Information Technology Reference
In-Depth Information
Figure 7.6: In 8-VSB the transmitter operates in eight different states enabling three bits to be sent per symbol.
Multi-level signalling systems have the characteristic that the bits in the symbol have different error probability.
Figure 7.7 shows that a small noise level will corrupt the low-order bit, whereas twice as much noise will be needed
to corrupt the middle bit and four times as much will be needed to corrupt the high-order bit. In ATSC the solution is
that the lower two bits are encoded together in an inner error-correcting scheme so that they represent only one bit
with similar reliability to the top bit. As a result the 8-VSB system actually delivers two data bits per symbol even
though eight-level signalling is used.
Figure 7.7: In multi-level signalling the error probability is not the same for each bit.
Multi-level signalling can be combined with PSK to obtain multi-level quadrature amplitude modulation (QUAM).
Figure 7.8 shows the example of 64-QUAM. Incoming six-bit data words are split into two three-bit words and each
is used to amplitude modulate a pair of sinusoidal carriers which are generated in quadrature. The modulators are
four-quadrant devices such that 2 3[ 3 ] amplitudes are available, four which are in phase with the carrier and four
which are antiphase. The two AM carriers are linearly added and the result is a signal which has 2 or 64
combinations of amplitude and phase. There is a great deal of similarity between QUAM and the colour subcarrier
used in analog television in which the two colour difference signals are encoded into one amplitude- and phase-
modulated waveform. On reception, the waveform is sampled twice per cycle in phase with the two original carriers
and the result is a pair of eight-level signals. 16-QUAM is also possible, delivering only four bits per symbol but
requiring a lower SNR.
Search WWH ::




Custom Search