal to generate pseudo noise, and the pseudo noise is output from the first microphone in the subtractor. Another feature is that it is subtracted from the transmitted voice signal.

[0012]

As a result, according to the present invention, even if ambient noise components such as noise are mixed in the transmitted voice signal collected by the first microphone of the headset type handset, the ambient noise components are , And is subtracted and removed by the noise removal circuit based on the ambient noise collected by the second microphone. Therefore, the S / N ratio of the transmitted voice signal is kept high, which enables clear and high-quality communication. Also, by having the function of canceling ambient noise components in this way, the microphone installation position in the headset type handset is not limited to the speaker's mouth, but it can be separated from the mouth, such as near the chest. It is possible to freely set the position. Therefore, the usability of the headset type handset can be improved, and the aesthetic appearance can be improved.

If the wireless telephone is provided with a third microphone for handset communication, the third microphone is used.
If the microphone is used as a second microphone for collecting ambient noise, a new microphone for collecting ambient noise is not required, which makes the device configuration simple and inexpensive. It becomes possible to

Furthermore, when noise is removed, the impulse response between the first microphone and the second microphone is estimated to generate pseudo noise, and this pseudo noise is subtracted from the transmitted voice signal. Therefore, the ambient noise can be canceled efficiently, and it becomes possible to transmit the transmission voice signal having a better S / N.

[0015]

EXAMPLE An example of the present invention will be described below.

FIG. 1 is a circuit block diagram showing the configuration of the wireless telephone device in the embodiment. The wireless telephone device of this embodiment includes a portable wireless telephone PSS and a headset type handset HS.

First, the headset type handset HS is used for hands-free communication.
, A speaker 52, and a connector group 53 for connecting the portable radio telephone PSS.

On the other hand, the portable radio telephone PSS has a transmission system,
It is roughly divided into a receiving system and a control system. The transmission system includes a microphone 11 used for handset transmission and ambient noise detection during hands-free communication, and an analog / digital converter (A /
D converter) 13, a voice encoder (SPCOD) 14,
Error correction encoder (CHCOD) 16, digital modulator (MOD) 17, mixer 18, power amplifier (P
A) 19, a high frequency switch circuit (SW) 20, and an antenna 21.

In the voice encoder 14, the transmission voice signal output from the microphone 11 and then digitized by the A / D converter 13 is encoded. The error correction encoder 16 performs error correction encoding on the encoded transmission signal output from the speech encoder 14 and the control signal output from the control circuit 31 described later. In the digital modulator 17, the intermediate frequency signal is, for example, π by the encoded transmission signal output from the error correction encoder 16.
It is modulated by the / 4 shift QPSK method. Mixer 18
Then, the intermediate frequency signal output from the digital modulator 17 is mixed with the local oscillation signal output from the frequency synthesizer 32 and frequency-converted into a radio frequency signal. The power amplifier 19 amplifies the radio frequency signal output from the mixer 18 to a predetermined transmission power. The high frequency switch 20 becomes conductive only during a transmission time slot designated by the control circuit 31, and during this period the radio frequency signal output from the power amplifier 19 is supplied to the antenna 21 and the antenna 21 supplies a base (not shown). Send to the station.

On the other hand, the receiving system is the receiver (RX) 22.
, Digital demodulator (DEMOD) 23, error correction decoder (CHDEC) 24, speech decoder (SPDE)
C) 25, a digital / analog converter (D / A converter) 26, a reception amplifier 27, and a speaker 28 used for receiving a handset.

The receiver 22 performs a mixing process for frequency-converting the radio high frequency signal received by the antenna 21 and the high frequency switch 20 into the intermediate frequency signal in the reception time slot of the predetermined radio frequency. The digital demodulator 23 performs bit synchronization and frame synchronization on the reception intermediate frequency signal output from the receiver 22, and in this state, digital demodulation of the reception intermediate frequency signal is performed. The signals representing the states of bit synchronization and frame synchronization are input to the control circuit 31. The error correction decoder 24 performs error correction decoding on the encoded reception signal and the control signal output from the digital demodulator 23. Then, the error-correction-decoded coded reception signal is input to the voice decoder 25, and the control signal is input to the control circuit 31. The voice decoder 25 performs a decoding process of the encoded reception signal. Then, the reception signal returned to the original by this decoding processing is converted into an analog signal by the D / A converter 26, and then amplified by the reception amplifier 27 and output from the speaker 28 in a loud voice.

The control system includes, for example, a control circuit (CONT) 31 having a microcomputer as a main control section, a frequency synthesizer (SYN) 32, a reception electric field strength detection circuit (RSSI) 33, and a console unit 34. ing. The frequency synthesizer 32 includes a control circuit 31.
Generates a transmission local oscillation signal and a reception local oscillation signal corresponding to the radio channel frequency designated by. The received electric field strength detection circuit 33 detects the received electric field strength of the radio wave transmitted from the base station, and the detection signal is supplied to the control circuit 31 for free channel search and outside communication area monitoring. The console unit 34 is provided with switches such as a transmission switch and a dial key, and indicators such as a light emitting diode and a liquid crystal display. 3
5 is a power supply circuit, and in this power supply circuit 35, the battery 3
Based on the output voltage of 6, the predetermined operating voltage Vcc required in each of the above circuits is generated.

Incidentally, the portable radio telephone PS of this embodiment
S includes a circuit group for making a hands-free call using the headset type handset HS. That is, the first switching circuit 42 is inserted between the D / A converter 26 of the receiving system and the reception amplifier 27. This first
The switching circuit 42 operates in accordance with the switching instruction from the control circuit 31, and when the handset call mode using the portable wireless telephone PSS is used, the D / A converter 26 is used as the reception amplifier 27.
In addition, in the hands-free communication mode using the headset type handset HS, the D / A converter 26 is connected to the reception amplifier 46 for hands-free communication. The reception amplifier 46 amplifies the reception signal to a predetermined level and then, through the connector group 43, the headset type handset H
It is supplied to the S speaker 52.

A second switching circuit 41 is inserted between the A / D converter 13 of the transmission system and the voice encoder 14. The second switching circuit 41 performs a switching operation in accordance with a switching instruction from the control circuit 31, directly connects the A / D converter 13 to the voice encoder 14 in the handset call mode, and in the hands-free call mode, the A / D converter 13 described above. D converter 1
A noise canceller 15 is inserted between 3 and the voice encoder 14.

The noise canceller 15 includes an impulse response estimation circuit 15a and an adder 15b, and is composed of, for example, a DSP. Impulse response estimation circuit 15
a is an estimation of the impulse response between the microphone 51 and the microphone 11 of the headset type handset HS based on the ambient noise signal detected by the microphone 11 and the transmission signal input to the voice encoder 14. Then, pseudo noise is synthesized from the result, and an adaptive filter is used, for example.

FIG. 2 shows an example of the structure of the impulse response estimation circuit 15a. In the figure, the adder 15
The digital transmission signal output from b is band-limited to, for example, 500 Hz or less by the low-pass filter (LPF1) 152, and further, for example, 1/4 of the sampling frequency (for example, fs = 8 kHz) in the A / D converter 13 described above. After being re-sampled at the frequency (2 kHz), it is input to the adaptive filter (AF) 151. The digital transmission signal is also input to the silence detector (VDET) 153. The silence detector 153 detects a silent period in which neither transmission nor reception is performed. On the other hand, the ambient noise signal output from the microphone 11 is A / D
The low-pass filter (L
PF2) 154 is input. Then, the LPF 2 performs band limitation to remove frequency components other than noise components, and then inputs the result to the adder 156. In the adder 156, the noise signal output from the LPF 2 is subtracted from the pseudo noise signal output from the AF 151, and the noise signal after the subtraction is fed back to the AF 151.

The AF 151 is composed of, for example, a transversal type digital filter having 128 taps (32 taps × 4). And this AF1
In 51, the estimation of the transfer function between the microphone 51 and the microphone 11 of the headset type handset HS, that is, the impulse response of the impulse response is performed based on the transmitted voice signal and the noise signal during the period in which the VDET 153 detects silence. The estimation is performed, and the pseudo noise signal is synthesized based on the estimation result. A learning identification method is used as the learning algorithm of the AF 151. For the learning identification method, see “A Learning Idetification Method for Sy
stem Idetification ”, Trans on IEEE AC12 no3 JUN
1967, Nagumo et al. ”.

The pseudo noise signal synthesized by the AF 151 is inputted to the adder 15b after passing through the low pass filter (LPF3) 155. In the adder 15b, the transmission signal output from the microphone 51 of the headset type handset HS is digitized by an analog / digital converter (A / D converter) 45 including a low frequency amplifier 44 in the preceding stage. It has been entered later. The adder 15b performs arithmetic processing for subtracting the pseudo noise signal from the digital transmission signal, and outputs a digital transmission signal in which ambient noise components have been canceled.

FIG. 3 shows an external view of the device when the headset type handset HS is connected to the portable radio telephone PSS. The headset type handset HS has a pair of speakers 52 inwardly attached to both ends of a U-shaped elastic support member 54.
1 is attached to an intermediate portion of the U-shaped elastic support member 54. A connection cord CD is pulled out from an intermediate portion of the U-shaped elastic support member 54, and the headset type handset HS is connected to the portable radio telephone PSS via the connection cord CD.

Next, the operation of the apparatus configured as described above will be described. First, when the user is using the wireless telephone device in the handset call mode, the portable wireless telephone PS
Since the headset type handset HS is not connected to S, the control circuit 31 of the portable radio telephone PSS outputs a switching control signal for setting the call mode to the handset call mode. Therefore, the switching circuit 42 is provided on the side of the reception amplifier 27 for handset communication, and the switching circuit 4 is provided.
1 is switched to the A / D converter 13 side. Therefore, the receiving amplifier 27 for handset communication is connected to the D / A converter 26 of the receiving system. In addition, the transmission system A / D
The converter 13 and the voice encoder 14 are directly connected to each other, so that the noise canceller 15 is substantially in a non-operating state.

Therefore, when a call is made or a call is made in this state and a call is made, the user's transmitted voice signal collected by the microphone 11 is converted into a digital signal by the A / D converter 13. After that, it is encoded by the voice encoder 14c. Then, the encoded digital transmission signal is subjected to error correction encoding processing by the error correction encoder 16 and then input to the digital modulator 17. Therefore, the digital modulator 17 outputs an intermediate frequency signal modulated by the coded transmission signal, and the modulated signal is frequency-converted into a radio frequency and power-amplified by the mixer 18 and the power amplifier 19, respectively. , Is transmitted from the antenna 21 to the base station via the high frequency switch 20.

On the other hand, the radio signal arriving from the base station is received by the antenna 21 and the high frequency switch 20, converted into an intermediate frequency signal by the receiver 22, and then digitally demodulated by the digital demodulator 23. The demodulated received signal is subjected to error correction decoding by the error correction decoder 24, then decoded by the voice decoder 25, further D / A converter converted by the D / A converter 26 to receive the baseband signal. Become a signal. Then, this reception signal is input to the reception amplifier 27 for handset communication via the switching circuit 26, amplified here, and then amplified by the speaker 2.
A loud voice is output from 8 as a received voice.

Therefore, the user is required to use the portable radio telephone PS.
By putting the S microphone 11 and the speaker 28 on their mouths and ears, respectively, handset conversation can be performed.

On the other hand, it is assumed that the user connects the headset type handset HS to the portable radio telephone PSS in order to make a hands-free call. Then, in the portable wireless telephone PSS, the control circuit 31 outputs a switching control signal for setting the call mode to the hands-free call mode. Therefore, the switching circuits 42 and 41 are switched to the receiver amplifier 46 side and the noise canceller 15 side for hands-free communication, respectively. Therefore, the receiver D / A converter 26 is connected to the receiver amplifier 46 for hands-free communication, and the noise canceller 15 is inserted between the transmitter A / D converter 13 and the voice encoder 14. To be done. The user wears the headset type handset HS on the head as shown in FIG. 4, for example.

When a call is made in this state, the talker's transmitted voice is collected by the microphone 51, and the transmitted voice signal is transferred to the portable radio telephone PSS via the code CD. Then, after being converted into a digital signal by the A / D converter 45, it is input to the adder 15b of the noise canceller 15. Also, at this time, ambient noise is caused by the portable radio telephone PS.
Sound is collected by the S microphone 11, the ambient noise signal is converted into a digital signal by the A / D converter 13, and then input to the impulse response estimation circuit 15a of the noise canceller 15. In this impulse response estimation circuit 15a,
A pseudo noise signal is generated based on the ambient noise signal and the transmission voice signal output from the microphone 51.
Then, the pseudo noise signal is subtracted from the transmission voice signal from the microphone 51 in the adder 15b. Therefore, even if ambient noise such as noise is collected together with the transmitted voice of the speaker in the microphone 51 by the hands-free call, the ambient noise component included in the transmitted voice signal is canceled.

In this way, the transmitted voice signal from which the ambient noise has been canceled is the voice encoder 14 and the error correction encoder 1.
Each of them is subjected to encoding processing at 6 and then input to the digital modulator 17. Therefore, the digital modulator 17
Outputs an intermediate frequency signal modulated by the coded transmission signal, and the modulated signal is frequency-converted into a radio frequency and power-amplified by a mixer 18 and a power amplifier 19, respectively, and then is passed through a high-frequency switch 20. It is transmitted from the antenna 21 to the base station.

On the other hand, the radio signal arriving from the base station is received by the antenna 21 and the high frequency switch 20 and frequency-converted into the reception intermediate frequency signal by the receiver 22, and then demodulated by the digital demodulator 23. Then, the demodulated received signal is subjected to error correction decoding and decoding in the error correction decoder 24 and the speech decoder 25, respectively,
Further, the D / A converter 26 performs D / A converter conversion to form a baseband received signal. Then, this reception signal is passed through the switching circuit 42 to the reception amplifier 4 for hands-free communication.
6, is amplified to a level suitable for hands-free communication, is then led to the speaker 52 of the headset type handset HS via the connection code CD, and is output as loud sound from the speaker 52.

Therefore, the user can use the portable radio telephone PS.
It is possible to talk with the headset type handset HS, for example, while holding it in a pocket without holding S in hand.

As described above, according to this embodiment, the noise canceller 15 is provided in the portable radio telephone PSS, and the microphone 11 of the portable radio telephone PSS is used to detect ambient noise, and the headset type handset HS is used.
When making a hands-free call, the noise canceller 15 generates a pseudo noise signal based on the ambient noise detected by the microphone 11, and outputs the pseudo noise signal from the microphone 51 of the headset type handset HS. Since the ambient noise component is canceled by subtracting it from the generated transmitted voice signal, even if the microphone 51 is provided at a position away from the speaker's mouth in the headset type handset HS, the transmitted voice signal The S / N can be kept high. Therefore, it is possible to make a clear and high-quality hands-free call using the headset type handset HS. Further, in the headset type handset HS, the microphone 51 is not limited to the vicinity of the speaker's mouth and can be provided at a desired position that does not bother the speaker, so that it is easy to use and has an excellent appearance. It is possible to provide a wireless telephone device equipped with the headset type handset HS.

In the case of this embodiment, the portable radio telephone P
Since the SS microphone 11 is used for ambient noise detection, it is not necessary to provide a new microphone for ambient noise detection, and this has an advantage that the device can be configured with a relatively simple configuration at low cost.

The present invention is not limited to the above embodiment. For example, in the headset type handset HS, the microphone 51 'may be attached in the middle of the connection cord CD as shown in FIG. FIG. 6 shows a usage example of the apparatus in this case. Also, the microphone 51 '
May be fixed to the collar or chest pocket of clothes by using a clip or the like. Further, the headset type handset HS and the portable wireless telephone PSS may be connected via a wireless line. A microphone for detecting ambient noise may be provided on the headset type handset HS side.

In the above embodiment, the noise canceller generates a pseudo noise signal based on the ambient noise and the transmitted voice signal, and the pseudo noise signal is subtracted from the transmitted voice signal to be included in the transmitted voice signal. Although the ambient noise component included in the transmitted voice signal is canceled, the ambient noise component included in the transmitted voice signal may be canceled by subtracting the ambient noise signal from the transmitted voice signal.

In addition, the type and circuit configuration of the wireless telephone, the shape and configuration of the headset type handset, the configuration of the noise elimination circuit, etc. can be variously modified and implemented without departing from the scope of the present invention.

[0044]

As described above in detail, according to the present invention, a second type for collecting ambient noise and outputting a corresponding ambient noise signal to either one of the wireless telephone and the headset type handset. A microphone and a noise removing circuit are provided, and the noise removing circuit outputs the first microphone provided in the headset type handset based on the ambient noise signal output from the second microphone. The arithmetic processing is performed to remove the ambient noise component included in the transmitted voice signal.

Therefore, according to the present invention, it is possible to prevent the call quality from deteriorating even when the position of the microphone is set to a position away from the mouth, and thus, the hands-free call using the headset type handset. It is possible to provide a radiotelephone device that can be performed with high quality without feeling uncomfortable.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing a configuration of a wireless telephone device according to an embodiment of the present invention.

2 is a circuit block diagram showing a part of the configuration of a noise canceller in the device shown in FIG.

FIG. 3 is a diagram showing the appearance of a headset type handset and a mobile wireless phone.

FIG. 4 is a diagram showing an example of use of the headset type handset shown in FIG.

FIG. 5 is a diagram showing appearances of a headset type handset and a portable wireless telephone of a wireless telephone device according to another embodiment of the present invention.

FIG. 6 is a diagram showing an example of use of the headset type handset shown in FIG.

FIG. 7 is a schematic configuration diagram of a cellular mobile wireless telephone system.

[Explanation of symbols]

PSS ... Cellular radio telephone HS ... Headset type handset CD ... Connection cord 11 ... Cellular radio telephone microphone 12,44 ... Low frequency amplifier 13,45 ... A / D
Converter 14 ... Speech decoder 15 ... Noise canceller 15a ... Impulse response estimation circuit 15b ... Adder 16 ... Error correction encoder 17 ... Digital modulator 18 ... Mixer 19 ... Power amplifier 20 ... High frequency switch 21 ... Antenna 22 ... Receiver 23 ... digital demodulator 24 ... error correction decoder 25 ... voice decoder 26 ... D / A converter 27, 46 ... receiver amplifier 28 ... portable wireless telephone speaker 31 ... control circuit 32 ... frequency synthesizer 33 ... electric field strength detector 34 ... Console unit 35 ... Power supply circuit 36 ... Battery 41, 42 ... Switching circuit 43, 53 ... Connector terminal group 51, 51 '... Headset type handset microphone 52 ... Headset type handset speaker 54 ... U-shaped elastic support Element

Claims (3)

[Claims] 1. A headset comprising: a wireless telephone connected to a base station via a wireless communication line; and a headset type handset that is at least electrically connected to the wireless telephone. A speaker for outputting the received voice corresponding to the received signal transmitted from the wireless telephone to the handset, and a speaker for collecting the transmitted voice and transmitting the corresponding transmitted voice signal to the wireless telephone. A first microphone, and a second microphone for collecting ambient noise and outputting a corresponding ambient noise signal to one of the wireless telephone and the handset, and the second microphone On the basis of the ambient noise signal output from the first microphone, an arithmetic process for removing the ambient noise component included in the transmission voice signal output from the first microphone is performed. Radio telephone apparatus characterized by comprising a noise reduction circuit. 2. The wireless telephone comprises a third microphone for collecting transmission voice and outputting a corresponding transmission voice signal in the state where the headset type handset is not connected. 2. The wireless telephone device according to claim 1, wherein the third microphone operates as a second microphone for collecting ambient noise when the headset type handset is not connected. .. 3. The noise elimination circuit uses the first microphone and the second microphone based on the transmitted voice signal output from the first microphone and the ambient noise signal output from the second microphone.
From the transmission voice signal output from the first microphone, and an impulse response estimation circuit for estimating an impulse response between the microphone and
3. The radiotelephone device according to claim 1, further comprising a subtracter for subtracting pseudo noise generated by the impulse response circuit.