Telecommunications – Wireless distribution system
Reexamination Certificate
1999-12-20
2002-10-29
Congle, Thanh (Department: 2684)
Telecommunications
Wireless distribution system
C348S725000, C725S131000, C455S112000, C455S118000
Reexamination Certificate
active
06473593
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of video broadcasting, particularly in a cable television system. More specifically, the present invention relates to an improved, multiple channel upconverter for upconverting television signals to an appropriate transmission frequency prior to transmission, e.g., over a cable television system.
BACKGROUND OF THE INVENTION
Cable television systems allow subscribers access to dozens or even hundreds of channels of television programming. This wide variety of programming accounts for the great popularity of cable television. Additionally, the current trend is for cable television systems to provide additional services such as premium channels, pay-per-view programming, video-on-demand programming and even internet access. Providing video-on-demand programming, in which the subscriber can request transmission of a program, e.g., a movie, at any time, places particular burdens on the operation of a cable television system.
In a conventional cable television system, a cable television signal is transmitted from a central facility known as a headend. The signal from the headend travels over the cable television network to the homes or offices of individual subscribers where it is received and used. Typically, each subscriber will have a set-top terminal, a box of electronic equipment, through which the cable television signal is processed after being received through a co-axial cable wall outlet. The set-top terminal is connected to the subscriber's television set and provides the cable television signal to the television for use by the subscriber. In advanced cable systems, the set-top terminal can also signal or send messages “upstream” to the headend. Consequently, such a terminal can be used by the subscriber to electronically order a video-on-demand program directly from the headend.
Cable television signals are typically generated at an intermediate (IF) frequency that is unsuitable for transmission over the cable television network. Consequently, a device called an upconverter is used at the headend to upconvert the intermediate frequency signal to a signal in the very high frequency (VHF) or ultra high frequency (UHF) range suitable for transmission over the cable television system.
Conventionally, a separate upconverter is required at the headend for each television channel being transmitted over the cable system. Additionally, upconverter units are commonly built and sold as composite units that contain two independent upconverters. Thus, one upconverter unit may include two independent upconverting circuits and handle two of the channels of the cable television system.
In a simple upconversion technique, an incoming baseband or IF signal is multiplied in a mixer with a signal produced by a local oscillator, e.g., a carrier signal. The resulting output of the mixer is an upconverted output signal that occupies a desired VHF or UHF channel or frequency band. Typically, a filter is used to pass one sideband of the mixing process that falls within the desired VHF or UHF channel or frequency band, and to block any undesired sidebands or an oscillator signal at the output of the mixer from being transmitted.
The problem with this single mixer upconverter is that generally at least one of the upper and lower sidebands and the oscillator signal fall within the higher VHF or UHF channel. As a result, a special bandpass filter is needed to just pass, for example, the upper sideband signal and reject the lower sideband and oscillator signal. Alternatively, a well-known tracking filter can be used that only tracks the desired output signal. Both of the special bandpass and tracking filters are difficult and expensive to implement.
These problems can be overcome by implementing a two-stage upconverting technique with appropriate intervening filtering. As shown in
FIG. 1
, a basic two-stage upconverter includes two local oscillators, two mixing stages and appropriate intervening filters.
The input signal (
106
) is an intermediate frequency signal, typically 41 to 47 MHz. After filtering with a filter (
101
), the input IF signal is input to the first-stage mixer (
102
). The mixer (
102
) also receives a signal (
110
) from an oscillator (
108
). The mixer (
102
) multiplies the incoming IF signal (
106
) with the signal (
110
) from the oscillator (
108
) to produce a high intermediate frequency signal. Generally, the signal (
110
) output by the oscillator (
108
) is fixed as to its frequency. The oscillator (
108
) may, however, be able to accept and make small adjustments in the output frequency of the signal (
110
).
The mixed signal is output to a filter (
103
) and then provided to a second-stage mixer (
104
). The second-stage mixer (
104
) mixes the signal output by the first-stage with a signal (
111
) output by a second-stage oscillator (
109
). In this way, the second-stage mixer (
104
) converts the signal down to the appropriate frequency, VHF or UHF, at which it will be transmitted. This frequency will be different for each channel.
Consequently, the second-stage oscillator (
109
) should be controllable to output a signal (
111
) over the wide range of VHF and UHF channel frequencies, e.g., 50 to 1000 MHz, as needed. As illustrated in
FIG. 1
, a wide range voltage controlled oscillator (
109
) may be used for this purpose. This allows the system operator to set the upconverter shown in
FIG. 1
to upconvert a television signal for any desired channel in the cable system. As noted above, a different upconverter is used for each channel.
The output of the second-stage mixer (
104
) may be filtered again in the filter (
105
). The result is an output signal (
107
) in the VHF or UHF range that is ready for transmission over the cable system. The output signal (
107
) created in this way is cleaner than those available from a one-stage upconverter.
A more advanced two-stage upconverter is described in U.S. Pat. No. 5,390,346, issued Feb. 14, 1995 to Daniel J. Marz of General Instrument Corporation. This upconverter is illustrated in FIG.
2
. As shown in
FIG. 2
, the first and second-stage oscillators illustrated in
FIG. 1
are replaced by oscillator synthesizers (
201
,
202
). These synthesizers (
201
,
202
) use a phase looked loop to synthesize the output of an oscillator, i.e., oscillating signals (
110
,
111
). A reference signal (
204
,
205
) is provided respectively to each oscillator synthesizer (
201
,
202
) to which the phase of the output signal is locked by the phase looked loop circuitry of the synthesizer (
201
,
202
).
Both the synthesizers are wide range synthesizers that can be controlled to output oscillating signals (
110
,
111
) at any desired frequency over the VHF and UHF ranges, i.e., 50 to 1000 MHz. A control means (
203
) is provided to control the frequency of the output signal of the synthesizers (
201
,
202
).
An expensive aspect of prior art upconverters, such as those described above, resides in the requirements placed on the second-stage oscillator and oscillator synthesizer. Because the second-stage oscillator determines the frequency and, therefore, the television channel, on which the output signal (
107
) will be transmitted, the second-stage oscillator must be able to output a signal (
111
) at any one of many closely located frequencies over the extremely broad range of the VHF and UHF bands. This is required to allow the cable system operator to use the upconverter for any desired channel. However, placing these demands on the second-stage oscillator increases it costs and can degrade its performance.
These limitations on prior art upconverters are exacerbated in the video-on-demand field. With video-on-demand, essentially a separate channel is used to provide the programming requested by the subscriber at the time the subscriber requests it. Thus, a separate upconverter is used to supply each program requested by a subscriber through a video-on-demand service. Consequently, as the number of upconverters increases to meet subscriber demand, the cost of
Congle Thanh
General Instruments Corporation
Nguyen Thuan T.
Rader & Fishman & Grauer, PLLC
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