GB2042849A - Encoding of information - Google Patents

Abstract

Information encoding apparatus comprises at least two modulators 15 and 16, each having a carrier wave of a different frequency, means 19 for combining the outputs of the modulators, means 11 for applying an information signal to the modulators for modulating their carrier waves according to the information signal, a random data terminal 12 and means 10, 13 and 14 responsive to the random data signal to enable the combining means to receive a carrier wave modulated with said information from individual modulators in turn. The transmitter transmits the encoded information together with a signal from the random data terminal and the receiver receives and decodes the information under the control of the random data signal extracted from the received signal. <IMAGE>

Description

SPECIFICATION Encoding of information This invention relates to the encoding of information ready for transmission.

Encoding of information is necessary when it is desired that the information is only receivable by certain people, for example people who subscribe to the company providing the information.

When information is prepared for transmission, it is usually used to modulate a carrier wave, the frequency of the carrier wave being selected for good transmission characteristics and to avoid interference with other broadcasting channels.

According to the present invention there is provided information encoding apparatus comprising at least two modulators, each operating on a different channel, means for combining the outputs of said modulators, means for applying an information signal to the modulators for modulating the carrier waves in the channels according to the information signal, a random data generator and means responsive to the random data generator to enable the combining means to receive a carrier wave modulated with said information from individual modulators in turn.

The invention also includes information transmitting apparatus including the encoding apparatus as described above, and means for transmitting a signal from the random data generator together with the output of the combining means. The invention also includes a receiver for use with such a transmitter, the receiver including means for extracting the random data signal from the received signal, a demodulator for each of the channels of the encoding apparatus, means for combining the outputs of the demodulators and means for enabling the combining means to receive the demodulated information signal from the appropriate demodulator in response to the extracted random data signal.

With the apparatus described above, each channel will be modulated with information for certain periods separated by periods when it is not modulated by the information signal. These latter periods can be used for transmitting alternative information, or for transmitting noise signals as desired. A receiver tuned to a single channel used with a transmitter in which the channels are left unmodulated during the latter periods will receive bursts of information separated by silence. Since such information may still be intelligible, it is preferable that the latter periods are used for modulating the channels with different information or noise so that the signal at a receiver tuned to a single channel alternates between the desired information and other information or noise, which makes the desired information at the receiver output much harder to understand.

An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of a transmitter Figure 2 is a block diagram of a receiver for use with the transmitter of Figure 1 and Figures 3 and 4 are details of modifications to Figures 1 and 2 respectively.

In Figure 1, there are two main input terminals, the information input terminal 11 and a random data input terminal 12. The terminal 11 may, for example, be connected to a microphone or output of a tape recorder when the information is in the form of speech or music. The random data signal may be a pulse train of random frequency.

Two channels are provided in the apparatus of Figure 1, the channels comprising respective analog gates 1 3 and 14 respective frequency modulators 1 5 and 16. The gates 13 and 14 are connected to respective outputs of a flip-flop 10 responsive to the random data signal. The gates 1 3 and 14 are enabled alternately by successive pulses of the random data signal. The modulators 1 5 and 1 6 are connected to respective oscillators 1 7 and 1 8 providing carrier signals of different frequencies.The information input terminal 11 is connected to the input of both gates 13 and 14, and the outputs of both modulators 1 5 and 1 6 are connected to a combining device 1 9 whose output is applied to a transmitter 21. The random data signal from terminal 12 is also applied to the combining device 19 so that the composite signal transmitted comprises a carrier wave of one frequency modulated according to the information input at terminal 11 together with a signal representing the random data signal at terminal 12. In a television transmission system, a video signal, suitably encoded as desired, is also applied to the combining device 1 9 for transmission by the transmitter 21.The frequency of the carrier signal alternates in a random manner according to the random data signal at terminal 12. It is preferred that the random data signal should have a frequency which varies within the range of one to fifty hertz, and preferably within the range of three to ten hertz.

At the receiver, shown in Figure 2, the receiver signal is decoded in a corresponding manner.

There are two channels comprising respective demodulators 31 and 32 tuned to the frequencies of the respective oscillators 1 7 and 1 8 of the transmitter, and analog gates 33 and 34, whose outputs are connected to a combining device 35.

The received signal is first fed to an extracting device 36 which extracts the random data signal from the received signal and applies it to a flip-flop 37 whose outputs are connected to respective gates 33 and 34.

The extracting device 36 supplies the information modulated signal to the inputs of both demodulators and the appropriate channel will be enabled by the random data signal to supply demodulated information to the combining device 35.

In the arrangement where one channel is arranged to carry the desired information while the other channel is arranged to carry undesired information or noise, while one demodulator is supplying the desired information to the analog gate of its channel, the other demodulator will be supplying the undesired information or noise to the gate of its channel, but this latter gate will be disabled by the flip-flop 37, so that the undesired information or noise will not be combined by the device 35 in the receiver output.

Figures 3 and 4 show modified details of Figures 1 and 2, showing how a second information signal or noise can be transmitted on the other carrier wave from that carrying the first information signal.

In Figure 3, there is the first information signal input 11 and random data input terminal 12, as in Figure 1. A second information input terminal 41 is provided, which may be fed with further information or with noise, as desired. In place of the single pair of gates 13 and 14 of Figure 1 , two pairs of gates are provided in Figure 3, the input terminals 11 and 41 being connected to respective members of the two pairs. The random data signal from terminal 12 is connected directly to the gate 42 and through an inverter 43 to gate 44. The terminal 12 is connected directly to the gate 45 and through an inverter 46 to the gate 47.

The outputs of gates 42 and 47 are connected to respective inputs of an OR gate 48, and the outputs of gates 44 and 45 are connected to respective input terminals of OR gate 49. Input terminal 11 is connected to the inputs of gates 42 and 44, and input terminal 41 is connected to the inputs of gates 45 and 47.

Gates 42 and 44 are enabled alternately by the signal appearing at terminal 12 to connect either the terminal 11 or the terminal 41 to the OR gate 48, and gates 44 and 45 are enabled alternately by the signal appearing at terminal 12 to connect either the terminal 41 or the terminal 11 to the OR gate 49. When the terminal 11 is connected by gate 42 to the OR gate 48, terminal 41 will be connected by gate 45 to the OR gate 49. The modification of Figure 3 takes the place of the lefthand side of Figure 1 up to the outputs of the analog gates 1 3 and 14, the outputs of OR gates 48 and 49 being in place of the outputs of the gates 1 3 and 14 to operate the remainder of the apparatus of Figure 1.The output of the combining device 19 will thus be two carrier waves, modulated respectively with the signals applied at terminals 11 and 41 , the relationship of the input signals to the carrier waves being reversed in a random manner according to the signal appearing on terminal 12.

The action of the flip-flop 10 Figure 1 corresponds to that of the parallel direct connection and connection through the inverter 43 or 46 in Figure 3, and either arrangement can be used as convenient.

Figure 4 shows the corresponding modification to the decoder of Figure 2. The two modulated carrier waves are received from the extracting device 36 of Figure 2 and are applied to respective members of two pairs of complementary gates.

The first carrier wave is applied to gates 51 and 53, and the second carrier wave to gates 52 and 54. A decoding data signal is also generated from the output of the extracting device 36 and is applied directly to gates 52 and 53, and through inverter 55 to gate 51 and inverter 56 to gate 53.

The outputs of gates 51 and 52 are connected to respective inputs or OR gate 57 and the outputs of gates 53 and 54 are connected to respective inputs of OR gate 58.

During one state of the random data signal, gates 51 and 53 will be enabled, passing the first channel signal to OR gate 57 and the second channel signal to gate 58. During the other state of the random data signal, gate 58 will receive the first channel and gate 57 the second. Since the two information input terminals 11 and 41 are alternated between the two carrier waves by the random signal, the correspondence of the outputs of gates 57 and 58 to the terminals 11 and 41 remains constant. After suitable demodulation, the original audio information can be derived.

As is described in co-pending application 7905858, the audio signal may form part of a television signal and the random data generator may comprise a random number generator supplying a binary number and activated at the beginning of each field of the compositive video signal. In the apparatus described in the copending application, the random number generator is activated to produce a random number at the start of each field, which in the British television system occurs at the rate of 50 hertz. If the random number generator generates pulses representing a two-digit number, the pulse frequency will be up to 100 hertz which is above the preferable frequency range for the change between channels in the encoding device described above.Such a random number generator activated at the beginning of each field therefore has its pulses in serial form passed through a divide-by-ten counter, the output of the counter being applied to the terminal 12 in the apparatus described above. This will bring the maximum frequency of the random singal down to 10 hertz. Some of the binary digits will be zero, represented by the absence of a pulse, so that the pulses in the random data singal will occur at frequencies varying between different fractions of 10 hertz. To prevent the intelligibility of undecoded signals received on one channel, it is preferred that the alternation should occur at a frequency of at least 1 hertz, and ideally between 3 and 10 hertz. If the random number generator frequency is taken above 50 hertz, no extra benefit is gained to compensate for the additional cost of higher frequency operation. As described in the co-pending application, the random number is transmitted with the coded audio and video information, so that a similar divide-by-ten counter is provided in the decoding data input line of the receiver, as shown in Figure 4.

The channels corresponding to the modulators need not be completely separate. They could be provided respectively by the upper and lower side bands in an amplitude modulated transmission system.

Claims (6)

1. Information encoding apparatus comprising at least two modulators, each operating on a different channel, means for combining the outputs of said modulators, means for applying an information signal to the modulators for modulating the carrier waves in the channels according to the information signal, a random data generator and means responsive to the random data generator to enable the combining means to receive a carrier wave modulated with said information from individual modulators in turn.

2. Apparatus as claimed in claim 1 wherein said enabling means comprises gates for connecting said information signal to respective modulators.

3. Apparatus as claimed in claim 1 or 2 wherein said applying means is arranged to apply a plurality of information signals to the modulators, there being at least as many modulators as information signals, the enabling means enabling the combining means to receive carrier waves modulated with said information signals, the information signals modulating different modulators in turn in response to the random data generator.

4. Information encoding apparatus substantially as hereinbefore described with reference to Figure 1 alone or as modified by Figure 3.

5. Transmitting apparatus comprising information encoding apparatus as claimed in any one of claims 1 to 4 and means for transmitting a signal from the random data generator together with the output of the combining means.

6. Receiving apparatus for use with transmitting apparatus as claimed in claim 5, the receiver including means for extracting the random data signal from the received signal, a demodulator for each of the channels of the encoding apparatus, means for combining the outputs of the modulators and means for enabling the combining means to receive the demodulated signal from the appropriate demodulator in response the random data signal.