MXPA96002659A - Free channel selection system for a wireless telephone - Google Patents

Abstract

The present invention relates to a radio frequency (RF) transmitter / receiver system comprising a pair of transmitter / receiver units (100, 101). A microcomputer-based radiofrequency carrier detection system determines when a radio frequency signal is present or absent. The radio frequency carrier detection system also responds to noise interference signals on or near its currently tuned receiving channel. In periods of inactivity, the radiofrequency signal receiver monitors its currently tuned channel, and upon detecting noise interference, or upon determining that the currently tuned channel is busy, initiates a quick scan operation to identify an unoccupied channel and free of interference, and transmits a command to your transmitter / receiver unit in pairs, making it tune into the newly identified idle channel

Description

FREE CHANNEL SELECTION SYSTEM FOR A WIRELESS TELEPHONE Field of the Invention The present invention relates, in general, to the field of cordless telephones capable of operating on any one of at least two wireless telephone channels, and specifically relates to an apparatus for detecting and eliminating interference on a particular channel.

BACKGROUND OF THE INVENTION Some known wireless telephone systems employ a radiofrequency (RF) carrier detection system, wherein the base unit verifies the presence of a radio frequency carrier during the period in which the wireless telephone unit is in the state "hung" inactive. These known systems use an analog hardware carrier detection circuit. In the patent application of the United States of America with serial number RCA 87,448 (Fossaceca et al.), Filed on December 22, 1993, and assigned to the same transferee, a radiofrequency carrier detection system based on microcomputer In that request, it would be said that, unfortunately, during the manufacturing process, an adjustment procedure must be performed on the hardware analogue detection circuitry of the hardware in any wireless telephone apparatus in order to ensure a correct triggering in the thresholds specified. This adjustment procedure tends to be difficult, delayed, and often causes the detection of the radiofrequency carrier to be unreliable. Moreover, since the carrier detection circuitry is in the base unit of the wireless telephone apparatus, these systems are only capable of detecting interference at, or near, the handset transmitter frequencies (i.e. receiver of the base unit). Interference that occurs at, or near, the transmitter frequencies of the base unit will go undetected, and may lead to an altered communication between the base unit and the handset. One might think that the radio frequency carrier detection circuitry should also be installed in the headset to alleviate this problem, but so far there are two separate obstacles that have prevented this simple solution. First, another analog radiofrequency carrier detection circuit had to be added from the hardware to the headset, increasing the manufacturing cost, further complicating the design, and increasing the manufacturing assembly time, as well as the calibration time of each unit. In addition, the headset is a separate unit, and as such, exhibits poor ground reference, which adds to the difficulties of adjusting the reliability of the threshold potentiometer of the radio frequency carrier detector. Second, it is intended that wireless telephone headsets be portable, and consequently, be operated with battery. Ideally, one would like the headset to not need power in idle mode. Unfortunately, the headset receiver must be energized, and consequently, draw power, in order to receive the incoming calls by means of the transmission from the base unit. In order to conserve energy, a common solution to this problem is to repeatedly "press" the handset to "on" and "off" in a short duty cycle to search the radio frequency transmissions from the base unit. The duty cycle for the handset is typically 40 to 60 milliseconds (ms) "on" and several hundred milliseconds "off". However, not all of the "on" portion of the duty cycle is available for use, since 15 to 20 milliseconds are required for the secured phase cycle (PLL) and microprocessor circuitry to stabilize after " to be energized ", and up to 5 additional milliseconds are required for the hardware's analog radio frequency carrier detection circuitry to stabilize. In the worst case, the additional stabilization time of 5 milliseconds would dissipate unacceptably 25 percent of the active monitoring period available from the headset.

SUMMARY OF THE INVENTION A radio frequency (RF) signal transmitter / receiver system comprises a pair of transmitter / receiver units. Each of the units is capable of operating on one of a plurality of radiofrequency channels, and each receiver includes an input to receive a radio frequency signal, the radio frequency signal being subject to periods of absence. A microcomputer-based radiofrequency carrier detection circuit determines when a radio frequency signal is present or absent. The radiofrequency carrier detection circuit also responds to noise interference signals at or near its currently tuned receiver channel. In periods of inactivity, the radiofrequency signal receiver monitors its currently tuned channel, and upon detection of noise interference, or upon a determination that the currently tuned channel is occupied, initiates a quick scan operation to identify an unoccupied channel and relatively free of interference, and transmits a command to its transmitter / receiver unit in pair, causing it to tune into the newly identified idle channel.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows a simplified block diagram of a base unit and a headset of a wireless telephone apparatus suitable for use with the invention. Figures 2a-2c show waveforms useful in understanding the invention. Figures 3 to 6 are illustrations of flow charts showing relevant portions of the control program for the icrocontrollers of Figure 1.

DETAILED DESCRIPTION OF THE MODALITY Referring to Figure 1, a wireless telephone base unit 100 includes a Duplexer 105 for receiving radio frequency (RF) signals from, and applying the radiofrequency signals to, an antenna 106. The duplexer 105 may be a DPX 46/49-B10 type duplexer manufactured by Soshin Electric Ltd. The signals received from the duplexer 105 are applied to a receiver circuit 110 for detection, processing, and amplification. baseband to a compressor / extender 120 for spectral expansion, which in turn applies the audio signals to an audio amplifier 130. The audio amplifier 130 applies the amplified audio signals to the 5-point (T) terminals and ring (R) of a telephone network by means of a telephone interconnection unit 140. The incoming telephone signals are received by the telephone interconnection unit 140, and are applied to the compressor / extender 120 by means of the audio amplifier 130. The compressor / extender 120 compresses the amplitude of the audio signals to increase its immunity to noise, and applies the compressed signals to a transmitting circuit 175. transmitter 175 modulates the audio signals on a radio frequency carrier, and applies the signals of radio frequency to the duplexer 105 to be transmitted by means of the antenna 106. The interconnection with the telephone network, the dialing operations, the channel selection, and the communication with a headset 101 of the wireless telephone device 0, is under the control of a microcontroller 160. The microcontroller 160 can be a microprocessor or a microcomputer or a dedicated controller integrated circuit. The microcomputer 160 controls a secured phase cycle circuit (PLL) 165 for the selection of 5 frequencies, and receives the user data input by means of a keypad 170. The elements of the handset 101 of FIG. 1 carrying numerals of reference similar to those of the elements of the base unit 100 of Figure 1, serve the same function, and do not need to be explained again. The audio amplifiers 130 'of the headset 101 receive the input signals from a microphone 180 * and provide output audio signals to a speaker unit to listen to 190'. The United States patent application of North America with Serial Number 08 / 171,353, filed on December 21, 1993, discloses that the carrier detection function can be performed by the microcontroller in the base unit, by having the microcontroller examine the frequency content of the audio signal of baseband detected. At present, it is recognized that when both the base unit and the handset employ a microcomputer-based radio frequency carrier detection configuration, conveniently, the transmission frequencies of both units can be monitored during the inactive periods in order to move towards a different channel to avoid noise interference or the previous occupant of a wireless telephone channel. An additional 5-second delay is not required for the stabilization of a hardware carrier detection circuit because a hardware carrier detection circuit is not used. For simplicity, the operation of the receiver in the headset 101 will be described only, the operation of the receiver in the base unit 100 being essentially the same. The receiver 110 'includes a data computer circuit 111' that receives a portion of the baseband signal, and sorts it into amplitude levels previously determined to produce a binary signal. The data computer 111 'applies this binary signal directly to the microcontroller 160' by means of a line 115 '. Figure 2a is a simplified illustration of a waveform representing the band-limited white noise that would be present in the baseband signal received by the data computer 111 'in the absence of a radiofrequency carrier. Note that the waveform of Figure 2a contains random noise pulses that have frequencies generally higher than the frequency scale of the speech signals. Figure 2b is a simplified illustration of the waveform of Figure 2a after ordering the data. The waveform of Figure 2b is typical of a signal that would be present on line 115 'in the absence of a radiofrequency carrier. It has been discovered that the human voice tends to exhibit frequencies that cluster around 1 kHz with relatively long periods without signal, due to the silence caused by the cuts in the conversation. Accordingly, the simplified waveform of Figure 2c illustrates a signal with a lower frequency content (i.e., longer pulse amplitudes) than the random noise shown in Figure 2b. It is recognized herein that the microcontroller 160 'is capable of sampling the signal on the line 115' fast enough to make a determination of the presence or absence of noise, which indicates the ^ _. absence or presence of the radiofrequency carrier, J respectively. The microcomputer 160 'samples the baseband signal every 118 microseconds, which enables the microcontroller 110' to identify the audio frequencies up to about 8.5 kHz. The present invention combines a new method for selecting a channel that is likely to be free, with the characteristics of the microcomputer-based radiofrequency carrier detection configuration of the United States patent application with RCA serial number. 87,448, and with a 0"quick scan" configuration previously used in GE cordless phones that carry the model numbers 2-9632, 2-9626, 2-9615, and 2-9635, and manufactured by Thomson Consumer Electronics, Inc ., Indianapolis, Indiana, USA, to provide a configuration that detects and avoids previously occupied wireless telephone channels 5, and channels that are noisy due to interference on any of the receiving channels of the handset or the receiving channels of the telephone. the base unit. It has been found that the system in accordance with the present invention is more reliable than the prior art systems in that it is less likely to experience loss of communication between the handset and the base unit. In the prior art cordless telephone sets, a loss of communication is typically overcome when the handset is returned to the base unit holder, because the aforementioned fast scan configuration (described in detail below) operates to ensure that the handset and the base unit are tuned in. same channel. It is felt that this is an important feature in which it makes it possible for a cordless telephone to operate with a remote recharging cradle that does not require its own telephone outlet, and that consequently, can be placed in substantially any room of a typical house. As noted earlier, the handset is designed to conserve energy. In this regard, the handset detects interference in the standby operation mode, which is inactive, or in the idle mode where the handset turns on its receiver for a very short period (approximately 40 to 60 milliseconds), it looks for input signals, and then, finding none, it disables for several hundred milliseconds. The headset will only be fully energized upon determining that a carrier exists on its currently tuned channel. If there is a carrier, and if the carrier is a transmission from the base unit that is paired with the handset, then the handset transmits instructions (explained in detail later) to the base unit, and enters the quick scan mode, which will ensure a high probability of a successful link of the handset and the base unit on a new channel. The system is designed to stop trying to bind on a new channel after a previously determined number of unsuccessful attempts, and will only be reactivated after a system reset, or when the user causes a hang-up condition followed by a condition of strung up. The following describes three different systems to avoid interference, which differ only in the relative complexity of the program to select a new channel that is likely to be free. Preferably, each of these three systems uses a microcomputer-based radiofrequency carrier detection configuration; however, the use of a hardware carrier detection circuit, although not preferred, is still considered acceptable for practicing the invention.

Interference Avoidance System 1 In the passive WAIT state, the headset 101 periodically turns on its receiver to seek communication from the base unit 100. During that time, the microcomputer-based radio frequency carrier detection system 5 measures the amount of energy of white noise received. If the white noise content is low, the system increases a counter. After a predetermined number of these white noise detections j.ó low is reached (e.g., 5), the handset determines that there is interference present on its currently tuned receiving channel (i.e. the base unit currently selected). When detecting this interference, the headset simply assumes that the next consecutive channel is free, emits a channel change command to the base unit, and moves to the next channel. The channel to which the handset is moved is called the Auto-Free (AC) channel, and the designation of AC in this case should not be confused with the designation of AC which means "Current 0 Alternate". After moving to the Auto-Free channel, the handset issues a hang command to the base unit. The base unit must respond to that command on the Auto-Free channel. If the base unit does not respond in a predetermined period of time (for example, several milliseconds), then the headset 5 will perform a quick scan operation by issuing a fast-scan "go to" command over each channel, moving quickly from one channel to the next without waiting for a response from the base unit. It is expected that the "go to" commands cause the base unit to move to the Auto-Free channel. After issuing the "go to" command on all channels, the handset moves to the Auto-Free channel, and listens to a response from the base unit. If a response is received, then the fast scan operation succeeded, and if not, the handset returns to the original channel, increases the Auto-Free channel (because the originally selected Auto-Free may not be free), and starts again. If after increasing the Auto-Free channel through all the available channels, the system has failed to link, the system stops trying to reestablish communications until the next reset of the handset, or until a hanging condition is presented followed by a off-hook condition. In Figure 3 there is illustrated a flow chart showing the details of the system to avoid the interference 1 described above. Table 1 shows a high level language program to implement the flow chart of Figure 3.

Interference Avoidance System 2 As described above, during the operation WAIT mode, the headset 101 periodically turns on its receiver to look for a communication signal from the base unit 100. During that time, the radiofrequency carrier detection system based on microcomputer measures the amount of white noise energy received. If the white noise content is low, the system increases a counter. After a predetermined number of these low white noise detections is reached (e.g., 5), the handset determines that there is interference present on its currently tuned receiving channel (i.e., the transmission channel of the base unit). currently selected). Upon detecting this interference, the handset simply assumes that the next consecutive channel is free, issues a channel change command to the base unit, and moves to the next channel (ie, the Auto-Free channel). After moving to the Auto-Free channel, the headset 101 waits for a response from the base unit 100. The base unit 100, upon receiving a channel change command from the headset 101, moves to the Auto-Free channel, and monitors briefly the input signal. If the base unit 100 measures a high white noise content, then it determines that the Auto-Free channel is free, and sends a response to the headset 101. On the other hand, if the base unit 100 determines that the Auto-Free channel is busy, it returns to the original channel without issuing a response. The receiver 101, upon receiving a response on the Auto-Free channel, sends a final confirmation to the base unit 100 via the Auto-Free channel, to which the base unit responds 100. If no answer is received, then the headset 101 returns to the original channel, issues a hang command, and waits for a response. If a response is not received, then the handset 101 performs a quick scan operation by forcing the base unit 100 back to the original channel. In the same way, once the base unit 100 has moved to the Auto-Free channel and has responded, it waits to receive a final confirmation signal from the headset. 101. If a final confirmation signal is not received, then the base unit 100 returns to the original channel. Upon each subsequent attempt, after a failure to move to the Auto-Free channel, the headset 101 increments the Auto-Free channel. If after increasing the Auto-Book channel through all the available channels, the system has failed to link, the system stops trying to reestablish communications until the next reset of the handset, or until a hang condition is presented followed by a off-hook condition. In Figure 4 there is illustrated a flow diagram showing the details of the system to avoid the interference 2 described above. Table 2 shows a high level language program to implement the flow chart of Figure 4.

Interference Avoidance System 3 As described above, during the operation WAIT mode, the headset 101 periodically turns on its receiver to look for a communication signal from the base unit 100. During that time, the radiofrequency carrier detection system based on microcomputer measures the amount of white noise energy received. If the white noise content is low, the system increases a counter. After a predetermined number of these low white noise detections is reached (e.g., 5), the headset 101 determines that there is interference present on its currently tuned receiving channel (i.e., the unit's transmission channel). base currently selected). Upon detecting this interference, the handset briefly verifies that the next consecutive channel is free, issues a channel change command to the base unit, and moves to the next channel (ie, the Auto-Free channel). After moving to the Auto-Free channel, the handset 101 waits for a response from the base unit 100. If no answer is received, then the handset 101 performs a quick scan operation by issuing a "go to" command on each channel, and rapidly changing to the next channel without waiting for a response, in order to force the base unit 100 to move to the Auto-Free channel. If then a response is received on the Auto-Free channel, the link succeeded. Upon each subsequent attempt, after a failure to link over the Auto-Free channel, the headset 101 increments the Auto-Free channel. If after increasing the 5 Auto-Free channel through all available channels, the system has failed to link, the system stops trying to reestablish communications until the next reset of the handset, or until a hanging condition is presented followed by a condition of off-hook.

J Ú In the following off-hook condition, the headset 101 sends the Auto-Free channel embedded in the off-hook command to the base unit 100. If no response is received on the Auto-Free channel, to the off-hook command, then the handset issues a series of warning tones to user, returns to the original channel, and deactivates. In Figure 5 there is illustrated a flow chart showing the details of the system to avoid the interference 3 described above. Table 3 shows a high level language program to implement the 0 flow diagram of Figure 5.

Initiative of the Base Unit It is hereby recognized that the base unit 100 can also initiate an attempt to switch to the Auto-Free channel 5 by detecting the interference on its reception channel currently tuned while it is "hung". Then, the base unit 100 sends an "attention" signal to the headset 101, followed by a FreeAuto_Request command. The headset 101, if it is in WAIT mode 5, and if it is within the range, responds by activating its Auto_Libre routine (ie, one of the routines to avoid interference 1, 2, or 3) of the same way as if the headset 101 had detected the interference itself. Accordingly, the base unit 100 jftv can initiate a sequence that will result in communications being established between the base unit 100 and the headset 101 on an Auto-Free channel. As noted above, the use of the radiofrequency carrier detection configuration based on microcomputer facilitates the use of the invention in wireless telephone headsets by eliminating the adjustment of the hardware components. It is important to note that in a hardware based system, the threshold setting is a potentiometer set at the factory. This 0 necessarily limits the carrier detection circuitry to a single "wired" decision threshold value (i.e., presence of the carrier, or no carrier present). This limitation requires that the carrier detection circuitry use the same threshold value whether it is hanging or off-hook. It is recognized herein that the threshold value is a software variable that can be easily changed in a microcomputer-based radiofrequency carrier detection configuration according to the current operating mode of the system. For example, while hanging, the radio frequency carrier detection configuration is used to automatically select the free channel, and must be set to a low value to detect low levels of interference. While when off-hook, the configuration of _Í? Radio frequency carrier detection is used for out-of-scale detection and warning, and should be set at a higher threshold to detect the absence of the carrier when the signal strength drops to a level at which the conversation becomes difficult because at noisy audio signals. The computer-based radiofrequency carrier detection configuration of US Patent Application Number RCA 87,448 is briefly explained with reference to the 0 flow diagram of Figure 6. In normal operation, the microcontroller 160 ' it monitors the baseband signal flow to detect the signal data that may accompany the received analog audio signals. A frequency filtering algorithm varies the value of a counter of 5"noise energy" in response to the white noise content of the received audio signal. If the count of the noise energy meter exceeds a previously determined value, then a determination is made that there is no carrier on the currently selected receiving channel. The audio signal present on line 115 'is sampled, and the value is checked at a counter AMPLITUDE, to see if it is less than a previously determined high frequency threshold, higher than the highest voice frequency. If so, the value in AMPLITUDE indicates the detection of high audio frequencies (ie, white noise). If the count does not indicate a high frequency audio content, then it is checked to determine if the detected pulse amplitudes indicate frequencies greater than a previously determined low frequency threshold value, for example, 1 kHz (as noted above, the voice human tends to cluster around 1 kHz in frequency). If the detected frequencies are not above 1 kHz, then long pulse amplitudes (ie, low audio frequencies) have been detected, a condition indicating voice and / or silence, and the routine is exited to continue decoding the audio signal in the usual way. On the other hand, if the frequency content is not clearly high (ie, noise) or clearly low (ie, audio), then a counter_RB ENERGY (ie, white noise energy) is checked to see if it is equal to the minimum value. If so, the FLAG OF NO_HAY_PORTADORA is deleted, indicating the presence of a radiofrequency carrier, and the routine is exited. If ENERGIA_RB is not at its minimum value, it decreases, due to a longer pulse amplitude (ie, the lowest frequency has barely been detected). The lowest value in ENERGIA_RB is checked to see if it exceeds a previously determined threshold. If so, then a FLAG OF NO_HAY_PORTADORA is established. If not, the FLAG OF NO_HAY_PORTADORA is deleted. In any case, then the routine is exited by means of an instruction of RETURN. If a determination was made that there are high audio frequencies, then the SI path is taken, and the content of ENERGIA_RB is checked to see if it is at its maximum value. sets the FLAG OF NO_HAY_PORTADORA, and exits the routine If ENERGIA_RB is not at its maximum value, it increases, because a high frequency was detected The highest value in ENERGIA_RB is checked to see if it exceeds a threshold previously If this is the case, then the FLAG OF NO_HAY_PORTADORA is established, otherwise the FLAG OF NO_HAY_PORTADORA is deleted, in any case, then the routine is exited by means of a RETURN instruction. of free channel selection based on microcontroller useful in, but not limited to, the environment of cordless telephones In a convenient manner, through the use of the present invention, reliability is improved by virtue of the fact that both the receiving channels of the base unit and the receiving channels of the handset are monitored by interference signals, either from other cordless telephones or from other sources of radio frequency noise, such as radio stations, energy, or household appliances. The terms "microcontroller" and "microcomputer" are used interchangeably herein, and are intended to include microprocessors, microcomputers, dedicated control integrated circuits, and the like.

TABLE 1 2 (HIDE ALL) 3 Free Scan Channel System - IA1 Passive waiting status. 6 If the rx energy is activated? 7 (Yes) measure white noise energy. 8 9 If energy_of_white_rule < 16? 10 (Yes) increase the energy_count low. 11 (No) erase energy meter__down. 13 If low_power_count = 5? 14 (Yes) set interference flag. 15 (No) continue. 16? end 17? end 18? end Main Cycle 21 If the Flag_interference is established? 22 (Yes) send channel change to base for channel Self-Free 23 Go to Auto-Free channel. 24 Send command to hang to the base. 25 If the base answers? 26 (Yes) continue. 27 (No) perform Quick Scan. 28 If the base answers? 29 (Yes) continue. 30 (No) return to the old channel. 31 increase the Auto-Free channel. 32? end 33? end 34 (No) continue. 35? end TABLE 2 2 (HIDE ALL) 3 Free Scan Channel System - IA2 Passive waiting status. 6 If the rx energy is activated? 7 (Yes) measure white noise energy. 8 9 If energy_of_white_rule < 16? 10 (Yes) increase the energy_count low. 11 (No) clear energy_count low. 13 If low_power_count = 5? 14 (Yes) set interference flag. 15 (No) continue. 16? end 17? end 18? end Main Cycle 21 If the Flag_interference is established? 22 (Yes) go to the Auto-Free channel. 23 If the base responds? 24 (Yes) send final confirmation to base 25 to channel Auto-Libre 26 (No) return to the old channel. 27 Send command to hang to the base. 28 If the base answers? 29 (Yes) continue. 30 (No) perform Quick Scan, 31 increase the Auto-Free channel. 32? end 33? end 34 (No) continue. 35 end TABLE 3 2 (HIDE EVERYTHING) 3 Free Exploration Channel System -IA3. 5 Passive waiting status. 6 If the rx energy is activated? 7 (Yes) measure white noise energy, 8 9 If white_white_power < 16? 10 (Yes) increase the energy_count low. 11 (No) clear energy_count low. 13 If low_power_count = 5? 14 (Yes) set interference flag.

(No) continue 16? end 17? end 18? end Main Cycle 21 If the Flag_interference is established? 22 (Yes) If it disappears_bandera_de_esperar_interferencia? 23 (Yes) continue. 24 (No) go to the Auto-Free channel. 25? end 26 (No) continue. 27 If there is no interference on the Auto-Free channel? 28 (Yes) send the command to go to the Auto-Libre channel. 29 If the base responds? 30 (Yes) continue. 31 (No) perform Quick Scan. 32 If the base responds? 33 (Yes) continue. 34 (No) establish disappear_bandera_de_esperar_ interference. 35? end 36? end (No) return to the old channel, Increase Auto-Free channel. ? end ? end.

Claims (3)

1. A wireless telephone apparatus exhibiting a hung state to enable communication by means of an external telephone network, and a hung state, this wireless telephone apparatus comprising: a base unit and a manual unit operating on one of a plurality of wireless phone channels, providing each of these channels? < / cordless telephone communication between the manual unit and the base unit by means of a base unit transmission frequency and a manual unit transmission frequency; a monitoring circuit located in the base unit or in the hand unit for monitoring, during the hung state, the frequency content of a baseband signal derived from a respective one of the base unit's transmission frequency or transmission frequency of the hand-held unit, to produce an output signal indicating that that wireless telephone channel is either busy or has noise; causing the wireless telephone apparatus, in response to the output signal, to cause the base unit and the manual unit to switch to a second wireless telephone channel.

2. The wireless telephone apparatus of claim 1, wherein the monitoring circuit monitors the frequency content of a demodulated audio baseband signal to determine that a second channel is busy, has noise, or is suitable for use before 5 to cause the transmission of the command to change to the second channel.

3. A wireless telephone system, which comprises: a base unit to connect to a telephone JÍÚ network, and which exhibits a suspended status or an off-hook status; and a hand unit subject to being away from the base unit for a period in which the base unit is displaying the hung condition; 15 the base unit and the hand unit communicating by means of one of a plurality of communication channels, using each of these communication channels a pair of radiofrequency carrier frequencies, wherein one of the pair is a transmission frequency for the unit base, and the other one of the pair is a transmission frequency for the manual unit; during the period in which the base unit is displaying the hung state, and when the manual unit is away from the base unit, the base unit monitors the transmission frequency of the manual unit, and in determining that the transmission frequency of the the manual unit has noise or is busy, it issues a command to the manual unit to change to another of the communication channels; during the period in which the base unit is displaying the hung state, and when the manual unit is away from the base unit, the manual unit monitors the base unit's transmission frequency, and by determining that the base unit's transmission frequency The base unit has noise or is equipped, emits a command to the manual unit to change to another of the communication channels.