Comparison of Modulation Techniques

1. May 18, 2010

xreyuk

Hi guys, I'm new to the forums and hope someone can help with this.

1. The problem statement, all variables and given/known data

Compare the modulation techniques used by 2 different applications, giving reasons for their use. Use data tables, diagrams and any other sources to help

2. Relevant equations

None

3. The attempt at a solution

I originally put the following.

I am going to compare the use of frequency modulation and amplitude modulation in radio transmissions. Frequency modulation tends to be used in the transmission of music, where as amplitude modulation tends to be used for the transfer of just voice, and talk shows.

Firstly, amplitude modulation has a much further range than FM due to its longer wavelength. It can get through obstacles such as buildings and hills much more easily than FM. The main reason amplitude modulation is used for talk shows, is because even though it has a low signal to noise ratio as it picks up noise from amplitude modulation, the quality is still good enough to understand voice. Amplitude modulation would not be good enough quality for music. It’s the same way that telephone calls are no where near as good quality as mp3 files.

Frequency modulation is a much shorter wavelength than amplitude modulation, meaning it cannot travel as far, and struggles more than an AM signal to get through obstacles due to FM being line of sight. However, FM has a much higher signal to noise ratio, because the amplitude of the signal is not being modified, preventing it from picking up noise. This higher signal to noise ratio makes it suitable enough for transmitting music at a good quality. FM also uses both the sidebands on a signal, meaning it can transmit in stereo.

I was told I need to critically compare them more, and find some information on transmission distances, bandwidths, frequencies, costs etc. If someone could point me in the right direction I'd be grateful.

However, if someone has different modulation techniques they think would be easier to compare, I'd appreciate that to.

2. May 18, 2010

vk6kro

Most of that is true enough, although you need to decide which effects are caused by the modulation and which are caused by the frequencies in use.

AM can be used at any frequency, but the band from 550 KHz to 1600 KHz is commonly used for entertainment. That is 0.55 to 1.6 MHz roughly.
Analog TV pictures are a special type of AM, and aircraft still use AM to talk to control towers at frequencies like 118 MHz.

FM can also be used at any frequency, but the frequency often used for entertainment is about 88 MHz to 108 MHz. This is roughly 100 times the frequency of the AM band above.

Audio quality is not a direct product of the type of modulation and either type is capable of very adequate audio transmission.
However, because of limited spectrum space on the AM band, regulations had to be introduced to limit bandwidth so that adjacent stations did not interfere with each other.
This does put some limits on the audio quality, but with a good receiving setup, the quality of music from AM can be still very good.
The bandwidth limits on the FM band are much more generous, so the audio quality can be better.

3. May 19, 2010

xreyuk

I decided to re-write it about DAB and FM, and this is what I have so far.

The two applications I am going to compare are DAB and Frequency Modulation. FM works by making the frequency of a carrier wave higher or lower, depending on the voltage of the original signal. DAB works in a very similar method to Freeview television in that channels are modulated using QPSK and QAM, and then multiplexed together using Coded Orthogonal Frequency Division Multiplexing (COFDM).

Firstly, one of the main comparisons is the quality of the 2 signals, and their bandwidth. DAB receivers can only tune to 1 DAB multiplexer at a time currently, but each multiplexer can be carrying multiple channels. For instance, in the UK, the BBC DAB multiplex carries 12 channels, with data rates ranging from 80Kbits/s (used for speech radio on DAB) to 192Kbit/s (used for music radio), although, only one of those services runs at the higher threshold. However, experts say that because DAB uses the out date MP2 format for music, it requires a data rate of 224Kbps to reach the same sound quality as FM radio. You can see from the table below that the UK has no stations at that quality, and using the figures for the UK, the average bit rate is 128.65Kbps (129Kbps), which is no where near good enough.

Even though the audio quality of a radio station on DAB is lower compared to FM, it does make much better use of the bandwidth available, making it more cost-effective to run and more efficient. On the EM spectrum FM is assigned from 88MHZ to 108MHZ, and DAB is assigned 217.5-230 MHz range. So FM has a 20MHZ bandwidth to use, but it can only fit 1 audio channel per frequency. DAB however, has 12.5Mhz to use, and offers around 12 channels per frequency (depending on what bit rates the channels are at). Each DAB multiplex is 1.536Mhz, as there are 1536 carriers placed 1Khz apart. Using the example before of the BBC UK DAB, 12 channels are placed onto one of these multiplexes (carriers are merged to allow higher bit rates). 1.536Mhz/12 offers 103Khz per channel, compared to FM’s 150Khz per channel. By using QPSK in DAB, the bandwidth is effectively doubled, allowing you to put 2 bits per phase change. This shows that DAB makes better use of its available bandwidth by allowing more channels, even though it has a smaller bandwidth.

4. May 19, 2010

xreyuk

This is my new effort

The two applications I am going to compare are DAB and Frequency Modulation. FM works by making the frequency of a carrier wave higher or lower, depending on the voltage of the original signal. DAB works in a very similar method to Freeview television in that channels are modulated using QPSK and QAM, and then multiplexed together using Coded Orthogonal Frequency Division Multiplexing (COFDM).

Firstly, one of the main comparisons is the quality of the 2 signals, and their bandwidth. DAB receivers can only tune to 1 DAB multiplexer at a time currently, but each multiplexer can be carrying multiple channels. For instance, in the UK, the BBC DAB multiplex carries 12 channels, with data rates ranging from 80Kbits/s (used for speech radio on DAB) to 192Kbit/s (used for music radio), although, only one of those services runs at the higher threshold. However, experts say that because DAB uses the out date MP2 format for music, it requires a data rate of 224Kbps to reach the same sound quality as FM radio. You can see from the table below that the UK has no stations at that quality, and using the figures for the UK, the average bit rate is 128.65Kbps (129Kbps), which is no where near good enough.

Even though the audio quality of a radio station on DAB is lower compared to FM, it does make much better use of the bandwidth available, making it more cost-effective to run and more efficient. On the EM spectrum FM is assigned from 88MHZ to 108MHZ, and DAB is assigned 217.5-230 MHz range. So FM has a 20MHZ bandwidth to use, but it can only fit 1 audio channel per frequency. DAB however, has 12.5 MHz to use, and offers around 12 channels per frequency (depending on what bit rates the channels are at). Each DAB multiplex is 1.536 MHz, as there are 1536 carriers placed 1 KHz apart. Using the example before of the BBC UK DAB, 12 channels are placed onto one of these multiplexes (carriers are merged to allow higher bit rates). 1.536 MHz/12 offers 103 KHz per channel, compared to FM’s 150 KHz per channel (with 0.2 MHz separation between stations). This allows 100 stations on FM, and 97 stations on DAB, very similar amounts but DAB is obviously doing it over nearly half the bandwidth. Also, by using QPSK in DAB, the bandwidth is effectively doubled, allowing you to put 2 bits per phase change. If they were to use QAM this would allow more data to be sent over the same bandwidth, meaning audio quality could increase to better than FM, over less bandwidth. This shows that DAB makes better use of its available bandwidth by allowing more channels, even though it is a lot smaller than FM’s bandwidth.

FM also suffers more interference than DAB because DAB channels are far enough apart when multiplexed, they do not interact with each other. However, FM channels close together can suffer from crosstalk.

Finally, FM has a slightly further transmission distance than DAB, and degrades slowly. DAB will stay at the same quality no matter on your distance from the transmitter, however, if you are on the fringe of a signal, instead of degrading it just cuts off and stops receiving. This is because when the signal attenuates on FM, signal quality degrades because it’s analogue, however, because DAB is digital, even though it’s attenuated the equipment can still tell the difference between a 0 and a 1. It’s only when the attenuation becomes so much that the receiver can’t tell the difference between 0 and 1 that it cuts off.