Modulated And An Analogue Wave example essay topic

The world around us in a standard sense such as conversation and listening is analogue - the basic principals of audio its self is a sound wave and wave forms donate an analogue signal. But with regards to modern society and its computers - we have to conform to their language - this is called binary logic, or a digital signal a series of 1's and 0's that get sequenced in code form. We generally refer to a digital signal as having a bit rate, this means that a signal holds 8 1's and 0's for every piece of audio data. Analogue to digital conversion Analogue to digital conversion is the easier form of audio conversion for example lets use the theory of a microphone plugged into a computer sound card. Let's start with a very short piece of stereo audio: [Picture Here] As noted on the timeline this represents just slightly over 100 milliseconds of this piece of audio.

From this graphic we can apparently see deviations in amplitude over time, but we must also be aware of alterations in frequency over the same time span as well which are not so intuitively obvious unless we change the view of the waveform to something like this: This graphic is of the same piece of audio represented in a plot known as a Time Energy Frequency Analysis. Now let's begin the process of converting this audio from analogue continuously moving alternating current voltages to a digital representation. First, the wave is modulated with a very high frequency carrier pulse wave: [Picture Here] The frequency of the modulation carrier must be at least twice the expected bandwidth necessary to preserve the intended audio baseband. The necessity of doing this was first proposed by Harry Nyquist in Certain Factors Affecting Telegraph Speed, Bell System Technical Journal, April 1924, and later developed by Claude Shannon in 1948 with his Mathematical Communication Model: [Picture Here] (From The Bell System Technical Journal, Vol. 27, pp. 379423, 623656, July, October, 1948. A Mathematical Theory of Communication By C.E. SHANNON) Now we will begin to increase the zoom to select an even smaller portion of time: [Picture Here] Here is the area we will now begin to look at: [Picture Here] It should begin to be obvious that we are looking at discrete windows of the original analogue audio signal. Each of these windows represents a value to be calculated by the digital converter system.

If we were to be utilizing this audio for compact disc purposes, each of these sample windows would represent 1/44,100 of a second: [Picture Here] Now we will increase the zoom function of the microscope even further: [Picture Here] We will be examining this single pulse in some more detail. During the modulation process it is important to note that it is creating artifacts. These artifacts represent themselves as duplications of the pulse train at equidistant frequency locations above and below the harmonics of the carrier wave: [Picture Here] Assuming we have been able to filter this signal effectively in the analogue domain prior to conversion, then we can proceed to analyze the pulse to determine its value. We will do this with a process known as quantization: Each of these individual samples is now analyzed by subdividing the voltage levels. One method utilized to determine the binary value of an analogue voltage is to use a binary counting system called Two's Complement: Now we must zoom into the bit level [Picture Here] And that is how an analogue signal is processed into a digital signal. To convert back to a digital signal the digital wave form is reverted back through the same process - this is done using a complex algorithm known as a binary counting circuit - this device looks at the digital signal and separated by the bit rate of a sample it looks at each segment and creates a wave profile - this wave profile is then modulated and an analogue wave is formed.

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