University:
Boston University
Course:
CAS PY 105 | Physics 1
Academic year:

2024
Views:

185

Pages:

11
Author:

Ross Raymond

Analog Transmission Overview Boston Visible University Light Slideshow Communication Title Goes Here  Audio Signals  Amplitude Modulation  Digital Sampling and Binary Representation  Experiment  Analog Modulation with VLC Transceivers 2 Visible Light Communication (VLC)  Radio spectrum is congested, visible spectrum has potential! Boston University Slideshow Title Goes Here 3 Visible Light Communication Transceiver Boston University Slideshow Title Goes Here Link 1 Link 2 4 Audio Signals Boston Audio Signals University Slideshow Title Goes Here  Acoustic Input (e.g., microphone) converts air pressure variations into an electrical signal.  ADC converts to digital data representing the sampled electrical signal.  DAC converts digital samples back into a continuous time electrical signal  Acoustic output (e.g., speaker) converts voltage back to air pressure variations.  AM Radio? FM Radio?  AM: Amplitude Modulation  FM: Frequency Modulation  Sirius Satellite Digital Radio Electrical signal is transmitted via electromagnetic signals at a specified carrier frequency. 5 Amplitude Modulation Boston Carrier Modulation: “Placement” of information signal University Slideshow Title Goes Here onto a high frequency carrier for transmission.  Carrier Demodulation: Retrieving the baseband or low frequency signal from the carrier-modulated signal In VLC Signals, optical intensity is modulated. Carrier Frequency is NOT required. Technically, the signal is modulated in THz 6 Digital Sampling Boston ToUniversity convert an analog signal into a digital representation: Slideshow Title Goes Here  Discrete Time samples  Discrete value amplitude Sampling Range Sampling Resolution Sampling period 7 Binary Representation Boston We typically consider numbers in decimal, base-10 University Slideshow Title Goes Here 39512 = 3 10000 + 9 1000 + 5 100 + 1 10 + 2 1  Binary is a numeric system with base-2  Value of any digit (or bit) can either be ‘0’ or ‘1’  Each digit is a power of 2 rather than a power of 10 16 8 4 2 1 1 1 0 1 0 1 16 + 1 8 + 0 4 + 1 2 + 0 1 = 26  A byte is a set of 8 bits 8 Digital Sampling  Digital sample values are stored in binary Boston University Slideshow Title Goes Here Binary Form Decimal Form 16 8 4 2 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 0 2 0 0 0 1 1 3 0 0 1 0 0 4 0 0 1 0 1 5 0 0 1 1 0 6 0 0 1 1 1 0 1 0 0 0 7 ??? 8 1 0 0 0 0 1 1 1 1 1 16 ??? 31 ??? 9 Nyquist Sampling Theorem Boston Reconstruction of a signal is possible when sampling University Slideshow Title Goes Here frequency is greater than twice the maximum frequency of the signal being sampled  Human hearing can recognize frequencies up to 20kHz  The typical sampling rate for audio is 44.1kHz. Why?  Digital Storage Example:  How much space is needed for 70 minutes of music with 16 bit resolution? 2 channels 16 bits sample 44.1k samples second 60 seconds minute 70 minutes = 5,927,040,000 bits 10 Experiment Boston Generate analog signals between VLC transceivers University Slideshow Title Goes Here  Observe on Oscilloscope and Spectrum Analyzer 11