Abstract:
A learning remote “learns” both a digital code carried by an infrared operational signal as well as a timing characteristic (for example, time period) of a carrier used to modulate the operational signal. When the photodiode of the learning remote is close to the transmitter of the remote to be learned from, a low frequency saturation current is superimposed on the intelligence signal. Rather than using a fixed reference voltage to detect when the carrier component of the intelligence signal transitions, an adaptive reference voltage (VAR) is used. A comparator compares a photocurrent voltage to VAR. Because VAR is maintained between the envelope of positive peaks and the envelope of negative peaks of the photocurrent voltage despite changes in the low frequency current, the comparator detects each transition of the carrier component. A microcontroller timer determines the time between transitions output by the comparator and thereby determines the timing characteristic.

Description:
TECHNICAL FIELD 
     The present invention relates to a learning infrared amplifier circuit, and more particularly to a learning infrared amplifier circuit of a learning universal remote control device. 
     BACKGROUND 
     A typical household may have numerous electronic consumer devices that are controlled by remote control devices. A household may, for example, have multiple televisions, set-top boxes, tuners, home theatre systems, CD (compact disc) players, DVD (digital video disc) players, as so forth. Each such electronic consumer device typically is responsive to a different set of operational signals. Accordingly, each electronic consumer device is sold with its own remote control device that transmits appropriate operational signals for controlling the electronic consumer device. The coffee table in front of the television in the typical household can be covered with numerous such remote control devices to control the multitude of electronic consumer devices within the home. 
     A remote control device called a “universal remote control device” has an ability to output different types of operational signals for controlling different types of electronic consumer devices. When a user presses a key on the universal remote control device (hereafter “universal remote”), the universal remote uses an amount of information called a “codeset” to determine how to generate a corresponding operational signal that is transmitted from the universal remote. Operational signals for each different make and model of electronic consumer device are generated using a different codeset. Operational signals for a first device (for example, a television of a particular make and model) may, for example, be generated using a first codeset whereas operational signals for a second device (for example, a tuner of a particular make an model) may, for example, be generated using a second codeset. If a user wishes to control the first device, then the user first presses a device type key (for example, the “TV” device type key) on the universal remote. Thereafter, operational signals generated by the universal remote are generated using the first codeset. These operational signals control the first device (in this case, the “TV”). If the user then wishes to control the second device, then the user presses another device type key (for example, the “TUNER” device type key) on the universal remote. Thereafter, operational signals generated by the universal remote are generated using the second codeset. These operational signals control the second device (in this case, the tuner). 
     To program the universal remote to control a desired electronic consumer device, a user generally identifies the make and model of the electronic consumer device in a booklet supplied with the universal remote, finds an associated codeset number listed in the booklet, and then follows procedures to key the codeset number into the universal remote in association with the type of device with which the codeset is to be identified. Thereafter, when a key is pressed on the universal remote, the universal remote uses the identified codeset to generate operational signals. There are now thousands and thousands of various televisions and other types of electronic consumer devices that universal remotes are to be able to control. Nevertheless, universal remotes store many codesets and can accommodate the large number of devices. 
     Despite the large number of codesets typically stored in universal remotes, it is often desired to be able to control an exotic or unusual or new remote control device for which there is no codeset stored in the universal remote. Consequently, a feature sometimes called a “learning” feature has been incorporated into some universal remotes. The learning feature usually takes advantage of the fact that most remote controls transmit infrared light operational signals. An infrared detector is therefore incorporated into the learning universal remote. This infrared detector senses the infrared operational signal transmitted from another remote control device and converts the infrared operational signal into a photocurrent signal. Circuitry in the learning universal remote analyzes the photocurrent signal, and stores information about the signal. When a user later presses a key on the learning universal remote, the learning universal remote uses the stored information to generate and transmit a facsimile of the learned operational signal. Because the learning remote control device can now generate the operational signals of the other remote control device, the other remote control device is no longer needed. In this way, the learning remote control device can be made to “learn” operational signals from multiple other remote control devices so that the single learning remote control device can then be used in place of the numerous other remote control devices. 
       FIG. 1  (Prior Art) is a diagram of an infrared operational signal  1  being transmitted from a remote control device  2  to a learning remote control device  3 . The infrared operational signal  1  is received by an infrared photodiode  4  of learning remote  3 . When infrared operational signal  1  is received by infrared photodiode  4 , a reverse photodiode current is made to flow through the photodiode  4 . The reverse photodiode current carries the information being conveyed in operational signal  1 . 
       FIG. 2  (Prior Art) is a simplified diagram of the photodiode current in an example of  FIG. 1  where the operational signal  1  is mark-space modulated using RECS80 coding. The photocurrent signal includes mark times and space times. During a mark time, the photocurrent pulses on and off at a carrier frequency. The carrier frequency may range, depending on the modulation technique and codeset, from approximately 25 KHz to 500 KHz. In the illustrated example, a digital zero (BIT  0 ) is encoded as one such mark time followed by two space times. The mark time is an “on-time” and the two space times together form an “off-time”. A digital one (BIT  1 ) is encoded as a mark time followed by three space times. The mark time is an “on-time” and the three space times together form an “off-time”. The sequence of digital bits is detected in learning remote  3  and constitutes the information carried by operational signal  1 . 
     Electronic consumer device manufacturers often incorporate tuned detector circuits into electronic consumer device to detect such operational signals. The tuned detector circuit in a particular electronic consumer device may be sensitive to only a narrow range of carrier frequencies used by the remote control device supplied along with the electronic consumer device. Consequently, if a learning universal remote is to be capable of learning operational signals encoded using a wide range of different carrier frequencies, then the learning universal remote cannot use a tuned detector circuit. Rather, a detector circuit that is able to detect multiple different carrier frequencies is to be used. 
       FIG. 3  (Prior Art) is a simplified diagram of a non-tuned infrared photodetector circuit that is used in learning universal remote control device  3 . The reverse current  5  flowing through photodiode  4  depends on the intensity of the incoming infrared light of operational signal  1 . When the user wishes to teach the learning universal remote device to learn the unknown operational signal, then the learning universal remote device is placed with respect to the remote control device  2  to be learned from as shown in  FIG. 1  such that the infrared photodiode  4  of the learning universal remote device  3  can pick up the infrared operational signal  1  transmitted from the remote control device  2  to be learned from. Photocurrent  5  is made to flow through a fifty ohm resistor  6  to convert the photocurrent into a corresponding voltage signal. The voltage signal on node  7  is compared to a fixed reference voltage VREF on node  8 . If the magnitude of the photocurrent voltage signal is higher than VREF, then a digital high is output by comparator  9 . If the magnitude of the photocurrent voltage signal is lower than VREF, then a digital low is output by comparator  9 . Accordingly, each pulse of the carrier frequency modulation signal in the photocurrent signal is detected and is output as a pulse. 
     From learning attempt to learning attempt, the distance between the two remote control devices  2  and  3  can change. Devices  2  and  3  can be nose-to-nose in which case the infrared energy received by photodiode  4  can be large. Devices  2  and  3  can be several feet apart in which case the infrared energy received by photodiode  4  is small. The variations in infrared signal intensities as received gives rise to a problem. If the infrared transmitter on device  2  is too close to the infrared detector on device  3 , then the photodiode  4  may no longer recover to its original dark condition between light pulses. Photocurrent  5  may not decay fast enough because the photodiode is driven into saturation. This saturation is evident as a lower frequency saturation current that is superimposed on higher frequency (carrier frequency) intelligence signal being detected. The intelligence signal, as described above, is modulated at the carrier frequency. 
       FIG. 4  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3  when remote control devices  2  and  3  are too far apart for the learning feature to work properly. The positive peaks of the pulses  10  and  11  of the carrier frequency component of photocurrent  5  are below the reference voltage VREF. The pulses  10  and  11  are therefore not detected by comparator  9 . 
       FIG. 5  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3  when remote control devices  2  and  3  are separated by a good distance. The two photocurrent voltage pulses are large and the peaks of the pulses are off the chart. Nonetheless, the photocurrent voltage signal decays to zero voltage between pulses. The points  12 - 15  at which the photocurrent voltage signal crosses the reference voltage VREF are usable to detect the pulse times of the carrier frequency modulated pulses with adequate precision. 
       FIG. 6  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3 . The carrier frequency in the example of  FIG. 6  is, however, higher than the carrier frequency in the example of  FIGS. 4 and 5 . The points at which the photocurrent voltage signal crosses the reference voltage VREF are still usable to detect the pulse times of the carrier frequency modulated pulses. 
     If the remote control devices  2  and  3  in the higher frequency carrier example of  FIG. 6  are, however, brought close together (nose to nose), then the learning remote control device  3  does not properly detect the pulse time of the carrier frequency modulated pulses. 
       FIG. 7  (Prior Art) illustrates this situation. The intensity of the infrared operational signal  1  as received by photodiode  4  causes photodiode  4  to saturate. This gives rise to a lower frequency saturation current that is superimposed on the carrier frequency component of the photocurrent signal. Due to the low frequency saturation current, the valleys of the photocurrent voltage signal (see  FIG. 7 ) do not return to zero between pulses. Reference voltage VREF is below the envelope of the negative peaks of the photocurrent voltage signal. Comparator  9  therefore does not detect individual carrier frequency pulses. Although the learning feature of the remote  3  may be able to learn the individual bits encoded, remote  3  cannot properly learn the carrier frequency. Because the infrared detector of the electronic consumer device to be controlled is a tuned circuit, just learning the individual bits encoded does not generally enable the regeneration of the operational signal. As explained above, the receiver in the electronic consumer device to be controlled may be tuned so that it only receives operational signals having the proper carrier frequency. 
     In an attempt to solve the problem of not being able to detect the carrier frequency as illustrated in  FIG. 7 , conventional learning remote control devices may be supplied with instructions to place the two remotes a certain distance apart. Conventional instructions may also instruct the user to go through a trial and error process at different distances until the learning feature works. These cumbersome techniques are undesirable. An effective and inexpensive solution is desired. 
     SUMMARY 
     A learning infrared remote control device “learns” both a digital code carried by an infrared operational signal as well as a timing characteristic (for example, cycle time period) of a carrier signal used to modulate the on-time of the infrared operational signal. When the infrared photodiode of the learning remote is close to the transmitter of the remote to be learned from, a low frequency photodiode saturation current may be superimposed on the intelligence signal that is modulated at the higher carrier frequency. Rather than using a fixed reference voltage to detect when the photocurrent voltage transitions, an inexpensive adaptive reference signal circuit is provided. The adaptive reference signal circuit outputs an adaptive reference voltage (VAR) that is used in the detection. The adaptive reference signal circuit maintains VAR between the envelope of positive peaks of the photocurrent voltage signal and the envelope of negative peaks of the photocurrent voltage signal despite changes in the low frequency saturation current. 
     If the learning remote control device is close to the remote control device being learned from, then the low frequency saturation current may be large but the adaptive reference signal circuit nonetheless maintains VAR between the envelope of positive peaks of the photocurrent voltage signal and the envelope of negative peaks of the photocurrent voltage signal. If the learning remote control device is far away from the remote control device being learned from, then there may be no low frequency saturation current component to the photocurrent but the adaptive reference signal circuit nonetheless maintains VAR between the envelope of positive peaks of the photocurrent voltage signal and the envelope of negative peaks of the photocurrent voltage signal. 
     The photocurrent is converted into a photocurrent voltage by running the photocurrent through a resistor. A comparator compares a photocurrent voltage to the VAR. Because the VAR is maintained between the envelope of positive peaks of the photocurrent and the envelope of negative peaks of the photocurrent despite changes in the low frequency saturation current, the comparator properly detects each transition of the carrier component of the photocurrent voltage signal. A timer circuit of a microcontroller in the learning remote determines the amount of time between transitions output by the comparator and thereby determines the timing characteristic (for example, cycle time period) of the carrier signal. The determined timing characteristic information is then later usable by the learning remote control device to generate an infrared operational signal. The generated infrared operational signal is modulated using a carrier signal in the same way that the learned operational signal was. The carrier frequency of the carrier signal of the generated infrared operation signal is substantially the same as the carrier frequency of the infrared operational signal learned. 
     Other embodiments, methods, advantages and considerations are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  (Prior Art) is a diagram of an infrared operational signal  1  being transmitted from a remote control device  2  to a learning remote control device  3 . 
         FIG. 2  (Prior Art) is a diagram of a photocurrent signal that flows through the photodiode  4  in the learning remote control device of  FIG. 1  when the infrared operational signal  1  is being learned by learning remote control device  3 . The on-times of the photocurrent signal are modulated with a carrier signal. 
         FIG. 3  (Prior Art) is a simplified circuit diagram of learning remote control device  3 . 
         FIG. 4  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3  when remote control devices  2  and  3  are too far apart for the learning feature to work properly. 
         FIG. 5  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3  when remote control devices  2  and  3  are separated by a good distance. 
         FIG. 6  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3 . The carrier frequency in the example of  FIG. 6  is, however, higher than the carrier frequency in the example of  FIGS. 4 and 5 . 
         FIG. 7  (Prior Art) is a waveform diagram of a photocurrent voltage signal on node  7  in the detector of  FIG. 3  when the learning remote control device is brought closer to the learning remote control device being learned from. The carrier frequency in the example of  FIG. 7  is the same as in the example of  FIG. 6 . 
         FIG. 8  is a simplified circuit diagram of circuitry  99  within a learning remote control device  100  in accordance with a first novel aspect. 
         FIG. 9  is a waveform diagram of an operation of the circuit of  FIG. 8 . The waveform represents photocurrent voltage signal (VA) present on terminal  103  of microcontroller  102  under the same higher carrier frequency and proximity conditions of  FIG. 7 . The adaptive reference voltage signal (VAR) is between the envelope of positive peaks (VAPKP) of the photocurrent voltage signal and the envelope of negative peaks (VAPKN) of the photocurrent voltage signal. 
         FIG. 10  is another waveform diagram illustrating another example of operation of the circuit of  FIG. 8 . The learning remote control device is moved closer to the remote control device being learned from, and is then moved farther away. 
         FIG. 11  is a waveform diagram of an operation of the circuit of  FIG. 8 . The waveform represents photocurrent voltage signal (VA) present on terminal  103  of microcontroller  102  under the same higher carrier frequency and proximity conditions of  FIG. 6 . The adaptive reference voltage signal (VAR) is between the envelope of positive peaks (VAPKP) of the photocurrent voltage signal and the envelope of negative peaks (VAPKN) of the photocurrent voltage signal. 
         FIG. 12  is a simplified circuit diagram of circuitry within a learning remote control device  100  in accordance with a second novel aspect. The adaptive reference signal circuit includes both a positive peak detector circuit as well as a negative peak detector circuit. 
         FIG. 13  is a waveform diagram of an operation of the circuit of  FIG. 12 . The waveform represents photocurrent voltage signal (VA) present on terminal  103  of microcontroller  102  under the same higher carrier frequency and proximity conditions of  FIG. 7 . The adaptive reference voltage signal (VAR) is between the envelope of positive peaks (VAPKP) of the photocurrent voltage signal and the envelope of negative peaks (VAPKN) of the photocurrent voltage signal. 
         FIG. 14  is a waveform diagram of an operation of the circuit of  FIG. 8 . The waveform represents photocurrent voltage signal (VA) present on terminal  103  of microcontroller  102  under the same higher carrier frequency and proximity conditions of  FIG. 6 . The adaptive reference voltage signal (VAR) is between the envelope of positive peaks (VAPKP) of the photocurrent voltage signal and the envelope of negative peaks (VAPKN) of the photocurrent voltage signal. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 8  is a circuit diagram of circuitry  99  within a learning remote control device  100  in accordance with one novel aspect. Learning remote control device  100  is for controlling an electronic consumer device such as, for example, a television, set-top box, tuner, home theatre system, CD (compact disc) player, or DVD (digital video disc) player. 
     Circuitry  99  includes an infrared photodector  101  (for example, a photodiode) and a microcontroller integrated circuit  102 . Microcontroller integrated circuit  102  includes a terminal  103 . A first terminal of photodiode  101  is coupled to receive a supply voltage VCC whereas a second terminal of photodiode  101  is coupled to terminal  103 . An infrared operational remote control signal  104  is incident on photodiode  101 . As the intensity of infrared light of operational signal  104  changes, the magnitude of a photocurrent  105  flowing through photodiode  101  changes. Photocurrent  105  flows into microcontroller integrated circuit  102  through terminal  103  and through a resistor  106  to ground potential. Resistor  106  is used to convert photocurrent  105  into a corresponding photocurrent voltage signal (VA). 
     The noninverting input lead  107  of a comparator  108  is coupled to terminal  103  so that the photocurrent voltage signal (VA) is received onto the noninverting input lead  107 . An adaptive reference signal circuit  109  has an input lead  110  and an output lead  111 . Input lead  110  is coupled to terminal  103  so that input lead  110  receives the photocurrent voltage signal (VA). Output lead  111  is coupled to an inverting input lead  112  of comparator  108  so that an adaptive reference voltage signal (VAR)  113  output by adaptive reference signal circuit  109  is supplied onto the inverting input lead  112  of comparator  108 . An output lead  114  of comparator  108  is coupled to an input lead of a timer circuit  115  of microcontroller integrated circuit  102 . Timer circuit  115  may, for example, be a general purpose timer of microcontroller  102 . In the present example, adaptive reference signal circuit  109  includes a positive peak detector circuit  116  and a voltage dividing circuit  117 . Because the two resistors  120  and  121  that make up the voltage dividing circuit  117  have equal resistances, the voltage dividing circuit  117  outputs a voltage that is half the magnitude of the voltage it receives from positive peak detector circuit  116 . Positive peak detector circuit  116  includes a positive peak detector  118  and an amplifier circuit  119  that is connected as a voltage follower. In operation, adaptive reference signal circuit  109  generates the adaptive reference voltage signal (VAR)  113  so that the magnitude of the adaptive reference voltage signal is the positive peak voltage (VAPKP) divided by two. 
       FIG. 9  is a waveform diagram of an operation of the circuit of  FIG. 8 . Waveform  200  represents photocurrent voltage signal (VA) present on terminal  103  of microcontroller  102 . Photocurrent voltage signal (VA) has a zero millivolt amplitude from time zero until 8.0 microseconds in the diagram. This amount of time represents the end of the space times (an “off-time”) within operational signal  104 .  FIG. 9  illustrates fourteen pulses that occur between time 8.0 microseconds to time 40.0 microseconds. These pulses in the photocurrent voltage signal are due to pulses in photocurrent  105 . The pulses have a carrier frequency rate of approximately 500 KHz. The fourteen pulses illustrated in  FIG. 9  represent the beginning of a mark time (an “on-time”) within operational signal  104 . 
     The waveform of  FIG. 9  was generated when photodiode  101  of the learning remote control device  100  was placed close to the remote control device from which the carrier frequency is to be learned. The magnitude of the carrier frequency component of the signal (that varies between the envelope VAPKP of the positive peaks and the envelope VAPKN of the negative peaks) is therefore relatively large. In the present example, the negative peak envelope at time 36.0 microseconds is approximately 30 millivolts and the positive peak envelope at time 36.0 is approximately 130 millivolts. The magnitude of the carrier frequency component is therefore said to be approximately 100 millivolts. Due to the high intensity of infrared light received onto photodiode  101 , a lower frequency saturation current voltage signal is superimposed on the higher carrier frequency signal. This lower frequency saturation current voltage signal is also represented by the waveform labeled VAPKN in  FIG. 9 . At time 36.0 microseconds, the lower frequency saturation current voltage signal has a magnitude of approximately 30.0 millivolts. The negative peak value (30 mV) at time 36.0 microseconds is at least twenty percent of the positive peak value (130 mV). 
     Rather than supplying a constant voltage reference signal VREF onto non-inverting input lead  112  as in the conventional circuit of  FIG. 3 , adaptive reference signal circuit  109  generates adaptive reference voltage signal (VAR)  113  that is between the negative peak envelope VAPKN and the positive peak envelope VAPKP. Because the photocurrent voltage signal on noninverting input lead  107  of comparator  108  crosses the adaptive reference voltage (VAR) signal twice during each period of the carrier frequency, the DATA OUT signal output by comparator  108  onto output lead  114  also transitions twice. Timer circuit  115  times the amount of time between successive rising edges of the DATA OUT signal, and in this way determines the cycle time period of the carrier frequency component of photocurrent  105 . 
     In the embodiment of  FIG. 8 , adaptive reference signal circuit  109  works by generating a signal that approximates the envelope VAPKP of the positive peaks of the photocurrent voltage signal. A positive peak detector involving a diode and a leaky capacitor can be employed as peak detector  118 . The peak detect signal is supplied to voltage follower  119  that supplies the peak detector signal VAPKP to resistive voltage divider circuit  117 . As explained above, voltage divider circuit  117  divides the positive peak detect signal VAPKP in half. The positive peak detect signal VAPKP divided by two is therefore the adaptive reference voltage signal VAR that is supplied to the noninverting input lead  112  of comparator  108 . Accordingly, as the positive peak detect signal VAPKP increases in voltage amplitude, so too does the adaptive reference voltage signal VAR. As the positive peak detect signal VAPKP decreases, so too does the adaptive reference voltage signal VAR. 
       FIG. 10  is another waveform diagram illustrating another example of operation of the circuit  99  of  FIG. 8 . The learning remote control device is moved closer to the remote control device being learned from in the time period from zero to fifty microseconds, and is then moved back away again in the time period from fifty microseconds to one hundred microseconds. Note that the adaptive reference voltage signal VAR has a magnitude that is half of the positive peak detect signal. Despite that fact that the envelope of the negative peaks of the photocurrent voltage signal increases up until time 50 microseconds and then decreases to time 100 microseconds, the adaptive reference voltage signal VAR  113  is always between the positive peak envelope VAPKP and the envelope of the negative peaks. The upper waveform in  FIG. 10  illustrates the resulting DATA OUT signal that is output by comparator  108 . The duty cycle of the DATA OUT signal changes, but the period of the DATA OUT signal remains substantially constant. Because timer circuit  115  measures the period of the DATA OUT signal, the timer circuit  115  can measure the period of the carrier frequency component of the photocurrent signal  105  at any time in  FIG. 10  and the result will be a fairly constant measurement of the period. (The waveform of the adaptive reference voltage VAR in  FIG. 9  is for peak detector  118  being an ideal peak detector, whereas the waveform of the adaptive reference voltage VAR in  FIG. 10  is for peak detector  118  being an actual peak detector. The slight differences in the waveforms of the VAR signal in  FIGS. 9 and 10  are due to the different peak detector circuits.) 
       FIG. 11  is a waveform diagram that illustrates an operation of the circuit  99  of  FIG. 8  where the learning remote control device  100  is farther away from the transmitting remote control device than in the example of  FIG. 9 , but where the carrier frequency and other operating parameters are otherwise identical to those of  FIG. 9 . Due to the increased distance between the transmitting remote control device and the learning remote control device, the magnitude of the carrier frequency component of the photocurrent voltage signal is less. In the example of  FIG. 11 , the carrier frequency component varies between approximately 25 millivolts and zero millivolts. The magnitude of the carrier frequency component is therefore said to be approximately 25 millivolts. The envelope of the negative peaks of the photocurrent voltage signal is very close of ground potential throughout the amount of time illustrated in  FIG. 11 . The magnitude of the low frequency component due to photodiode saturation is therefore approximately zero. Because the adaptive reference signal circuit  109  outputs an adaptive reference voltage signal VAR  113  that has a magnitude that is half of the magnitude of the positive peak envelope VAPKP of the photocurrent voltage signal, the adaptive reference voltage signal VAR  113  during the mark time in  FIG. 11  has a magnitude of approximately 12 millivolts. Accordingly, despite the fact that there is essentially no low frequency saturation current component to photocurrent  105 , the adaptive reference voltage signal VAR  113  supplied to the inverting input lead  112  of comparator  108  is nonetheless between the positive peak envelope VAPKP of the photocurrent voltage signal and the negative peak envelope of the photocurrent voltage signal. The photocurrent voltage signal crosses the adaptive reference voltage signal VAR  113  twice during each period of the carrier frequency component of the photocurrent  105 . Comparator  108  therefore outputs a pulse each period as in the case of  FIG. 9 . Timer circuit  115  measures the time between successive rising edges of the DATA OUT signal, and therefore properly measures the period of the carrier frequency component of the photocurrent signal  105 . The processor of microcontroller  102  controls and reads timer circuit  115 , thereby obtaining information about the carrier frequency component. This information (for example, a period time) is then usable to generate operational signals when a key is later pressed on the learning remote control device. The operational signal generated by the learning remote control device  100  mimics the operational signal learned in that the generated operational signal has the same or substantially the same carrier frequency as the operational signal learned and in that the generated operational signal can control the electronic consumer device in the same way that the learned operational signal did. 
     The manufacture of remote control devices can be extremely cost competitive. Reducing the manufacturing cost of a remote control device by even a few cents can make a great difference in the ability to sell the remote control device in the market. The circuit  99  of  FIG. 8  advantageously includes only one discrete component (photodiode  101 ) external to microcontroller integrated circuit  102 . Avoiding the use of additional discrete components reduces manufacturing cost of the learning remote control device. 
       FIG. 12  illustrates another embodiment of circuit  99 . Adaptive reference signal circuit  109  includes an additional negative peak detector  122  and an additional voltage follower amplifier  123 . Rather than maintaining adaptive reference voltage VAR  113  halfway between the positive peak envelope signal VAPKP and ground potential as in the embodiment of  FIG. 8 , the voltage divider circuit  117  maintains adaptive reference voltage VAR  113  halfway between the positive peak envelope signal VAPKP and the negative peak envelope signal VAPKN. 
       FIG. 13  illustrates operation of the embodiment of  FIG. 12  when it is receiving the same photocurrent signal as in the situation described above in connection with  FIG. 9 . Whereas in the example of  FIG. 9  the adaptive reference voltage signal VAR  113  is halfway between the positive peak envelope VAPKP and ground potential, in the example of  FIG. 13  the adaptive reference voltage signal VAR  113  is halfway between the positive peak envelope signal VAPKP and the negative peak envelope signal VAPKN. 
       FIG. 14  illustrates operation of the embodiment of  FIG. 12  when it is receiving the same photocurrent signal  105  as in the situation described above in connection with  FIG. 11 . Because there is essentially no low frequency saturation current component in photocurrent  105  in the examples of  FIGS. 11 and 14 , the adaptive reference voltage signals VAR in  FIGS. 11 and 14  are essentially identical. 
     Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Although a circuit is described above that uses a voltage comparator to compare a photocurrent voltage signal to an adaptive reference voltage, a comparator or differential amplifier circuit can be employed that compares currents. An adaptive reference current can be compared directly to the photodiode reverse current into order to detect transitions at the carrier frequency. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.