10/13/2015 SINGLE SIDEBAND {SSB} MODULATION DSB modulation results in doubling the bandwidth of a given message signal. Thus both DSBSC and DSBLC are wasteful of channel bandwidth. It can be seen from the following figure that transmission of both side bands is unnecessary since the information in both sidebands is redundant. SSB MODULATION Two Methods • Frequency discrimination - filtered DSB • Phase discrimination Each sideband pair {whether upper or lower} contains the complete information or the original message. Frequency Discrimination The most straight forward way to generate SSB is to first generate DSB and the suppress the side bands by filtering. 1 10/13/2015 Frequency Discrimination Frequency Discrimination Difficulties In practice the construction of the sideband filter is difficult. To produce a perfect SSB spectrum requires an ideal filter characteristic at fc. Because this is impossible, we instead put a restriction on the message spectrum. IF M(f) contains no low frequency components then there will be no significant components in the vicinity of fc after modulation. Frequency Discrimination Difficulties This condition permits use of a less than perfect sideband filter. Frequency Discrimination Difficulties The need to have a message signal without low frequency content limits this method to voice transmissions. Since voice signals have little or negligible frequencies below 300Hz. 2 10/13/2015 Phase Discrimination SSB Generation It is possible to generate SSB by proper phasing of signals. The method essentially uses two product modulators to create two DSB-SC signals that are phase shifted by 90°. The two DSB signals are then combined to cancel one of the side bands. Phase Discrimination SSB Generation Phase Discrimination SSB Generation Continuing.. Phase Discrimination SSB Difficulties A major problem in the design of phase-shift SSB systems is the construction of the 90° phase shift network for m(t), because all frequency components must be shifted by exactly 90°. Such systems can work efficiently for only a very restricted message bandwidth. Likewise; 3 10/13/2015 Demodulation of SSB Signals To recover the baseband signal m(t) from the SSB signal we have to shift the spectrum by +/- fc. This requires synchronous detection similar to that of DSBSC. The process is; Vestigal Sideband Modulation {VSB} VSB is a compromise between SSB and DSB modulation. In VSB one sideband is passed completely and just a trace or ‘vestige’ of the other sideband is retained. Demodulation of SSB Signals The spectrums; VSB Generation Start with a DSBSC signal and then pass through a sideband shaping filter. Advantages: • Improved BW over DSB modulation • Can be recovered using and envelope detector. 4 10/13/2015 VSB Generation VSB Demodulation LPF VSB Demodulation Complimentary Filters The VSB filters Hv(f) are required to be ‘complimentary’ in the vicinity of fc. These type filters have many applications in electronics and their designs are well understood. Therefore, VSB filter design (unlike SSB) does NOT pose special difficulties. 5 10/13/2015 B/W Television Spectrum In order to transmit a two dimensional (picture) information via a one dimensional coordinate system ( time), it is necessary to employ a scanning technique. The TV camera is designed to focus on an image on a two dimensional photo- sensitive array. B/W Television Spectrum A rough video band width can be found as; B/W Television Spectrum The signal resulting from scanning the array is a voltage vs time waveform that varies with the brightness of the elements. This is a bw video signal. In bw TV; the raster scan consists of - 525 vertical lines scanned 30 times/sec. - 4:3 aspect ratio means there are 525 x 700 picture elements [pixels] - So 525 x 700 x 30 = 11,025,000 picture elements [one frame] must be transmitted every second. B/W Television Spectrum In practice a bandwidth of 4 MHz is typical. If DSB was used 2 x 4 MHz = 8 MHz would be required. However, by employing VSB, channel allocations of only 6 MHz are required. 6 10/13/2015 Comparison of AM Systems 1. Large carrier systems have the advantage of simple detection at the receiver. The disadvantage is that they are less power efficient at the transmitter when compared to a suppressed carrier system. 2. Suppressed carrier systems are more power efficient than large carrier systems. The disadvantage is the complex receivers required to perform synchronous demodulation of DSBSC signal. Comparison of AM Systems 3. SSB requires minimum bandwidth and minimum transmitted power possible. Thus is long distance communication where signal repeaters are necessary SSB is a good choice since reducing the number of repeaters is paramount to reduce the system cost. The added transmitter complexity is thus justified. 4. VSB requires a bandwidth less than that of DSB and the bandwidth savings is justified when modulating signals with large bandwidths. COMMERICIAL BROADCAST AM Broadcast AM stations are assigned at 10kHz spacing from 540-1600 kHz. Each transmitter sends a DSBLC signal at it’s assigned carrier frequency. An AM receiver can pick up any of these signals by tuning to the proper carrier frequency. The selected station is then demodulated using an envelope detector. The problem is that production of stable tuning amplifiers and filters that cover the entire AM band are hard to build. COMMERICIAL BROADCAST AM This gave rise to the heterodyne receiver used today. In the heterodyne receiver, the incoming modulated signal is translated to a new center frequency that is independent of the station selected. This intermediated frequency (IF) is fixed. The signal is then amplified at the IF before demodulation. 7