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3.1 Vestigial Sideband Modulation (VSB)
3.1.1 Principle of VSB
In television broadcasting, a DSB modulation technique takes too much bandwidth for the television channel, and an SSB technique is too expensive to implement, although it takes half the bandwidth. In this case a compromise between DSB and SSB, called vestigial sideband (VSB), is often chosen.
VSB is obtained by partial suppression of one of the sidebands of a DSB signal while preserve the other sideband. The DSB signal maybe either an AM signal or a DSB-SC signal. The expressions of a VSB signal in time-domain and frequency-domain is as follows:
 (3-1a)
 (3-1b)
The ’–‘ in 3-1b denotes Upper-VSB while the ‘+’ denotes Lower-VSB. ‘ωc‘ is the radian frequency of the carrier.  , ‘*’ denotes convolution. hs(t) is the impulse response of the orthogonal filter  .
Figure 3.2 illustrates the amplitude spectrum and the phase spectrum of the orthogonal filter Hs(ω).
Figure 3-2 The amplitude spectrum and the phase spectrum of the orthogonal filter Hs(ω)
According to the expression of the VSB modulation both in time-domain and frequency-domain, 2 methods could be used to generate VSB signal:
The first method is to generate VSB signals using VSB filter, the diagram of the process is as follows:
Figure 3-3 Generate VSB signals using VSB filter The second method is to generate VSB signal using phase-shifting, the diagram of this method is as follows:
Figure 3-4 Generate VSB signals using phase-shifting
3.1.2 Implement of VSB modulation in HDTV
In practical HDTV systems VSB is often obtained by Filtering and phase compensation method, the diagram is as follows:
Figure 3-5 Generate VSB signals using Filtering and phase compensation
Compared to Figure 3-3 and Figure 3-4, we can find out that the Filtering phase compensation method can be obtained by replacing the filtering method of phase-shifting phase shift in the broadband network.
Suppose 
Then 
So 
Then modulate the orthogonal signal to the IF wavefrom:
Finally, we can get the VSB signal:
The typical VSB transmission system is depicted in Figure 3-3 and consists of a Data Randomizer, Reed-Solomon encoder, data interleaver, trellis encoder, VSB modulator and an RF upconverter.
Figure 3-3 Typical Vestigal Sideband Broadcast transmission system.
Purpose of a Data Randomizer
The Data Randomizer XORs all incoming data bytes with a 16-bit maximum length pseudo random sequence that is initialized at the beginning of each data field. By doing this, the data randomizer guarantees a flat, noise-like spectrum, which permits the DTV receiver’s recovery loop to work optimally. This noise-like spectrum minimizes interference with NTSC signals by appearing as white noise to the NTSC receivers.
Purpose of Reed-Solomon Coding
Reed-Solomon (RS) codes achieve the largest possible code minimum distance for any linear code having the same encode input and output block lengths. Reed-Solomon codes are particularly useful for burst-error correction (channels having memory) and where the input symbols are large. Reed-Solomon coding increases the space between transmitted bytes allowing for data recovery even after loosing some bytes.
Purpose for using a Data Interleaver
Interleaving the coded message before transmission and de-interleaving after reception causes bursts of channel errors to be spread out in time and thus to be handled by the decoder as if they were random errors. Since channel memory decreases with time separation, the idea behind interleaving is to separate the codeword symbols in time. The interleaving times are similarly filled by the symbols of other codewords. Separating the symbols in time (time diversity) effectively transforms a channel with memory to a memoryless one, and thereby enables the random-error-correcting codes to be useful in a burst-noise channel.
Purpose of the Trellis Encoder
Trellis-coded modulation achieves coding gain without any bandwidth expansion. However, this benefit results in decoder complexity. The objective of Trellis coding is to increase the minimum distance between the signals that are the most likely to be confused, without increasing the average power. It accomplishes this feat by exploiting the repetitive structure of trellis diagrams.The block diagram for the 8-VSB trellis encoder is shown in Figure 3-4.
Figure 3-4 8-VSB trellis encoder with precoder and symbol mapper.
Purpose of the RF Upconverter
After the signal has left the trellis encoder, it is upconverted and the lower sideband removed for transmittal. Figure 3-5 shows graphical the procedure. The signal is split and sent to an I-channel and Q-channel filter. The I and Q channel filters prevent intersymbol interference (ISI) from occurring. Each output of the filter is mixed with a cosine and sine signal then summed. At the output, a VSB spectrum results with the lower sideband removed. The absence of the lower sideband is a result of the difference in phase between the I and Q channels.
Figure 3-5 Block diagram of RF upconverter showing lower sideband removal
The terrestrial 8-VSB modulation has a net data rate of 19.29 Mbps and a transmission clock of 19.39 Mbps within a 6 MHz channel. As mentioned in the previous section, the data carried by the 8-VSB signal is divided into Data Frames consisting of two Data Fields, which are further divided into 313 Data Segments. Each Data Segment contains 187 bytes of data, a sync byte, and 20 Reed-Solomon parity bytes, adding up to 208 bytes and consisting of 832 symbols. Four symbols are used for segment synchronization leaving 828 symbols for data transmission as shown in Figure 3-6 below.
Figure 3-6 VSB data frame
These symbols are transmitted using the 8-VSB-modulation scheme. As a result of the 8-level signal, there are three bits per symbol. Doing the math, 828 x 3, yields 2484 bits of data in each Data segment. The exact symbol rate (Sr) of the system is calculated using the NTSC horizontal sweep frequency fHand a 684 constant resulting as follows:
Sr = fH×684 = (4.5/286)x684 = 10.76223776...MHz
The chosen modulation, 8-VSB, consists of a 4-level AM vestigal sideband input signal with trellis coding that produces an 8-level output signal. As shown in Figure 3-7 below, the basic spectrum occupies a 6 MHz channel and is flat over most of the channel with roll-off regions on both spectrum edges where a nominal square root raised cosine response results in 620 KHz transition regions (310 KHz on either side). There is a small suppressed-carrier located 310 KHz from the lower band edge of the spectrum. The channel bandwidth of 6 MHz was chosen to match the existing bandwidth of NTSC channel as well as abide with Nyquist requirements. A Nyquist bandwidth of one-half the symbol rate is required to recover the transmitted data. This results in a passband (excluding sidebands) of
Sr/2=5.3811888...MHz
Excess bandwidth amounting to 6.000 – 5.381 = 0.619 MHz is required for data transmission beyond the Nyquist minimum. These frequency requirements are graphical demonstrated in Figure 3-8.
Figure 3-7 DTV Spectral response of 6 MHz channel
There are several features added to the 8-VSB signal to aid receivers in acquiring and locking to the signal in extreme propagation conditions. One such feature is the inclusion of the small pilot carrier at the edge of the band. This pilot is placed on the Nyquist slope of NTSC receivers, in order to effectively minimize co-channel interference between DTV and NTSC signals. This pilot can be acquired by the receiver to a signal-to-noise ratio of zero (0) dB.
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