Abstract:
A method for detecting a media device comprises generating a waveform on a bias node of a demonstration system. The waveform has a fundamental frequency that is greater than a maximum frequency of a media frequency range of the demonstration system. The waveform is rectified to produce a detection signal. An impedance between the bias node and a ground is modified in response to a coupling of the media device to the bias node. An output signal changes state in response to a change in the detection signal due to the modification of the impedance.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to electronic systems. More specifically, the invention relates to the detection of a media device when connected to an electronic demonstration system. 
     BACKGROUND 
     Audio demonstration systems are commonly found in retail environments to demonstrate to a prospective consumer the qualities of the system. Such systems typically have a sample of audio content (e.g a playlist) chosen to demonstrate the system capabilities (e.g. audio range, bass or treble response) and to entice the consumer to purchase the system. These demonstration systems are also capable of accepting an external audio device so that that consumer can play the audio content they are most familiar with and to hear how the system responds to their favorite audio device. When the consumer plugs their device into the system, the system will automatically detect the device and play content from it. 
     U.S. Pat. No. 6,321,278 discloses a computer system audio circuit that is connected to a sound device, a combination game/MIDI/line-in/line-out/headphone jack, and internal loudspeaker. The circuit routes audio signals to a headphone, when the headphones are plugged into the headphone connector of the combination jack. In addition, the audio circuit switches and does not route audio signals to the internal loudspeaker nor external loudspeakers when the headphones are plugged in. Furthermore, if the external loudspeakers are plugged into the line-out connector of the combination jack, the circuit provides audio to the external loudspeakers but not the internal loudspeaker. This system detects the connection of a headset using an audio switch to change the outputs used to play back content (e.g. from internal speaker to a headset), but does not teach a solution to switching the source of audio content. Thus, there remains a requirement for a demonstration system to switch audio sources upon detection of a consumer audio device in a safe and reliable manner. 
     BRIEF SUMMARY 
     In one aspect, a method for detecting a media device includes generating a waveform on a bias node of a demonstration system. The waveform has a fundamental frequency that is greater than a maximum frequency of a media frequency range of the demonstration system. The waveform is rectified to produce a detection signal. An impedance is modified between the bias node and a ground in response to a coupling of the media device to the bias node. A state of an output signal is changed in response to a change in the detection signal due to the modification of the impedance. 
     Embodiments may include one of the following features, or any combination thereof. At least one spurious signal is removed from the output signal. A one or more source inputs of the demonstration system is connected to the media device in response to the switching of the comparator output signal. The waveform is monotone. The waveform comprises a plurality of tones, each tone having a frequency that is greater than a maximum frequency of a media frequency range of the demonstration system. Modifying an impedance includes decreasing an impedance. The coupling of the media device to the bias node is a capacitive coupling. The coupling of the media device to the bias node is a lower impedance coupling than a coupling of a waveform generator to the bias node. The waveform is buffered prior to rectifying the waveform. 
     In another aspect, a media device detection system includes an impedance module configured to modify a bias impedance between a bias node and a ground when a media device is coupled to the bias node. A waveform generator is in electrical communication with a bias node of a demonstration system and is configured to generate a waveform having a fundamental frequency greater than a maximum frequency of a media frequency range of the demonstration system. A rectifier is in electrical communication with the bias node. The rectifier is configured to produce a detection signal from the waveform responsive to the modification of the impedance. A comparator is in electrical communication with the rectifier. The comparator is configured to switch a state of an output signal when the detection signal is less than a threshold of the comparator. 
     Embodiments may include one of the above and/or below features, or any combination thereof. A filter is in electrical communication with the comparator and is configured to remove at least one spurious signal from the output of the comparator. A filter is in electrical communication with the rectifier and is configured to remove at least one spurious signal from the detection signal. A media switch is configured to connect one or more source inputs of the demonstration system to the media device in response to the switching of the comparator. The media frequency range is an audio range. The media frequency range is a video range. The comparator includes a hysteresis voltage greater than a ripple voltage of the detection signal. Only one channel of the media device is in electrical communication with the bias node. The impedance module further comprises a first capacitor in electrical communication with the bias node and the rectifier. A first resistor is in electrical communication with the bias node and the waveform generator. A second capacitor is in electrical communication with the bias node and an electrical connector capable of receiving the media device. A buffer is in electrical communication with the bias node and the rectifier. 
     In another aspect, a method for detecting a media device includes generating a waveform on a bias node of a demonstration system. The waveform has a fundamental frequency that is greater than a maximum frequency of a media frequency range of the demonstration system. The waveform is rectified to produce a detection signal. An impedance is decreased between the bias node and a ground in response to a coupling of the media device to the bias node. A state of an output signal is changed in response to a change in the detection signal due to the decrease of the impedance. At least one spurious signal is removed from the output signal. A one or more source inputs of the demonstration system is connected to the media device in response to the switching of the comparator output signal. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a functional block diagram view of an embodiment of a media device detection system according to the present invention. 
         FIG. 2  is a schematic view of an embodiment of an impedance module according to the present invention. 
         FIG. 3  is a schematic view of an embodiment of a buffer according to the present invention. 
         FIG. 4  is a schematic view of an embodiment of a rectifier according to the present invention. 
         FIG. 5  is a schematic view of an embodiment of a comparator according to the present invention. 
         FIG. 6  is a schematic view of an embodiment of a transient filter according to the present invention. 
         FIG. 7  is a plot of the output of a waveform generator. 
         FIG. 8  is a plot of the input and output waveforms of the buffer shown in  FIG. 3 . 
         FIG. 9  is a plot of the output of the rectifier shown in  FIG. 4 . 
         FIG. 10  is a plot of the input and output waveforms of the comparator shown in  FIG. 5  and the filtered output from the filter shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of systems described herein provide for the automatic detection of a consumer media device in a safe and reliable manner, and the change of a media source used by a demonstration system from preprogrammed content to that provided by the consumer. Accordingly, the consumer can experience the qualities of the system with their preferred media content. Preprogrammed content includes any content different than that provided by the consumer and used to demonstrate the qualities of the demonstration system. Examples of preprogrammed content include stored playlists on the demonstrations system and streaming media from the Internet. 
     In one example, the consumer media device is an audio entertainment device (e.g. an MP3 player). In another example, the consumer media device is a video device used for playing movies or streaming video. The scope of this disclosure is envisioned to include the detection of any device, whether provided by the consumer or not, that includes media content capable of being demonstrated on the demonstration system. 
     The auto-detection of a media device must not cause harm to the device. Accordingly, the embodiments described herein avoid exposing the media device to direct current (DC) levels. The auto-detection of the media device detects the presence of the device rather than the signal emanating from it. More specifically, the media device is detected by sensing a change in impedance caused by the connection of the media device to the demonstration system. Advantageously, this approach provides reliable device detection even if the device is not playing media content or has periods during which the device output signal is very weak. In contrast, relying on the signal from the device to detect the presence of the device can cause sporadic switching between the preprogrammed content from the demonstration system and that of the media device. In the various embodiments described herein, the reliable detection of the media device is not adversely affected by excessive media device volume or output signal. 
       FIG. 1  illustrates an embodiment  10  of the functional blocks of the media device auto-detection system. An out-of-band waveform generator  12  generates a signal with substantially all of the frequency components being above the range of a media content generated by either a media source in the demonstration system or the consumer media device. Specifically, the fundamental frequency (e.g. first harmonic) of the generated waveform is above a maximum frequency of the frequency range of the media content. In one embodiment, the generated waveform is monotonic (e.g. a pure sinusoid). In another embodiment, the generated waveform is a damped square wave, shaped to remove lower frequencies that can mix with the media content and degrade the fidelity of the output of the demonstration system. In addition, the use of a higher frequency waveform from the waveform generator  12  substantially eliminates direct current (DC) signals from feeding back to the consumer media device and potentially harming the device. 
     The output  13  of the waveform generator  12  connects to an impedance module  14  with an output  16 . In one embodiment, the left audio channel  20  of a consumer media device electrically communicates with the impedance module  14 , thereby modifying the Thevenin equivalent impedance of the impedance module  14  as measured between a bias node within the impedance module  14  (shown in  FIG. 2 ) and a ground. In another embodiment, the right audio channel  22  of a consumer media device electrically communicates with the impedance module  14 . In another embodiment, the media device provides a video channel in electrical communication with the impedance module  14 , wherein the out-of-band waveform generator  12  generates a waveform above the video frequency range to be viewed on the demonstration system. 
     Optionally, the impedance module output  16  is buffered by a buffer  24 . The buffer  24  outputs a signal  26  to a rectifier  28 . The buffer  24  isolates the impedance at the output  16  from the rectifier  28 . The rectifier  28  outputs a detection signal  30  whose DC level shifts depending on whether and how a consumer media device is connected to the impedance module  14 . For example, if the left channel  20  of the consumer media device is referenced to ground, the waveform at the output  16  will have a root mean square (RMS) value that will be closer to ground when the media device is connected to the media device auto-detection system  10 . In a preferred embodiment, the impedance of the consumer media device is lower than the impedance of the out-of-band waveform generator  12 . 
     The comparator  34  compares the output  30  of the rectifier  28  to a threshold  32 . In one embodiment, a consumer media device connected to the media device auto-detection system  10  reduces the impedance of the impedance module  14 , thereby decreasing the RMS value of the waveform at the output  16  and the voltage of the detection signal  30 . When the detection signal  30  is decreased to less than the threshold  32  the comparator output  36  transitions to high thus providing an enabling signal. The output  36  is filtered by a transient (e.g. low pass) filter  38  that removes spurious signals due to excessive signal voltage from the consumer media device and provides a stable enable signal  40  to a media switch  41 . In one embodiment, the transient filter  38  filters the detection signal  30  before it is compared against the threshold  32  by the comparator  34 . 
     When the enable signal  40  is low, indicating that there is no consumer media device connected to the media device auto-detection system  10 , the media switch  41  connects the left and right channels  42  and  44  respectively of a preprogrammed content to the left and right channel outputs  48  and  50  respectively. For example, when the consumer media device is an audio device, the left and right channel outputs  48  and  50  respectively further connect to a speaker in one example. When the consumer media device is a video device, the left channel output  48  connects to a speaker and the right channel output  50  connects to a video monitor. In another example, the left channel output  48  is a video channel, whereby the out-of-band waveform generator  12  generates a waveform with a fundamental frequency above the range of frequencies used by the video channel. Upon receiving a positive enable signal  40 , indicating that a consumer media device is connected to the media device auto-detection system  10 , the media switch connects the left and right channels  20  and  22  respectively of the consumer media device to the left and right channel outputs  48  and  50  respectively. 
       FIG. 2  shows an embodiment of the impedance module  14  as shown in  FIG. 1 . The output  13  of the waveform generator  12  electrically connects to a bias node  64  through a resistor  66 . The left channel output  20  of the consumer media device connects to the bias node  64  through a capacitor  70 . The bias node  64  connects to an output  16  through a capacitor  72 . In a preferred embodiment, the resistor  66  is 10 KΩ, the capacitor  70  is 0.01 uF, the capacitor  72  is 0.01 uF and the media device is an iPod®. 
     In another embodiment, the impedance module  14  connects to a right channel output  22  of the consumer media device. In another embodiment, the impedance module  14  connects to a video channel or a consumer media device. In addition to audio and video content, it is envisioned that that the concepts disclosed herein apply to other frequency ranges whereby the out-of-band waveform generator  12  provides a waveform with a fundamental frequency above the frequency range. 
     Coupling of a ground referenced media device to the impedance module  14  reduces the RMS voltage of the waveform on the bias node  64 . In a preferred embodiment, an impedance of the media device measured between the bias node  64  and ground is substantially less than an impedance of the waveform generator  12  measured between the bias node  64  and ground. In another embodiment, the media device is reference to the supply voltage and results in an increase in the voltage of the bias node  64  when the media device is coupled to the impedance module  14 . With reference to  FIG. 1 , the output of the rectifier  28  connects the positive input of the comparator  34  and the threshold connects to the negative input of the comparator  34  when used with a media device that is referenced to the supply voltage instead of ground. 
       FIG. 3  shows an embodiment  24  of the buffer as shown in  FIG. 1 , used to isolate the impedance of the impedance module  14  from the rectifier  28 . The buffer  24  includes an operational amplifier (op-amp)  82  with an output  26 . A feedback resistor  86  connects the output  26  to the negative input  88  of the op-amp  82 . A resistor  90  is connected between the negative input  88  and a ground  92 . The ratio of the resistor  86  and the resistor  90  establishes the gain or amplification of the buffer  24 . In a preferred embodiment, resistors  86  and  90  are 9.09 KΩ and 10 KΩ respectively, thus providing a slight attenuation of the output  16  of the impedance module  14 . In another embodiment, the buffer  24  is a simple voltage follower with no attenuation. In another embodiment, the buffer  24  amplifies the output  16 . A resistor  94  is connected between the supply voltage  96  and the output  16 . A resistor  98  is connected between the output  16  and the ground  92 . The output  16  connects to a positive input of the op-amp  82  and has a DC operating point established at substantially half of the supply voltage  96 . In a preferred embodiment, resistors  94  and  98  are each 20 KΩ. 
       FIG. 4  describes a preferred embodiment of the rectifier  28  in  FIG. 1 . The output  26  of the buffer  24  capacitively couples to a summing node  104  through a capacitor  102 . The summing node connects to the anode of a diode  110 , whose cathode connects to an output forming a detection signal  30 . The summing node also connects to a cathode of diode  106 , whose anode connects to the ground  92 . Current will conduct through the diode  110  when the summing node  104  has a positive voltage (plus a diode threshold) relative to the voltage of the detection signal  30 . Current will conduct through the diode  106  when the summing node  104  has a negative voltage (minus a diode threshold) relative to the ground  92 . A capacitor  114  connects between the detection signal  30  and ground  92 . A resistor  116  also connects to the detection signal  30  and ground  92 . The capacitor  114  and the resistor  116  form a time constant which serves to reduce a ripple voltage on the detection signal  30 . A ripple voltage is the difference between the maximum and minimum values of the detection signal  30 . In a preferred embodiment, the capacitor  102  is 0.01 uF (e.g. same value as the capacitor  72 ), the capacitor  114  is 0.01 uF and the resistor  116  is 20 KΩ. 
       FIG. 5  describes a preferred embodiment of the comparator  34  in  FIG. 1 . The comparator  34  includes an op-amp  122  with an output  36 . A feedback resistor  126  connects the output  36  to a positive input  128  of the op-amp  122 . A resistor  130  is connected between the positive input  128  a ground  92 . A resistor  134  is connected between the positive input  128  and the supply voltage  96 . The detector signal  30  from the output of the rectifier connects to the negative input of the op-amp  122 . In a preferred embodiment, the resistors  126 ,  130  and  134  are 61.9 KΩ, 27.4 KΩ and 12.1 KΩ respectively. The comparator  34  is configured to be a Schmitt trigger with hysteresis. Preferentially, the hysteresis value is larger than the worst-case ripple of the detection signal  30 . The worst-case ripple is defined by the voltage variation of the detection signal  30  when a worst-case consumer device is either connected or disconnected from the media device auto-detection system  10 . 
       FIG. 6  shows an embodiment of a transient filter  38  as shown in  FIG. 1 . The output  36  of the comparator  34  connects the anode of a diode  142 . The cathode of the diode  142  is an enable signal  40  that enables the switching of inputs on a media switch  41 . A capacitor  146  connects between the enable signal  40  and ground  92 . A resistor  148  connects between the enable signal  40  and ground  92 . The diode  142  ensures negative transients do not propagate to the media switch  41 . The capacitor  146  and the resistor  148  removes positive spurious transients, for example due to excessive media device volume or output signal. 
     An embodiment of the media device auto-detection system  10  was tested with successful results as shown in  FIGS. 7 ,  8 ,  9  and  10 .  FIG. 7  shows the output  13  of the out-of-band waveform generator as a 45 KHz square wave, well above the typical audio range of 20 Hz to 20 KHz.  FIG. 8  shows the impedance module output  16  with an iPod connected to the impedance module  14 . The output  16  is an input the buffer  24 .  FIG. 8  also shows the buffer output  26  as an amplified signal.  FIG. 9  shows the detection signal  30  relative to ground  92  with an iPod connected to the impedance module  14 . The detection signal  30  has a ripple voltage  174 , which preferentially is less than the hysteresis of the comparator  34 . 
       FIG. 10  illustrates the behavior of the media device auto-detection system  10  while an iPod is connected during the connection phase  182 , and after the iPod is disconnected during the disconnection phase  184 . When a media device is connected to the media device auto-detection system  10  and the media device volume level is very high, large transient voltages exist on the detection signal  186 , which cause the comparator  34  to falsely trip. The large transient voltages effectively increase the effective ripple voltage, (shown as  174  in  FIG. 9 ), causing a transient range  188 . The transient range  188  exceeds the hysteresis of the comparator  34  set by a lower threshold  190  of 440 mV and an upper threshold  192  of 1.32V. The transient range  188  causes false transitions  194  on the comparator output  36  when the detection signal  186  is greater than the upper threshold  192  of the comparator  34 . The false transitions  194  caused by excessive transient voltages are removed by the transient filter  38  to produce a stable enable signal  196 . 
     When the media device is removed from the media device auto-detection system  10  and the disconnection phase  184  is entered, the detection signal  198  rises above the upper threshold  192  of the comparator  34 . In addition, the detection signal  198  no longer contains excessive transient voltages because the media device is removed. The detection signal  198  results in a low value on the output  36  of the comparator, which is subsequently filtered by the transient filter  38  to produce a low value enable signal  202 . The time constant formed by the capacitor  146  and the resistor  148  in the transient filter  38  causes a smooth decay  204  of the enable signal  196  over 200 msec during the transition between the connection phase  182  and the disconnection phase  184 . 
     A number of implementation have been described. Nevertheless, it will be understood that additional modifications may be made without departing from the scope of the inventive concepts described herein, and, accordingly, other embodiments are within the scope of the following claims: