Signals

1. Transmission media work by conducting energy along a physical path. So a data stream of 1s and 0s must be turned into energy in the form of electromagnetic signals.

2. Analog vs Digital
   • Analog refers to something that is continuous -- a set of specific points of data and all possible points between. It is a continuous wave form that changes smoothly over time. As the wave moves from value A to value B, it passes through and includes an infinite number of values along its path.

   • Digital refers to something that is discrete -- a set of specific points of data with no other points in between. A digital signal therefore changes instantaneously. It can only have a limited number of defined values, often as simple as 1 and 0.

3. Period and Aperiodic Signals
   • A signal is a periodic signal if it completes a pattern within a measurable time frame, called a period, and repeats that pattern over identical subsequent periods. The completion of one full pattern is called a cycle. A period is defined as the amount of time (expressed in seconds) required to complete one full cycle.

   • An aperiodic, or nonperiodic, signal changes constantly without exhibiting a pattern or cycle that repeats over time.

4. Analog Signals
   • A simple analog signal, or a sine wave, cannot be decomposed into simpler signals.
     • The amplitude of a signal is the value of the signal at any point on the wave. It is equal to the vertical distance from a given point on the wave form to the horizontal axis.

     • Period refers to the amount of time, in seconds, a signal needs to complete a cycle. Frequency refers to the number of periods in one second. The frequency of a signal is the number of cycles per second. Frequency is expressed in hertz (Hz). It is a general measurement of the rate of change of a signal with respect to time. Mathematically, the relationship between frequency and period is that they are the multiplicative inverse of each other, Frequency = 1/Period.

     • The term phase describes the position of the waveform relative to time zero. Phase is measured in degrees or radians (360 degrees or 2(3.14) radians.

     • Changes in all three attributes can be introduced into a signal and controlled electronically.

     • Frequency-domain plots show the relationship between amplitude and frequency.

   • A composite analog signal is composed of multiple sine waves.
     • The frequency spectrum of a signal is the collection of all the component frequencies it contains and is shown using a frequency-domain graph.

     • The bandwidth of a signal is the width of the frequency spectrum. It is the range of component frequencies, and frequency spectrum refers to the elements within that range.

5. Digital signals
   • Most digital signals are aperiodic and, thus, period or frequency is not appropriate.

   • The bit interval is the time required to send one single bit.

   • The bit rate is the number of bit intervals per second. It is the number of bits sent in one second, usually expressed in bits per second (bps). Mathematically, the bit interval = 1/(bit rate).

   • A digital signal can be decomposed into an infinite number of simple sine waves called harmonics, each with a different amplitude, frequency, and phase.

   • Although the frequency spectrum of a digital signal contains an infinite number of frequencies with different amplitudes, if we send only those components whose amplitudes ae significant (above an acceptable threshold), we can still recreate the digital signal with reasonable accuracy at the receiver (minimum distortion). We call this part of the infinite spectrum the significant spectrum, and its bandwidth the significant bandwidth.