The basic idea of transmission is, given a bit-stream, to transform it to a signal according to some modulation scheme and to send this signal through some medium. The medium can change the shape of the signal (i.e. distort it), how much it is destorted depends on the signal properties and the medium properties. The receiver demodulates the signal into a bit-stream again. In this way, we manage to bridge distance so that systems have a way to exchange 'information'. With the simulator, which you are about to use, you can investigate:
Start the Transmission Simulator. (It is opened in a separate window).
The simulator window is organized in five 'cells', this is shown in schematic form in the below.
| Medium Properties | Signal / Modulation Properties |
| Time Signals and Bit Values | Signal Spectra |
| General Simulator Control | |
Figure 1: Layout of the Transmission Simulator
In the left-bottom area of the simulator window, you see the time signals and bit values.
As time passes by, you see the lines moving to the right.
In the same area you also can see (a bit hidden though) the bit-stream:
Note: in case mutiple 0's or 1's occur in a row, the lightblue or yellow lines are drawn longer. The bit-stream is automatically genenatedit consists of a sequence of bit-values. To have the simulator work automatically, these bit-values are chosen randomly.
As you can see, the output signal is similar to the input signal. But they are not exactly the same. To explain that, we need the signal spectra and medium properties...
Key to analyse the signals and media are harmonic signals (see Fourier). And we know that with these harmonic signals, we can built periodic signals and non-periodic signals. Also, given a signal we can calculate how this signal is built out of harmonic signals. In case of a periodic signal we can write is as a sum of harmonic signals. In case of a non-periodic signal we can write is as an integral of harmonic signals.
The frequencies of harmonic signals that are present in the signal under consideration is called the spectrum of the signal. We can make a graph of the signal's spectrum:
The spectrum of the input signal and of the output signal are shown in the 'Signal Spectra' area of the simulator.
You may recall that for a periodic signal we have a discrete spectrum. And, for a non-periodic signal we have a continuous spectrum (say a continuous line in our spectrum graph). In the simulator we see a continuous spectrum both for the input and the output signal. Suggesting that we have a non-periodic signal. Is this true? The answer is yes!!! Another question: Why? There are two ways to answer this:
For the input signal only a portion of the spectrum is shown (from 0 to 5500 Hz). (Note: some bounds had to be chosen here for building the simulator). However, the real spectrum of the input signal is somewhat larger (there are also hamonic signal with frequencies higher than 5500 Hz that contribute to building the real signal).
A medium is used to pass signals. In the analysis of a medium we mainly consider harmonic signals
(we know that these signals can be used to built other periodic and non-periodic signals).
Some harmonic signals pass a medium better than others. That is, for some frequencies the attenuation
of the harmonic signal is low, hence the amplitude of these signals is not affected much. For
other frequencies the attemuation is high, hence the amplitude of those signals are affected
a lot.
A common way of telling for which frequencies harmonic signals pass a well and which
frequencies pass poorly is to specify in the range of frequencies the harmonic signals pass well
and for which not (hence we specify a bandwidth).
In the simulator we can specify this range using the 'Passband Frequencies'.
Passband Frequencies
In this simulator, the maximum range of frequencies of harmonic signals that pass the medium well, is from 0 Hz to 5500 Hz. Recall that the spectrum of the input signal contains frequencies of harmonic signals that go beyond this limit. So, at the other end of the medium (the output signal), we do not have these harmonic signals any more in the outpt signal. This causes the output signal to be similar to the input signal, but not exactly the same.
You can change the values of these 'Passband Frequencies' and see how this effect can be made even stronger. Try the following:
Note that the purple lines in the 'Signal Spectra' area move accordingly (for details see Section 'Signal Spectra'). Moreover, you see that the spectrum of the output signal changes as well: the output signal now only consists of harmonic signals that have passed the medium well and all harmonic signals that are outside the range of passband frequencies, but that were present in the input signal, have disappeared. As a result, also the shape of the output signal has changed.
Another source of distortion is noise. An electromagnetic signal may get distorted due to external influences (for instance thunder, or other media closely located to the medium under consideration). As a result the shape of the signal sent changes.
Noise Level
To see the influence of noise, do the following
As a result you see the shape of the output signal change.
In the 'Signal Properties' area you can select the modulation scheme (i.e. select the mapping of bit values to signal elements). There simulator knows about 5 different modulation schemes:
Finally, you can change the speed at which the modulation takes place: i.e. how many bits per seconds to you want to modulated? In relation to the modulation scheme what you do is the following: in case you increase the bitrate you decrease the time-period T of the signal elements.
The simulator has some additional control elements: