Hello, and welcome to this presentation, Understanding Single Sideband. In this presentation, we'll provide a general technical introduction to single sideband modulation, how it's used, and the primary variants found in radiofrequency communication systems. Single sideband is a special type of amplitude modulation, so we'll start with a quick review of AM.
AM signals are usually created using a mixer, the inputs to which are an unmodulated RF carrier and a low-frequency modulating signal. The result is a modulated RF carrier whose time domain envelope is a copy of the modulating signal. In the frequency domain, the information in an AM signal is contained in so-called sidebands that appear both above and below the carrier frequency. If we use a simple 1000 Hz sine wave signal as a modulation source, this modulating signal appears as two tones in spectrum, 1000 Hz above and 1000 Hz below the carrier. Our total signal width is twice the modulating frequency.
Human speech, on the other hand, contains very broad and non-uniform spectral content. So if we use human speech as our modulation source, our upper and lower sidebands will also be broad and non-uniform, and the total width of our AM modulated signal will be twice the width of the highest modulating frequency. Note that the information contained in the upper and lower sidebands is identical. Single sideband is a type of amplitude modulation, in which both the carrier and one of the sidebands is suppressed.
Recall that both sidebands contain the same information, so there's no loss of information if we suppress one of the sidebands. Which sideband should we suppress? There are actually two different types of single sideband. Upper sideband, which is what we see here, suppresses the lower sideband, whereas lower sideband suppresses the upper sideband. The main application for single sideband is the transmission of voice, although data is also sometimes sent using single sideband.
Sideband is also primarily used at HF, where spectral efficiency is very important, but can sometimes be found in the low VHF range as well. Single sideband offers several advantages compared to standard or double sideband AM. As we've already seen, sideband is more spectrally efficient, that is, it uses less bandwidth.
since we're eliminating approximately half of the signal. Using single sideband also allows us to put all of our power into one sideband, and not waste power transmitting the carrier and the other redundant sideband. There's also a reduction in the received noise, due to a reduction in the bandwidth of the received signal.
With regards to which sideband should be used, there's no real difference or advantage in using upper versus lower sideband. This is usually decided by mutual agreement or by convention. For example, in amateur radio, lower sideband is used for most transmissions below 9 MHz, and upper sideband is used above 9 MHz. On the transmit side, a single sideband transmitter suppresses the carrier and one sideband. On the receive side, the carrier must be reinserted before the signal can be demodulated.
This is done using a so-called carrier insertion oscillator, or beat frequency oscillator. In the ideal case, The carrier would be reinserted at the precise frequency that was originally used. Any offset from the original frequency will change the pitch of the received audio.
If this offset becomes too large, speech can become unintelligible. There are numerous ways of implementing single sideband. This presentation has been describing single sideband suppressed carrier, which is by far the most common variant.
You may also find single sideband full carrier or reduced carrier. In both of these cases, the carrier is not completely suppressed, which makes carrier reinsertion easier at the received side. The disadvantage to these schemes is that some of the transmit power goes into the carrier instead of into the sideband. There are also two somewhat specialized variants.
The first is independent sideband, in which there are two sidebands, but each carries different information. One of the early proposals for AM stereo transmission used this variant. In the vestigial sideband variant, Both sidebands are present, but one of the sidebands is reduced or limited. This variant was used for older analog television broadcasts.
Let's summarize what we've learned. Single sideband is a type of amplitude modulation, in which the carrier and one of the sidebands is suppressed. This is done for two main reasons. First, single sideband uses less spectrum than conventional amplitude modulation. And second, power that would have gone into the carrier and the redundant sideband can instead be placed into the transmitted sideband.
On the received side, the carrier must be reinserted to demodulate the signal. Differences between the frequency of the original carrier and the reinserted carrier can lead to pitch changes in the received audio, or even unintelligibility if the error is too great. This presentation discussed single sideband suppressed carrier, which is the most common form of single sideband, but other variants are possible. Some of these variants preserve either a full or a reduced carrier to facilitate carrier reinsertion.
Special variants, such as independent sideband and vestigial sideband, were often used in various types of broadcast transmissions, but are much less common today. This concludes our presentation, Understanding Single Sideband. Thanks for watching.