CA2239274A1 - Common high-power amplification of adjacent-channel signals - Google Patents

Abstract

Use of a single high-power amplifier, for common amplification of adjacent-band television signals, is not only possible, but practical and economical. Such adjacent-band signals can include analog-format signals such NTSC signals, and digital-format signals such as DTV signals, in all combinations (NTSC/DTV, DTV/NTSC, NTSC/NTSC and DTV/DTV). The amplifier preferably includes a tetrode-class tube (such as a diacrode) and a properly-chosen cavity resonator. Circuits provide analog-format (e.g., NTSC) and digital-format (e.g., DTV) signals under control of a common frequency reference. The amplifier input receives a signal from a device that cleanly and linearly combines the NTSC and DTV signals at broadcast frequency, and amplifies the combined signal for transmission through a single, common antenna. Thus, not only may a single antenna be used for transmission of adjacent-band analog-format and digital-format signals, but interference between the transmitted analog-format and digital-format signal components is avoided by virtue of the substantial linearity of the combiner and amplifier components.

Description

COMMON HIGH-POWER AMPLIFICATION
OF ADJACENT-CHANNEL SIGNALS
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to television transmitters. More specifically, the invention relates to television transmitters in which a single high-power amplifier amplifies adjacent-channel television signals (especially adjacent-channel NTSC and DTV digital television signals), in common, for broadcast on a single antenna.

2. Related Art Using a single amplifier to amplify visual and aural components of an NTSC
television signal has been performed since the 1980s, when high-power UHF
tetrodes were developed and klystrode~/IOT (inductive output tube) devices were introduced.
The IOT, as well as the tetrode and its derivative, the diacrode~, have been used in common amplification (that is, amplification using a single amplifier) of elements within the NTSC signal.
With the advent of DTV (digital television), the Federal Communications Commission (FCC) has allocated television channels in a manner that has resulted in many broadcasters having their respective DTV channels assigned immediately above their NTSC channels in the frequency spectrum (called the "n+1" situation), or immediately below their respective NTSC channels (the "n-1 " situation).
Broadcasters desire to add DTV broadcasting capability to their existing NTSC broadcasting ability as inexpensively as possible. However, the immediate adjacency of related NTSC
and DTV channels has raised technical problems that were not present in simple NTSC
channels. The problems raised by the adjacent-channel NTSC-DTV arrangement are based largely on a combination of demanding requirements, including flatness of antenna response across a wider bandwidth (12 MHZ in the United States, 16 MHZ in most countries outside the U.S.), and power-combining two transmitters with a channel combiner where there is little or no guard band.
The presence of n+1 and n-1 situations under FCC channel assignments has conventionally been thought to require separate amplifiers for the NTSC and DTV
signals, even if a single antenna were used to transmit both signals.
Moreover, if the NTSC and DTV signals were generated separately, without using a common frequency reference to keep the signals in phase, additional problems would arise in the transmitted signals.
In the n-1 situation, in which a small guard band is present between the (lower) DTV channel and the (higher) NTSC channel, it is possible to provide a channel combining filter system to allow both signals to be transmitted on a common antenna.
But in the n+1 situation, there is virtually no guard band whatsoever between the (lower) NTSC channel and the (higher) DTV channel: there is a meager (200 KHz) separation between the NTSC signal's deviated aural carrier (at the higher end of the NTSC channel) and the lower edge of the DTV channel. Especially in the n+1 situation, therefore, use of a filter combiner implementation at the input to a common NTSC/DTV
broadcast antenna is not practical.
Various systems involving NTSC, DTV, or combination signals, are known in the art, including U. S. Patent No. 5,532,748 (Naimpally), U. S. Patent No.
5,412,426 (Totty), U.S. Patent No. 5,200,709 (Saito et al.), U.S. Patent No. 5,148,279 (Gabor), U.S. Patent No. 5,127,021 (Schreiber), U. S. Patent No. 5,019,788 (Fischer et al. ), U. S.
Patent No. 4,423,386 (Gerard), U. S. Patent No. 4,227,156 (Mattfeld), U. S. Patent No. 4,117,413 (Moog), and U.S. Patent No. 4,045,748 (Filliman).
Totty's patent discloses simulcast transmission of NTSC video information with HDTV information, but achieves this by digitizing the originally-analog NTSC
signals before combining the digital result with the HDTV data. Naimpally discloses a hybrid digital and analog television signal in which an analog signal is combined with a digital signal before the combination is transmitted (see FIG. 4 frequency domain diagram, and the two parallel paths in the circuit diagram in FIG. 5.) More generally, Gabor discloses a television transmitter that is said to be able to transmit fifty channels simultaneously using a common amplifier and a single antenna (see power amplifier module in FIG. 3). Schreiber discloses a more complex television transmission system in which a television signal is first divided into various frequency components before being combined again, the combined signal being transmitted.
Fischer et al. and Saito et al. appear to illustrate the general approach of providing separate amplification paths for signals of different respective frequency ranges.
Finally, the Mattfeld, Gerard, Filliman and Moog patents disclose systems that amplify signals of different frequencies. MattFeld provides a common AM/FM
amplifier at IF; Gerard provides a distributed amplifier said to reduce intermodulation products;
Filliman is concerned with separation and re-combination of frequency-based audio signal bands; and Moog discloses an amplifier with various filters for emphasizing different (especially, audio) frequency ranges.
However, none of these documents solve the problem described above, relating to efficient transmission of adjacent-band NTSC and DTV signals. Thus, there has been a need in the art to provide a system that would enable broadcasters to broadcast acceptably clean, adjacent-band NTSC and DTV signals, and to do so at minimum cost.
It is to fulfill this need that the present invention has been developed.
SUMMARY OF THE INVENTION
Applicant has recognized that use of a single high-power amplifier, for common amplification of adjacent-band signals, is not only possible, but practical and economical.
Such adjacent-band signals can include analog-format signals such NTSC
signals, and digital-format signals such as DTV signals, in all combinations (NTSC/DTV, DTV/NTSC, NTSC/NTSC and DTVlDTV).
More specifically, the invention provides an amplifier that preferably includes a tetrode-class tube (such as a diacrode) and a properly-chosen cavity resonator. Circuits provide analog-format (e.g., NTSC) and digital-format (e.g., DTV) signals under control of a common frequency reference. The amplifier input receives a signal from a device that cleanly and linearly combines the NTSC and DTV signals at broadcast frequency, and amplifies the combined signal for transmission through a common antenna.
Thus, the invention not only allows a single antenna to be used for transmission of adj acent-band analob format and digital-format signals, but avoids interference between the transmitted analog-format and digital-format signal components by virtue of the substantial linearity of the combiner and amplifier components.
Other objects, features and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is better understood by reading the following Detailed Description of the Preferred Embodiments with reference to the accompanying drawing figures; in which like reference numerals refer to like elements throughout, and in which:
FIG. 1 is a high-level functional block diagram that schematically illustrates important elements of an exemplary, preferred embodiment of the present invention.
FIG. 2 illustrates details of an exemplary NTSC signal modulator 120 of the embodiment of FIG. 1 FIG. 3 illustrates details of an exemplary DTV signal modulator 130 of the embodiment of FIG. 1 FIG. 4 illustrates details of an exemplary combiner 140 and amplifier 145 of the embodiment of FIG. 1.
_5_ FIG. 5 shows an alternative to the embodiment of FIGS. 2-4, emphasizing an alternative implementation of the power level AGC feedback connections.
$ DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose. Moreover, components and design procedures that are readily known to those skilled in the art are omitted for the sake of clarity.
Referring to FIG. 1, a high-level block diagram of a preferred embodiment of the present invention is illustrated schematically, with conceptually less important components omitted for the sake of clarity. An NTSC signal modulator 120 receives an NTSC video signal (in the DC-to-4.2 MHZ frequency range) and an audio signal, and provides a broadcast-frequency NTSC signal (an RF signal in an assigned channel "N") to a first input of a combiner 140. Analogously, a DTV (digital television) signal modulator 130 receives a bit stream (such as an SMPTE 310M-compliant MPEG bit stream at 19.39 MHZ), and provides a broadcast-frequency, 8-VSB-compliant DTV
signal to a second input of the combiner 140. The NTSC signal modulator 120 and the DTV signal modulator 130 operate under the timing of a common frequency reference 100 that helps to keep the combined signals from undesirably interfering with each other as they would if not phase locked.
The combiner 140 linearly combines the broadcast-frequency signals and provides a combined NTSC/DTV signal to a single high-power amplifier 145. The single amplifier 145 drives a single transmitting antenna 150 that transmits both the NTSC and DTV signals.
The characteristics of the amplifier are important to the invention, because, as introduced in the Background of the Invention, common wisdom would require two separate amplifiers and (possibly) two antennas, in an arrangement that would have undesirable interference characteristics between the two signals. Applicant's invention avoids the need for an additional amplifier and the undesirable interference characteristics that would be characteristic of conventional approaches to the n+1 and n-1 problems.
FIG. 2 shows certain details of an exemplary embodiment of the NTSC signal modulator of FIG. 1. Conventional features that are well understood to be necessary or desirable to implementation of the circuit are omitted for the sake of clarity.
The illustrated NTSC signal modulator 120 receives an NTSC video signal (in the DC-to-4.2 MHZ frequency range) and an audio signal, and provides a broadcast-frequency NTSC signal (an RF signal in an assigned channel "N") to the combiner (not shown in FIG. 2). This overall function is readily implemented by those skilled in the art, and it is to be understood that the illustrated embodiment is shown by way of example and is not intended to limit the scope of the invention in any way.
Additional details need not be provided, as various vendors provide commercially available products that perform this overall function.
Referring to FIG. 2, a video processor 210 manipulates the input video signal to compensate, in advance, for distortion that is expected to occur at intermediate frequency (IF) and broadcast radio frequency (RF) to the modulated visual signal. Video processor $ 210 provides such compensation functions as differential gain compensation and differential phase compensation and luminance non-linearity compensation, to achieve linearity of response.
An NTSC modulator 220 receives the compensated video signal from the video processor 210, along with an associated audio signal. Essentially, the modulator modulates the video and audio signals onto an intermediate frequency carrier from a phase lock loop (PLL) 225 that is phase locked to common frequency reference 100. The modulator may comprise, for example, a vestigial side band (VSB) filter implemented with surface acoustic wave (SAW) technology. In any event, NTSC modulator 220 outputs an intermediate-frequency signal with NTSC-standard visual and aural carriers at 45.75 MHZ and 41.25 MHZ carrier frequencies, respectively.
Frequency reference 100 may include, for example, an oscillator 101 that provides a stable-frequency reference signal (such as 10 MHZ) to various circuit components via a signal sputter 102. Oscillator 101 may be implemented as a temperature-controlled crystal oscillator, an oven-controlled crystal oscillator, a GPS (global positioning system) reference signal, and the like. Signal sputter 102 may be any circuit that fans out the reference signal, ensuring a constant phase relationship throughout the circuits it drives.
The NTSC IF signal from modulator 220 is pre-distorted by an NTSC IF
_g_ processor 230 to compensate for distortion that is expected to occur at radio frequency (RF) to the modulated NTSC signal. NTSC IF processor 230 may compensate for such undesirable phenomena as intermodulation distortion, cross-modulation distortion, and incidental carrier phase modulation distortion, and the like, resulting in a compensated, purely amplitude-modulated signal that is desired. The pre-compensated NTSC IF
signal is provided to IF-to-broadcast-frequency converter 240.
IF-to-broadcast-frequency converter 240 also receives a sinusoidal carrier of a frequency determined by the desired broadcast frequency of the particular broadcast channel "N", such as in the UHF' range, that is allocated to the broadcast site involved.
The carrier is provided by a phase lock loop (PLL) 250, which is phase-locked to an output from the frequency reference 100. IF-to-broadcast-frequency converter includes a mixer that provides a low-power (for example, one watt) modulated NTSC
signal at Channel N's broadcast frequency. Converter 240 reverses the frequency order of the aural and visual components of this low-power, broadcast-frequency, modulated NTSC signal, so that the visual component carrier is now below the aural component carrier, in accordance with broadcast standards.
A series of amplifiers, shown by exemplary intermediate power amplifier (IPA) 260 and driver amplifier 270, amplify the low-power, broadcast-frequency, modulated NTSC signal from converter 240 to a power level closer to broadcast power levels. For example, driver amplifier 240 may output a signal of 2.5 kW peak average power at sync, with 125 W aural power. This signal is provided to the first input of the combiner 240 shown in FIGS. 1 and 4.
_g_ Preferably, the signal output to the combiner is subject to automatic gain control (AGC). For this purpose, one example of an AGC feedback path is provided from the output of driver amplifier 270 to the IF-to-broadcast-frequency converter 240.
Gain control circuitry that may be of conventional design, and located within converter 240, ensures that an NTSC signal of substantially constant power level is provided to the combiner.
Referring now to FIG. 3, details of an exemplary DTV signal modulator 130 (FIG. 1 ) are shown by way of illustration and not to limit the scope of the invention.
Like the description of the NTSC signal modulator of FIG. 2, the following description omits conventional elements known to those skilled in the art, with the understanding that commercially-available products perform the same overall function performed by FIG.
3. Further, the present description is abbreviated because the functions performed by FIG. 3 elements 320, 325, 330, 340, 350, 360, 370, 375 and 377 perform functions that are analogous to the functions of FIG. 2 elements 220, 225, 230, 240, 250, 260, 270, 275 and 277.
Referring again to FIG. 3, a modulator 320 receives a carrier frequency signal that is phase locked by PLL 325 to a reference carrier from frequency reference element 100 (FIGS. 1 and 2). Modulator 320 further encodes a 19.39 MHZ, SMPTE 310M-compliant MPEG bit stream, and modulates a pilot carrier at 46.69 MHZ in accordance with (for example) the 8-VSB standard accepted by the Federal Communications Commission for terrestrial broadcast. Modulator 320 outputs an intermediate-frequency analog signal with a pilot carrier at 46.69 MHZ, at the upper edge of the 41--ZO-band allocated for television signals at IF.
A DTV IF processor 330 processes the IF signal from the modulator, performing pre-compensation and pre-conditioning functions generally analogous performed by processor 230 (FIG. 2) for NTSC signals. However, DTV IF processor 330 is preferably implemented as a digital signal processor (DSP) to perform the pre-compensation and pre-conditioning functions on a digital-content signal, using techniques (such as finite impulse response filters) that are better suited to processing of such signals. In any event, the DTV IF processor 330 provides a pre-compensated and pre-conditioned signal to an IF-to-broadcast-frequency converter 340.
IF-to-broadcast-frequency converter 340 converts the analog IF signal from the DTV IF processor to a broadcast-frequency signal. In the preferred application of the invention, in which the DTV channel is immediately adjacent the corresponding NTSC
channel in the frequency spectrum in accordance with FCC channel assignments, two situations are encountered. The "n-1" situation involves a DTV channel that is immediately below the NTSC channel, and the "n+1" situation involves a DTV
channel that is immediately above the NTSC channel. In FIG. 3, the IF-to-broadcast-frequency converter 340 is therefore illustrated as providing a signal on Channel N-1 or Channel N+l, where "N" is the channel assigned the corresponding NTSC channel in FIG.
2.
Essentially including a mixer, converter 340 receives a sinusoidal carrier signal from a phase lock loop 350 that is driven by frequency reference element 100 (FIGS.
1, 2) to modulate the DTV IF signal.
The converter's operation results in a reversal of the frequency order of the DTV

signal from the upper end of the channel (46.69 MHZ is near 47 MHZ) to the lower end of the channel at broadcast frequencies. It is noteworthy that, in the n+1 situation, this placement of the DTV signal at the lower end of the DTV channel places it only 200 kHz away from the deviated NTSC aural carrier.
Converter 340 provides a low-power, broadcast-frequency signal to a series of amplifiers, shown in FIG. 3 as including an intermediate power amplifier (IPA) 360 and a driver amplifier 370. Driver amplifier 370 provides to the FIG. 1 combiner, an 8-VSB-compliant DTV signal, in Channel N-1 for the "n-1" channel allocation situation or in Channel N+1 for the "n+1" channel allocation situation. The signal from driver amplifier 370 is of a power level sufficient to drive the high power amplifier 145 to provide the desired broadcast output power. An AGC feedback path 375 is provided from the drivers's output back to the converter 340, which ensures that the signal provided to the combiner is of substantially constant power level.
Referring now to FIG. 4, a preferred implementation of the combiner 140 and high-power amplifier 145 are illustrated. Combiner 140 is preferably implemented as a conventional quadrature hybrid combiner of the type discussed in detail in commonly-assigned U.S. Patent No. 4,804,931, issued in 1989, and which, for completeness, is incorporated herein by reference.
As is readily appreciated by those skilled in the art, hybrid combiners are four-port devices that have two outputs, each one of which receives half the signal power from each of the combiner's two inputs. Thus, an undesirable characteristic of hybrid combiners is that they halve the power of the sum signal. In the present use of hybrid combiners, half the power from each input signal is provided to the high-power amplifier 145, while the other half of the power from each input signal is wasted through dissipation in resistance 141 to ground. Despite the power loss to resistance 141, the desirable linearity of the hybrid combiner, and the isolation of the input signals from each other to thereby avoid undesirable mixing of the two inputs, make it a preferred implementation for combiner 140.
High-power amplifier 145 is an essential element of the invention, as it fulfills the demand of flatness of response (less than 1 dB) across a two-channel-wide bandwidth (12 MHZ in the United States, 16 MHz in most countries outside the U.S.), and the requirement for meaningful power to the NTSC+DTV signals with minimal inter-channel interference. The invention provides that a tetrode-class device, and especially a diacrode amplifying device such as a Thomson TH-680, provide optimum performance for this application. Tetrode and diacrode implementations are preferred because of their ability to operate with cavity sections tunable to wide (two-channel wide) bandwidths, to exhibit sufficient linearity so that cross modulation and intermodulation distortion may be corrected with established methods, and to provide meaningful broadcast power levels.
Of course, the scope of the invention should not be limited to tetrode and diacrode solutions; alternative implementations, such as those involving solid state amplifiers, also lie within the contemplation of the invention. The diacrode or tetrode power amplifier may be replaced by a suitable broadband solid state amplifier using an appropriate number of power RF transistors to get to the required power, and advantageously can operate in both the UHF' and VHF bands.

In an exemplary embodiment to which the invention should not be limited, the TH-680 can provide 104 kW of peak envelope power, which may (as a non-limiting example) include the following allocation of power levels. To reduce interference with channels outside the adjacent-channel pair, a suitable two-channel-wide (12 MHZ in the U.S.) band pass filter (BPF) is provided at the output to the amplifier 145.
To comply with broadcast power standards, the amplifier 145 must amplify the combined NTSC+DTV signal so that the BPF provides a signal 25 kW average peak-of sync power (NTSC), 1.25 kW NTSC average aural power, and 2.5 kW average DTV power. Of course, variation of the above particulars in accordance with commonly-known principles lies within the ability of those skilled in the art.
As is readily appreciated by those skilled in the art, such an amplifier involves a tube that performs the power amplification, as well as a resonator cavity that limits the frequency range in which the tube amplifies signals. For any assigned adjacent-channel pair (either Channel N-1 through N, or Channel N through N+1), one skilled in the art, upon reading this specification, is readily capable, without undue experimentation, of implementing a properly-tuned amplifier using a suitable tetrode-class device and resonator cavity. The implementation is different for each adj acent-channel pair, but the design principles remain the same regardless of the particular assignment, and further details need not be provided here to illustrate the implementation and operation of the invention.
For many applications, high-power amplifier 145 comprises a tetrode-class device, especially a Thomson TH-680 diacrode, available from Thomson Tubes Electronique. Some of the specifications of the TH 680 relevant to an exemplary NTSC
application are presented in Table 1. It is to be understood that the specifications are for the exemplary NTSC application only, and that the scope of the invention should not be limited to a particular component or to a specific set of signal types.

TABLE 1: Thomson TH- 680 (NTSC application example) Frequency 754 MHz Peak of Sync Average output 63 KW
power Sound output power 6.3 kW

Anode voltage 8.5 kV

Control grid bias voltage -110 V

Screen grid voltage 700 V

Anode current at zero signal 3 A

Control grid current 190 mA

Screen grid current 210 mA

Gain 14.7 dB

Third order Intermodulation 52 dB
Ratio Sync compression 4 Differential gain 10 Differential phase 2 Efficiency at black level 43 The teachings of the invention may also be employed using Thomson's TH-563 (with power levels one-half those of the TH 680 in Table 1), and the TH-582 (with power levels one-quarter those of the TH 680 in Table 1).
The TH-610 is an air-cooled diacrode (with power levels one-sixth those of the TH 680). Tentative data showing performance of the TH-610 is presented in Table 2:

TABLE 2: Thomson TH- 610 (NTSC application example) Frequency UI~' Range Peak of Sync Avg output power 10.5 KW

Sound output power I .5 kW

Anode voltage 4.8 kV

Control grid bias voltage -50 to -80 V

Anode current at zero signal 1.5 A

Anode current at (black level+sound)3.8 A

Control grid current 30 mA

Screen grid current 60 mA

Gain 15.5 to 16 dB

Third order intermodulation > SO dB
ratio Air flow 15 m3/minute Air pressure 22 millibar Of course, the particular device employed may be chosen for a particular power application (especially average power at peak of sync) and frequency response (preferably less than 1 dB variation across the entire two-channel frequency band of interest) are met.
FIG. 5 illustrates an alternative implementation of the embodiment of FIGS. 2-4, emphasizing an alternative implementation of the power level AGC feedback arrangements, which ensure that output power levels are maintained substantially constant. In FIG. S, feedback paths 276 and 376 are shown leading from the broadcast signal output by the band pass filter 146, back to respective IF-to-broadcast-frequency converters 240 and 340. Paths 276 and 376 are provided in lieu of paths 275 and 375 (FIGS. 2 and 3, respectively). NTSC channel bandpass filter 277, and DTV
channel band pass filter 377, are provided in feedback paths 276, 376, respectively, so that only in-channel frequency components are returned to converters 240, 340.
The alternative feedback arrangements operate on similar principles of feedback control, known to those skilled in the art. When average power varies from a desired steady-state power level, either at the outputs of driver amplifiers 270, 370 or at the output of BPF 146, feedback arrangements within converters 240, 340 act to correct the variation to return the power level at the sensed point back to the desired steady-state power level. The gain factor that converters 240, 340 apply to the feedback signals on paths 275, 375 or 276, 376 are different, and are determined by the differences in magnitude of power between the outputs of driver amplifiers 270, 370 and of BPF 146.
However, the principles remain the same.
In the embodiment shown in FIGS. 2 and 3, gain correction is achieved locally (within the respective NTSC and DTV paths), and compensates only for variations that occur through amplification paths 260, 270 and 360, 370. However, the alternative implementation shown in FIG. 5 achieves a more comprehensive gain correction over the entire path between the IF-to-broadcast-frequency converters 240, 340 and the ultimate output of BPF 146.
Modifications and variations of the above-described embodiments of the present invention are possible, as appreciated by those skilled in the art in light of the above teachings. For example, the choice of components for achieving the described functions, the particular tuning (or programming) methods for tunable (or programmable) elements, and the type and frequency of signals to which the invention is applied, may be varied in accordance with principles known to those skilled in the art without departing from the scope of the present invention. It is therefore to be understood that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described.

Claims (30)

1. An apparatus for simultaneously transmitting, via a single antenna, first and second television signals that respectively occupy first and second channels that are immediately adjacent in frequency, the apparatus comprising:
first means for providing a first broadcast-frequency signal in response to a first baseband signal;
second means for providing a second broadcast-frequency signal in response to a second baseband signal;
means for linearly combining the first broadcast-frequency signal and the second broadcast-frequency signal to provide a combined broadcast-frequency signal that occupies the first and second immediately-adjacent channels; and a single, high-power amplifier for amplifying the combined broadcast-frequency signal with a substantially flat frequency response over the two immediately-adjacent channels, and for outputting a high-power combined signal to the single antenna for broadcast.

2. The apparatus of claim 1, wherein:
the first broadcast-frequency signal is a broadcast-frequency NTSC signal;
the first baseband signal includes baseband NTSC video and audio signals;
the second broadcast-frequency signal is a broadcast-frequency DTV

signal;
the second baseband signal is a baseband digital television signal;
the combined broadcast-frequency signal is a broadcast-frequency NTSC+DTV signal; and the high-power combined signal is a high-power combined NTSC+DTV
signal.

3. The apparatus of claim 2, further comprising:
a frequency reference by which the first and second means respectively provide the broadcast-frequency NTSC signal and broadcast-frequency DTV signal phase locked.

4. The apparatus of claim 2, wherein the high-power amplifier includes:
a tetrode-class device and a resonant cavity.

5. The apparatus of claim 4, wherein:
the tetrode-class device is a diacrode.

6. The apparatus of claim 2, wherein the combining means includes:
a hybrid combiner.

7. The apparatus of claim 2, wherein:

the two immediately-adjacent broadcast channels are UHF channels.

8. The apparatus of claim 2, wherein:
the first channel is beneath the second channel in frequency.

9. The apparatus of claim 2, wherein:
the first channel is above the second channel in frequency.

10. The apparatus of claim 2, wherein:
each of the first and second channels is 6 MHz wide.

11. An apparatus for simultaneously transmitting, via a single antenna, first and second television signals that respectively occupy first and second channels that are immediately adjacent in frequency, the apparatus comprising:
means for linearly combining a first broadcast-frequency television signal and a second broadcast-frequency television signal to provide a combined broadcast-frequency signal that occupies the first and second immediately-adjacent channels; and a single, high-power amplifier for amplifying the combined broadcast-frequency signal with a substantially flat frequency response over the two immediately-adjacent channels, and for outputting a high-power combined signal to the single antenna for broadcast.

12. The apparatus of claim 11, wherein:
the first broadcast-frequency signal is a broadcast-frequency NTSC signal;
the second broadcast-frequency signal is a broadcast-frequency DTV
signal;
the combined broadcast-frequency signal is a broadcast-frequency NTSC+DTV signal; and the high-power combined signal is a high-power combined NTSC+DTV
signal.

13. The apparatus of claim 12, further comprising:
a frequency reference for ensuring that the broadcast-frequency NTSC
signal and broadcast-frequency DTV signal are phase locked.

14. The apparatus of claim 12, wherein the high-power amplifier includes:
a tetrode-class device and a resonant cavity.

15. The apparatus of claim 14, wherein:
the tetrode-class device is a diacrode.

16. The apparatus of claim 12, wherein the combining means includes:
a hybrid combiner.

17. The apparatus of claim 12, wherein:
the two immediately-adjacent broadcast channels are UHF channels.

18. The apparatus of claim 12, wherein:
the first channel is beneath the second channel in frequency.

19. The apparatus of claim 12, wherein:
the first channel is above the second channel in frequency.

20. The apparatus of claim 2, wherein:
each of the first and second channels is 6 MHz wide.

21. A method for simultaneously transmitting, via a single antenna, first and second television signals that respectively occupy first and second channels that are immediately adjacent in frequency, the method comprising:
linearly combining a first broadcast-frequency television signal and a second broadcast-frequency television signal to provide a combined broadcast-frequency signal that occupies the first and second immediately-adjacent channels; and amplifying, with a single, high-power amplifier, the combined broadcast-frequency signal with a substantially flat frequency response over the two immediately-adjacent channels, so as to output a high-power combined signal to the single antenna for broadcast.

22. The method of claim 21, wherein:
the first broadcast-frequency signal is a broadcast-frequency NTSC signal;
the second broadcast-frequency signal is a broadcast-frequency DTV
signal;
the combined broadcast-frequency signal is a broadcast-frequency NTSC+DTV signal; and the high-power combined signal is a high-power combined NTSC+DTV
signal.

23. The method of claim 22, further comprising:
ensuring, with a common frequency reference, that the broadcast-frequency NTSC signal and broadcast-frequency DTV signal are phase locked.

24. The method of claim 22, wherein the amplifying step includes:
amplifying the combined broadcast-frequency NTSC+DTV signal using a tetrode-class device and a resonant cavity.

25. The method of claim 24, wherein:
the tetrode-class device is a diacrode.

26. The method of claim 22, wherein the combining step includes:

combining the broadcast-frequency NTSC signal and the broadcast-frequency DTV signal using a hybrid combiner.

27. The method of claim 22, wherein:
the two immediately-adjacent broadcast channels are UHF channels.

28. The method of claim 22, wherein:
the first channel is beneath the second channel in frequency.

29. The method of claim 22, wherein:
the first channel is above the second channel in frequency.

30. The method of claim 22, wherein:
each of the first and second channels is 6 MHZ wide.