Electrical audio signal processing systems and devices – Binaural and stereophonic – Broadcast or multiplex stereo
Reexamination Certificate
1999-06-09
2004-03-16
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Broadcast or multiplex stereo
C381S001000
Reexamination Certificate
active
06707917
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a monaural and stereo audio signal control system for use in television signal distribution systems and, more particularly, to such audio control systems in cable television distribution systems.
Various systems have been developed for the distribution of television signals to viewers. These systems include conventional local-area radio frequency broadcast, distribution by cable, and wide-area satellite broadcast. In general, television signals that include content that is considered objectionable are subject to government regulation and/or industry self-regulation to prevent viewing by unauthorized persons, particularly children.
Conventional television signals include a video and an audio component; the audio component is presented via a modulated FM carrier for both monaural sound and/or stereo sound. In the earliest days of cable television, the cable TV provider distributed TV signals from a headend facility into the subscriber's location via a set-top box. The cable television provider typically transmitted a number of channels greater than that provided by the area broadcast stations. The subscriber selected the desired channel which was then passed through the set-top box to the television receiver. The set-top box functioned essentially as a tuner for the many signals provided from the headend.
Later, with the advent of channels that carried objectionable content, the cable television provider scrambled the video component by changing the timing/position of the horizontal synchronization pulses or otherwise interfering with the video feed sent to the set-top box. The set-top box was provided with a subscriber processing section that enabled the authorized subscriber to control the set-top box to descramble the video signal for presentation to the television receiver. However, unauthorized viewers, e.g., children of the subscriber, could disconnect the cable from the set-top box and then reconnect the cable directly to the signal input of the television receiver and then tune the television receiver to the desired channel. While the scrambled video signal could not be viewed, the audio content was available. Oftentimes, the audio content was also considered objectionable when heard by children.
In order to prevent unauthorized users from listening to the audio component of the objectionable television signal, the cable television supplier also altered the audio component of the television signal. In the case of a monaural audio component, the carrier frequency of the FM audio signal was shifted. In the case of a stereo audio component, a second FM carrier with the audio component was added to the television signal. In either case, the audio component could not be demodulated within a conventional television receiver. Thus, disconnecting the cable from the set-top box and directly connecting the cable to the input of the television and tuning to an objectionable channel would cause the receiver to display the scrambled, unviewable video component without any meaningful audio component.
The set-top boxes were manufactured in accordance with the type of audio signal to be processed. During the time period in which monaural sound was dominant and stereo sound was only a small portion of the market, set-top box manufacturers would manufacture two separate product lines, one for monaural sound and the other for stereo sound.
FIG. 2A
represents the audio processing path in a prior art monaural-only set-top box, and
FIG. 2B
represents the audio path in a prior art stereo-only set-top box. In both FIG.
2
A and in
FIG. 2B
, the audio component of the television signal can be transmitted from the headend in either a ‘clear’ mode or a ‘masked’ mode. In the clear mode, the audio component can be demodulated and processed by a conventional NTSC television receiver. In the masked mode, the audio component of the television signal is modified at the headend by the cable TV operator so that it is not demodulatable by a conventional TV receiver. The masked form of the signal has been commonly referred to as a “privacy” mode.
In the monaural-only system of
FIG. 2A
, the audio component of the television signal from the headend can be transmitted to the set-top box in either the clear form or in the masked form. The clear signal conforms to the applicable signal standard (i.e., NTSC, PAL, etc.) in which the audio component is modulated on the sound carrier according to the signal standard or specification. The “masked” frequency corresponds to a frequency twice the horizontal sweep frequency (2H). The clear signal can be processed normally in a conventional TV receiver while the masked 2H signal cannot be properly demodulated and processed.
As shown in
FIG. 2A
, the monaural-only set-top box includes processing, designated by the reference character
10
, designed to process the monaural audio associated with a clear signal and to process the monaural audio associated with a 2H masked audio signal.
The monaural processing system
10
includes a RF input
12
that provides the cable-supplied signals to a tuner
14
, which, in turn, provides the selected signal to an intermediate frequency section
16
. The output of the intermediate frequency section
16
is then provided to a down converter
18
and to a FM detector
20
that recovers the modulation envelope from the transmitted audio and presents that audio component to a clear signal path or a 2H masked path. The clear path includes an amplifier
22
. The 2H path includes a 2H band-pass filter
24
that conveys the filtered signal to a detector
26
, to a low-pass filter
28
, and to an amplifier
30
. The respective outputs of the clear path amplifier
22
and the masked path amplifier
30
are provided to a clear/masked select switch
32
. The clear/masked select switch
32
is under the control of a microprocessor
34
that receives control information via a DATA input. The control information provided to the DATA input is provided by digital information impressed on the incoming video signal, typically on the portion of the retrace subsequent to the vertical synchronization pulse. The selected output of the clear/mask switch
32
is provided to an output amplifier
36
which provides its output to the audio-out connector
38
of the set-top box for connection to the subscriber's television receiver.
During operation in which the signal from the headend has not been masked by the headend operator, the output of the FM detector
20
is provided through the amplifier
22
to the clear/masked select switch
32
. The microprocessor
34
, in response to control information provided on its DATA input, selects the clear signal for presentation through the amplifier
36
to the audio output connector
38
.
During operation in which the signal from the headend has been masked, the output of the FM detector
20
is provided to the 2H band-pass filter
24
which conveys the filtered signal to the detector
26
, to the low-pass filter
28
, and to the amplifier
30
. The output of the amplifier
30
is then presented to the clear/masked select switch
32
. The microprocessor
34
, in response to the control information provided to its DATA input, selects the output of the amplifier
30
for inputting to the amplifier
36
and presentation to the audio output connector
38
.
In the event an unauthorized user, e.g., the children of the subscriber, attempts to circumvent the set-top box by disconnecting the cable input from the RF input
12
and connecting the cable directly to the television receiver, no intelligible audio will be provided when a masked audio component is being supplied by the headed operator.
The stereo system of
FIG. 2B
functions in a manner similar to that of the monaural system of
FIG. 2A
, but processes a clear or masked stereo signal in accordance with the BTSC (Broadcast Television System Committee) standard for multi-channel television sound (MTS). Under these standards, a “masked” stereo signal is transmitted on a second FM carrier.
As shown
Landgraf Henry S.
Patel Dipakkumar R.
Troiano Anthony
General Instrument Corporation
Harvey Minsun Oh
Kananen Ronald P.
McChesney Elizabeth
Nichols Steven L.
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