JPH0464217B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a PCM signal transmission system for CATV,
In particular, 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 panel code-modulated through a CATV (cable television) line.

[Conventional technology]

As a method of transmitting PCM signals to CATV lines,
The modulation signal of multiple PCM signals is modulated by performing multiphase phase modulation (or multilevel amplitude modulation) on the subcarrier with the PCM signal, and then amplitude modulating the local carrier with this modulation signal to transmit the desired sideband. Frequency division multiplexing and transmission over CATV lines.

[Problem that the invention seeks to solve]

In the conventional method described above, when the television signal is multiplexed and transmitted together with the modulated signal of the PCM signal, it is necessary to prevent the local carrier wave for amplitude modulation described above from mixing into the band of the television signal and causing noise interference. In order to achieve this, 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 reproduce the local carrier wave from the received signal, and it is necessary to generate a local carrier wave of the same frequency as that on the transmitting side and perform amplitude demodulation. Differences in environmental conditions such as temperature between the transmitter and receiver cause differences in the frequency of local carrier waves, but this difference in frequency of local carrier waves between the transmitter and receiver is kept within the range that allows the system to operate normally. Therefore, the circuit scale of the oscillator that generates the local carrier wave increases, resulting in a problem that the device becomes larger and more expensive.

For example, if the pull-in frequency range of the subcarrier recovery circuit in the phase demodulation circuit on the receiving side is (center value) ±250 Hz, and the local carrier is 125 MHz, the center value of the frequency on the transmitting side and receiving side is 125 MHz.
Therefore, a local carrier oscillator with an accuracy of within ±2×10 ⁶ is required. To meet this accuracy, a temperature-compensated crystal oscillator (TCX0) must be used on both the transmitting and receiving sides, making it considerably large and expensive.

The purpose of the present invention is to solve the above-mentioned problems and to provide a PCM for CATV that can make the device smaller and cheaper than before.
The objective is to provide a signal transmission method.

[Means for solving problems]

The system 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 first local carrier wave;
modulating a second local carrier with the fundamental frequency to
a first modulating means for generating a modulated signal;
After phase modulating the subcarrier with the signal, the first
a second modulation means having a two-stage modulation circuit that generates a second modulation signal by amplitude modulating a local carrier wave of a transmitting device that transmits a multiplexed signal to a CATV line; a first demodulating means that demodulates the first modulated signal included in the multiplexed signal received from the CATV line to reproduce the fundamental frequency; The first
a multiplier for reproducing the first local carrier wave by frequency-multiplying the fundamental frequency regenerated by the demodulating means; and a multiplier for reproducing the second modulated signal included in the multiplexed signal by the multiplier. and a second demodulating means having a two-stage demodulation circuit that performs phase demodulation using the subcarrier after demodulating using one local carrier wave to reproduce the PCM signal.

〔Example〕

Next, 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 transmitting end side includes an oscillation circuit (OSC) 1 that generates a fundamental frequency of a local carrier wave, a modulation path including a modulator 4 and a band pass filter (BPF) 5 for modulating the carrier wave with this fundamental frequency, and a PCM. Phase modulation circuit 2- for modulating signal (1) (or (2))
1 (or 2-2), a modulator 4-1 (or 4-2), and a band pass filter (BPF) 5-1 (or 5-2). A phase-locked loop (PLL) 3-1 (or 3-2) that generates each local carrier wave by frequency-multiplying the sub-fundamental frequency), as well as transmission signals C and D for each modulation path.
1 and D2 and sends the resultant signal to the CATV transmission line 12 as a frequency division multiplexed signal.
Equipped with:

PCM signal (1) or (2) input to the transmitting end
(Other than this, illustrations are omitted to avoid duplication of explanation) are all signals obtained by pulse code modulating (time division multiplexing as necessary) audio signals (or data signals). For example, the PCM signal (1) is
After the subcarrier is subjected to polyphase differential phase modulation in the phase modulation circuit 2-1, it passes through a roll-off filter, undergoes predetermined roll-off shaping, and is sent to the modulator 4-1 as a phase modulation signal. Modulator 4-1 is PLL3
The local carrier wave sent from -1 is amplitude-modulated using the above-mentioned phase modulation signal. The BPF 5-1 blocks unnecessary sideband components included in this amplitude modulation signal, passes the desired sideband components, and converts it into the signal D1.
The signal is sent to the coupling circuit 6 as a signal. 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. In BPF5-1 and 5-2,
In order to prevent each local carrier wave from entering the television signal band, modulators 4-1 and 4-4 are used, respectively.
- Sufficiently attenuate the local carrier leakage from -2. On the other hand, the OSC 1 sends the fundamental frequency (or sub-fundamental frequency) to the PLLs 3-1 and 3-2, and also sends the fundamental frequency to the modulator 4. Modulator 4 is an amplitude modulator, which modulates a carrier wave of a predetermined frequency.
Amplitude modulates the fundamental frequency sent from OSC1 and sends it to BPF5. The BPF 5 suppresses out-of-band components other than the double-side band included in this amplitude modulation signal, and sends this as a signal C to the coupling circuit 6.
The coupling circuit 6 connects the signals C, D1 and D2 (etc.)
The multiplexed signal on which the signals are superimposed is sent to the CATV transmission line 12.

The device on the receiving end side includes a distribution circuit 7 that distributes the multiplexed signal sent through the CATV transmission line 12 to each demodulation path, a bandpass filter (BPF) 8, and a demodulator 9 (and a multiplier 10). demodulation path of signal C, bandpass filter (BPF) 8-1 (or 8
-2), a demodulation path for the signal D1 (or D2) including a demodulator 9-1 (or 9-2), and a phase demodulation circuit 11-1 (or 11-2), and a demodulation path for the signal C. PLL 3-1 (or 3
-2).

The BPF 8 is a bandpass filter that passes the signal C included in the multiplexed signal and suppresses other unnecessary components. The demodulator 9 is an envelope detector, and detects the envelope of 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 OSC 1 to the PLLs 3-1 and 3-2 at the transmitting end with respect to the fundamental frequency. Therefore, if the fundamental frequency is sent as is from OSC1 to PLL3-1 and PLL3-2 on the transmitting side without being multiplied,
This multiplier 10 is also unnecessary. The output signal of the demodulation path of signal C is sent to PLL 3-1 and 3-2, and frequency-multiplied by demodulator 9-1.
and sent to 9-2. On the other hand, for example, signal D1
In the demodulation path, the signal D1 included in the multiplexed signal passes through the BPF 8-1 and then the demodulator 9-
1, the PCM signal (1) is demodulated by the local carrier wave sent from the PLL 3-1, and further demodulated by the phase demodulation circuit 11-1 to reproduce the PCM signal (1). Similarly,
The signal D2 passes through the demodulation path and undergoes a two-stage demodulation process to reproduce the PCM signal (2).

FIGS. 2a and 2b are block diagrams each showing an example of the configuration of the OSC 1 in this embodiment. Oscillator 13 generates a fundamental frequency of a predetermined frequency.
Figure a shows a case in which the fundamental frequency generated by the oscillator 13 is sent to the modulator 4 and also sent as is to the PLL 3-1 (and 3-2, etc.). In this case, a local carrier wave having a frequency that is an integral multiple of the fundamental frequency can be used. FIG. 1b shows a case where a fundamental frequency is sent to the modulator and a sub-fundamental frequency obtained by frequency-multiplying the fundamental frequency by a multiplier 10 is sent to each PLL. 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 is a block diagram showing an example of the configuration of PLL 3-1 (or 3-2, etc.) in this embodiment. A phase detector (PD) 31 divides the fundamental frequency (or sub-fundamental frequency) sent from the OSC 1 and the output signal of the voltage controlled oscillator (VCO) 33 into a frequency divider 3.
The phase difference with the signal frequency-divided by 4 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 that passes through the LPF 32 and has its high frequency components suppressed is sent to the VCO 33. The VOC 33 responds to the detection signal and changes its oscillation frequency in a direction that reduces the absolute value of the voltage. The output signal of the VCO 33 is transmitted to the modulator 4-1
as a local carrier wave to the frequency divider 34.
is being sent to. In other words, the PLL 3-1 (or 3-2, etc.) operates so as to converge to a synchronous state, that is, a state in which the voltage of the detection signal becomes zero, and transmits the fundamental frequency (or sub-fundamental frequency) to the frequency divider 3.
A local carrier wave whose frequency is multiplied by a multiplication ratio equal to a division ratio of 4 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 double-sided bands that are separated by the frequency of the carrier wave input to the modulator 4 and the fundamental frequency above and below the carrier wave, and thus occupies a bandwidth of twice the fundamental frequency. Signals D1 and D2 each have a desired sideband spectrum of amplitude modulation control 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, the receiving end Each local carrier wave of the same frequency as the transmitting end side can be generated at the transmitting end side. Compared to the conventional method, which requires the use of oscillators with highly accurate oscillation frequencies, such as TCXOs, on both the transmitting end and the receiving end, in this embodiment, the OSC
As the oscillator 13 in the oscillator 1, an oscillator with lower frequency accuracy than conventional ones, such as a crystal oscillator without temperature compensation, can be used, and the means for generating local carrier waves can be made smaller and cheaper than conventional ones. 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 embodiment shows the case where the amplitude modulation signal of the fundamental frequency is additionally multiplexed, the modulation method of the fundamental frequency need not be limited to amplitude modulation (AM), and may be frequency modulation (FM) or frequency shift modulation ( It is clear that even if FSK) is applied, additional multiplexing of the fundamental frequency can be performed, and the same effect as this embodiment can be obtained.

As is clear from the above description, the present invention includes:
By multiplexing and transmitting the modulation signal of the local carrier wave together with the modulation signal of the PCM signal, and generating a local carrier wave with the same frequency at the receiving end as that at the transmitting end, the frequency accuracy is high, and therefore a large and expensive oscillator is used. This has the effect of making it possible to realize a PCM signal transmission system for CATV that can be made smaller and cheaper than before without having to do so.

[Brief explanation of the drawing]

FIG. 1, FIG. 2 a and b, and FIG. 3 are block diagrams showing an embodiment of the present invention, and FIG.
The figure is a spectrum diagram showing the frequency arrangement 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,
5, 5-1, 5-28, 8-1, 8-2... Bandpass filter (BPF), 6... Combining circuit, 7... Distribution circuit, 9, 9-1, 9-2... Demodulator , 10...
Multiplier, 11-1, 11-2...phase demodulation circuit.

Claims (1)

[Scope of Claims] 1. Oscillating means for generating a fundamental frequency of a predetermined frequency, multiplication means for frequency-multiplying the fundamental frequency to generate a first local carrier wave, and a second local carrier wave at the fundamental frequency. a first modulating means that generates a first modulated signal by modulating the subcarrier; and 2 that further amplitude modulates the first local carrier after phase modulating the subcarrier with the PCM signal to generate a second modulated signal. a transmitting device having a second modulating means having a stage modulation circuit, and transmitting a multiplexed signal obtained by superimposing the first and second modulated signals and frequency division multiplexing to a CATV line; and receiving from the CATV line. a first demodulating means for demodulating the first modulated signal included in the multiplexed signal to reproduce the fundamental frequency;
a multiplier for reproducing the first local carrier wave by frequency-multiplying the fundamental frequency regenerated by the demodulating means; and a multiplier for reproducing the second modulated signal included in the multiplexed signal by the multiplier. and a second demodulation means having a two-stage demodulation circuit that demodulates the PCM signal using one local carrier wave and then further phase demodulates the PCM signal using the subcarrier wave. PCM signal transmission method.