WO2008048102A1

WO2008048102A1 - Distributing digital terrestrial broadcast signals

Info

Publication number: WO2008048102A1
Application number: PCT/NL2007/050504
Authority: WO
Inventors: Johannes Marinus Maria De Nijs
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 2006-10-20
Filing date: 2007-10-18
Publication date: 2008-04-24

Abstract

A method of transmitting digital terrestrial broadcast signals using a plurality of transmitters (12-15) comprises the steps of: • converting a digital terrestrial broadcast signal (DTBS) into a corresponding radio frequency signal (RFS) having a transmission frequency, • feeding the radio frequency signal (RFS) into a cable television network (2) without modification or frequency conversion, • transmitting the radio frequency signal (RFS) at the said transmission frequency through the cable television network (2) to a set of transmitters (12-15) coupled to the cable television network, and • transmitting the radio frequency signal (RFS), using the set of transmitters, through the air at the said transmission frequency. The radio frequency signal may also be fed into a main transmitter, in which case the set of transmitters coupled to the cable television network may be constituted by auxiliary transmitters. The radio frequency signal (RFS) preferably lies within band III or band IVAV.

Description

Distributing digital terrestrial broadcast signals

The present invention relates to distributing digital terrestrial broadcast (DTB) signals. More in particular, the present invention relates to a method and system for distributing digital terrestrial radio, video and/or data broadcast signals using a cable network, such as a cable television network.

Digital signals, such as digital radio and video signals, can be broadcast using a satellite. However, satellites are expensive and in particular video signals, which require a large bandwidth, take up a relatively large proportion of the available bandwidth of a satellite. For this and other reasons, terrestrial digital broadcast systems have been developed which do not require satellites. Such systems are often referred to by the acronyms DVB-T (Digital Video Broadcasting - Terrestrial), DVB-H (Digital Video Broadcasting - Handheld) and T-DAB (Terrestrial Digital Audio Broadcasting). Many known systems use a cable network to distribute digital broadcast signals. However, all existing solutions are relatively complicated and involve complex circuitry.

International Patent Application WO 94/29977 discloses a digital broadcast system in which a spread-spectrum, time and frequency hopping waveform is used to broadcast a plurality of digital channels to remote receivers. A number of relatively low power radio broadcast stations are provided for range extension. A separate program distribution system has a transmission path between the master broadcast station and a range extension station. This transmission path may comprise a microwave link or a coaxial cable. The radio broadcast stations locally store data to transmit the data at the appropriate time. A GPS receiver may be used to provide timing synchronisation in overlapping cells. This known system is quite complex and involves modulation, frequency hopping, frequency dehopping, demodulation and local intermediate storage of the digital broadcast signal.

European Patent Application EP 1 492 369 discloses a DVB-T network in which a single broadcast channel is shared between transmitters on a time division basis. A common time reference, such as a GPS receiver or an atomic clock, is required to indicate for each transmitter when transmission is allowed. Any timing errors will inevitably lead to signal degradation.

International Patent Application WO 01/93475 discloses an upgraded bidirectional cable network adapted to carry mobile radio signals. A so-called network coupling demultiplexer, which comprises high-pass and low-pass filters, is added to the existing network. This additional circuitry provides an extended frequency range from 750 MHz (or 860 MHz) to 2 GHz in which cellular signals (that is, mobile telephone radio signals) can be transmitted without requiring a frequency conversion. The unmodified cellular signals are therefore transmitted at frequencies which lie outside the frequency range of a conventional cable television network. The additional circuitry required for this extension of the frequency range adds to the cost of the network. WO 01/93475 does not disclose, nor suggest distributing digital terrestrial broadcast signals using a cable television network.

It is an object of the present invention to overcome these and other problems of the Prior Art and to provide a method of and system for transmitting terrestrial digital broadcast signals which is simple and economical. More in particular, it is an object of the present invention to provide a method of and system for transmitting terrestrial digital broadcast signals which involves no time division, frequency hopping or intermediate data storage techniques, which requires no time synchronisation or GPS receivers, and which allows using an existing cable network without extending its frequency range.

Accordingly, the present invention provides a method of transmitting digital terrestrial broadcast signals using a plurality of transmitters, which method comprises the steps of: • converting a digital broadcast signal into a corresponding radio frequency signal having a transmission frequency,

• feeding the radio frequency signal into a cable television network,

• transmitting the radio frequency signal at the said transmission frequency through the cable television network to a set of transmitters coupled to the cable television network, and

• transmitting the radio frequency signal, using the set of transmitters, through the air at the said transmission frequency. By passing the radio frequency (RF) signal through the cable television network and transmitting the RF signal through the air at the same frequency, any frequency conversions are avoided. As a result, any frequency conversion apparatus coupled between the cable network and the transmitters can be omitted. In addition, transmitting the RF signal through the cable television network at the same frequency as through the air ensures that all transmitters use the same frequency and that any fading effects due to frequency discrepancies between the transmitters are avoided.

Digital (terrestrial) broadcast signals distributed in accordance with the present invention may comprise video signals, radio signals and/or data signals.

It is noted that the step of converting a digital broadcast signal into a corresponding radio frequency signal having a certain transmission frequency may be omitted if the digital broadcast signal is provided in a suitable format and already has the transmission frequency (that is, is centred around the transmission frequency) used by the transmitters. Typically, however, conversion into a suitable format and frequency will be required. The format conversion may involve conversion into a quasi-analogue signal: a signal that is converted into a mode suitable for transfer via an analogue channel, such as the transmission channel through the "ether". The frequency conversion will typically involve transforming the (central) frequency of the (digital or quasi-analogue) broadcast signal into a frequency suitable for antenna transmission.

Accordingly, digital terrestrial broadcast (DTB) signals are distributed via the cable television network at appropriate RF frequencies. Those skilled in the art will understand that any DTB signal will comprise more than a single frequency, and that the transmission frequency mentioned above is equal to the carrier frequency of the signal, or to the central frequency of a frequency band containing the signal. It will further be understood that the transmitters for transmitting the DTB signals through the air or "ether" are radio wave transmitters operating at a suitable radio wave transmission frequency or "radio frequency" (RF). More in particular, the transmission frequency preferably lies within band III or within band IV/V. Band III, which extends from 174 to 230 MHz and band IV/V, which extends from 470 to 862 MHz, are particularly suitable for transmitting (suitably converted) digital broadcast signals as these bands were previously used for analogue terrestrial broadcasts. As analogue video broadcasts are being phased out, these bands become available for digital broadcasts. Accordingly, using bands III and/or IWV ensures an efficient use of the available bandwidth. In an advantageous embodiment, the set of transmitters comprises auxiliary transmitters, and the step of feeding the radio frequency signal into a cable television network further comprises feeding the radio frequency signal into a main transmitter. In this embodiment, therefore, the (converted) RF signal fed into the cable television network is also fed into a main transmitter. This main transmitter, which may be more powerful than the auxiliary transmitters, therefore receives the RF signal directly from a signal conversion apparatus, while the auxiliary transmitters receive the RF signal from the cable television network and thus indirectly from a signal conversion apparatus. In accordance with the invention, no frequency translation is carried out at either the main transmitter or the auxiliary transmitters. The method of the present invention may advantageously further comprise the step of band-pass filtering the radio frequency signal in a signal path between the cable television network and the transmitters. By band-pass filtering the RF signal received from the cable television network, it is ensured that only the desired signal reaches the transmitters. Band-pass filters will preferably pass only a selected frequency range of band III or band IWV.

Similarly, the method of the present invention may advantageously further comprise the step of band-pass filtering the radio frequency signal in a signal path between the conversion unit and the cable television network. By band-pass filtering the RF signal fed into the cable television network, it is ensured that the (converted) digital broadcast signal, when transmitted through the cable television network, does not interfere with signals in other frequency ranges.

Although the cable television network could be laid out especially for the purpose of transmitting digital terrestrial broadcast signals, it is preferred that the cable television network is constituted by or comprises an existing cable television network laid out before the transmitters were installed. That is, an existing cable television network originally laid out for the distribution of analogue television signals only can, in accordance with the present invention, be used for transmitting digital terrestrial broadcast signals. The present invention allows the existing cable television network to be used without any alterations, in particular without extending the bandwidth of the network, only a signal conversion apparatus (if necessary) and the transmitters have to be coupled to the existing network to allow the transmission of digital terrestrial broadcast signals. It will be understood that only a single signal conversion unit is required, and that no frequency translation is carried out in the signal path between the main transmitter and the auxiliary transmitters.

Accordingly, it can be seen that the preferred embodiments of the present invention are characterised by the absence of bandwidth extension, time division, frequency hopping and local intermediate data storage techniques.

The present invention additionally provides a system for transmitting digital terrestrial broadcast signals using a plurality of transmitters, the system comprising:

• a conversion unit for converting a digital broadcast signal into a corresponding radio frequency signal having a transmission frequency and for feeding the radio frequency signal into a cable television network,

• a set of transmitters coupled to the cable television network for receiving the radio frequency signal from the cable television network and transmitting the radio frequency signal through the air at the said transmission frequency.

In other words, in the system of the present invention the RF signal supplied by the conversion unit is fed directly into the cable television network, without any frequency conversion and preferably also without any format conversion.

The transmission frequency preferably lies within band III and/or within band

I V/ V, and accordingly the transmission of the radio frequency signal through the cable television network preferably takes place at a frequency (or at frequencies) within band III and/or within band TVfV,

The set of transmitters may comprise auxiliary transmitters, and the conversion unit may further be arranged for feeding the radio frequency signal into a main transmitter.

The present invention will further be explained below with reference to exemplary embodiments illustrated in the accompanying drawings, in which: Fig. 1 schematically shows a digital broadcast system according to the present invention.

Fig. 2 schematically shows frequency ranges which may advantageously be used in the present invention.

The digital broadcast system 1 shown merely by way of non-limiting example in Fig. 1 comprises a cable network (CN) 2, a conversion (C) unit 3, a head-end (HE) unit 6, transmitters 11-15, filter (F) units 41-45, and splitter (S) units 52-53. The cable network 2 may be an existing cable television network which was designed for analogue video distribution. The present invention allows this existing, analogue cable television network to be used for digital terrestrial broadcasts, such as video broadcasts, radio broadcasts and data broadcasts. A suitable cable network is a Hybrid Fibre Coaxial (HFC) cable network, although the present invention is not limited to such networks. The cable network 2 preferably is an unextended network, that is, no apparatus has been added to extend the frequency range of the cable network beyond its original range of approximately 0 to 900 (or 862) MHz.

The head-end unit 6 may be constituted by a conventional head-end unit and serves to receive the digital terrestrial broadcast signal(s) DTBS from a network distribution station (not shown). Such a network distribution station is located at a hierarchically higher network layer. In the case of video broadcasting, the network distribution station may gather television channels and/or video content, process video signals so as to make them suitable for terrestrial broadcasting (for example by encoding and/or bit rate conversion), and/or multiplex television channels and/or video streams into appropriate Internet Protocol (IP) or transport streams, which are then multicast to various head-end units. In the example shown, these IP streams or transport streams are assumed to comprise one or more digital terrestrial broadcast signals DTBS which are transmitted using the cable television network 2.

The signal conversion unit 3 carries out any format and/or frequency conversions that may be required to transmit a digital terrestrial broadcast signal

DTBS through the air ("ether"). The format conversion may involve a conversion of the original digital signal into a so-called quasi-analogue signal, that is, a basically digital signal which has a suitable mode for transmission over an analogue channel. The frequency conversion may involve a modulation of the quasi-analogue signal using a suitable radio frequency (RF) carrier frequency.

As will later be explained in more detail, the carrier frequency and hence the transmission frequency of the RF signal preferably lies within band III or band FVTV. As is well known by those skilled in the art, the so-called band III radio frequency range is usually defined as extending from 174 MHz to 230 and is primarily used for radio and television broadcasting. Similarly, the so-called band IV/V radio frequency range is usually defined as extending from 470 MHz to 862 MHz and is also primarily used for radio and television broadcasting.

The transmitters 11-15 comprise, in the embodiment shown, a main transmitter 11 and auxiliary transmitters 12-15. Each transmitter comprises an antenna and may further comprise a signal amplifier, but contains no frequency converter and will preferably contain no signal processor either. It will be understood that in actual embodiments the number of transmitters will typically be greater than is shown in Fig. 1. For example, dozens of auxiliary transmitters may be used, and possibly more than one main transmitter.

The main transmitter 11 serves to broadcast the converted digital terrestrial broadcast signal in a relatively large area, while the auxiliary transmitters 12-15 serve to broadcast the signal RFS in smaller areas. For this reason, it is preferred that the main transmitter 11 has a larger transmission power and/or a higher antenna than the auxiliary transmitters 12-15. In other embodiments, the transmitter 11 may be similar to the other transmitters or may even be omitted.

In the embodiment shown, the transmitter 11 is present and is directly coupled to the (signal) conversion unit 3 so as to receive the RF signal RFS to be transmitted. The conversion unit 3 is coupled, via a band-pass filter 41, to the cable network 2. Accordingly, the cable network 2 receives the same signal RFS as the transmitter 11, as the band-pass filter 41 merely ensures that no frequencies are fed into the cable network that may interfere with other signals in the network. More in particular, the signal RFS fed into the cable network has the same frequency as the signal transmitted by the transmitter 11. The (auxiliary) transmitters 12-15 are coupled to the cable network 2 via filter units and, in some instances, splitter units. Transmitter 15, for example, receives the signal RFS from the cable network 2 via filter unit 45 only, as filter unit 45 is directly connected to the cable network 2. Transmitter 14, however, receives the signal RFS via filter unit 44, splitter unit 53 and splitter unit 52. The filter units 42-45 serve to select the frequency range of the signal RFS from the frequency range of the cable network 2. For example, if the signal RFS is centred around 600 MHz, then the band-pass filters will also be centred around that frequency.

In some embodiments, the three filters 42-44 located between the splitters 52- 53 and the transmitters 12-14 are replaced with a single filter located between the cable network 2 and the first splitter 52. In other embodiments, all filters 42-45 may be incorporated in their respective transmitters. In these or alternative embodiments, the head-end unit 6 may be incorporated in the signal conversion unit 3.

As stated above, the DTB signals are preferably transmitted through the cable television network 2 in quasi-analogue mode, that is, as quasi-analogue signals. It will be understood by those skilled in the art that quasi-analogue signals are digital signals which have been converted into a mode suitable for transfer via an analogue channel. This conversion may be carried out using a modem or similar device.

The frequencies at which these signals are preferably transmitted is described below with reference to Fig. 2, which shows preferred frequency ranges.

Band III is shown to extend between 174 and 230 MHz, while band IVTV extends between 470 and 862 MHz. A preferred RF transmission frequency fj is, in the example shown, 600 MHz, although all frequencies within bands III and IV/V are preferred. In some embodiments of the present invention, however, RF frequencies outside bands III and FVTV may be used, for example a frequency of 360 MHz.

Accordingly, a preferred embodiment of the present invention uses the terrestrial radio spectrum more efficiently by using existing cable networks for the distribution of DTB (Digital Terrestrial Broadcast) signals. To that end, the invention provides a geographical decoupling of the DTB network distribution station from which the DTB signals originate and the signal conversion unit where these signals are converted on the one hand, and the associated transmitters which broadcast these signals into the "ether" on the other hand. The present invention is based upon the insight that digital broadcast signals may be conveyed to a set of transmitters at RF transmission frequencies (for example band III or band IV/V frequencies) via a cable television network. As no down- conversion or up-conversion of the frequencies takes place, all transmitters will transmit the signals at the same RF frequencies and hence any fading effects due to frequency differences are avoided.

In an advantageous embodiment of the present invention, a number of geographically spread transmitters are fed by a signal conversion unit via a cable network, connecting the signal conversion unit with all distributed (local) transmitters. The cable network provides the transport of the (typically quasi- analogue) DTB signals from the signal conversion unit to the transmitters, which emit those quasi-analogue DTB signals into the local air. The transmitters are placed in the vicinity of local DTB receivers and therefore the transmission power of the transmitters only needs to be a fraction of the transmission power of the transmitting antennas in centralised Prior Art systems. Moreover, the transmitting antennas can be much lower than in comparable Prior Art systems.

The DTB signals can be distributed via the cable network to all locations in a region. By means of a transmitter comprising e.g. a band filter circuit, a signal amplifier and an antenna, and which is coupled to a distribution point of the cable network (servicing a number of houses or offices etc.) or coupled to an in-house connection point of the cable network (servicing one house or office etc.), the signals can be emitted locally in the terrestrial air. By placing transmitters with restricted power at a great number of locations and at low heights, the terrestrial distribution of DTB signals may be provided at a low and homogeneous signal strength and having a good coverage. This is in contrast to Prior Art DTB networks such as DVB-T, in which the field strength show strong peaks due to the fact that transmitters used are placed at high positions and have a relative high transmission power.

By using a plurality of transmitters which are located at relatively low positions and which each have a relatively low transmission power, the various frequency channels can be reused within shorter distances. This means that the same frequency channel can be used within a larger geographical area. Suppose that the reuse distance reduces with a factor equal to ^2 (approx. 1.4), then the geographical area where this frequency channel can be used will double.

It is noted that any terms used in this document should not be construed so as to limit the scope of the present invention. In particular, the words "comprise(s)" and "comprising" are not meant to exclude any elements not specifically stated. Single (circuit) elements may be substituted with multiple (circuit) elements or with their equivalents.

It will be understood by those skilled in the art that the present invention is not limited to the embodiments illustrated above and that many modifications and additions may be made without departing from the scope of the invention as defined in the appending claims.

Claims

1. A method of transmitting digital terrestrial broadcast signals using a plurality of transmitters (12-15), which method comprises the steps of: • converting a digital terrestrial broadcast signal (DTBS) into a corresponding radio frequency signal (RFS) having a transmission frequency,

• feeding the radio frequency signal (RFS) into a cable television network (10),

• transmitting the radio frequency signal (RFS) at the said transmission frequency through the cable television network (10) to a set of transmitters (12-15) coupled to the cable television network, and

• transmitting the radio frequency signal (RFS), using the set of transmitters, through the air at the said transmission frequency.

2. The method according to claim 1, wherein the transmission frequency lies within band III.

3. The method according to claim 1 , wherein the transmission frequency lies within band IV/V.

4. The method according to any of the preceding claims, wherein the set of transmitters (12-15) comprises auxiliary transmitters, and wherein the step of feeding the radio frequency signal (RFS) into a cable television network (2) further comprises feeding the radio frequency signal into a main transmitter.

5. The method according to any of the preceding claims, further comprising the step of band-pass filtering (42-45) the radio frequency signal in a signal path between the cable television network (2) and the set of transmitters (12-15).

6. The method according to any of the preceding claims, further comprising the step of band-pass filtering (41) the radio frequency signal in a signal path between the conversion unit (3) and the cable television network (2).

7. The method according to any of the preceding claims, wherein the cable television network (2) comprises an existing cable television network laid out before the transmitters (11, 12-15) were installed.

8. A system (1) for transmitting digital terrestrial broadcast signals using a plurality of transmitters (12-15), the system comprising:

• a conversion unit (3) for converting a digital terrestrial broadcast signal (DTBS) into a corresponding radio frequency signal (RFS) having a transmission frequency and for feeding the radio frequency signal (RFS) into a cable television network (2),

• a set of transmitters (12-15) coupled to the cable television network for receiving the radio frequency signal (RFS) from the cable television network and transmitting the radio frequency signal (RFS) through the air at the said transmission frequency.

9. The system according to claim 8, wherein the transmission frequency lies within band III.

10. The system according to claim 8, wherein the transmission frequency lies within band IWV.

11. The system according to claim 8, 9 or 10, wherein the set of transmitters (12- 15) comprises auxiliary transmitters, and wherein the conversion unit (3) is further arranged for feeding the radio frequency signal (RFS) into a main transmitter (11).

12. The system according to any of the claims 8-11, further comprising at least one band-pass filter (42-45) arranged between the cable television network (2) and a transmitter (12-15).

13. The system according to any of the claims 8-12, comprising a further bandpass filter (41) arranged between the conversion unit (3) and the cable television network (2).

14. The system according to any of the claims 8-13, wherein the cable television network (2) comprises an existing cable television network laid out before the transmitters (11, 12-15) were installed.

15. The system according to claim 14, wherein the existing cable television network was designed for distributing analogue television signals.

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