Method for the common transmission of digital and analogue modulated radio broadcasting and/or television broadcasting signals

[0002] The present invention relates to a method for common transmission of digital-modulated and analog-modulated radio and/or television signals, especially over a broadband cable system.

[0003] The journal “Funkschau” [Broadcasting Review] No. 9, 1995, p. 46, discloses a transmission method in which a flexible multiplexer at the output of a video server combines a plurality of program signals with different data rates into a transmission package; the digital summation signal is QAM-modulated (QAM=quadrature amplitude modulation) and is then transmitted parallel to the analog TV signals already fed into a cable. The digital signal then occupies one entire TV channel of the kind that was previously needed for a single analog television program.

[0004] It is an object of the present invention to provide an improved method for common transmission of digital-modulated and analog-modulated radio and/or television signals, especially over a broadband cable system, which provide increased transmission capacity without significantly increasing side channel interference.

[0005] According to the invention the method for common transmission of digital-modulate and analog-modulated radio and/or television signals comprises the steps of:

[0006] a) transmitting at least one digital signal together with an analog television signal in a common channel of at least one communications channel;

[0007] b) controlling a signal level of the at least one digital radio and/or television signal so that it does not exceed a predetermined digital signal level;

[0008] c) controlling a signal level of the analog television signal so that it does not drop below a predetermined analog signal level which is much greater than the predetermined digital signal level of the at least one digital radio and/or television signal;

[0009] d) controlling the at least one digital radio and/or television signal so that an amplitude of a frequency spectrum of the at least one digital radio and/or television signal does not exceed a predetermined amplitude value, which is much lower than an amplitude of a video carrier of the analog television signal; and

[0010] e) controlling the at least one digital radio and/or television signal so that the frequency spectrum of the at least one digital radio and/or television signal is limited to a frequency range smaller than a width of the common channel.

[0011] The method of the invention has the advantage over the prior art that the transmission of digital-modulated radio and/or television signals with an analog television signal in one and the same channel leads to a marked increase in transmission capacity, so that substantially more radio and/or television programs can be transmitted.

[0012] In a preferred embodiment of the method the data quantity of the at least one digital radio and/or television signal is reduced by coding so that additional digital radio and/or television signals can be transmitted together with the analog television signal in the one channel, thus further increasing the transmission capacity.

[0013] It is also advantageous to utilize the channel for transmitting further supplementary data. Once again, still a further increase in transmission capacity is thereby achieved.

[0014] By suitably disposing the frequency range of the at least one digital radio and/or television signal between two carrier frequencies, possible influences of the digital and analog signals on one another can advantageously be kept slight and below a threshold of perceptibility.

[0015] By transmitting audio carrier frequency components of the analog television signal with predetermined attenuation it is possible in a preferred embodiment to obtain an improvement in the signal to noise ratios for the digital signals by attenuation of interfering video and audio carrier frequency components of the transmitted analog radio and/or television signals.

[0016] In a preferred embodiment of the method the individual digital radio and/or televisions signals are combined into blocks in the at least one channel and the frequency ranges of at least two of the transmitted digital radio and/or television signals are separated from one another by a protective frequency spacing. The advantage of this feature is that mutual interference between the signals is avoided.

[0017] An advantage in the modulation of one or more digital radio and/or television signals combined into a multiplex signal in accordance with a preferred embodiment of the method is the selection of carrier frequencies that for at least one audio carrier frequency, color auxiliary carrier frequency or video carrier frequency of the analog television signal do not fall below a predetermined frequency spacing. In this way, because critical video and audio carrier frequencies are blanked out, interference of carrier frequencies on the analog television signal with the spectrum of the digital multiplex signal is prevented.

[0018] The frequency range of one or more digital radio and/or television signals, combined into a multiplex signal, is advantageously split between video and audio carrier frequencies of the analog television signal, and the corresponding video and audio carrier frequencies of the analog television signal are advantageously separated by one protective frequency spacing each. In this way, even digital multiplex signals that because of the width of their frequency range can be accommodated in the channel of the analog television signal only with difficulty without interfering superposition with video and audio carrier frequencies of the analog television signal, can be accommodated entirely without problems between the individual interfering video and audio carrier frequencies of the analog television signal because of the described splitting of their frequency range, for instance by blanking out carrier frequencies of the multiplex signal, thus averting their superposition on one another, which would cause interference.

[0019] In a preferred embodiment advantageously the transmission of one or more digital radio and/or television signals, can be combined into a multiplex signal, in an arbitrary direction. In this way, via the broad band cable system, interactive radio or television and/or telecommunications can be carried out, above all including the utilization of the transmission of digital supplementary data in accordance with the preferred embodiment described above.

[0020] Another advantage of this preferred embodiment is also the realization of reverse data channels in broad band cable systems, without requiring additional frequencies or without having to change the conventional frequency allocation of the broad band cable system.

[0021] An apparatus transmitting and receiving digital multiplex signals modulated according to an OFDM method, preferably digital radio and/or television signals, comprises, for reception, a tuner circuit for selecting a channel of the transmitted frequency band, a demodulator for demodulating the OFDM modulated signals, a decoder for error evaluation of the received digital signals, a demultiplexer for splitting the corresponding multiplex signal into individual digital signals and an interface device for connection of data playback devices and/or optical broadband distribution network, especially using fiber optic devices; and, for transmission, a channel adaptation unit for coding and combining individual digital signals transmitted via the interface device from a data playback device and/or from the optical broadband distribution network connected to the interface device into a multiplex signal; a modulator for modulating the multiplex signal at carrier frequencies in frequency channels, especially for transmission of analog television signals and a coupler for connection to a broadband cable system, which is connected to the tuner circuit and the modulator. The apparatus according to the invention has the advantage that interactive radio or television and/or telecommunications using digital multiplex signals modulated by an OFDM method is enabled by coupling both receiving and transmitting components to the broad band cable system.

[0022] Advantageous preferred embodiments of this apparatus are possible. An especially advantageous feature is the reduction of the data rate of the digital multiplex signal, in that signal components of the multiplex signal to be transmitted are to be modulated to the carrier frequencies which exceed a predetermined frequency spacing from at least one audio carrier frequency and/or one color auxiliary carrier frequency and/or one video carrier frequency of an analog television signal. In this way, with the apparatus, multiplex signals that are not interfered with by the video and audio carrier frequencies of an analog television signal can be generated.

[0023] The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying figures in which:

[0024] FIG. 1 is a block diagram of an apparatus for feeding digital and analog radio and/or television signals into a broadband cable system;

[0025] FIGS. 2 to 4 are graphical illustrations of respective examples of signal spectra in an analog channel;

[0026] FIG. 5 is a block diagram of an apparatus for receiving transmitted data from the broadband cable system;

[0027] FIG. 6 is a block diagram of a transmitting and receiving apparatus according to the invention for OFDM-modulated digital multiplex signals;

[0028] FIG. 7 is a graphical illustration showing a superposition of the spectrum of an analog television signal with a frequency range, split into three frequency blocks, of an OFDM-modulated digital multiplex signal; and

[0029] FIG. 8 is a graphical illustration showing protective frequency spacings of FIG. 7.

[0030] In FIG. 1, reference numeral 5 designates a multiplexer, to which via a first encoder 1 a digital television signal and via a second encoder 2 a digital radio signal are delivered. Via an enciphering unit 10, a modulator 12 and a first amplifier 13, the multiplexer 5 is connected to a mixer 15, to which an analog television signal is delivered via a second amplifier 14. The signal generated in the mixer 15 is fed into a broad band cable system 30, of which only one cable input is shown in FIG. 1, via a third amplifier 20 and a filter 25 for audio carrier frequency components of the analog television signal. The output of the first amplifier 14 represents the output of a first transmitter for transmitting digital-modulated radio and/or television signals. The output of the second amplifier 14 then represents the output of a second transmitter for transmitting analog or analog-modulated radio and/or television signals. The digital- and analog-modulated radio and/or television signals may also be transmitted from a single transmitter, which then includes both the output of the first amplifier 13 and the output of the second amplifier 14, and whose output is the output of the filter 25.

[0031] The analog television signal delivered to the second amplifier 14 is amplified in the amplifier 14 to a signal level, so that the peak level of the analog television signal exceeds a predetermined value, for instance of 65 dB μV. The digital signals delivered to the multiplexer 5 via the first encoder 1 and the second encoder 2 are combined, frequency-multiplexed, into a digital signal in the multiplexer 5 and then enciphered in the enciphering unit 10. The two encoders 1 and 2 reduce the quantity of data in the digital television signal and the digital radio signal, so that a limitation of the frequency spectra of the digital signals is accomplished, each to a predetermined frequency range that is less than the width of the channel of the analog television signal. To reduce the data quantity in the digital television signal, data compression algorithms are suitable, such as MPEG1, MPEG2 or MPEG4 (MPEG=motion picture expert group). For audio data reduction, the MPEG standard is equally suitable. The enciphered digital signal is then, in the modulator 12, subjected to a modulation process, such as the COFDM process (COFDM=coded orthogonal frequency division multiplexing), the PSK process (PSK=phase shift keying), or the QAM process. The goal of this provision is to limit the amplitude of the frequency spectrum of the digital signal to a predetermined value that is markedly less than the amplitude of the video carrier of the analog television signal, so that if at all possible cross-modulation with the analog television signal does not occur. Next, in the first amplifier 13, the modulated digital signal is adjusted in level to a value that is 20 to 30 dB lower than the value predetermined for the peak level of the analog television signal. In the mixer 15, the digital signal is superimposed on the analog television signal. The resultant signal is then adjusted in its level to the predetermined values via the amplifier 20. Finally, the filter 25 is used to lower the signal level in the range of the audio carrier frequencies of the analog television signal by a predetermined value that has no significant influence on the playback or reproduction quality of the received sound for the user. In the exemplary embodiment described, such attenuation can amount to up to 10 dB compared with the typical audio levels in broad band cable systems. This provision improves the reception of the digital signal or its signal to noise ratio. This plays a role above all whenever the frequency range of the digital signal is superimposed on the frequency range of the audio carrier. The task of the modulator 12, finally, is also to convert the frequency ranges of the digital signals into the channel of the analog television signal. Depending on the range of the broad band cable system 30, this broad band cable system has additional amplifiers and filters for audio carrier frequency components, so that the transmitted signal will remain adjusted in its level to the predetermined values.

[0032] It is also possible for a digital signal additionally to be impressed, as described above, on the analog television signal that is broadcast via a transmission system; the digital signal can be broadcast either by the same transmission system or some other transmission system.

[0033] Along with a digital radio or television program, it is also possible to transmit digital supplementary data. These supplementary data include traffic reports, transportation schedules, weather reports, or theater and movie schedules.

[0034] FIG. 2 shows one example of a frequency spectrum of the channel of the analog television signal. The amplitude A is plotted over the frequency f. The channel is defined by a lower limit frequency f₀₁and an upper limit frequency f₀₂. The spectrum 35 of the analog television signal is located in this channel and has a video carrier at a frequency f₁and a color auxiliary carrier at a frequency f_FHTwhich is greater than the video carrier frequency f₁. The spectrum 35 also has a first audio carrier at a frequency f_T1and a second audio carrier at a frequency f_T2. Both audio carrier frequencies f_T1and f_T2are greater than the color auxiliary carrier frequency f_FHT. The second audio carrier frequency f_T2is greater than the first audio carrier frequency f_T1. The video carrier at the frequency f₁has the highest amplitude, followed by the amplitudes of the two audio carriers at the frequencies f_T1and f_T2and the color auxiliary carrier at the frequency f_FHT. Between the video carrier at the frequency f₁and the color auxiliary carrier at the frequency f_FHT, the spectrum of the analog television signal has a relatively high amplitude in comparison with the range between the color auxiliary carrier at the frequency f_FHTand the upper limit frequency f₀₂, except for the two audio carriers at the frequencies f_T1and f_T2. Above the color auxiliary carrier frequency f_FHT, a spectrum 40 of a digital radio and/or television signal between a lower limit frequency f_D1and an upper limit frequency f_D2is superimposed on the spectrum 35 of the analog television signal. In this way, the spectrum 40 of the digital signal is accommodated in a range of the channel in which, with the exception of the two audio carriers at the frequencies f_T1and f_T2, the spectrum of the analog television signal has a relatively small amplitude, so that interference of the digital signal by the analog television signal is kept as slight as possible. The lower limit frequency f_D1of the spectrum 40 of the digital signal is lower than the two audio carrier frequencies f_T1, and f_T2, and the upper limit frequency f_D2of the spectrum 40 of the digital signal is higher than the two audio carrier frequencies f_T1and f_T2. The two audio carriers are thus superimposed on the spectrum 40 of the digital signal. The lower limit frequency f_D1of the spectrum 40 of the digital signal is higher than the color auxiliary carrier frequency f_FHT, so that there is no impairment of the spectrum 40 of the digital signal by video and color carriers of the analog radio signal. The upper limit frequency f_D2of the spectrum 40 of the digital signal is lower than the upper limit frequency f₀₂of the channel, so that there is no impairment of the spectrum 40 of the digital signal by a video carrier at a frequency f₂, which is higher than the upper limit frequency f₀₂of the channel and is thus located in a neighboring channel. Since the amplitudes of the two audio carriers at the frequencies f_T1and f_T2are greater than the amplitude of the spectrum 40 of the digital signal, the two audio carriers do not interfere with the digital signal. By feeding the audio carriers into the cable system 30 at predetermined attenuation, this interference is already lessened. A further reduction in interference takes place in the receiver.

[0035] In a further exemplary embodiment in accordance with FIG. 3, the spectrum 40 of the digital signal is superimposed on the otherwise unchanged spectrum of the analog television signal, in such a way that it is located between the second audio carrier frequency f_T2and the upper limit frequency f₀₂of the channel and is thus also no longer impaired by the two audio carriers.

[0036] In a further exemplary embodiment in accordance with FIG. 4, along with the spectrum 40 of the digital radio and/or television signal, a second spectrum 45 of a second radio and/or television signal is superimposed on the otherwise unchanged spectrum 35 of the analog television signal. The two spectra 40 and 45 may for instance belong to the transmitted digital radio signal or the transmitted digital television signal of FIG. 1. The frequency ranges of the two spectra 40 and 45 are introduced via the first amplifier 13 and the mixer 15 into the channel of the analog television signal in such a way that they are separated from one another by a protective frequency spacing f_Sand therefore do not impair one another, and are superimposed on the spectrum of the analog television signal between the color auxiliary carrier frequency f_FHTand the video carrier frequency f₂of the neighboring channel. The first spectrum 40 has the two audio carriers superimposed on it and interfering with it while conversely the second spectrum 45 is located between the second audio carrier and the video carrier frequency f₂of the neighboring channel.

[0037] In an analog television signal in accordance with the PAL B standard, the width of the channel is 7 MHz. This is also equivalent to the spacing between two video carrier frequencies f₁and f₂of adjacent channels. For a spacing of 4.43 MHz between the video carrier frequency f₁and the color auxiliary carrier frequency f_FHT,2.57 MHz thus remain available for occupation by one or more spectra of digital radio and/or television signals. When the aforementioned encoding algorithms MPEG2 or MPEG4 are used, the bandwidth for the spectrum of a digital television signal can be limited to 1.5 MHz. In this way, a digital television signal can be transmitted between the color auxiliary carrier of the frequency f_FHTand the video carrier, located in the neighboring channel, having the frequency f₂. Alternatively, up to six digital radio signals can be transmitted in a frequency band 1.5 MHz wide, with data rates normally of up to 256 kbit/s. For a channel of an analog television signal having a width of 8 MHz in accordance with the PAL G standard and a spacing of the second audio carrier frequency f_T2from the video carrier frequency f₁of 5.75 MHz, 2.25 MHz are available between the second audio carrier frequency f_T2and the video carrier frequency f₂in the neighboring channel, so that a digital television signal with a spectrum 1.5 MHz wide can be transmitted between these two frequencies with relatively little impairment by the analog television signals. If the spacing between the color auxiliary carrier frequency f_FHTand the video carrier frequency f₁remains constant at 4.43 MHz, the spacing between the color auxiliary carrier frequency f_FHTand the video carrier frequency f₂in the neighboring channel is 3.57 MHz, so that between the color auxiliary carrier and the video carrier in the neighboring channel in accordance with FIG. 4, two digital radio and/or television signals, each with a spectrum 1.57 MHz wide and a protective frequency spacing of 0.2 MHz, for instance, can be transmitted. As an alternative, instead of a digital television signal, up to six digital radio signals, with or without a protective frequency spacing, can be transmitted, so that in the channel of the analog television signal, either two television programs or one television program and six audio programs, or twelve audio programs can be accommodated. When encoding algorithms that lead to a further reduction in the data quantities are employed, the number of television and/or audio programs that can be transmitted can be increased.

[0038] In FIG. 5, a cable output of the broad band cable system 30 is connected via a converter 50, which includes a decoder 55, to an analog input 105 of a television set 100 and also to a filter circuit 60, accommodated in the decoder 55, for video carrier frequency components. The filter circuit 60 for video carrier frequency components is connected to a demultiplexer 80 via a filter circuit 65 for audio carrier frequency components, a demodulator 70, and a deciphering unit 75. The demultiplexer 80 is connected to a first decoder 85 and a second decoder 86. The filter circuit 65 for audio carrier frequency components, the demodulator 70, the deciphering unit 75, the demultiplexer 80, and the two decoders 85 and 86 are likewise accommodated in the decoder 55. The first decoder 85 is connected to a digital input 110 of the television set 100, and the second decoder 86 is connected to a speaker 95 via an audio amplifier 90.

[0039] The signal received from the converter 50 via the broad band cable system 30 includes the analog television signal, the digital television signal, and the digital radio signal, which were fed into the broad band distribution network by the circuit in accordance with FIG. 1. In the analog input 105 of the television set 100, the received digital signals have the effect of noise and lead to a reduction in the signal-to-noise ratio for the analog television signal. In a realization in accordance with predetermined level values for the analog television signal and the digital signals, however, the resultant signal-to-noise ratio is tolerable. In the filter circuit 60 for video carrier frequency components, interfering video carrier frequency components, especially from neighboring video carriers, are suppressed by up to 20 dB, for instance, so that the signal-to-noise ratio for the digital signals is increased. A corresponding suppression of audio carrier frequency components of the analog television signal serves the same purpose and is effected by the filter circuit 65 for audio carrier frequency components. The two filter circuits 60 and 65 can be realized by active and/or passive components. The filtered digital signal is demodulated in the demodulator 70, deciphered in the deciphering unit 75, and finally split, in the demultiplexer 80, into the digital television signal and the digital radio signal. The two decoders 85 and 86 expand the digital radio or television signal. The digital television signal is then converted into an analog signal at the digital input of the television set 100 and played back in the form of video and audio. The digital radio signal is converted in the audio amplifier 90 into an analog signal, amplified, and delivered to the speaker 95 for audio playback.

[0040] In a further exemplary embodiment, the digital-to-analog conversion already takes place in the decoder 55, so that no digital input 110 or audio amplifier 90 is needed in the television set 100.

[0041] FIG. 7 shows the spectrum of an analog television signal, for instance in accordance with the PAL B standard, and the width of the channel is 7 MHz. Between the color auxiliary carrier frequency f_FHTand the frequency f₂of the video carrier in the upper neighboring channel, one or more digital radio and/or television signals, combined into a multiplex signal, are superimposed on the analog television signal with the spectrum 35. The frequency range of this multiplex signal is split into three frequency blocks 245, 250 and 255 that are separate from one another. This is accomplished by providing that in the modulation in the modulator 12, for instance by an OFDM modulation process, carrier frequencies are selected that do not fall below a predetermined frequency spacing for the audio carrier frequencies f_T1, f_T2, the color auxiliary carrier frequency f_FHT, and the video carrier frequency f₁of the analog television signal as well as the video carrier frequency f₂of the analog television signal of the upper neighboring channel. In this way, a first portion 245 of the frequency range of the multiplex signal is transmitted between the video carrier frequency fFHT and the first audio carrier frequency f_T1. It can be seen from FIG. 8 that the first portion 245 of the frequency range is separated from the color auxiliary carrier frequency f_FHTby a first protective frequency spacing f_S1and from the first audio carrier frequency f_T1by a second protective frequency spacing f_S2. A second portion 250 of the frequency range of the multiplex signal is transmitted between the first and second audio carrier frequencies f_T1and f_T2of the analog television signal. The second portion 250 of the frequency range is separated from the first audio carrier frequency f_T1by a third protective frequency spacing f_S3and from the second audio carrier frequency f_T2by a fourth protective frequency spacing f_S4. A third portion 255 of the frequency range of the multiplex signal is transmitted between the second audio carrier frequency f_T2of the analog television signal and the video carrier frequency f₂of the upper channel, that is, the channel neighboring it toward higher frequencies. The third portion 255 of the frequency range is separated from the second audio carrier frequency f_T2by a fifth protective frequency spacing f_S5and from the video carrier frequency f₂of the upper neighboring channel by a sixth protective frequency spacing f_S6. In this way, the aforementioned video and audio carrier frequencies f_FHT, f_T1, f_T2and f₂are prevented from being superimposed on the frequency range of the multiplex signal, so that resultant interference in reception of the digital multiplex signal is avoided.

[0042] It is also contemplated according to the invention that the described frequency range of the multiplex signal be used both in the forward and the reverse directions, with the goal of using existing broad band cable systems for interactive services as well as remote polling and/or remote control tasks. The described capability of transmitting digital supplementary data along with a digital radio or television program can then also be used in setting up interactive and/or telecommunications services.

[0043] Finally, it is also possible to utilize the splitting up of the frequency range into three frequency blocks 245, 250 and 255 in accordance with FIG. 8 in such a way that one or two frequency blocks are used for transmitting digital data in the forward direction, and the remaining frequency blocks are used to transmit digital data in the reverse direction.

[0044] In FIG. 6, an apparatus 201 is shown for transmitting and receiving digital multiplex signals modulated by an OFDM process. The multiplex signals are preferably radio and/or television signals, but may be any possible other digital signals, in particular those suitable for realizing interactive and/or telecommunications services. Here both frequency and multiplexing can be employed. In the apparatus shown in FIG. 6, a multiplexing process is employed. In FIG. 6, 240 designates a coupler for coupling the apparatus 201 to the broad band cable system 30. Connected to the coupler 240 are a tuner circuit 205 and a modulator 235. Connected to the tuner circuit 205 in turn is a demodulator 210, which is connected to the decoder 215. A first channel adaptation unit 230 is connected to the demodulator 235. The decoder 215 is connected via a demultiplexer 260 to an interface device 225, which is also connected to the channel adaptation unit 230. The aforementioned components are all contained within the apparatus 201.

[0045] Digital multiplex signals arriving at the apparatus 201 via the broad band cable system 30 are carried on to the tuner circuit 205 by the coupler 240, which carries out a directional separation between signals received and signals to be transmitted. In the tuner circuit 205, a channel of the transmitted frequency band is selected, and the frequency range of the digital multiplex signal to be received is filtered out. If an analog television signal is also superimposed on this frequency range as described above, then it is also possible for a suppression of corresponding video and/or audio carrier frequency components of the analog television signal to be done in the tuner circuit 205, so as to avert resultant interference upon reception. The tuned and filtered digital multiplex signal is also OFDM-modulated and is subjected to a corresponding demodulation in the demodulator 210. After the demodulation, a digital, error-protected, encoded base band data stream is available, which in the decoder 215 is subjected to error evaluation and decoding, which eliminates the data redundance that is jointly transmitted, particularly for error protection purposes, and the result is that a decoded digital net data stream is present at the output of the decoder 215. The net data stream at the output of the decoder 215 is split in the demultiplexer 260 into individual digital signals, which in turn are passed on to the interface device 225. Three double-headed arrows at the interface device 225 in FIG. 6 indicate that data playback equipment, such as radio receivers, television sets, video cassette recorders, telephones, personal computers, and so forth, can be connected to the apparatus 201 via the interface device 225. An optical broad band distribution network can also be connected to the interface device 225, preferably using fiber optics, and after electrooptical conversion in the interface device 225, this network passes on the signals received from the apparatus 201. In the reception situation, the data playback equipment serve the purpose of optical or acoustical playback of the digital signals transmitted via the broad band cable system 30 and received by the apparatus 201. To realize interactive and/or telecommunications services, the apparatus 201 must also be prepared for transmitting digital signals, which are transmitted to the apparatus 201 by data playback equipment via the interface device 225, on to the broad band cable system 30. The same is true for digital data which are carried on to the apparatus 201 via the interface device 225 from an optical broad band distribution network connected to the interface device 225. To that end, the corresponding optoelectrical conversion of the digital signals from the optical broad band distribution network must be done in the interface device 225. For transmitting digital data from to the broad band cable system 30, the channel adaptation unit 230 is provided in the apparatus 201; this unit is used for encoding and combining the individual digital signals, transmitted to the apparatus 201 via the interface device 225, into a multiplex signal. The digital multiplex signal formed in the channel adaptation unit is ofdm-modulated in the modulator 235. In this process, carrier frequencies are used that are located in frequency channels intended for transmission of analog television signals. At the places where the analog television signal to be superimposed has video and audio carrier frequencies that interfere with reception, no carrier frequencies are provided in the modulator 235, and thus the aforementioned six protective frequency spacings f_S1, f_{S2 fS3}, f_S4, f_S5and f_S6of FIG. 8 are observed. The omission of the corresponding carriers is also known as frequency blanking. Because of this frequency blanking, the data rate of the digital multiplex signal is reduced. Via the coupler 240, the thus-modulated digital OFDM multiplex signal is supplied to the broad band cable system 30.