JP3009025U - Transmitter - Google Patents

Abstract

(57) [Abstract] [Purpose] To multiplex analog signals and digital signals, and to make effective use of idle channels in the transmission path. [Structure] A first modulator 10 modulates a digital signal using a frequency direct displacement method. The second modulator 20 amplitude-modulates the output signal of the first modulator 10 with an analog signal. The energy of the output signal of the first modulator 10 is concentrated at both ends of the band. The analog signal component in the output signal of the second modulator 20 has energy distributed in the central portion of the band sandwiching the frequency of the carrier wave. Therefore, both can be easily separated and demodulated. 96 digital signals
If it is set to 00 bps and the analog signal is used as voice data, a digital signal having a high transfer speed and a voice signal having sufficient clarity can be multiplexed and transmitted.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a transmitter that multiplexes and transmits a digital signal and an analog signal on a transmission path.

[0002]

[Prior art]

 Signals are multiplexed in order to effectively utilize communication lines and other transmission paths. FIG. 2 shows a connection diagram of an example of a general CATV system. This figure shows an example of a line configuration of a CATV system, and a transmission line 3 composed of a coaxial cable or an optical fiber cable is connected to a head end 2 connected to an antenna 1. A relay amplifier 4 and a distributor 5 are inserted in the transmission path 3, and the distributor 5 is connected to a television serving as a terminal.

In the CATV system as described above, the television signal received by the antenna 1 is transmitted to each distributor 5 via the head end 2 and distributed to the television of each house. Here, in order to make more effective use of the transmission line 3 used in such a system, it has been attempted to transmit data, voice, etc., using an empty channel.

FIG. 3 shows an explanatory diagram of empty channels. As shown in this figure, one channel is divided for every 6 MHz of the television signal. For example, when 4 channels and 6 channels are used for broadcasting, 5 channels become empty channels to prevent interference. A band with an appropriate width can be assigned to such an empty channel, and various signals can be transmitted and received using this band. The transmitter is provided on the side of the head end 2 and the television 6 which is a terminal.

[0005]

[Problems to be solved by the device]

 By the way, in the conventional system as described above, if the digital data and voice signals are directly assigned to the limited available channels, the number of signals that can be transmitted is limited in order to prevent mutual interference. The present invention has been made in order to meet such a demand, and an object thereof is to provide a transmitting apparatus that increases transmission capacity by multiplexing and efficiently uses a transmission line.

[0006]

[Means for Solving the Problems]

 In the transmitter of the present invention, the bandwidth allocated on the transmission path is W, the frequency of the carrier wave is F, the transfer speed of the digital signal to be transmitted is fd, and the high cutoff frequency of the analog signal to be transmitted is fa. Then, fd is set in the range of W> 2fd, a first modulator that modulates the carrier wave by the frequency direct displacement method with the digital signal, and fa with fd or less and the first modulation with the analog signal. A second modulator for amplitude-modulating the output of the device, and a transmitter for sending the output of the second modulator to the transmission path.

[0007]

[Action]

 This device modulates a digital signal with a first modulator using the frequency direct displacement method. Further, the second modulator amplitude-modulates the output signal of the first modulator with an analog signal. The energy of the output signal of the first modulator is concentrated at both ends of the band. The energy of the analog signal component in the output signal of the second modulator is distributed in the central part of the band sandwiching the frequency of the carrier wave. Therefore, both can be easily separated and demodulated. If the digital signal is 9600 bps and the analog signal is voice data, the digital signal with a high transfer speed and the voice signal with sufficient clarity can be multiplexed and transmitted.

[0008]

【Example】

 Hereinafter, the present invention will be described in detail with reference to the embodiments shown in the drawings. FIG. 1 is a block diagram showing an embodiment of the transmitting apparatus of the present invention. This device comprises a first modulator 10 and a second modulator 20, wherein the first modulator 10 receives the output of a digital signal source 31 and the second modulator 20 receives the output of an audio signal source 32. Has become. The first modulator 10 includes a voltage controlled oscillator (VCO) 11, a low pass filter (LPF) 12, a phase comparator (PLL) 13, and a channel selector 14. The second modulator 20 is composed of an amplitude modulator (AM-MOD) 21, a low pass filter (LPF) 22, an automatic gain controller 23 and a gate switch 24. The digital signal source is, for example, various kinds of data transmitted and received between computers, and the transfer speed thereof is selected to be, for example, 9600 bps (bits / second) in this embodiment.

The audio signal source 32 is composed of a telephone transmitter and a microphone. The output of the digital signal source 31 is configured to be input to the voltage controlled oscillator 11 together with the output of the low pass filter 12. Further, the output of the audio signal source 32 is configured to be input to the automatic gain controller 23 of the second modulator 20. The output of the first modulator 10 is input to the amplitude modulator 21 of the second modulator 20, and the output of the amplitude modulator 21 is sent out to the transmitter 40. The transmitter 40 is composed of a circuit for transmitting the output of the second modulator 20 to the transmission line 3 provided in the CATV or the like.

The channel selector 14 provided in the first modulator 10 is a circuit for selecting the frequency of the carrier wave. For example, in this embodiment, a plurality of bandwidths for multiplexing and transmitting a digital signal and a voice signal are selected to be about 20 to 30 KHz. Therefore, the center frequency of any one of the bands is selected by the channel selector 14. The low pass filter 12 and the phase comparator 13 form a phase locked loop. As a result, the voltage-controlled oscillator 11 controls to modulate and transmit the carrier wave set accurately. When a digital signal is input from the digital signal source 31, the first modulator 10 operates as a so-called FSK (Frequency Shift Keying) modulator. That is, for example, a modulation method for directly displacing the frequency of an oscillator in a mark and a space of a digital signal which is widely used for transmission of RTTY (printing telegraph code) is adopted.

On the other hand, the gate switch 24 of the second modulator 20 converts the input signal from the audio signal source 32 into an audio signal of an appropriate level by the automatic gain controller 23 and sends it to the low pass filter 22. The low pass filter 22 sets the high cutoff frequency fa of the audio signal. This is set to 3 KHz, for example, as described later. The amplitude modulator 21 amplitude-modulates the output of the first modulator 10 by the audio signal received from the low-pass filter 22. Then, the modulated signal is sent to the transmitter 40.

FIG. 4 is an explanatory diagram of an output signal spectrum of the transmitter of the present invention. The vertical axis of this graph represents amplitude and the horizontal axis represents frequency. As shown in the figure, for example, in the first modulator 10, the frequency of the carrier wave generated by the voltage controlled oscillator 11 is F. Here, the carrier wave is FSK modulated by the digital signal received from the digital signal source 31, and the transfer speed of the digital signal source in this case is fd. In that case, the FSK modulated signal is mostly concentrated in the vicinity of the frequencies F + fd and F-fd. That is, the signal energy is concentrated on the upper side and the lower side of the carrier wave F at a position separated by the frequency fd.

For example, if the transfer speed of digital data is set to 9600 bps, the frequency is converted to 4.8 KHz. Therefore, the modulated signal is concentrated near F + 4.8 KHz and F-4.8 KHz. On the other hand, since the high cutoff frequency of the audio signal in the second modulator is selected to be about 3 KHz, the signal energy is concentrated in the upper and lower ± fa range around the frequency F of the carrier as shown in this figure.

As described above, in the present invention, when a digital signal is FSK-modulated, there is room to insert a signal amplitude-modulated within a certain range around the frequency F of the carrier wave, and the digital signal and the analog signal are left. We are trying to multiplex. When this analog signal is a voice signal, the validity of the band will be explained.

FIG. 5 shows an intelligibility explanatory diagram of an audio signal. In FIG. 5A, the vertical axis represents the sound pressure level in the unit of db, and the horizontal axis represents the frequency. The horizontal axis represents the audio frequency of 100 Hz to 7000 Hz on a logarithmic scale. As shown in this figure, when an audio signal is received at a distance of about 1 m from the lips, the minimum audible value of the ear is as shown by the lower broken line. The average power spectrum of voice is shown by the upper solid line.

Further, in (b), the vertical axis represents clarity and the horizontal axis represents the frequency of the audio signal in the unit [KHz]. The solid line shows the high cutoff frequency, and the broken line shows the low cutoff frequency. That is, for example, when the high cutoff frequency is set to 3 KHz, the intelligibility becomes a value sufficiently exceeding about 80%. Therefore, the intelligibility of the voice signal is considered to be 100%. Therefore, even if an audio signal is taken out and transmitted through a bandpass filter with a high cutoff frequency of 3 kHz, the content can be sufficiently recognized on the receiving side.

As a result, the energy distributions of the digital signal and the analog signal as shown in FIG. 4 can be arranged in frequency bands that do not overlap each other, and the signals can be multiplexed. It is possible to set the high-frequency cutoff frequency of the audio signal to about 2500 KHz because the voice can be sufficiently recognized if the intelligibility is 75% or more.

FIG. 6 shows a block diagram of a receiver suitable for implementing the present invention. The signal transmitted by the transmitter as described above is received and reproduced by the receiver as shown in this figure. In FIG. 6, this device includes a receiver 51, a bandpass filter 52, a mixer 53, a local oscillator 54, a phase comparator (PLL) 55, a channel selector 56, a bandpass filter (BPF) 57, and a mixer 58. , A local oscillator 59, a bandpass filter (BPF) 60, an FM detector 61, a waveform shaper 62, a data processor 63, an AM detector 64, an audio signal extraction filter 65 and a speaker 66.

When the receiver 51 receives the signal through the transmission path 3, the bandpass filter 52 extracts the signal in the corresponding band. Specifically, when the band width W is selected to be 20 KHz, the band pass filter 52 is selected to the pass band of 20 KHz. The output of the bandpass filter 52 is input to the mixer 53. The channel selector 56 selects a channel according to the frequency of the carrier wave, a PLL circuit is constituted by the phase comparator 55 and the local oscillator 54, and the mixer 53 converts the signal into an intermediate frequency signal of 10.7 MHz. The band pass filter 57 passes the signal in the 10.7 MHz band. The mixer 58 receives the 10.25 MHz signal output from the local oscillator 59 and outputs an intermediate frequency 455 KHz signal. The band pass filter 60 is a filter for passing this signal. The output of the bandpass filter 60 is input to the FM detector 61 and the AM detector 64. In the FM detector 61, normal FM detection is performed, and when it is shaped by the waveform shaper 62, the digital signal modulated on the transmitting side is restored, and this is input to the data processor 63 and subjected to a certain processing. It Further, the AM detector 64 AM-detects the input signal, and the unnecessary signal is removed by the audio signal extraction filter 65 to obtain an audio signal. This audio signal is output from the speaker 66.

According to the receiving apparatus as described above, since the energy distributions of the digital signal and the voice signal are sufficiently separated as viewed from the frequency spectrum, they can be easily separated and demodulated. Focusing on the influence from voice to data, the modulation / demodulation system of digital data is the FM system, and even if AM components are mixed, suppression of about 40 dB is possible. Therefore, even if amplitude modulation of about 30 to 50 percent is applied on the transmitter side using the second modulator, it is technically possible to remove noise and clearly demodulate. Therefore, there is almost no occurrence of reception error after receiving the digital data.

Further, focusing on the influence of data on voice, the method is such that the data transfer speed is set to a relatively high speed of 9600 bps and the voice signal energy is inserted into the gap, so that interference is generally caused. Is more likely to occur. Therefore, in order to prevent the high cutoff frequency of the voice signal from overlapping with the digital signal, in the present invention, the high cutoff frequency fa is set below the transfer speed fd of the digital signal to increase the degree of separation. On the receiving side, an S / N ratio of about 40 db can be secured by increasing the degree of separation using the audio signal extraction filter 65.

As shown in FIG. 4, when the bandwidth W is allocated on the transmission line, the transfer speed fd of the digital signal is set within the range of the following equation. W> 2fd In this case, 2fd is selected to be a value close to the band width W as long as the FSK modulated signal does not extend outside the band. As a result, the high cutoff frequency of the AM modulated signal can be made relatively high, and the distortion of the analog signal can be reduced.

The present invention has a great effect in that the idle channels of the CATV system are effectively used as described above to achieve multiplexing of signal transmission / reception, but the present invention can also be used in various transmission lines. Further, in order to further improve the quality of voice, it is sufficient to use a subcarrier obtained by shifting the carrier frequency of the voice signal by a quarter wavelength with respect to the carrier frequency of the digital signal. In this case, a 90-degree phase shifter, a synchronous oscillator, etc. may be added to the transmitting side.

[0024]

[Effect of device]

 The transmission apparatus of the present invention described above has the bandwidth allocated on the transmission line as W, the frequency of the carrier wave as F, the transfer speed of the digital signal as fd, and the high cutoff frequency of the analog signal as fa. Then, fd is set in the range of W> 2fd, the first modulator that modulates the carrier wave by the frequency direct displacement method with the digital signal, and fa is set to fd or less and the analog signal is used. Since the second modulator that amplitude-modulates the output of the first modulator and the transmitter that sends the output of the second modulator to the transmission path are provided, a digital signal and an analog signal can be easily separated. Can be multiplexed. As a result, even when the allocated bandwidth is relatively narrow, it becomes possible to transfer a digital signal at a high speed and multiplex a voice signal that can be recognized sufficiently clearly, for example. Moreover, mutual interference between the digital signal and the analog signal can be suppressed sufficiently low, the circuit configuration can be simplified, and the transmitter and the receiver can be configured at low cost.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a transmitting device of the present invention.

FIG. 2 is a CATV system example connection diagram.

FIG. 3 is an explanatory diagram of empty channels.

FIG. 4 is a graph showing an output signal spectrum of the transmitter of the present invention.

FIG. 5 is a graph illustrating the intelligibility of an audio signal.

FIG. 6 is a block diagram of a receiver suitable for implementing the present invention.

[Explanation of symbols]

 3 Transmission Line 10 First Modulator 20 Second Modulator 31 Digital Signal Source 32 Voice Signal (Analog Signal) Source 40 Transmitter

Claims (3)

[Scope of utility model registration request] 1. When the bandwidth allocated on the transmission line is W, the frequency of the carrier is F, the transfer speed of the digital signal to be transmitted is fd, and the high cutoff frequency of the analog signal to be transmitted is fa, fd Is set in the range of W> 2fd, and the carrier wave is modulated by the frequency direct displacement method by the digital signal.
A modulator; a second modulator that sets fa to be equal to or less than fd and amplitude-modulates the output of the first modulator by the analog signal; and a transmitter that sends the output of the second modulator to the transmission path. A transmitting device characterized by the above. 2. The transmission line comprises a CATV system, the digital signal comprises data transmitted using an empty channel, and the analog signal comprises a voice signal also transmitted using the empty channel. The transmitter according to claim 1. 3. The second modulator amplitude-modulates a sub-carrier, which is obtained by shifting the carrier wave processed by the first modulator by a quarter wavelength, with an analog signal.
The transmission device described.