Electrical audio signal processing systems and devices – Binaural and stereophonic – Broadcast or multiplex stereo
Reexamination Certificate
1997-04-02
2001-08-07
Lee, Ping W. (Department: 2747)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Broadcast or multiplex stereo
C381S003000, C381S004000, C381S015000, C381S016000
Reexamination Certificate
active
06272226
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to an apparatus and method for transmitting “masked” stereo audio signals in a signal distribution system such as a cable TV network. More particularly, the invention prevents the unauthorized reception of narrowband signals while minimizing bandwidth requirements needed for masking such signals.
2. Related Information
FIG. 1
shows the frequency spectrum of a conventional BTSC (Broadcast Television Standards Committee) encoded audio signal. Stereo signals consist of a left and a right speaker component, each of which is band-limited between 50 Hz and 14.5 Khz. The BTSC signal transmits the sum of the two components (L+R)
101
at baseband. The difference of the components (L−R) is amplitude modulated onto a suppressed carrier signal at twice the horizontal line frequency (2f
h
, where the line frequency is 15.73 KHz) thus producing two sidebands
103
and
104
. The L−R signal is usually DBX encoded before AM modulation in order to provide greater immunity to noise introduced by FM demodulation at the receiver. A pilot tone signal
102
, which is phase-locked to the carrier signal, is transmitted at the horizontal line frequency (f
h
).
At the receiving end, the L−R signal is AM demodulated and recombined with the L+R signal to generate the L and R audio signals. Conventional BTSC audio decoders lock on to the pilot tone
102
and re-generate the suppressed carrier signal; this pilot tone also indicates the presence of a stereo signal to BTSC decoders. In the absence of the pilot tone, BTSC decoders assume that only a monaural signal is transmitted.
It is often desirable in a cable television system to scramble or otherwise render unintelligible portions of the transmitted signal, including the audio signal, in order to prevent unauthorized reception. Various techniques for providing secure access to the audio portion of the transmitted signal have been devised. As one example, U.S. Pat. No. 4,956,862 to Robbins et al. describes a technique wherein the audio portion of the television signal is modulated at a frequency that is offset from the standard carrier frequency (e.g., 4.75 MHZ instead of 4.5 MHZ). Unfortunately, this technique has the disadvantage of increasing the amount of bandwidth required to carry the signal, and suffers from other side effects as well.
Other techniques, such as those exemplified by U.S. Pat. No. 5,159,631 to Quan et al., vary the frequency of the modulation carrier in a pseudo random fashion or use other means of scrambling the audio signal. Other methods make use of digital encryption techniques. However, such conventional approaches undesirably increase the overall bandwidth required to transmit the audio signal and require complicated circuitry. Accordingly, conventional techniques have been found to be unsatisfactory.
SUMMARY OF THE INVENTION
In certain embodiments of the present invention, an audio “masking” operation is performed at a headend along with video scrambling. The scrambled video signal is AM-VSB modulated, and the masked audio signal is FM-modulated and combined with the TV signal at IF. The TV IF signal is then upconverted to a desired channel for transmission across a network. At a customer's premises, a settop terminal descrambles the video signal and “demasks” the stereo audio signal.
At the transmitting end, the BTSC signal is split into a baseband L+R signal and a baseband (DBX encoded) L−R signal. These two signals are used to form an independent sideband (ISB) modulated signal containing information from the L+R signal on the lower sideband and information from the L−R signal on the upper sideband. The ISB modulated signal is combined with a video signal at a headend and transmitted over a network such as a cable TV distribution system. At the receiving end, the “masked” audio signal is demodulated and provided as an output to a user's television.
It is thus one object of the present invention to provide for the secure transmission and reception of BTSC encoded stereo audio signals over a cable TV network or other distribution system using a minimum of bandwidth.
It is a further object of the present invention to transmit narrow bandwidth signals in a secure manner within the constraints of the NTSC frequency allocation scheme.
In various aspects of the invention, a Hilbert transformation operation is used to perform the independent sideband modulation. A jamming or low bandwidth data signal may also be transmitted at a frequency location normally used for transmitting a pilot signal.
Other advantages and benefits of the present invention will become apparent through the following detailed description, figures, and the appended claims.
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Khan Raheel
LaFay William P.
Barnhardt III Hubert J.
Lee Ping W.
Massaroni Kenneth M.
Scientific-Atlanta, Inc.
West John Eric
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