By Jon P. GrosJean, DataVision
Two-way data transmission on CATV cables is growing rapidly. Unfortunately, TV and F/1 signals occupy most of the available frequencies in current cable systems leaving very little for data signals. Return channels, however, are usually vacant but often very noisy and suffer from outside signal leakage. Most, if not all, current CATV data systems use FSK because of its inherent signal-to-noise improvement properties. This is helpful in the return path but not necessary in the path into the home where signals are very good quality. Furthermore, if signal-to-noise improvement is desired, FSK must have a deviation greater than the signal bandwidth so it uses up the available spectrum with fewer data channels. A better approach would be to use different modulation schemes in different directions while choosing them to keep both the home unit cost and data error rate low.