|Sunday, 14 January 2007|
Portable communication devices such as cellular phones have achieved widespread market distribution, and are commonly used by many people, particularly in metropolitan areas. Portable telephones generally have a compact size so that the user may more easily carry the telephone, and typically include a housing containing a transceiver circuit, a microphone, a low-level speaker, and a user interface. The user interface includes a keypad and a display. A rechargeable battery attached to the housing typically powers the portable telephone. The battery has a limited life, and therefore the portable telephone is designed to operate at low power to increase the time period between battery recharging. Mobile phones often include a speakerphone mode in which users can use the telephones in a hands-free configuration without the use of headsets. The mobile phones have attachments that allow hands-free operation. Some such attachments include headset systems and also speakerphone attachments. Speakerphone operation combines a relatively high power speaker and a high gain microphone with control circuitry and software so that the user can speak and hear at a distance from the speakerphone device while talking with a remote party telephonically. Speakerphone attachments allow the user to turn the portable telephone into a speaker-phone by tapping the audio signal from the portable telephone, amplifying it and sending it to a speaker. Such attachments usually require connecting a jack on the telephone to the speaker-phone attachment via a loose wire terminated by a plug. Many portable and notebook computers currently offered include internal sound systems which incorporate a microphone and a speaker which could, if properly interfaced, be used as the audio input and output for speakerphone operation. In addition to portable phones, speakerphones are also used in a conference room setting, with conference call participants seated around a table, and generally include at least one speaker and at least one microphone. The speakerphone used for a telephone conversation using a speaker and microphone without using a handset has been applied widely to a teleconference system that connects plural locations, and to the automobile telephone system wherein the driver cannot free his hands from the steering for obvious safety reasons.
A speakerphone defines a speaker for converting electrical signals representative of speech to audible sound, and a microphone for converting an audible input such as speech to electrical signals to enable telephone communications without a conventional telephone handset. The speaker is designed to produce an audio level sufficient to be heard by the user at some distance from the speakerphone device, and the microphone is designed to produce sufficiently large electrical signals on the telephone line even when the user talks at some distance from the speakerphone device. Typically, the speaker and microphone are incorporated at a relatively close distance in a same housing. In a typical speakerphone, the gain setting of a channel is dependent upon the detection of speech within that channel. If a far end talker is speaking, the receive signal is greater than the transmit signal, and the transmit attenuator should be set to maximum loss while the receive attenuator is set to maximum gain. The reverse is true if the mirror-end talker is speaking. By monitoring the amplitudes of the signals in both channels, a control circuit may be developed to determine which channel is active and adjust the gains accordingly. The control circuit monitors electrical signals supplied to the speaker and received from the microphone in order to independently activate either the speaker or the microphone. A speakerphone switch may be provided to activate the speakerphone microphone and speaker while deactivating the telephone handset microphone and earpiece. A speakerphone circuit also has a transmit channel coupled between a microphone and the telephone line, and a receive channel coupled between the telephone line and a speaker. Each of the channels has an attenuator or amplifier whose gain can be adjusted by a control circuit. When used in combination with a digital answering machine, the speakerphone microphone may be employed for recording outgoing messages and the speakerphone speaker may be employed for playing back recorded outgoing messages or incoming messages. To prevent outbound audio from the telephone's output transducer from feeding back into the telephone's input transducer, and thus and back to the original source of the audio, a voice activity detector (VAD) is typically used to mute the telephone's input audio when vocal patterns are being reproduced by the output audio transducer. Speakerphones may take many shapes and forms, including as a land line telephone or as a hands free wireless or cellular telephone.
Speakerphones may be designed to operate in either half-duplex or full-duplex mode. A full duplex speakerphone refers to a telecommunications system capable of simultaneously transmitting audio signals in two directions (that is, it can simultaneously transmit and receive audio signals). In a full duplex speakerphone mode of a system, the speaker and microphone of a speakerphone are acoustically coupled such that sound waves from the speaker travel to the microphone. Acoustic echoes occur because sounds generated by the loudspeaker are detected by the microphone and transmitted back over the telephone line. The sounds generated by the speaker reach the microphone either directly or by reflections from the walls of the room in which the phone is located. In order for a full duplex speakerphone to avoid undesirable audio feedback, a sophisticated process of adaptive echo cancellation of the near end telephone line and acoustic signals is required. Substantially all full duplex speakerphones employ digital signal processors (DSPs) in order to implement adaptive echo cancellation. In half-duplex mode, a conversation between two parties is carried in one direction at a time, essentially requiring the connected parties to take turns speaking. 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. A full duplex speakerphone is preferable to a half duplex speakerphone. Full duplex speakerphones allow both parties to speak and be heard at the same time. 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. Systems capable of handling voice communication applications such as audio conferencing, teleconferencing, or telephony are increasingly integrating full duplex speakerphone functionality.