JPS6193785A - Pcm signal transmission system for catv - Google Patents

Abstract

PURPOSE:To increase the frequency accuracy and to miniaturize the device by multiplexing a modulation signal of a local carrier together with a modulation signal for a PCM signal, transmitting it and allowing a reception side to generate a local carrier having the same frequency as that of the transmission side. CONSTITUTION:The device at the transmission side is provided with a generating circuit OSC1 generating a fundamental frequency of a local carrier and a modulating path comprising a modulator 4 modulating the carrier at the fundamental frequency and a BPF5. Further, the device at the reception side is provided with a demodulation path comprising a distributing circuit 7 distributing a multiplex signal transmitted through a CATV transmission line 12 to each demodulation path, a BPF8, a demodulator 9 and a multiplier 10 for the signal C. A transmission signal of the demodulation path for the signal C is fed to PLL3-1 and PLL3-2, where the signal is subject to frequency multiplex, the result is transmitted to demodulators 9-1, 9-2 to form a local carrier at demodulation.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a PCM signal transmission system for CATV, and particularly to a PCM signal transmission system for CATV.
The present invention relates to a PCM signal transmission system for CATV that is suitable for transmitting PCM signals in which voice and data are pulse code modulated through ATV (cable television) WA.

[Conventional technology]

By using a method of transmitting PCM signals over a CATV line, the transmitted wave is subjected to multiphase phase modulation (or multilevel amplitude modulation), and then the local carrier wave t@width modulation is performed using this modulation signal, and the transmission *@Ik is transmitted. , a plurality of (DPCM signals) are frequency-divided into lengths and transmitted through CATV circuits.

[Problem that the invention seeks to solve]

In the conventional system described above, when the television signal is multiplexed and transmitted together with the modulated signal of the PCM signal, the local carrier wave for amplitude modulation mentioned above mixes into the band of the television signal. In order to prevent noise interference, the level of the local carrier wave leaked from the amplitude modulation circuit and transmitted is suppressed to a sufficiently low level. Therefore, on the receiving side, it is not possible to extract and exogenously generate a local carrier wave from the received signal, and it is necessary to generate a local carrier wave with the same frequency as the transmitting side and perform amplitude demodulation. Differences in environmental conditions such as temperature between the transmitting side and the receiving side will cause a difference in the frequency of the local carrier wave, but this difference in the frequency of the local carrier wave between the transmitting side and the receiving side 1 must be kept within the range in which the system operates normally. In order to maintain this level, the circuit scale of the oscillator that generates the local carrier wave must be large, resulting in a problem that the device becomes larger and more expensive.

For example, if the pull-in frequency range of the subcarrier regeneration circuit in the phase demodulation circuit on the receiving side is (center 1 series) ±250 Hz', and the local carrier is 125 MHz, the center value of the frequency on the transmitting side and receiving side is A local carrier oscillator with an accuracy of ±2×10@ at 125 MHz is required. To meet this accuracy, a temperature compensated crystal oscillator (TCXO) is required.
) must be used on both the sender and receiver,
Kanashi is large and expensive.

SUMMARY OF THE INVENTION An object of the present invention is to provide a PCM signal transmission system for CATV that solves the above-mentioned problems and allows equipment to be made smaller and cheaper than the conventional equipment.

[Means for solving problems]

The method of the present invention includes an oscillation means for generating a fundamental frequency of a predetermined frequency, a multiplication means for frequency-multiplying the fundamental frequency to generate a tenth local carrier wave, and a method for modulating a second local carrier wave with the fundamental frequency. a tenth modulated signal;
and a second modulation means having a two-stage modulation circuit that phase-modulates the subcarrier wave with a PCM signal and further amplitude-modulates the tenth frequency carrier wave to generate a second modulation signal. , a transmitting device that transmits a multiplexed signal obtained by superimposing the '#J1 and a second modulated signal to have a frequency division length of 1 to a CATV line; fgl(Q demodulation means for demodulating the tenth modulation signal to reproduce the fundamental frequency; a multiplication means for frequency-multiplying the fundamental frequency reproduced by the tenth demodulation means to reproduce the tenth local carrier wave; 7z included in multiplex signal
After demodulating the modulated signal of the recitation 2 with the tenth local carrier wave reproduced by the multiplier, and a receiving device having.

(Example〕 Next, one embodiment of the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention.

The device on the transmission side includes an oscillation circuit (08C) that generates a fundamental frequency of a local carrier wave, a modulation oil path that includes a modulator 4 and a bandpass filter (BPF) 5 for modulating the carrier wave with this fundamental wave. , phase modulation circuit 2-1 (or 2-2 in some animals) for modulating with PCM signal (1) (or (2)).
), modulation 54-1 (4-2), and bandpass filter (
BPF) 5-1 (or frequency) is frequency-multiplied to generate each local carrier wave. Phase-locked loop (PLL) 3-1 (
Or 3-2).

The transmission signals C, DI and D2 of each modulation path are converted into a frequency division multiplexed signal and sent to the CATV transmission line 1.
It is equipped with a coupling circuit 6 for sending data to 2.

PCM signal (1) or (2) manually transmitted to m units
(Other than this, illustrations are omitted to avoid duplication of explanation) are all signals obtained by pulse code modulation and time division multiplexing of audio signals (a certain 7h is a data signal). For example, the PCM signal (1) is transmitted to the phase modulation circuit 2-1.
In order to perform polyphase differential phase modulation on the subcarrier, the subcarrier is passed through a roll-off filter and subjected to predetermined roll-off shaping, and then sent to the f modulator 4-1 as a phase modulation signal. The modulator 4-1 is
The local carrier wave sent from PLL 3-1 is amplitude-modulated using the above-mentioned phase modulation signal. The BPF 5-1 blocks unnecessary sideband components from this amplitude modulation signal, passes the desired sideband components, and sends it to the coupling circuit 6 as a signal D1.
send to Similarly, the PCM signal (2) also undergoes a two-stage modulation process of phase modulation and amplitude modulation, and its desired sideband component, signal D2, is sent to the coupling circuit 6. BPFs 5-1 and 5-2 sufficiently attenuate leakage of local carrier waves from modulators 4-1 and 4-2, respectively, in order to prevent each local carrier wave from entering the television signal band. On the other hand, 08C1 is the fundamental frequency (or sub-fundamental frequency)
is sent to the PLLs 3-1 and 3-2, etc., and its fundamental frequency is sent to the modulator 4.The modulator 4 is an amplitude modulator, and the carrier wave of a predetermined frequency is sent to the fundamental frequency sent from 08Cl. 0BPF5, which is amplitude modulated and sent to BPF5, is
Out-of-band components other than the 7h double-side band component included in this amplitude modulation signal are suppressed and sent as signal C to the coupling circuit 6. The coupling circuit 6 connects the signals C9D1 and D2 (etc.)
f: The device on the receiving end side that sends out the superimposed multiplexed signal to the CATV transmission line 12 includes a distribution circuit 7 that distributes the multiplexed signal sent through the CATV transmission line 12 to each demodulation path. Bandpass filter (BPF) 8 and demodulator 9 (and multiplier 1
0) and the demodulation path of the signal C.

Bandpass filter (BPF) 8-1 (or 8-2), demodulator 9-1 (or 9-2), and phase demodulation circuit 11-1
(or 1l-2)
is a PLL that generates each local carrier wave by frequency-multiplying the output signal of the return WA route of D2) and the return p4 route of signal C.
3-1 (or 3-2).

BPFs are bandpass filters that pass the signal C included in the multiplexed signal and suppress other unnecessary components. The demodulator 9 is an envelope detector and performs envelope detection on the signal C to reproduce the fundamental frequency of the local carrier wave. The multiplier 10 is a frequency multiplier, and has a multiplication ratio equal to the multiplication ratio of the frequency of the signal sent from the 08C1 to the PLLs 3-1 and 3-2 with respect to the fundamental frequency on the transmitting end side. Therefore,
Send m#"t'osc 1 to PLL3-1 and 3-2
If the fundamental frequency is sent as is without being multiplied, this multiplier 10 is also unnecessary. The output signals of the demodulation path of signal C are sent to PLLs 3-1 and 3-2, frequency-multiplied, and sent to demodulators 9-1 and 9-2, respectively. On the other hand, for example, in the signal D10 multiplex path, the signal D1 included in the multiplexed signal passes through the BPF 8-1, and then is demodulated by the local carrier wave sent from the PLL 3-1 in the demodulator 9-1. , and is further demodulated by a phase demodulation circuit 11-1 to reproduce the PCM signal (1). Similarly, the signal D2 passes through a two-stage demodulation process (through the y4 path), and then
PCM signal (2) is reproduced.

FIGS. 2(a) and 2(b) Fi are block diagrams each showing a configuration example of 08CI in this embodiment. The incubator 13 generates a fundamental frequency of a predetermined frequency.

In the figure (a), the fundamental frequency generated by the oscillator 13 is transferred to the modulator 4.
PLL3-1 (and 3-2) as is.
etc.). In this case, it is possible to use a local carrier wave having a frequency that is lI times the frequency of the fundamental frequency. Figure (b) shows how the fundamental frequency is sent to the modulator.
A case is shown in which each PLL receives a sub-fundamental frequency obtained by frequency-multiplying its fundamental frequency by a multiplier 10. In this case, a local carrier wave having a frequency that is an integral multiple of the frequency of the sub-fundamental frequency can be used.

FIG. 3 shows PLL 3-1 (or 3-3) in this embodiment.
FIG. 2 is a block diagram illustrating an example of the configuration. The phase detector (PD) 31 detects the fundamental frequency (
sub-fundamental frequency) and voltage-controlled oscillator (VCO) 33
The phase difference between the output signal and the signal frequency-divided by the divider 34 is detected, and a voltage detection signal proportional to the phase difference is generated and sent to the low-pass filter (LPF) 32. The detection signal whose high frequency components have been suppressed through the LPF 32 is sent to the VCO 33. The VCO 33 responds to the detection signal by decreasing the absolute value of the voltage and at the same time changing the oscillation frequency.

let The output signal of the VCO 33 is sent to the modulator 4-1 as a local carrier wave, and is also sent to the frequency divider 34. In other words, PLL 3-1 (or 3-2, etc.) operates so that the voltage of the detection signal becomes zero and converges to this state, and the fundamental frequency (or sub-fundamental frequency) is
A local carrier wave whose frequency is multiplied by a multiplication ratio equal to the frequency division ratio of the frequency divider 340 is obtained.

FIG. 4 is a spectrum diagram showing the frequency arrangement of signals C, D1 and D2 in this embodiment. The signal C has a spectrum of a carrier wave inputted to the modulator 4 and double-sided bands above and below the carrier wave, each separated by the frequency of the fundamental frequency, and therefore occupies a bandwidth twice the frequency of the fundamental frequency. Signals D1 and D2 each have the desired sideband spectrum after amplitude modulation and therefore occupy the same bandwidth as the phase modulated and roll-off shaped signal.

□ As explained above, in this embodiment, the fundamental frequency for obtaining each local carrier wave is frequency division multiplexed and transmitted together with the phase modulation signal of each PCM signal, so that even if frequency fluctuations occur in the fundamental frequency, , each local carrier wave can be generated at the receiving end at the same frequency as at the transmitting end. Compared to the conventional system, in which oscillators with high oscillation frequency accuracy, such as TCXO%, must be used on both the transmitting end and the receiving end, in this embodiment, the oscillator 13 in the 08CI is Also, oscillators with low frequency accuracy,
For example, a crystal oscillator without temperature compensation may be used, and the means for generating the local carrier wave can be made smaller and cheaper than in the past. Furthermore, if the frequency of the fundamental frequency is selected to be low, the occupied bandwidth of the signal C can be narrowed, and the fundamental frequency can be added and multiplexed without significantly impairing the utilization efficiency of the transmission frequency bandwidth.

Note that although this example shows the case where the fundamental frequency amplitude modulation signal is additionally multiplexed, the fundamental frequency modulation method is amplitude modulation (
AM) There is no need to limit K, and it is obvious that additional multiplexing of the fundamental frequency can be performed even if frequency modulation (FM) or frequency shift modulation (F8K) is applied, as in this embodiment. You can get the effect of

As is clear from the above description, the present invention includes multiplexing and transmitting a modulated signal of a local carrier wave together with a modulated signal of a PCM signal, and generating a local carrier wave of the same wave number on the receiving end side as on the transmitting end side. If we can realize a PCM signal transmission system for CATV that has high frequency accuracy and therefore does not require the use of large and expensive oscillators, making iron equipment smaller and cheaper than before, it will have a V-like effect. .

[Brief explanation of the drawing]

FIG. 1, FIG. 2 (a) and (b), and FIG. 3 are block diagrams each showing an embodiment of the present invention, and FIG. 4 is a spectrum diagram showing the frequency distribution fill of the embodiment of the present invention. . 1... Oscillation circuit (OSC), 2-1.2-2.
...Phase modulation circuit, 3-1.3-2...
Phase-locked loop (PLL), 4, 4-1, 4-'2
...Modulator, color, 5-1.5-2.8.8-1
．． 8-2... Bandpass filter (BPF), 6...
... Combined circuit, 7... Distribution circuit, 9.9-
1.9-2... Demodulator, 10... Multiplier, 11-1.11-2... Phase demodulation circuit.纂 2 Figure $ 3 Senya Koshu - $ 4 Figure

Claims (1)

[Claims] oscillation means for generating a fundamental frequency of a predetermined frequency; multiplication means for multiplying the frequency of the fundamental frequency to generate a first local carrier wave; and modulating a second local carrier wave with the fundamental frequency to generate a first local carrier wave. a first modulating means for generating a modulated signal; and a PC.
a second modulation means having a two-stage modulation circuit that phase-modulates the subcarrier with the M signal and then amplitude-modulates the first local carrier to generate a second modulation signal; a transmitter that transmits a multiplexed signal on which a second modulated signal is superimposed and frequency division multiplexed to a CATV line;
a first demodulating signal that demodulates the first modulated signal included in the multiplexed signal received from the V line and reproduces the fundamental frequency;
a demodulating means, a multiplying means for frequency-multiplying the fundamental frequency reproduced by the first demodulating means to reproduce the first local carrier wave, and a multiplying means for reproducing the first local carrier wave, After demodulating with the tenth local carrier wave reproduced by the multiplier, the phase demodulation is further performed with the subcarrier wave to transmit the signal to the PC.
A CATV characterized by comprising: a receiving device having a second demodulating means having a two-stage demodulating circuit for reproducing the M signal;
PCM signal transmission method.