Pulse or digital communications – Apparatus convertible to analog
Reexamination Certificate
1998-10-08
2003-01-21
Vo, Don N. (Department: 2631)
Pulse or digital communications
Apparatus convertible to analog
C455S143000
Reexamination Certificate
active
06510175
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates, in general, to broadcasting information from a plurality of sources to one or more receivers. More particularly, the present invention is directed to broadcasting a digital information waveform in the same band and on the same channel with a conventional analog waveform.
At present, the sound quality of audio programming over commercial analog frequency modulation (FM) broadcast facilities is significantly poorer than that of more modern digital signal sources such as the compact disc. A number of attempts have been made to bring the quality of digital audio to FM broadcasting, but these attempts have usually given rise to other problems which rendered them unworkable.
For example, U.S. Pat. No. 5,038,402 to Robbins discloses an apparatus and method for broadcasting digital audio over the FM broadcast band and suggests that such digital broadcasts might be interspersed with analog broadcasts, across the band. This patent allows the use of the FM band for digital broadcast, but forces the individual broadcaster to choose between broadcasting in digital, with better audio but a listener base of only the new relatively scarce digital receivers, or conventional analog, with poorer audio quality but available to all listeners with conventional analog FM receivers. The only other alternative is for the broadcaster to broadcast on two frequencies, one for digital and a second for analog; however, this presents a potential problem in obtaining a license for such broadcasting from the Federal Communications Commission (FCC). The broadcaster may or may not be able to acquire a license to broadcast on two frequencies in a given FM radio broadcasting market.
The FCC, in addition to licensing individual frequency bands to individual broadcasters, is charged with allocation of frequency spectrum to all users for all uses. One other problem presented by the system of the Robbins patent and others like it is that spectrum must be allocated especially for digital FM broadcast if the number of analog broadcasters presently on the air is to remain unchanged. This problem is exacerbated by the fact that, for most of the United States and at attainable frequencies, there is no unused spectrum. In order for spectrum to be re-allocated for a new use, the FCC and the petitioners desiring use of a frequency band must go through a protracted, uncertain, political process which will culminate in an FCC decision on how the spectrum in question should be used.
For these reasons, there has been a need for digital audio broadcasting over the FM band which does not require a broadcaster to abandon its investment in analog FM transmission equipment, require a new frequency assignment in the existing FM band, require the listening audience to discard existing analog FM receivers, or force the FM program broadcasters to undergo a protracted, uncertain and expensive process to obtain a new frequency allocation.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a system for digital audio broadcasting on the existing FM broadcast band. More particularly, the purpose of this invention is to provide a system for In-band, On-channel, FM Digital Audio Broadcast (IBOC FM-DAB) which would allow simultaneous transmission of DAB and FM over existing allocations without interfering with the conventional analog FM signals. Such a system not only would be of great value to the broadcast industry, but the ability to multiplex supplemental message information over conventional analog FM transmissions would be of general interest and importance from a perspective of efficient spectrum utilization. In addition, a system which solves the problems raised for FM DAB will inherently find applications in many other communications' environments.
Once the requirement for an in-band, on-channel digital audio broadcast is defined, a number of subsidiary problems arise in executing that broadcast. In particular, the digital signal and the FM program must be modulated together in such a way that they can be demodulated and used by various end users on the receiver end. This causes problems of receiver design, which are made more complex since allowance must be made for disturbances which occur in the communications channel between the transmitter and receiver. For example, in FM transmission, demodulation is often inhibited by multipath, which applies fast, undesired phase changes to FM signals thereby causing a loss of phase lock in conventional receivers which normally use “phase locked loops” (PLLs). Reacquisition of lock with a conventional PLL causes an undesired spurious time response at the demodulation output of the receiver, which adversely affects audio quality.
In one aspect, this invention is directed to a method for introducing supplemental programming (more messages) to a given FM broadcast by amplitude modulating the FM waveform. The supplemental amplitude modulation is orthogonal to the initial frequency modulation so that both the AM and FM programs can be demodulated, either independently or together, without interfering with one another. This method thus provides a vehicle for the simultaneous transmission of supplemental programming, such as high bit rate DAB, with existing FM over the same spectral allocation at the same time without degrading the analog FM transmission. This invention is applicable to any system in which additional programming or an additional message is to be added to enhance or supplement another program which frequency modulates a carrier.
More specifically, the present invention is directed to a method and apparatus for modulating a DAB signal, consisting of 21 digital carriers, onto an analog FM carrier such that mirror images of the 21 digital carriers are spaced in frequency on either side of the analog FM carrier. These DAB carriers slew through the frequency band at the instantaneous rate of the analog FM carrier. This slewing in frequency is accomplished without causing interference between the analog FM audio and digital subcarrier signals. The analog FM program signal then becomes, in a sense, a carrier for the 21 digital subchannels. By slewing through frequency, the composite signal becomes resistant to multipath distortion. Further resistance to multipath is derived through the addition of a continuously transmitted wideband reference signal to the 21 digital subchannel modulation waveforms. This reference waveform is used at the receiver as a training system for adaptive multipath equalization with quick and continuous updating. Additional resistance to multipath distortion comes from the use of data interleaving and data coding systems which are customarily used to detect and correct errors in digital signal systems. As a result, the FM DAB transmission system of the invention provides unusually good resistance to multipath induced distortion. The FM DAB signal is modulated at a significantly lower peak power and is positioned within the allocated spectrum mask which is licensed to each broadcaster. This allows each broadcaster to transmit the digital signal within the licensed frequency on a single FM channel.
An important consideration in the development of an in-band, on-channel FM digital audio broadcasting (FM-DAB) system is the requirement that the digital signals not interfere with the analog FM signals occupying the same frequency allocation. An in-band, on-channel FM-DAB signal simultaneously occupies the same frequency allocation as a conventional analog FM broadcast signal. The characteristics of the digital signal must therefore be designed to prevent degradation of the analog signal. One approach to minimizing interference is to reduce the amplitude of the digital signal relative to the analog signal. Of course, the amplitude of the digital signal cannot be made arbitrarily small, because interference from the analog signal and thermal noise will ultimately degrade the digital signal. Once the digital signal amplitude has been reduced to the smallest possible level whic
Hunsinger Billie J.
Solie Leland P.
Vigil Armando J.
Ibiquity Digital Corporation
Lenart, Esq. Robert P.
Pietragallo Bosick & Gordon
Vo Don N.
LandOfFree
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