referring now to fig1 a tdm communication system in accordance with the present invention is illustrated . basically , a two - way radio ( or station ) 12 communicates ( optionally , if the radios are out of range , via a control station 13 ) with a second radio ( or station ) 14 using radio frequency ( rf ) physical or logical channels that are divided into frames having at least two time slots : a forward slot and a return slot . the radio 14 may be identical to the radio 12 and the control station 13 may be a repeater or a fixed station , each comprising a network controller to command system operation . to more clearly illustrate the efficient utilization of the frequency spectrum , a wide - band example will be used for emphasis . assume first a conventional tdm - tdma ( time division multiple access ) wide - band physical channel having a channel capacity of 160 kb / s . if speech is encoded at 10 . 8 kb / s each for the talker and the listener for quality speech in both directions , a duplex logical channel would require 21 . 6 kb / s for the channel bit rate ( or data transmission rate ). if 10 % of the 160 kb / s channel rate is used for synchronization and control , 144 kb / s remain for the voice logical channels . dividing the remaining 144 kb / s by the 21 . 6 kb / s channel bit rate , yields 6 full - duplex logical channels which may not all be used during off - peak operation . however , during peak hours , the present invention allocates 10 . 8 kb / s only the talking party and 3 . 6 kb / s , a third less , for example , to the listener . as a result , a duplex logical channel requires 14 . 4 kb / s for the channel bit rate and 4 more logical channels result ( dividing the channel rate of 144 kb / s by the channel bit rate of 14 . 4 kb / s yields 10 logical channels ). with more logical channels available , the original tdma physical channel may efficiently accommodates more users . in a tdma application , the present invention envisions a selection from encoding conventionally to encoding speech at unequal bit rates to transmit in a smaller bandwidth when a predetermined number of logical channels are already in used . of course , the present invention may also be implemented on a tdm - fdma ( frequency division multiple access ) physical or logical channel . however , the benefit in this case will be simpler voice coding or better speech quality rather than spectrum efficiency improvement since the federal communication commission ( fcc ) specifies the number of physical channels available and pre - assigns the bandwidth allocation on a fdma physical channel . to use the increased fdma logical channels made available under the present invention , approval from the fcc is required . the present invention contemplates the use of the current standard for land - mobile communication physical or logical channels , but provides efficient duplex operation by varying the slot allocations among users by varying the coding rates between the talker and listener . to accommodate two voice signals within a frame for spectrally efficient duplex operation , the present invention contemplates vocoding the brief feedback voice signal at a slower coding rate using , for example , linear predictive coding ( lpc ), to hold the speech data rate to approximately 3 . 6 kb / s in the return slot . for the talker , the invention contemplates vocoding the voice at a higher coding rate of approximately 10 . 8 kb / s to obtain a channel bit rate less than 16 kb / s ( assuming a 10 % overhead ). with less bandwidth used , two voice signals may be transmitted on the same physical or logical channel , while maintaining voice quality for the dominant speaker . other coding rate differentials may also be used with lpc or another coding scheme . fig2 illustrates a block diagram of the radio 12 ( or 14 ) having a controller 22 to oversee total radio operation . operationally , audio energy impressed upon a microphone 24 is digitized continuously at normal speed ( speech information rate ) by a voice digitizer 26 , which is enabled by the controller 22 via a connection 27 . the voice digitizer is activated by a voice operated relay ( vox ) location within the voice digitizer 26 . when voice is present , the voice digitizer 26 encodes according to a vocoding scheme such as lpc , sub - band coding ( sbc ) or another scheme . the controller 22 feeds the digitized voice signal into a tranmsitter buffer 28 via a data line 29 . to achieve time compression suitable to properly tdm the physical or logical communication channel , information is retrieved from the transmitter buffer 28 by the controller 22 ( via a connection 32 ) at a selected rate exceeding that at which it was stored ( channel transmission rate or channel bit rate ). to provide a digitized voice tranmsit burst , the retrieved information is routed to a transmitter 34 . additionally , the controller 22 attaches a preamble which constitutes a predetermined control signal to the speech information . the duration and timing of the slots are defined according to the invention and may be changed under command of the control station 13 during peak conditions as described later for tdma or other applicable systems . redefining the slots permits the more dominant speaker to be accorded the longer slot length , while the listener is provided with a shorter slot for acknowledgements . reversal occurs whenever the listener speaks longer than a predefined brief acknowledgement . the controller 22 couples the transmitter 34 to an antenna 36 via a switch 38 . alternatively , the switch 38 could be replaced with a duplexer ( or the like ) to continually couple the transmitter and receiver to the antenna . data information ( already in digital form ) may be transmitted in the same manner via data source 64 . to receive information from the physical or logical channel , the controller 22 couples the antenna 36 to a receiver 42 via the switch 38 . the receiver 42 is coupled both to the controller 22 , and a clock recovery means 44 , which may be any suitable means that will synchronize the controller 22 to the received information using the control signals . once synchronized , the controller 22 couples the received digitized voice signal into a receiver buffer 46 via a connection 48 at the higher rate ( transmission rate ) during one or more receive slots . the contents of the buffer 46 are applied continuously via a decoder 54 ( via a connection 52 ) to the controller 22 operating at normal speed ( speech information rate ). the decoder is enabled by the controller 22 via a connection 56 and includes means to detect the presence or absence of voice . to generate speaker audio , the analog information is routed to a speaker 58 . if , however , data was transmitted during a tdm slot , a data sink 62 , which may be a printer or monitor device , accepts the data and displays it for the operator . referring to fig3 the control station 13 ( or repeater ) of fig1 suitable for use in the tdm system of the present invention is shown . the control station 13 is controlled by a controller 302 , which takes a master clock signal from a system reference 304 . operationally , a vo - coded signal is received from at least one time slot on a first physical or logical channel via an antenna 306 and routed via a duplexer 308 to a receiver 310 . the receiver 310 supplies signals to the clock recovery means 312 and the controller 302 . the controller 302 accepts the received data signal from the receiver 310 at the data rate determined by the clock recovery means 312 and supplies it to the transmitter 314 . the transmitter 314 repeats the signal including the overhead in at least one time slot on a second physical or logical channel ( at a transmission data rate determined by the controller 302 ) through the duplexer to the antenna . those skilled in the art will appreciate that either the multiple frequency repeater as illustrated or a single frequency repeater ( sfr ) may be used alternately or in combination in any particular tdm system . accordingly , in the sfr , the transmitter 314 and receiver 310 are multiplexed to the antenna 306 and the duplexer is not required . referring to fig4 the steps executed by the control station 13 are illustrated for changing from equal to unequal bit rates in a tdma application . the routine begins in step 402 , which monitors the usage of a plurality of logical channels . with this system loading information , decision 404 determines whether the total number of logical channels in use is less than or equal to a predefined threshold . usually , the threshold is the number of logical channels available when the speech of the talker and listener are encoded at equal rates . if the total number of logical channels in use is less than or equal to the number of total logical channels available , decision 406 checks the status of the users waiting in queue . assuming the queue is empty , the control station 13 transmits command for the radios of the talker and listener to encode at equal rates in step 408 . however , if users are waiting step 407 assigns another logical channel to each user in queue before returning to the monitoring step of 402 . but if the total number of logical channels in use is greater than the threshold number , the control station 13 transmits a different command for the radios of the talker and listener to encode at unequal rates in step 410 . to continue monitoring the logical channels , the routine returns to step 402 . referring to fig5 a , a communication logical channel is illustrated having a bandwidth sufficient to allow two encoded voice signals to be transmitted within slots 72 and 74 of a frame 66 . areas between the slots 76 , 78 , 82 , are reserved for functions other than voice transmission or reception . some are gaps to allow for the proper operation of the switch 38 , and for the transmitter and receiver to stabilize and synchronize . in addition , for a tdma application for example , some of the areas are filled as slots with data and channel control bits inserted by the control station 13 to command the radios to switch slot allocations depending on the physical system channel capacity . the talker radio 12 transmits to the listening radio 14 in a slot 72 having a first duration to accommodate information encoded at a first rate . in a slot 74 having the same first duration to accommodate information encoded at the same rate , radio 12 detects an information signal from the radio 14 . this provides equal audio quality for both radios . in accordance with the invention for a tdma application , equal audio quality is provided during off - peak operational hours , when not all the logical channels are in use or the queues are not too long . as is known , the control station 13 determines how many logical channels are in used as compared against a normal capacity threshold . the radios continue transmission at equal coding rates until commanded by the control station 13 to change . when all the present logical channels are used , the sequence of operations follows fig5 b . in accordance with the invention , two voice signals coded at unequal rates are transmitted more efficiently within a smaller frame 86 . the voice signal in the return slot 92 is encoded at a slower rate resulting in a smaller slot 92 . as a result , more logical channels can be accommodated on a tdma system . upon command of the control station 13 , the controller 22 of the listener unit varies the voice digitizer 26 to reduce the coding rate in and the duration of the return slot . meanwhile , the controller 22 of the talker unit maintains the voice digitizer 26 at the higher encoding rate to keep the longer slot duration for the forward voice signal . since the return voice signal normally contains only brief feedback messages , the decrease in audio quality does not significantly impact the conversation . hence , the invention balances spectral efficiency against voice quality . under the direction of the controller 22 , the radio 12 transmits to the radio 14 in a slot 88 having the first duration to accommodate information encoded at the first rate . meanwhile , radio 12 detects transmission fram radio 14 in a slot 92 having a reduced duration to accommodate information encoded at the reduced second rate . if the listening radio 14 transmits a voice transmission longer than a predefined period ( indicating a desire to talk instead of just acknowledge ) in slot 94 , the radio 12 will detect the increased transmission from radio 14 , and upon detecting an absence of transmission from its operator , the radios exchange slot allocations . using the smaller slot 96 , the radio 12 acknowledges the slot exchange . on the other end , the radio 14 transmits information in a slot 98 having the longer duration to accommodate information encoded at the higher rate . basically , whoever is the dominant talker will have the longer slot by encoding at the higher rate . the unequal slot allocation will continue until the physical channel is no longer operating at peak capacity as determined by the control station 13 .