NTSC in-band data broadcasting using OFDM.

The Teletext standard sends data during the VBI (Vertical Blanking Interval) portion of an analogue NTSC (National Television System Committee) video signal and has a bit rate in the order of 150 kbps. More recently, digital data broadcasting systems using a modulation scheme from the PSK family have been implemented. In these systems, the digital carrier is strategically placed within the active portion of the video signal in order to minimise the interference into the video signal. These techniques offer bit rates up to 525 kbps. This thesis introduces a new technique for transmitting data using the Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme, within the NTSC television signal. This technique allows a greater data throughput, while still maintaining the picture quality at an acceptable level. With OFDM, the spectrum can be shaped according to a specific set of rules, for example, the subcarriers that overlap with the NTSC spectral components can be set to zero. The data signal is inserted in the vestigial sideband of the video signal. The advantages of this implementation are that the shape of the data spectrum minimises the degradation of the picture quality and increases the noise margin of the data signal. Two approaches are taken to evaluate the performance of this system. In the first one, computer simulations are used to determine the available bit rate by varying the number of carriers. Bit error rate measurements are also done to evaluate the performance of the system when the data signal is combined with the NTSC television signal. In the second approach, the perceptual impact is determined, using accepted subjective assessment methodology, when varying levels of OFDM carriers are injected into the NTSC signal. The results show that the proposed system has the potential to offer higher data rate throughput than existing system, with negligible impact on the NTSC picture quality