The invention relates generally to the transmission of a digital data signal combined with an analog TV signal, in particular to a method and system for encoding the digital signal using Orthogonal Frequency Division Multiplexing (OFDM) and combining it with an analog TV signal.
With the evolution of digital technologies and the demand for increased bandwidth for digital services, considerable research efforts have been done to widen the information superhighway reaching the end user. With the advent of digital radio and digital television each system will offer additional bandwidth to end-users. However, there will be a long transition period until a substantial number of end-users receive digital radio or digital television. Since the mid 1980""s, television broadcasters have been inserting additional data into a portion of the analog television signal. The content of this information has been used for applications, such as stock market updates, weather updates, news services, and closed-captioned programs allowing hearing impaired users to watch and follow the transmitted television programs.
With recent developments in the area of digital technologies, newer systems have been proposed to improve the available bit rate for such information. In these systems, the information is transmitted over digitally modulated carriers. One system designed by Digideck Inc. uses a low level Differential Quadrature Phase Shift Keying (DQPSK) carrier to transmit the data. The carrier is located in the vestigial side band, approximately 1.0 MHz below the video carrier and has a bandwidth of 450 kHz. Another system developed by WavePhore Inc. uses a phase-modulated subcarrier that is located between the color subcarrier and the audio subcarrier. Both of these systems inherently cause some degradation to the picture quality by inserting a data carrier in the active portion of the video signal. To ensure that the impact of the data carriers would not be clearly visible to non-expert viewers, both systems where studied using subjective evaluation methodologies. However, both systems provide only a low bit rate for the transmission of a digital data signal and have, furthermore, a substantial impact on the video signal quality.
U.S. Pat. No. 4,884,139 issued Nov. 28, 1989 to Pommier and U.S. Pat. No. 5,291,289 issued Mar. 1, 1994 to Hulyalkar et al. disclose a method and system for digital broadcasting in an analog TV environment. Pommier and Hulyalkar et al. teach multi carrier modulation in the form of an OFDM of a digital data signal and transmission of the same in a channel where analog television transmission and reception are impossible, i. e. transmission of the digital signal in so-called xe2x80x9ctaboo-channelsxe2x80x9d. In order to reduce co-channel interference the digital signal is interlaced with odd multiples of half the line frequency of the analog TV spectrum. However, with the introduction of ever more TV programs the number of available taboo-channels for such a digital data transmission is substantially decreasing. Furthermore, these methods do not allow, for example, a TV station to introduce digital data signal broadcasting using its allotted analog TV spectra for simultaneously broadcasting the digital data signal and the analog TV signal.
It is, therefore, an object of the invention to provide a method and system for encoding and transmitting a digital data signal at a higher bit rate than the prior art within an analog TV signal.
It is further an object of the invention to provide a method and system for encoding a digital data signal using OFDM and spectral shaping in frequency domain in order to minimize the impact of the digital data signal on the analog TV signal.
In accordance with the present invention there is provided a method for transmitting digital data within an analog TV signal comprising the steps of:
receiving the digital data from a data source;
multilevel encoding the digital data and providing a plurality of data symbols in dependence thereupon;
modulating in frequency domain each of the plurality of data symbols onto a carrier of a plurality of carriers such that each carrier carries a different data symbol by using a serial to parallel converter for providing a digital data signal in dependence thereupon, wherein some carriers of the plurality of carriers are turned off to shape the spectrum of the digital data signal such that the spectrum of the digital data signal comprises notches at locations coinciding with locations of spectral components of the analog TV signal, and wherein the width of the notches is determined such that degradation of the analog TV signal due to the transmission of the digital data within the analog TV signal is substantially reduced;
transforming the digital data signal into time domain;
combining the digital data signal with the analog TV signal; and,
transmitting the combined digital data and analog TV signal.
In accordance with the present invention there is further provided a method for receiving digital data within an analog TV signal comprising the steps of:
receiving the combined digital data and analog TV signal;
separating the digital data signal from the analog TV signal;
transforming the digital data signal into frequency domain; and,
multilevel decoding the digital data signal using a parallel to serial converter in order to obtain the digital data.
In accordance with an aspect of the present invention there is provided a method for transmitting digital data within an analog TV signal comprising the steps of:
receiving the digital data from a data source;
multilevel encoding the digital data and providing a plurality of data symbols in dependence thereupon;
modulating in frequency domain each of the plurality of data symbols onto a carrier of a plurality of carriers such that each carrier carries a different data symbol using a serial to parallel converter for providing a digital data signal in dependence thereupon, wherein some carriers of the plurality of carriers are turned off in order to shape the spectrum of the digital data signal such that the spectrum of the digital data signal comprises notches at locations coinciding with locations of spectral components in the vestigial sideband of the video carrier of the analog TV signal, and wherein the width of the notches is determined such that degradation of the analog TV signal due to the transmission of the digital data within the analog TV signal is substantially reduced;
transforming the digital data signal into time domain using IFFT;
D/A converting the digital data signal;
combining the digital data signal with the analog TV signal by inserting the digital data signal in the vestigial sideband of the video carrier;
RF upconverting the combined digital data and analog TV signal to a broadcaster""s RF frequency; and,
transmitting the upconverted combined digital data and analog TV signal.
In accordance with another aspect of the present invention there is provided a system for transmitting digital data within an analog TV signal comprising:
a port for receiving the digital data from a data source;
a processor for encoding the digital data and providing data symbols, for modulating in frequency domain each of the data symbols onto a carrier of the plurality of carriers such that each carrier carries a different data symbol using a serial to parallel converter for providing a digital data signal, wherein some carriers of the plurality of carriers are turned off in order to shape the spectrum of the digital data signal such that the spectrum of the digital data signal comprises notches at locations of spectral components of the analog TV signal, and for transforming the digital data signal into time domain using IFFT;
a D/A converter for converting the digital data signal;
a combiner for combining the digital data signal with the analog TV signal; and,
a RF upconverter for upconverting the combined signal to a broadcaster""s RF frequency for transmission.
In accordance with the other aspect of the present invention there is further provided a system for receiving digital data within an analog TV signal comprising:
a second other port for receiving the transmitted signal;
a tuner for downconverting the received signal and for separating the digital data signal from the analog TV signal;
an A/D converter for converting the digital data signal; and,
a processor for low pass filtering the digital data signal, for transforming the digital data signal into frequency domain using FFT, and for multilevel decoding the digital data signal using a parallel to serial converter in order to obtain the digital data.