Television – Receiver circuitry – Sound circuit
Reexamination Certificate
1998-10-21
2002-11-05
Miller, John (Department: 2614)
Television
Receiver circuitry
Sound circuit
C348S736000, C348S555000, C348S632000, C381S094500, C455S286000, C455S266000, C455S307000
Reexamination Certificate
active
06476878
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to controlling audio anomalies and in particular to controlling audio anomalies in signals which have an auxiliary audio signal or dual language audio signal adjacent to a main audio signal.
BACKGROUND OF THE INVENTION
Audio anomalies occur in a variety of signals such as in the television signals of a television system which has an auxiliary audio signal adjacent to a main audio signal. An understanding of the system in which a signal occurs can help define the causes of the audio anomalies. A television system is used to illustrate the audio anomalies which occur with a signal having an auxiliary audio signal adjacent to a main audio signal.
Television system
10
as illustrated in
FIG. 1
, includes a headend
14
which receives input programming from input sources
12
(
a-c
). Headend
14
combines the programming from the various input sources
12
(
a-c
) into a television signal with multiple channels and modulates and distributes the television signal to subscriber locations (e.g., subscriber location
17
) via distribution system
16
. The audio options associated with the multiple channels will vary. Some channels will be available in either monophonic or stereo sound, others will only be available in monophonic sound. Some channels may have an optional second audio language. At subscriber location
17
an intermediate device, such as a home communication terminal (HCT)
18
or a set-top, interfaces to distribution system
16
and to receiver
19
. Receiver
19
can be a monophonic television, a stereo television, an audio only stereo receiver, or a computer. HCT
18
allows access to the auxiliary audio options as an additional service of television system
10
. The options are unique to each television system
10
. For example, a television system
10
may have auxiliary audio signaling on all channels, such as a second audio language for every channel. Another television system
10
may have no auxiliary audio signaling, while another may have auxiliary audio signaling on one or more channels. Television signal
20
contains multiple channels.
FIG. 2
is a graphic representation illustrating the components of a single channel within multi-channel television signal
20
having auxiliary audio signal
21
adjacent to main audio signal
22
. The components also include video signal
24
and color signal
26
. The auxiliary audio signal
21
is provided because additional languages and stereo sound are available and in demand by subscribers paying for access to television signal
20
. The auxiliary audio signal
21
may be, for example, Near Instantaneously Companded Audio Multiplex (NICAM) audio signals or dual language audio signals. It is important to note that, even in systems designed for multiple audio signals, auxiliary audio signal
21
may not be present on every programming channel in television signal
20
. A set-top, HCT
18
, or any television can process main audio signal
22
. The reception or processing of auxiliary audio signal
21
can be limited by several factors such as the type of receiver
19
, system controlled conditional access within television system
10
, and the availability of additional audio on auxiliary audio signal
21
.
The multiple channels occur at a set spacing based on the format of television signal
20
. In Phase Alteration (by) Line type B (PAL-B) and PAL-G formats the channel spacing is 7 MHz. The location of television signal
20
components is also fixed within the total channel allocation based on the format. The location of main audio signal
22
is offset from the video signal by a fixed frequency. For example, the National Television System Committee (NTSC) format uses a frequency offset of 4.5 Megahertz (MHz), a PAL-I format uses a 6.0 MHz frequency offset, and PAL-B and PAL-G formats use a 5.5 MHz frequency offset.
Audio anomalies are most prevalent in PAL-B, -PAL-G, and PAL-B/G hybrid formats due to the close channel spacing, but have occurred in other formats such as PAL-I. A PAL-B signal format is used for all examples unless stated otherwise. In a PAL-B or PAL-G system additional limitations such as a total channel allocation of only 7 MHz impacts the audio signals. The main audio signal
22
is frequency offset from the video carrier by 5.5 MHz. The auxiliary audio signal
21
is frequency offset from the video carrier by 5.85 MHz. With limited space within the total channel allocation, the auxiliary audio signal
21
is located at only 350 Kilohertz (KHz) above the main audio signal
22
. Conversely, in a PAL-I system the auxiliary audio signal
21
is located at 552 KHz above the main audio signal
22
. In theory the close spacing of the two audio signals is not a problem as the main audio signal
22
would occur right at 5.5 MHz, however when the main audio signal
22
is modulated there is a spread around 5.5 MHz as shown by the shaded region in FIG.
2
. The close proximity of the two audio signals in a PAL-B or PAL-G system creates audio anomalies when filtering out one audio signal to receive the other.
One audio anomaly created in the filtering process is a constant tone generated on a television using the main audio, a monophonic audio. The constant tone is referred to as “audio whistle”. Audio whistle occurs when a portion of the main audio signal
22
is processed through the auxiliary audio signal processing circuitry or when signal interference occurs due to the layout of the components within HCT
38
. Due to the close proximity of the two signals, filters designed to isolate the auxiliary audio signal
21
capture a small portion of the spread main audio signal
22
, for example a portion of the main audio signal
22
at 5.5 MHz. Thus, when the signals are recombined in the output of HCT
18
, there are two components of the 5.5 MHz main audio signal
22
with different amplitudes and only slightly different or the same frequencies. One of the components is generated by processing main audio signal
22
. The other component is the portion of the main audio signal
22
that was processed through the auxiliary audio signal
21
circuitry. The additional component results in the generation of a constant tone signal, the audio whistle. Attempts to eliminate audio whistle have resulted in the creation of additional problems.
One proposed solution involves locking the main audio signal
22
output frequency to the main audio signal
22
input 5.5 frequency. This solution was unsuccessful as the main audio signal
22
input when modulated had signal spread and the resultant signal lock constantly shifted resulting in a loss of volume control.
A second proposed solution involved designing a deeper notch separating the two signals. Created using devices such as sound traps or filters, a deeper notch or gap between the main audio signal
22
and the auxiliary audio signal
21
allows easier separation of the two signals. Easier separation allows for an easier removal of the main audio signal
22
in the auxiliary audio signal
21
processing circuitry, thus no additional main audio signal
22
component remains in the recombined signal. A deeper notch while currently used in the art, creates additional problems such as audio pop. Audio pop is an instantaneous crackle or popping sound on the stereo or dual language audio.
Attempts to solve these audio anomaly problems included expensive sophisticated filters which still often result in audio popping. Modifying the response of the auxiliary audio signal
21
circuitry to create a deeper notch is also currently being done in the art, but has been found to still result in both some level of audio whistle and some audio pop. Indeed, because the causes and solutions of the audio whistle and audio pop are not easily understood, many cable television systems having multiple audio signals suffer from these audio anomalies.
Thus, what is needed is a better understanding of what is causing the audio anomaly problems and a method, apparatus, and system for eliminating them in signals which may have an auxiliary
Cheng Robert C.
Hsu Benny
Lafay William P.
Taylor Martin
Barnhardt III Hubert J.
Couturier Shelley L.
Massaroni Kenneth M.
Miller John
Natnael Paulos
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