Abstract:
A device which uses an input of speech and measures latency between stimulus and response. The device generally includes an input transducer for converting a stimulus speech sound into an electrical signal and transmits the electrical signal to an electric circuit. In the preferred embodiment, the electric circuit includes a central processing unit which utilizes delay time counters to measure the length of time between signals. A second input transducer is used to convert a response speech sound into an electrical signal and transmits the electrical signal to the electric circuit. Each input transducer operates on a separate channel, so that the central processing unit may easily distinguish between stimulus sounds and response sounds.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of speech-measuring devices. More specifically, the present invention comprises a device which takes an input of speech and measures the time lapse or latency between the stimulus and response. 
   2. Description of the Related Art 
   Being able to determine the “latency” of an individual&#39;s response to a speech stimulus is significant in many fields including audiology, speech pathology, psychometry, and motor testing of all kinds. For example, one theory holds that the longer it takes someone to perceive a speech unit correctly, the less clear or focused their perception is. Inversely, the shorter the temporal latency between stimulus and response, the higher the quality the perceptive event at the moment of perception is. This theory is based on the well-studied strong central component of psycho-acoustic ability. Short latency indicates “quickness of response” in auditory perception, cognitive recognition, and other aspects relevant to human measurement. Accordingly, it would be beneficial to have a device that is capable of accurately measuring the latency between an auditory stimulus and an individual&#39;s response. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention comprises a micro-controller based device which uses an input of speech and measures latency between stimulus and response. The device generally includes an input transducer for converting a stimulus speech sound into an electrical signal and transmitting the electrical signal to an electric circuit. A second input transducer is used to convert a response speech sound into an electrical signal and transmit the electrical signal to the electric circuit. In the preferred embodiment, the electric circuit includes a central processing unit which utilizes delay time counters to measure the length of time between signals. Each input transducer operates on a separate channel, so that the central processing unit may easily distinguish between stimulus sounds and response sounds. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1A  is a top view, illustrating a control panel used in the present invention. 
       FIG. 1B  is a back view, illustrating the input/output panel of the present invention. 
       FIG. 2  is a schematic, illustrating the present invention. 
       FIG. 3  is a transmission signal diagram, illustrating the present invention. 
       FIG. 4  is a transmission signal diagram, illustrating the present invention. 
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 REFERENCE NUMERALS IN THE DRAWINGS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 latency measuring device 
                 12 
                 input transducer 
               
               
                 14 
                 input transducer 
                 16 
                 A/D converter 
               
               
                 18 
                 central processing unit 
                 20 
                 power button 
               
               
                 22 
                 trigger level adjustment 
                 24 
                 trigger level adjustment 
               
               
                 26 
                 input level indicator 
                 28 
                 input level indicator 
               
               
                 30 
                 trigger level indicator 
                 32 
                 trigger level indicator 
               
               
                 34 
                 run/stop command button 
                 36 
                 command button LED 
               
               
                 38 
                 message screen 
                 40 
                 auto prompt rate adjustment 
               
               
                 42 
                 “get set” LED 
                 44 
                 “ready” LED 
               
               
                 46 
                 preamble LED 
                 48 
                 key word LED 
               
               
                 50 
                 response LED 
                 52 
                 gain adjustment 
               
               
                 54 
                 gain adjustment 
                 56 
                 talker microphone jack 
               
               
                 58 
                 subject microphone jack 
                 60 
                 audio in jack 
               
               
                 62 
                 earphone out jack 
                 64 
                 computer serial port 
               
               
                 66 
                 audio player 
                 68 
                 alternate response source 
               
               
                 70 
                 audio in jack 
                 72 
                 microphone one jack 
               
               
                 74 
                 microphone two jack 
                 76 
                 auxiliary in jack 
               
               
                 78 
                 preamplifier 
                 80 
                 talker input 
               
               
                 82 
                 preamplifier 
                 84 
                 response input 
               
               
                 86 
                 multiplexer 
                 88 
                 metronome rate adjustment 
               
               
                 90 
                 trigger level adjustment 
                 92 
                 trigger level adjustment 
               
               
                 94 
                 metronome 
                 96 
                 post-amplifier 
               
               
                 98 
                 Channel One output 
                 100 
                 mixing amplifier 
               
               
                 102 
                 audior recorder output 
                 104 
                 Channel Two output 
               
               
                 106 
                 earphone output 
                 108 
                 “ready” LED 
               
               
                 110 
                 “get set” LED 
                 112 
                 audio recorder start/stop 
               
               
                 114 
                 message display 
                 116 
                 program 
               
               
                 118 
                 memory 
                 120 
                 lamps 
               
               
                 122 
                 bad test command button 
                 124 
                 good test command button 
               
               
                 126 
                 automatic good/bad determiner 
                 128 
                 talker mic/audio command buttons 
               
               
                 130 
                 run/idle command buttons 
                 132 
                 audio manual/auto command buttons 
               
               
                 134 
                 select mic1/mic2 command buttons 
                 136 
                 test/command command buttons 
               
               
                 138 
                 metro/auto push buttons 
                 140 
                 metronome clock signal 
               
               
                 142 
                 white LED signal 
                 144 
                 green LED signal 
               
               
                 146 
                 metronome signal 
                 148 
                 time interval 
               
               
                 150 
                 ready signal 
                 152 
                 window signal 
               
               
                 154 
                 Channel Two trigger level 
                 156 
                 Channel Two signal 
               
               
                 158 
                 Channel One trigger level 
                 160 
                 Channel One signal 
               
               
                 162 
                 sample exceeds trigger function 
                 164 
                 Channel One trigger 
               
               
                 166 
                 short delay 
                 168 
                 short delay 
               
               
                 170 
                 Channel One end 
                 172 
                 Channel Two trigger 
               
               
                 174 
                 Channel Two start 
                 176 
                 good test end 
               
               
                 178 
                 failed test end 
                 180 
                 count/latency display buttons 
               
               
                 182 
                 transmission path 
                 184 
                 transmission path 
               
               
                   
               
             
          
         
       
     
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  shows a control panel used in the present invention, latency measuring device  10 . The preferred embodiment of the present invention generally comprises a series of delay timers which measure the “timing out” of a series of timer-clock circuits. Short timers are used to measure the differences in delay between the phonemic elements within a word. For example, the words “street” has nearly imperceptible pauses which occur between the “s” and the “tree” and the final “t”. 
   In a hearing aid evaluation, a speech discrimination test utilizes a series of words to test speech understanding. In this test, the tester says something such as “Say the word . . . street.” The ellipsis is used in the present case to denote a short pause between the word “word” and the word “street.” In this test, the subject responds with the word he or she understands. A long delay timer is set to time a delay between the preparatory phrase “say the word” and the test word “street.” Another long delay timer measures the time between the stimulus and the response of the subject. 
   It should be noted that the “test word” (in the above example, “street”) may be replaced by a picture representing the test word. For example, the test word “street” may be shown to the subject either in text form or as a picture of a street. The subject may either repeat the test word they perceive or touch a picture on an electronic touchpad. If an electronic touchpad is used, the subject may be presented with an array of pictures with the “correct answer picture” included in the array. Accordingly, the present invention may be used for many different subject populations including pediatric populations or people who cannot verbalize responses. 
   Latency measuring device  10  may be provided in many forms. For example, the device might be a stand-alone unit as illustrated in  FIG. 1A  and  FIG. 1B . Alternatively, the device may interface with a personal computer (with the control settings being made by mouse clicks, as an example). 
   The aforementioned delay timers are activated by a trigger circuit which operates on a “one-shot” type algorithm imbedded in the firmware of the circuit. The trigger circuit only responds to signals which “spike” or “flicker” above a pre-programmed target voltage. The target voltage may be set above the background noise by the tester using a sensitivity potentiometer, adjustable noise gate, or computer-setting. The trigger circuit begins the first delay timer at the onset of the speech input (in the aforementioned example, when the tester says “Say”). An amber signal light may be provided to indicate that the trigger circuit has been activated and the tester may begin the test. 
     FIG. 2  shows the signal flow in the device. Input transducer  12  transmits the speech stimulus to Channel One. From Channel One, the signal is converted from analog to digital using A/D converter  16 . Input transducer  14  transmits a speech response to Channel Two, where the signal is again converted from analog to digital using A/D converter  16 . Digital signals from Channel One and Channel Two are then transmitted to central processing unit  18  for analysis of temporal and amplitude aspects of the signals. 
   Central processing unit  18  monitors Channel One for a stimulus signal which exceeds the trigger level. Channel One is then monitored for longer time intervals. Central processing unit  18  observes Channel One for the actual cessation of trigger-level signals. Accordingly, a short delay timer rapidly samples Channel One to know when speech begins and a long delay timer samples at longer time intervals to determine the “cessation of speech.” The cessation of speech is noted by a separate timer or system clock. The system clock counts down at a set rate from an arbitrary maximum value. The current countdown value corresponding with the cessation of speech is stored in memory associated with central processing unit  18  for future comparison. 
   Central processing unit  18  then begins monitoring for the response on Channel Two. Initially, central processing unit  18  monitors Channel Two rapidly with a short delay timer. When a speech response is detected over the trigger level, central processing unit  18  stores the time of the onset of the response relative to the current value of the system clock in the memory associated with central processing unit  18 . In addition, the cessation of speech on Channel Two may also be noted using the long delay timers (as used in Channel One) when the trigger level is no longer exceeded. Central processing unit  18  may store the current value of the system clock corresponding to the cessation of speech on Channel Two in the memory. 
   If the system clock registered a value of 10000 at the cessation of speech on Channel One, and a value of 5000 when the onset of speech is observed on Channel Two, a total of 5000 time units would have elapsed between the two points. If each unit of time on the system clock corresponds to 5 microseconds, then 5000 time units equates to a real time latency of 25 milliseconds between stimulus and response. 
   In addition, the calculations may be further refined to take into account the length of time it takes for the stimulus to reach the subject&#39;s ear after leaving the speaker&#39;s mouth. For example, by entering the distance of the speaker to the subject, the device can calculate the time it takes for speech to travel from the speaker to the subject by dividing the distance between the speaker and subject by the speed of sound. Accordingly, if the speaker is 10 feet from the listener, the time it takes for speech to reach the subject is 9 milliseconds (since sound travels at approximately 1100 feet per second). This value may be subtracted from the measured latency to determine the actual latency. In the previous example, 9 milliseconds should be subtracted from the measured latency of 25 milliseconds to obtain the actual latency of 16 milliseconds. 
   An alternate embodiment of the present invention utilizes a recorded stimulus instead of a live speaker. In this case, the stimulus may be played through earphones, making the aforementioned distance factor calculation moot. 
   In some cases it may be important to control the metronome-rate or rhythm at which the speech stimulus is provided. Different color lights, such as green and white, may be employed on the device to assist the administrator of the test in controlling the rhythm. For example, the device may flash a green light at the onset of speech to indicate that the trigger level of speech has been observed by central processing unit  18 . A white light may then flash contemporaneously with or just after the stimulus word is stated by the test administrator. The green light may then flash again indicating the expectation of the onset of the response. The white light may then be configured to flash again when the subject provides the response. A variable window of time may then be set by the device or the administrator before the administrator is to provide the next stimulus. 
   In the previous example, the green lights may be either voice activated or may occur at a set metronome rate to indicate to the test administrator when and how to keep within the rhythm of the test (if rendered by live voice). For prerecorded test stimuli, the metronome rate for the delivery of the test stimuli may also be integrated with the recorded stimuli. In this case, the green and white lights may become indicators of the metronome rate of the recorded stimulus as well. Using this feature, the time intervals between and among the various stimuli and response, as well as the intervals of time between the stimuli themselves can be measured and/or varied as needed. 
   The device may also be programmed to wait on the response whether it occurs within the prescribed tempo of the test or not. Alternatively, the device may be programmed to deliver stimuli at a set rate regardless of the response. Using an “automatic” mode, whereby the metronome rate of the test is set to a “relentless” rate (where the stimulus presentation rate and inter stimulus rate are pre-set), the response may be judged as “incorrect” if it does not occur within the prescribed temporal interval between the stimuli. A red light may also flash to indicate a failed response. 
   With the general features and functionalities of the present invention in mind, the particulars of the preferred embodiment may now be considered in greater detail.  FIG. 1A  and  FIG. 1B  show a possible configuration for latency measuring device  10 . A top view of latency measuring device  10  is shown in  FIG. 1A . 
   The user of the device may user trigger level adjustment  22  to set the trigger level for the input transducer or microphone which corresponds to input one/Channel One. Another trigger level adjustment  24  is provided to set the trigger level for the input transducer to input two/Channel Two. In the present example, Channel One corresponds to the test administrator&#39;s microphone and Channel Two corresponds to the test subject&#39;s microphone. Trigger level adjustment  22  and trigger level adjustment  24  are used to calibrate the device so that the device may differentiate stimuli and responses from background noise. Accordingly, the trigger levels should be set just above background noise levels but below the normal speech sound levels. Trigger level indicator  30  and trigger level indicator  32  are provided so that the user may see where the trigger levels are set in relation to the signals transmitted via Channel Two and Channel One respectively. Input level indicator  26  and input level indicator  28  illustrate the intensity of the signal that is currently being transmitted in Channel Two and Channel One respectively. These allow the user to visually set the appropriate trigger level. 
   A series of command buttons are provided so that the user may utilize the various functions of the device. For example, run/stop command button  34  is provided for activating the latency measuring program. Each command button also has command button LED  36  which indicates the status of each function. The LEDs that appear on the command buttons are not necessarily directly controlled by the switch corresponding to the command button. For example, run/stop command button  34  is pressed to start a test run. After the processor determines that it is prepared to run the test, the LED on the button is lit. If the processor determines that something is wrong, the LED stays dark and a message is displayed in message screen  38 . Power button  20  is also provided for powering up the device. 
   The back of the device is illustrated in  FIG. 1B . Gain adjustment  52  and gain adjustment  54  are used to amplify the stimulus and response signals respectively. The amount of gain provided to each signal may be adjusted by turning the appropriate knob. A series of input jacks are also provided along the back of the device so that it can be connected to various input transducers and auxiliary sources. Talker microphone jack  56  is provided for the test administrator&#39;s microphone and subject microphone jack  58  is provided for the test subject&#39;s microphone. In addition, audio in jack  60  is provided so that a prerecorded stimulus may be played. Earphone out jack  62  may be used for connecting earphones. Earphones may be used by the subject if a prerecorded stimulus is used or if the stimulus is provided by a live test administrator. Computer serial port  64 , which may also be a USB port, is provided so that the device may interface with a personal computer for enhanced analysis and storage. 
   The schematic illustrating the circuitry of the preferred embodiment of the present invention is provided in  FIG. 2 . Input transducer  12  and input transducer  14  are the principal inputs to the device. Input transducer  12  is connected to microphone one jack  72 , which transmits signals from input transducer  12  to Channel One. Input transducer  14  is connected microphone two jack  74 , which transmits signals from input transducer  14  to Channel Two. The signals from input transducer  12  and input transducer  14  are amplified by preamplifier  78  and preamplifier  82  respectively. Preamplifiers  78  and  82  may be adjusted by gain adjustments  52  and  54  as described previously. Once amplified, the stimulus signal is transmitted to talker input  80  and the response signal is transmitted to response input  84 . If a prerecorded stimulus is used, audio player  66  may be connected to the device via audio in jack  70 . The prerecorded stimulus signal is transmitted to Channel One via talker input  80 . 
   In addition, alternate response source  68  may be provided if the test subject is to provide a nonverbal response to the stimulus. For example, the subject may be asked to press a button when the test administrator says the name of a type of animal. Alternate response source  68  may be connected to the device at auxiliary in jack  76  and the alternate response source signal is transmitted to Channel Two via response input  84 . 
   From talker input  80 , the stimulus signal is split. One signal is sent to Channel One output  98  (after amplification by post-amplifier  96 ) and the other signal is sent to multiplexer  86  via transmission path  182 . Likewise, from response input  84 , the response signal is split. One signal is sent to Channel Two output  104  (after amplification by a post-amplifier) and the other signal is sent to multiplexer  86  via transmission path  184 . In addition to being sent to Channel Two output  104 , the response signal is also transmitted to earphone output  106 . Although it is not illustrated in  FIG. 2 , the stimulus signal may also be sent to earphone output  106  in addition to being sent to Channel One output  98  similar to the response signal. 
   Multiplexer  86  also receives as its inputs metronome rate adjustment  88  (which is adjusted by the user with auto prompt rate adjustment  40  shown in  FIG. 1A ), trigger level adjustment  90  (corresponding to trigger level adjustment  22  in  FIG. 1A ), and trigger level adjustment  92  (corresponding to trigger level adjustment  24  in  FIG. 1A ). Multiplexer  86  transmits the signals to A/D converter  16  where the signals are converted from analog to digital. From A/D converter  16 , the signals are transmitted to central processing unit  18 . 
   The stimulus signals and response signals along with other information transmitted from multiplexer  86  is analyzed by central processing unit  18 . The operating instructions for central processing unit  18  are provided in object code format from program  116  which is stored in memory associated with central processing unit  18 . The analysis of the stimulus signals, response signals, and latency therebetween is performed using the method that was generally described previously. This method will be described in greater detail subsequently. 
   Central processing unit  18  utilizes memory  118  for storing relative time values for response and stimulus signals and other information needed for its analysis. Central processing unit  18  can transmit data regarding the response and stimulus signals to a personal computer via computer serial port  64  (shown in  FIG. 1B ) for further analysis or storage. Universal Serial Bus (“USB”) type connections may also be provided for increased comparability. In addition, central processing unit  18  can display information about the response and stimulus via message display  114 . Although numeric symbols are illustrated in  FIG. 2 , message display  114  may be configured to display other symbols as well. 
   Central processing unit  18  also communicates with metronome  94 . Metronome  94  may both be used as an internal clock for the device and may be used to provide rhythm signals to the test administrator or prerecorded stimulus feed to prompt the stimuli. When used as an internal clock, metronome  94  acts as an input to central processing unit  18  so that central processing unit  18  may associate the various transmitted signals with relative time. Metronome  94  may provide this rhythm information to the test administrator via “ready” LED  108  (corresponding to “ready” LED  44  in  FIG. 1A ) and “get set” LED  110  (corresponding to “get set” LED  42  in  FIG. 1A ). These lamps act to prompt the test administrator when to deliver the stimuli to the test subject. 
   Central processing unit  18  also communicates with audio player  66  or other device used to provide prerecorded stimuli. Central processing unit  18  may be configured to either start audio player  66  when the administrator selects to run the program, or it may be configured to start and stop the device providing the prerecorded stimuli at various times based on the program. Although reference has been made to a audio player in the current example, the reader will appreciate that compact discs or other mediums which are configured to play recorded sounds may also be used. 
   Central processing unit  18  may create an audio copy of the test for archive purposes. If this function is desired, central processing unit  18  operates audio recorder start/stop  112  to begin and end recording. The audio recorder records the test via a signal feed from audio recorder output  102 . Audio recorder output  102  receives its input from mixing amplifier  100 . Mixing amplifier mixes the stimulus signals received from Channel One, the response signals received from Channel Two, along with a beep tone provided by metronome  94  (where the beep tone corresponds to the prompt of “ready” LED  108 ). 
   The series of command buttons illustrated in  FIG. 1A  also interface with central processing unit  18  as illustrated in  FIG. 2 . For example, the test administrator may press bad test command button  122  if the subject responds incorrectly to the stimulus. If the subject responds correctly, the administrator may press good test command button  124 . Central processing unit  18  associates these command button inputs with the signals it receives and registers the signals in memory  118 . If the subject fails to respond to the stimulus in a set period of time, central processing unit  18  may determine that the test was failed utilizing automatic good/bad determiner  126 . In addition, central processing unit  18  interfaces with talker mic/audio command buttons  128  (which inform central processing unit  18  the input source of the stimulus), run/idle command buttons  130  (which inform central processing unit  18  when the administrator is ready to begin and pause the test), audio manual/auto command buttons  132 , select mic1/mic2 command buttons  134 , test/command command buttons  136 , metro/auto push buttons  138 , and count/latency display buttons  180  (which prompt central processing unit  18  to display count and latency information in message screen  38 ). In turn, central processing unit  18  activates lamps  120  (corresponding to various command button LEDs  36 ) and analyzes the test as prescribed by program  116 . 
   Transmission signal diagrams illustrating the device&#39;s rhythm and time keeping functions are provided in  FIGS. 3 and 4 . As illustrated in  FIG. 3 , metronome clock signal  140  oscillates periodically at a very short time interval. The metronome clock sets the minimum time between tests. White LED signal  142  causes “get set” LED  42  to flash three times in close succession. This prompts the test administrator to prepare to deliver the stimulus. After, white LED signal  142  flashes three times, green LED signal  144  causes “ready” LED  44  to flash once. “Ready” LED  44  indicates that the device is prepared for the administrator to begin the test. Metronome signal  146  represents the rhythm of metronome  94 . As shown in  FIG. 3 , metronome  94  maintains a periodic signal based on metronome clock signal  140 . 
   A sample of a test is provided in  FIG. 4  to illustrate the time-keeping and the latency-analysis functionalities of the device. Time interval  148  from  FIG. 3  is reproduced in part in  FIG. 4 . Activity on both channels is ignored until the device is “ready.” The “ready” state is indicated by the flash of “ready” LED  44  corresponding to green LED signal  144 . The device stays in the ready state for a period of time as signified by ready signal  150 . 
   The first sample on Channel One that exceeds the trigger level starts the sampling process and begins the long delay (triggers long delay timer). As illustrated in  FIG. 4 , Channel One signal  160  exceeds Channel One trigger level  158  when the administrator says the word “say.” Sample exceeds trigger function  162  illustrates the instances where the sampling process detects an “above trigger level” signal. Each sample exceeding the trigger level continues the long delay. This delay time should be long enough to cover any natural pauses during and between words. Also, window signal  152  is started when Channel One signal  160  first exceeds Channel One trigger level  158 . Window signal  152  defines a period of time for the test. Any response falling outside window signal  152  may be designated a “failed” test. 
   Also, when the long delay timer times out, the next sample on Channel One starts the short delay time (short delay  166 ). This delay time is only long enough to cover any natural pauses within a word. When the short delay times out, the relative time of the time out is registered in memory  118  for the cessation of speech on Channel One. This also causes the sampling process to switch to Channel Two. 
   The first sample on Channel Two that exceeds the trigger level starts the long delay again. As illustrated in  FIG. 4 , Channel Two signal  156  exceeds Channel Two trigger level  154  when the subject says the word “street.” After the administrator says the first “street”, any sample exceeding the trigger level continues the delay. When Channel Two signal  156  exceeds Channel Two trigger level  154 , the relative time is stored in memory  118  and associated with the onset of speech on Channel Two. When the short delay times out (short delay  168 ), the relative time of the time out is registered in memory  118  for the cessation of speech on Channel Two. This also clears the ready signal  150 . 
   If the end of the “ready” period is beyond the end of the “window” period, that test is failed and no data is saved and no calculations are made. If the “ready” period overlaps a metronome pulse, that metronome pulse is “lost” and the device waits for the next metronome pulse to restart the “ready” period. 
   The analysis and measurement of latency will now be considered in greater detail. Channel One trigger  164  illustrates the time period of “activity” on Channel One. Channel One end  170  signifies the point in time where sampling ceases on Channel One and is switched to Channel Two. Channel Two trigger  172  illustrates the time period of “activity” on Channel Two. Channel Two start  174  corresponds to the onset of speech on Channel Two and good test end  176  indicates the end of “activity” on Channel Two. The example test provided in  FIG. 4  is a “good” test because the response was provided in the “window” period. If the response does not occur prior to failed test end  178 , the test is “failed” as described previously. 
   The reader will note that the period of activity include the last short delay before cessation of speech was acknowledged. These periods of time are illustrated in  FIG. 4  as short delay  166  and short delay  168 . Accordingly, subtracting the delay time from the cessation of speech times which were registered in memory  118  gives the actual times of the last sample of each channel. 
   “Latency” may be measured from different perspectives. In one example, latency may be determined as follows: (1) subtract the time of short delay  166  from the cessation of speech time (Channel One end  170 ) registered for the cessation of speech on Channel One; (2) subtract that value from the relative time stored for the onset of speech on Channel Two (Channel Two start  174 ). This measurement of latency describes the amount of time between the cessation of the stimulus to the onset of the response. Latency may also measured from the cessation of the stimulus to the cessation of the response. This calculation may be made by subtracting the two values of cessation of speech registered for each channel since the short delay period is constant (Good test end  176  minus Channel One end  170 ). All latency times and test results may be saved in memory  118  (which may be RAM). The results may optionally be displayed on message screen  38 . 
   The preceding description contains significant detail regarding the novel aspects of the present invention. It should not be construed, however, as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. As an example, the device may be entirely implemented on a personal computer. For example, analogous measurement and analysis logic may be programmed onto the test administrator&#39;s computer. The stimulus and response signals may also be illustrated on the computer screen. This enables the test administrator to capture the stimulus and response waveforms for more detailed analysis. Such a variation would not alter the function of the invention. Thus, the scope of the invention should be fixed by the following claims, rather than by the examples given.