Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1998-08-31
2004-07-27
Gary, Erika (Department: 2681)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S570000
Reexamination Certificate
active
06768914
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to telephony. More particularly, the present invention relates to speakerphones that have a wireless connection to their microphone.
2. Description of Related Art
Telephones equipped with a loud speaker and microphone (“speakerphones”) allow one or more talkers in a room with the speakerphone to participate in a telephone conversation without using their hands. Freeing the hands during telephone conversations has many advantages, including facilitating note-taking, decreasing neck, arm, and shoulder fatigue, and allowing the talker to freely move about the room.
Speakerphones may be designed to operate in either half-duplex or full-duplex mode. In the half-duplex mode, when conducting a telephone call using a speakerphone, only one party involved in the telephone call can have his voice transmitted at one time. If both parties try to speak simultaneously, a choppy sound effect known as clipping results. Having to speak in turn can be unnatural and can make conversation difficult and laborious. Thus, many speakerphones are designed to operate in full-duplex mode. The full-duplex mode allows incoming and outgoing parties' voices to be simultaneously transmitted so that there is no clipping or choppyness. This results in much more natural and spontaneous flowing conversation. Full-duplex is even more important when more than one person is in a room using the speakerphone for a conference call. It can be difficult for the person on the other end of the conference call, connected via the telephone network, to break into the ongoing conversation. In addition, noise or commentary in the conference room can cut off the person on the other end.
FIG. 1
shows a conventional full-duplex speakerphone
100
. The speakerphone
100
includes a base station
102
and a handset
104
that is connected to the base station
102
by a cable
105
. The base station
102
includes a keypad
106
, a speaker
108
, and a microphone
110
. A person using the speakerphone
100
can conduct a telephonic conversation with another person by using the handset
104
to speak and hear, or by using the speaker
108
to hear the other person and the microphone
110
to speak.
However, conventional full-duplex speakerphones like the one shown in
FIG. 1
operate under severe constraints that adversely effect the audio quality of the telephonic conversation. The speaker
108
and microphone
110
are both located in the base station
102
, in close proximity to one another. The close proximity results in acoustic coupling between the speaker
108
and microphone
110
by which sound emanating from the speaker
108
is picked-up by the microphone
110
. This causes an echo signal to be transmitted by the speakerphone
100
to a listener connected to the speakerphone
100
over the telephone network.
This effect becomes more pronounced when the room talker is much farther from the microphone
110
than the speaker
108
. The microphone
110
receives the speaker
108
signal as a very loud signal, while the voice of the distant room talker is received by the microphone
110
as a very quiet signal. Moreover, the room talker's voice will typically echo as a result of impedance mismatches in the telephone network, from the point where the speakerphone
100
is connected to the telephone network to the termination at a central office of the telephone service provider. To mitigate these echoes, full-duplex speakerphones have echo cancelers. While echo cancelers can reduce the echo, they do not fully cancel the echo, leaving residual echos. When the residual echoes are audible, this significantly degrades the quality of a conversation.
In addition, full-duplex speakerphones like that illustrated in
FIG. 1
tend to pick-up substantial amounts of unwanted noise. With the microphone
108
fixed in the base station
102
, the person talking may be a significant distance from the microphone
108
. The farther away the talker is from the microphone
108
, the more difficult it becomes to hear the talker's voice. To make the talker audible, the microphone signal must be amplified. But this also amplifies the system background noise, which includes the residual echo, quantization noise in digital systems, environmental background noise, and the noise floor of the microphone
108
. The more noise the microphone
108
picks-up, the greater the degradation in sound quality during the call.
FIG. 2
shows a full-duplex speakerphone
200
that is designed to reduce the acoustic coupling problem. The speakerphone
200
includes a speaker
202
and microphone
204
. The speakerphone
200
has a line side
206
, which is connected on one end to the public switched telephone network (PSTN)
208
, and a room side
210
. The speakerphone
200
also has a transmit signal path
212
and a receive signal path
214
. A line echo canceler (LEC)
216
is located between the transmit signal path
212
and receive signal path
214
on the line side
206
, and an acoustic echo canceler (AEC)
218
is located between the transmit path
212
and receive path
214
on the room side
210
.
A line talker signal
220
incoming to the speakerphone
200
from the PSTN
208
is converted from analog to digital form, and any line echo
221
excited by the LEC reference input signal (LEC
ref
)
222
that is estimated by the LEC
216
is subtracted from the LEC echo input signal (LEC
echo
)
224
. A residual line signal
226
, which ideally contains only the line talker signal
220
, is then amplified by the receive automatic gain control (AGC)
228
, which is designed to maintain its output power at a specified level. The receive AGC
228
outputs a signal that is converted into an analog form and output to the speaker
202
, completing the receive signal path
214
.
Similarly, in the transmit direction, the microphone
204
picks-up a room talker signal
232
, which is converted into an AEC echo input signal (AEC
echo
)
234
. The room echo
236
, as estimated by the AEC
218
, is then subtracted from the AEC
echo
signal
234
, and the residual transmit signal
238
, which ideally contains only the room talker signal
232
, is amplified by the transmit AGC
240
to a specified level. The amplified signal is then converted to analog and output to the PSTN
208
, completing the transmit signal path
212
.
In practice, however, the AEC
218
and LEC
216
do not provide perfect cancellation, and the transmit and receive signals leak into one another. As a result, a potential loop exists in the signal path, shown by gain loop
242
. The AGCs
228
,
240
and other analog or digital gains may, at some frequencies, cause gain around the loop
242
to be greater than unity, especially if the standard deviations of the input line
244
and/or room talker signal
232
are small, and the gains of the AGCs
228
,
240
are high. When the loop gain
242
is greater than unity at any frequency, the full-duplex speakerphone
200
is prone to oscillate or feed-back.
To correct these problems, expensive circuitry and software must be designed and provided in the full-duplex speakerphone
200
. Even with the added circuitry and software, the speakerphone
200
may experience feedback due to the close proximity of the speaker
202
and microphone
204
and resultant acoustic coupling between them.
FIG. 3
shows another type of speakerphone
300
that is designed to provide a reduction in acoustic coupling and feedback. The speakerphone
300
has a base station
302
, keypad
304
, speaker
306
, and base station microphone
308
. Speakerphone
300
also includes a remote microphone housing
312
, which is connected to the base station
302
by a cable
310
. The remote microphone housing
312
has a remote microphone
314
. Locating the remote microphone
314
remote from the speaker
306
reduces coupling between the remote microphone
314
and speaker
306
.
The use of cable
310
to connect the remote microphone housing
312
to the base station
302
, however, limits the
Akin Gump Strauss Hauer & Feld & LLP
Gary Erika
Rourk Christopher J.
Skyworks Solutions Inc.
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