WO2001003440A1

WO2001003440A1 - Method and apparatus for generating multiple independent qam channels

Info

Publication number: WO2001003440A1
Application number: PCT/US2000/008679
Authority: WO
Inventors: Mark Kolber; Shlomo Ovadia
Original assignee: General Instrument Corporation
Priority date: 1999-07-06
Filing date: 2000-04-03
Publication date: 2001-01-11
Also published as: AU4187300A; TW469741B

Abstract

A method and apparatus allow the combination of IF QAM signals for two or more different channels in a cable television system so that the resulting composite signal can be upconverted to an RF frequency by a single upconversion circuit for broadcast. The result saves both expense and space at the signal headend. A QAM modulator for each channel modulates the video signal for that channel to a different IF carrier frequency than other modulators in the system. The output of the modulators is further processed and then combined to generate an IF composite signal for multiple QAM channels that can be upconverted to RF for broadcast by a single upconverter.

Description

TITLE OF THE INVENTION

Method and Apparatus for Generating Multiple Independent QAM Channels

FIELD OF THE INVENTION

The present invention relates to the field of digital signal processing. More specifically, the present invention relates to the field of generating multiple, independent QAM channels at a signal headend, for example, a cable television system headend.

BACKGROUND OF THE INVENTION

Cable and other proprietary television systems currently provide subscribers with a vastly extended array of programming than is available through conventional over- the-air broadcasting. In a cable or other proprietary television system, such as a community antenna (CATV) or satellite system, the service provider establishes a facility at which the television signals are processed and distributed to subscribers over the cable or satellite network. This central facility is known as the headend of the system or central office.

When television signals are originally generated, they are typically generated as intermediate frequency (IF) signals with a frequency around 44 or 45 MHz in the electromagnetic spectrum. In order for television signals to be broadcast, either over-the-air or through a cable network, and used by standard television sets, the television signals must be converted to higher frequency signals which are radio frequency (RF) signals in either the very high frequency (VHF) band or the ultra-high frequency (UHF) band.

This conversion is accomplished by a device known as an upconverter. The upconverter is located at the headend station where the television signals originate. The upconverter takes the IF television signal, converts it to a VHF or UHF signal, and provides the converted signal for broadcast. At a television signal headend, a separate upconverter or upconversion circuit is conventionally used for each channel being broadcast by the service provider. The trend of the future is for the system headend to process and distribute television signals in a digital, rather than an analog, format. Digital signals can convey a greater amount of data to subscriber's television sets more quickly. This allows service providers to offer subscribers higher quality picture and sound as well as an increased number of channels .

Conventional over-the-air television signals are analog signals. To broadcast a television program using over-the-air analog signals, a carrier wave with a particular frequency is established for each channel. Changes in the amplitude of the carrier wave, called modulations, are then made to convey the information of the television program encoded in the over-the-air signal. The modulated carrier wave is then upconverted from an intermediate frequency (IF) signal to a radio frequency

(RF) signal, i.e., a VHF or UHF signal, for broadcasting. An antenna receives the modulated carrier wave and feeds it to a television set. Television sets respond to the modulations of the carrier wave to generate the display of the television program being broadcast.

Digital television signals operate on the same principle of modulating a carrier wave. However, digital signals are not merely amplitude modulated, as analog signals are, but use modulation systems such as quadrature amplitude modulation ("QAM") in which both amplitude and phase are modulated. As noted before, this allows a service provider to pack significantly more information into a digital signal than a conventional analog signal.

While there are great advantages in the digital signal format, the cost and amount of equipment required to produce the more sophisticated digital QAM signals is also increased. In particular is the problem of producing multiple, independent QAM channels at different carrier frequencies in the radio frequency range.

Fig. 1 illustrates a conventional system for providing two different digital channels that are modulated using QAM and then combined for broadcast to subscribers. As shown in Fig. 1, the video data signals for two different television programs (104 and 105) are separately provided to two QAM modulators (102 and 103) . Each of the QAM modulators modulates a 44 MHz carrier signal in accordance with the data from the incoming video data signal (104 or 105) .

The result is a modulated IF signal at 44 MHz that is output by each of the QAM modulators (102 and 103) . Each of the output IF signals is respectively provided to a separate upconverter (100 or 101) which upconverts the IF signal to a UHF or VHF signal for broadcast. The upconverter (100) will output a UHF or VHF signal centered at a different carrier frequency than the other upconverter (101) or any other upconverters in the system. The resulting UHF or VHF signals at different carrier frequencies are feed to a combiner (107) where they are combined into a single signal (106) for broadcast over the cable or other proprietary television system. Consequently, a separate QAM modulator and upconverter are required for each channel that the cable television system includes. This is a costly solution that also consumes valuable space at the headend facility. This is particularly true if many different QAM channels are needed for such services as video-on-demand.

Therefore, there is a need in the art for a simplified and less expensive method and apparatus for generating multiple, independent QAM channels at different RF frequencies .

SUMMARY OF THE INVENTION It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide a simplified and less expensive method and apparatus for generating multiple, independent QAM channels at different RF frequencies .

Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims .

To achieve these stated and other objects, the present invention may be embodied and described as an apparatus for processing multiple QAM signals for independent television channels prior to broadcast. A preferred embodiment of the invention includes at least two quadrature amplitude modulators, each receiving, respectively, a video signal for a separate channel, and each outputting respectively a quadrature amplitude modulated signal at different IF carrier frequencies; and a combiner for combining the quadrature amplitude modulated signals output by the at least two quadrature amplitude modulators. A single upconvertor is then required for upconverting the composite intermediate frequency signal output by the combiner to a radio frequency. In this way, the need for a separate upconverter for each channel is eliminated.

In more detail, the apparatus of the present invention may include an x/sin(x) filter for filtering the composite intermediate frequency signal output by the combiner. Next, a digital-to-analog converter converts the composite intermediate frequency signal from digital to analog after the x/sin(x) filtering. Finally, a surface acoustic wave filter may be used to filter the composite intermediate frequency signal after the digital-to-analog conversion. The signal is then ready for upconversion.

Alternatively, multiple x/sin(x) filters, digital-to- analog converters and optional surface acoustic wave filters may be provided to separately process the modulated signals output by the QAM modulators prior to the combiner which combines the signals for the various channels. In this embodiment, the output of the combiner may be provided directly to the upconverter. The present invention also encompasses the method of making and operating the apparatus described above. For example, the present invention includes the method of processing multiple QAM signals for independent television channels prior to broadcast by combining intermediate frequency quadrature amplitude modulated signals output by at least two quadrature amplitude modulators to generate a composite intermediate frequency signal, where the at least two quadrature amplitude modulators each receive a different video signal for a channel, and each output a quadrature amplitude modulated signal at a different intermediate frequency carrier frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present invention.

Fig. 1 a block diagram of a conventional system for generating multiple independent QAM channels in a cable television signal showing multiple upconverters 100 and 101.

Fig. 2 is a diagram of a first embodiment of the present invention for generating multiple independent QAM channels at different RF frequencies in a cable television signal showing a single upconverter 210. Fig. 3 is a diagram of a second embodiment of the present invention for generating multiple independent QAM channels at different RF frequencies in a cable television signal with a single upconverter 210. In figure 2, the two QAM channel combination is done in the analog domain, while in Fig. 3 it is done in the digital domain. This means that only one digital to analog converter is needed for fig. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Using the drawings, the preferred embodiments of the present invention will now be explained.

Fig. 2 illustrates a system according to the present invention in which each of the QAM modulators (102 and 103) uses a different IF carrier frequency to which an incoming video signal (104 or 105) is modulated. The output of the QAM modulators (102 and 103) is then further processed and combined by a combiner (209) . By combining the two modulated IF signals prior to upconversion to RF frequencies, a single wide-band upconverter (210) can upconvert the combined signal for two or more QAM channels.

This eliminates the need for a separate upconverter for each channel as is required in the prior art. (Fig. 1) .

In fact, modern upconverter units are packaged as dual units comprising two upconversion sub-systems which are designed to function independently to handle two separate QAM channels. With the present invention, such a dual upconverter unit could instead handle, for example, the processing of four QAM channels, each sub-unit upconverting two channels for broadcast.

As shown in Fig. 2, the digital video signals (104 and 105) for two separate television channels are provided respectively to two QAM modulators (102 and 103) which output a quadrature amplitude modulated signal, each at a different IF carrier frequency. The QAM modulators (102, 103) preferably include a forward error correction (FEC) encoder (214 ) .

In quadrature modulation, there are two independent orthogonal modulated signal components referred to as the in-phase (I) and quadrature (Q) signals. The I and Q signals are modulated onto a single carrier. The I signal is multiplied by a cosine wave (Cos ωt) at the carrier frequency, and the Q signal by a sine wave (Sin ωt) carrier at the carrier frequency. After the FEC encoder, the I signal and Q signal of the QAM signal are separated. The I signal is processed along signal path (212) while the Q signal is process along signal path (213) . The resultant signals are then combined by a combiner (215) to form the composite I/Q modulated signal. It is important that each of the modulators (102 and 103) output a QAM signal modulated to a different IF carrier frequency. In the example of Fig. 2, the outputs of the QAM modulators (102 and 103) are, respectively, a QAM signal at an IF frequency of 44 MHz from modulator (102) and a QAM signal at an IF frequency of 50 MHz from modulator (103).

Each of the QAM signals output by the modulators (102 and 103) is respectively processed through an x/sin(x) correction filter (203, 206) and then converted from a digital to an analog signal (204, 207).

The analog signals may additionally be passed through respective surface acoustic wave (SAW) filters (205, 208) at 6 MHz for example. However, the SAW filtering may be omitted for further savings on components and expense. Finally, the signals for the two channels are combined in a combiner (209) to produce a dual QAM channel signal at in the IF frequency range. This signal can then be upconverted to an RF frequency signal (211) for broadcast. A single upconversion circuit (210) handles the upconversion for the composite signal, thereby upconverting two channels at once. It should be noted that under the principles of the present invention, more than two independent QAM channels can be combined in the IF frequency range and then upconverted by a single wide-band upconverter. The invention is not limited to the example provided above in which two QAM channels are combined for upconversion. In either case, the approach of the present invention is more economical and space-efficient than the previous system requiring a separate upconverter for each QAM channel being broadcast.

Fig. 3 illustrates an alternative embodiment which can eliminate even more equipment. As shown in Fig. 3, the output of the QAM modulators (102 and 103) are immediately combined in a combiner (301) still in the digital domain. Again, each of the QAM modulators (102 and 103) modulates the incoming video signal (104 or 105) to a different IF carrier frequency.

The composite signal from the combiner (301) is then processed through a single x/sin(x) filter (203), converted to an analog signal (204) and then filtered through a SAW wide-band filter (205) . A wide-band SAW filter has a band width greater than 6-MHZ. The IF output of the SAW filter is then provided to the upconverter (210) . The upconverter (210) then outputs an RF signal (211) representing the two QAM channels. The RF signal (211) is then ready for broadcast .

The preceding description has been presented only to illustrate and describe the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.

The preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for processing multiple QAM signals for independent television channels prior to broadcast, comprising : at least two quadrature amplitude modulators, each receiving, respectively, a video signal for a separate channel, and each outputting respectively a quadrature amplitude modulated signal at different intermediate frequency carrier frequencies; and a combiner for combining said quadrature amplitude modulated signals output by said at least two quadrature amplitude modulators.

2. The apparatus of claim 1, further comprising a single upconvertor for upconverting to radio frequency a composite intermediate frequency signal output by said combiner .

3. The apparatus of claim 1, further comprising an x/sin(x) filter for filtering a composite intermediate frequency signal output by said combiner.

4. The apparatus of claim 3, further comprising a digital-to-analog converter for converting said composite intermediate frequency signal from digital to analog after said x/sin(x) filtering.

5. The apparatus of claim 4, further comprising a surface acoustic wave filter for filtering said composite intermediate frequency signal after said digital-to-analog conversion .

6. The apparatus of claim 5, further comprising a single upconvertor for upconverting to radio frequency said composite intermediate frequency signal.

7. The apparatus of claim 1, further comprising first and second x/sin(x) filters for filtering respective modulated signals from first and second quadrature amplitude modulators of said at least two quadrature amplitude modulators .

8. The apparatus of claim 7, further comprising first and second digital-to-analog converters for converting respective outputs of said x/sin(x) filters from digital to analog after said x/sin(x) filtering.

9. The apparatus of claim 8, further comprising first and second surface acoustic wave filters for filtering respective outputs of said digital-to-analog converters after said digital-to-analog conversion, outputs of said first and second surface acoustic wave filters being provided to said combiner.

10. A method of processing multiple QAM signals for independent television channels prior to broadcast, comprising combining intermediate frequency quadrature amplitude modulated signals output by at least two quadrature amplitude modulators to output a composite intermediate frequency signal, wherein said at least two quadrature amplitude modulators each receive a different video signal for a channel, and each output a quadrature amplitude modulated signal at a different intermediate frequency carrier frequency.

11. The method of claim 10, further comprising upconverting to radio frequency said composite intermediate frequency signal.

12. The method of claim 11, wherein said upconverting is performed with a single wide-band upconverter.

13. The method of claim 10, further comprising x/sin(x) filtering said composite intermediate frequency signal .

14. The method of claim 13, further comprising digital-to-analog converting said composite intermediate frequency signal from digital to analog after said x/sin(x) filtering .

15. The method of claim 14, further comprising surface acoustic wave filtering said composite intermediate frequency signal after said digital-to-analog conversion.

16. The method of claim 15, further comprising upconverting to radio frequency said composite intermediate frequency signal.

17. The method of claim 10, further comprising x/sin(x) filtering respective modulated signals from said at least two quadrature amplitude modulators prior to said combining.

18. The method of claim 17, further comprising digital-to-analog converting respective modulated signals from said at least two quadrature amplitude modulators from digital to analog after said x/sin(x) filtering.

19. The method of claim 18, further comprising surface acoustic wave filtering respective modulated signals from said at least two quadrature amplitude modulators after said digital-to-analog conversion and prior to said combining .

20. An apparatus for processing multiple QAM signals for independent television channels prior to broadcast, comprising: at least two digital modulating means, each receiving, respectively, a video signal for a separate channel, and each outputting respectively a modulated signal at different intermediate frequency carrier frequencies; a combining means for combining said modulated signals output by said at least two modulating means; and an upconverting means for upconverting a composite intermediate frequency output by said combining means to radio frequency.