Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1997-10-09
2001-01-23
Hunter, Daniel S. (Department: 2749)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S296000, C455S277200, C455S501000, C455S063300, C381S003000, C381S004000
Reexamination Certificate
active
06178317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention directs itself to a system and method of mitigating the effects of signal fades, temporary blockages or severe channel impairments in an audio broadcasting system. More particularly, the system and method employs the transmission of a primary broadcast signal along with a redundant signal, the redundant signal being delayed by a predetermined amount of time, on the order of several seconds, with respect to the primary broadcast signal. A corresponding delay is incorporated in the receiver for delaying the received primary broadcast signal. Still further, this invention is directed to the concept of detecting degradation in the primary broadcast channel that represents a fade or blockage in the RF signal, before such is perceived by the listener. In response to such detection, the delayed redundant signal is temporarily substituted for the corrupted primary audio signal, acting as a “gap filler” when the primary signal is corrupted or unavailable. More in particular, this invention directs itself to use of a blend function for smoothly transitioning from the primary audio signal to the delayed redundant signal.
2. Prior Art
In fixed receiver installations, such as home receivers, the fading statistics are generally stationary, except for occasional temporary fades caused by passing vehicles or aircraft, and so effective mitigation of fades and blockages for these applications can be as simple as installing a better antenna or repositioning the existing antenna. In automotive applications, however, fading and blockage statistics are not stationary, being dependent on the vehicle location and velocity, and effective mitigation requires more sophisticated methods.
Digital Audio Broadcasting (DAB) techniques are being proposed to improve the quality of broadcasting over conventional AM and FM analog signals. In-Band-On-Channel (IBOC) DAB is a digital broadcasting scheme, likely to be adopted in the United States, in which analog AM or FM signals are simulcast along with the DAB signal The digital audio signal is generally compressed such that a minimum data rate is required to convey the audio information with sufficiently high fidelity. Terrestrial DAB systems generally have the characteristic that fades and blockages have a more deleterious effect on received audio than they do on analog modulated systems, such as commercial AM or FM broadcasts because these DAB systems do not degrade gracefully. This effect is exacerbated for in-band on-channel (IBOC) systems which are constrained to have orders of magnitude lower transmit power than the analog broadcast signals whose frequency band they share. IBOC DAB systems transmit both the analog and DAB signal simultaneously within the required spectral mask of the analog signal alone. Therefore, the IBOC DAB concept allows a station to offer digital audio while retaining its analog listeners, but the digital broadcast will not gain acceptance unless the audio loss due to temporary fades and blockages is mitigated.
SUMMARY OF THE INVENTION
A system for mitigating intermittent interruptions in an audio radio broadcast system is provided. The system includes a source of an audio signal and a transmitter subsystem having a first input coupled to the audio source for modulating at least one first carrier signal with the audio signal to broadcast a primary radio signal. The system also includes a first delay circuit having an input coupled to the audio source for adding a first predetermined time delay to the audio signal to form a delayed redundant audio signal at an output thereof, the output being coupled to a second input of the transmitter subsystem for modulating at least one second carrier signal with the delayed redundant audio signal to broadcast a delayed redundant radio signal simultaneously with the primary radio signal. The system further includes a receiver subsystem for receiving the primary radio signal and the delayed redundant radio signal, the receiver subsystem demodulating the primary radio signal to provide the audio signal to a first output thereof and demodulating the delayed redundant radio signal to provide the delayed redundant audio signal to a second output thereof. The receiver subsystem includes a circuit for detecting degradation of the received primary radio signal, the circuit for degradation detection providing a quality measurement signal to a third output of the receiver subsystem. The system includes a second delay circuit having an input coupled to the first output of the receiver subsystem for adding a second predetermined time delay to the audio signal to form a delayed primary audio signal at an output thereof, the second predetermined time delay being substantially equal to the first predetermined time delay. Still further, the system includes a blending circuit having a first input coupled to an output of the second delay circuit and second and third inputs respectively coupled to the second and third outputs of the receiver subsystem for combining a first weighting factor with the delayed primary audio signal and a second weighting factor with the delayed redundant audio signal and combining the weighted delayed primary audio signal with the weighted delayed redundant audio signal to form a composite audio signal. The first weighting factor is smoothly transitioned between a first value and a second value responsive to the quality measurement signal being less than a predetermined threshold value. The second weighting factor is smoothly transitioned between the second value and the first value responsive to the quality measurement signal being less than the predetermined threshold value. Additionally, the system includes an audio output circuit coupled to the blending circuit for converting the composite audio signal to an aural signal.
From another perspective, a method of mitigating intermittent interruptions in an in-band on-channel digital audio broadcast system is provided. Each channel includes at least one carrier signal modulated with an analog audio signal and a plurality of subcarriers modulated with a digital representation of the analog signal, wherein the method comprises the steps of:
(a) adding a predetermined first time delay to the analog audio signal prior to modulation of the at least one carrier signal, the analog audio signal being delayed relative to the digital representation of the analog audio signal;
(b) providing a receiver for receiving both the at least one modulated carrier signal and the plurality of modulated subcarriers to recover the delayed analog audio signal and the digital representation of the analog audio signal;
(c) detecting a predetermined level of degradation in the digital representation of the analog audio signal;
(d) adding a predetermined second time delay to the digital representation of the analog audio signal and converting the delayed digital representation of the analog audio signal to form a primary audio signal; and,
(e) substituting the delayed analog audio signal for the primary audio signal when the predetermined level of degradation is detected.
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John G. Proakis, “Digital Communications”, 1983, pp. 470-479.
Kroeger Brian W.
Stehlik Roy R.
Eckert Seamans Cherin & Mellott , LLC
Hunter Daniel S.
ibiquity Digital Corporation
Lenart Robert P.
Zewdu Meless N
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