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67 lines
3.8 KiB
Plaintext
67 lines
3.8 KiB
Plaintext
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"# Appendix A: Programming the Audio Beacon\n",
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"\n",
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" This appendix describes the audio beacon signal parameters. The signal is generated by the M0 microcontroller of the LPC4357 ARM, an amplifier circuit (LM4752) and a loudspeaker. The microcontroller runs a dedicated program that produces continuously this signal on one of its PWM output pins which is amplified by an op-amp circuit and made audible by the loudspeaker.\n",
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"\n",
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"The generated signal is similar to the audio signals you have used in the EE2T11 Telecommunication A practicum (Labday 4). The parameters that define the signal can be set through the command interface; in contrast to the earlier practicum, now it is possible to use an arbitrary carrier frequency, bit frequency, and repetition count.\n",
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"\n",
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"## Audio Beacon signal parameters\n",
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"\n",
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"The audio beacon transmits a binary code sequence using ``on-off keying'' (OOK). If a bit in the sequence is 0, nothing is transmitted; if the bit is 1, a modulation carrier frequency is transmitted during a certain period. The number of bits is fixed at 32.\n",
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"\n",
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"Besides the actual bit sequence (a 32 bit code word), the parameters that determine the signal are:\n",
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"\n",
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"- Modulation carrier frequency (parameter *Freq0* specified in Hz), at most 30 kHz, although this is probably beyond the specs of the loudspeaker and microphones; \n",
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"- Bit freqeuncy (parameter *Freq1* specified in Hz), this defines the rate at which the modulation carrier signal is switched on or off by bits in the code word, i.e., determines the duration of a single bit (and indirectly the bandwidth of the generated signal).\n",
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"- Repetition count of the bit sequence (parameter *Count3*, an integer), this specifies the number of bits the beacon waits before transmitting the code again. The minimum value is 32 (otherwise a new code is transmitted before the previous one is finished). The resulting repetition frequency is, \n",
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"\n",
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"<p align=\"center\">\n",
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" <img src=\"https://latex.codecogs.com/svg.latex?\\color{red}{repetition\\_frequency=\\frac{bit\\_frequency}{repetition\\_count}}\" alt=\"Equation\">\n",
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"</p>\n",
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"\n",
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"For example, with a value of 5 kHz of the bit frequency, a repition count of 2500 gives a repetition count of 2500 gives a repetition freqeuncy of 2 kHz. You will probably want to have a higher repetition freqeuncy.\n",
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"\n",
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"[An example of pulses generated by the audio beacon](figaudiocode.pdf)\n",
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"\n",
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"\n",
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"These parameters are set by sending them to the microcontroller on the car over the Bluetooth interface. The corresponding commands are described in *Controlling KITT* section in Module 1.\n",
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"\n",
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"The model *refsignal* that is used EE2T11 Telecommunications A practicum has similar parameters, but were limited to a specific set of values.\n",
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"\n",
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"The default setting of the audio beacon is a 32 bit code sequence bit-stream with:\n",
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"\n",
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"<center>\n",
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"\n",
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"| | | | | |\n",
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"|-------|--------------------|---|-------------|----------|\n",
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"| Freq0 | Carrier frequency | = | 15000 | Hz |\n",
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"| Freq1 | Bit frequency | = | 5000 | Hz |\n",
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"| Count3| Repeat counter | = | 64 | samples |\n",
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"| | Code word | = | `0x92340f0f`| (hex) |\n",
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"\n",
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"</center>\n",
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"\n",
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"There is not guarantee at all this default setting is of high quality!\n",
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"\n"
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