JPS6129285A - Two-way cable transmission system - Google Patents

Abstract

PURPOSE:To increase the transmission channel capacity in the up direction by generating the 2nd carrier wave for synchronizing with the 1st carrier wave in phase, modulating the 2nd carrier wave and forming a transmission channel in the up direction of a transmission line. CONSTITUTION:A head end 10 has a reference oscillator for generating a pilot carrier wave fp, and serves as a center unit for transmitting it in the down direction of a transmission line 12 and receiving a carrier wave fci from an arbitrary tap (i). The transmission line 12 from the head end is branched in several directions by a splitter 14, and some part 16 is led in a tap 20 through a directional coupler 18. Two-way amplifier 22 is inserted into the transmission line 16, and a transmitter 24 is connected to, for instance, an i-numbered tap branched by an amplifier 18. Each tap uses the pilot carrier wave fp transmitted in the down direction from a basic oscillator of the head end 10, and each tap controls of a phase of the carrier wave fci by means of said transmitter in the up direction with the carrier wave fp as reference.

Description

[Detailed Description of the Invention] Pei Yinggong! The present invention relates to a bidirectional cable transmission system, particularly a bidirectional CA
This invention relates to a bidirectional cable transmission system in which video signals are transmitted in one direction through multiple transmission channels in TVs and CGTVs.

The main factor that limits the transmission channel capacity in bidirectional cable transmission systems is cross-modulation distortion in the repeater amplifiers in the transmission system. In the case of video signals, cross-modulation distortion appears as beat disturbance in the image.

Conventionally, in one-way CATV, there has been a technique for visually reducing the influence of this beat interference on the downlink broadcast from the head-end. For example, a headend with a system called ``Harmonically Related Carrier'' (HRC) has a 6MHz master clock and has to phase synchronize all channels to maintain the same frequency spacing. This reduces third-order cross modulation distortion. In addition, in a headend using a system called Interval Related Carrier J (HRC), all channels except two specific channels are normally operated on the assigned frequencies, and these two channels are shifted to other channels. By.

Third-order cross modulation distortion is reduced.

In both of these conventional techniques, carrier waves for each channel must be generated centrally at the head end. Therefore, although it is effective in the downstream direction of the line, it cannot be used in the upstream direction.

SUMMARY OF THE INVENTION It is an object of the present invention to overcome these drawbacks of the prior art and to provide a bidirectional cable transmission system that has a higher transmission channel capacity in the upstream direction than the prior art.

Structure The structure of the present invention will be described below based on one embodiment.

Before explaining the embodiments, cross-modulation distortion, particularly third-order cross-modulation distortion, will be explained. The carrier frequency fci of the i-th transmission channel among the plurality of transmission channels is, assuming that its occupied band is fB (for example, 8MHz in Japan and the United States), 1fci = fo +
is fB+Δfi (
1). Here, the frequency deviation 1Δfil is generally sufficiently small, but not zero.

Third-order intermodulation distortion is the result of three channels x,! and 2, it occurs in the form fcx±fcy±fcz. For example, the third-order intermodulation distortion re I+fcm-fan for three channels l, m and n is fcl+fcm-f
an = fo+ (1+m-n) a fB+ (Δ
fl+Δfm−Δfn) (2). On the other hand, the carrier wave fc(1+m-n) of channel I+m-n is expressed by formula (1)
Yori, fc (, +, n) = fo+ (++m-n
) e fB+Δ'l+m□ (3)
It is.

The problem here is when the frequencies in equations (2) and (3) are not the same but slightly different. That is, Δf x Δf1 + Δfm - Δfn
(4) If 1+m-n, the difference in frequency on both sides of equation (4) becomes a beat and is mixed into the video signal. When reproduced on a television receiver, this third-order coupling distortion may have an adverse effect on visible images, resulting in, for example, unsightly stripes known as so-called windshield wipers.

Therefore, in this embodiment, this negative visual effect is removed by establishing the equality sign in equation 1 (0. In other words, Δf = Δfl + Δf + a - Δfn (5
), in equation (1), Δf
It is sufficient to set i'=o (8). In this case, both sides of equation (5) become 0.

fci = fo+ is fB
(la) □ means a phase-locked loop (PLL)
) technology, as in the prior art described above, CATV
In broadcasting downstream from the head end, a reference oscillator is provided at the head end, so it is possible to form a large number of phase-synchronized carrier waves fci, thereby reducing third-order cross modulation in the downstream direction. , easily. However, in the conventional bidirectional CATV system, it is difficult to control the carrier waves sent out from multiple taps in the upstream direction.
It was difficult. This is because there is an independent oscillator for each tap, which serves as a reference for transmitting carrier waves in the upstream direction.

In uplinks, it is difficult to maintain transmission loss strictly constant using methods such as the pilot AGC7+ method used in downlinks. Therefore, the transmission loss from each tap of the uplink to the head end varies. Therefore, the signal level of each carrier wave in the upstream direction is input to the relay amplifier in a state where the signal level varies. Therefore, sometimes too strong a signal is input, which tends to cause cross-modulation distortion. Therefore, conventionally, it has not been possible to increase the transmission channel capacity in the uplink direction.

Therefore, in this embodiment, the biloft carrier wave f transmitted in the downward direction from the reference oscillator provided at the head end is
p is used at each tap, and in each tap, the phase of the carrier wave fci is controlled using this pyro in the transmitter in the upstream direction with reference to the throat frequency fp.

Referring to Figure 1, a single-cable bidirectional CAT
An example in which the present invention is applied to V is shown. It goes without saying that the present invention can also be advantageously applied to a dual cable system. In the figure, the head end 10
is a center unit that has a reference oscillator that generates the pilot carrier wave fp described above, sends it out in the downward direction of the transmission line 12, and receives a carrier wave fci from an arbitrary tap i. A transmission line 12 from the head end 10 is split in each direction by a splitter 14, and a portion 16 thereof is drawn into a tap 20 through a directional coupler 18 as shown. Bidirectional amplifiers 22 are inserted into the transmission line 1θ at appropriate distances.
For example, a transmitter 24 is connected to the i-th tap branched by the bidirectional amplifier 18 as shown in the figure, and its input/output port 26 is connected to a video signal system such as a television camera or a television receiver. 28 are connected.

As an example of the configuration of the transmitter 24 is shown in FIG. 2, a modulator 50 and a directional coupler 52 are connected between the tap 20 and the input/output port 28. The modulator 50 is a circuit that modulates the carrier wave fci with the video signal input from the video signal system 28. The directional coupler 52 is a modulator 50
The carrier wave fci from is sent to tap 20, and tap 20
This is a combiner that sends the pilot carrier wave rp received from the head end IO through the divider 54. Divider 54
is a frequency dividing circuit that divides the pilot frequency rp by an integer a.

carrier frequency fci supplied to modulator 50 of tap i
is generated by reference oscillator 56 at output θ2. This carrier frequency fci is selected such that, in each tap i, the frequency intervals of its own channel with respect to mutually adjacent transmission channels on the frequency axis are substantially equal. The phase of the carrier wave fci is controlled by the output 80 of the phase comparator 58. The output 62 of the oscillator 56 is also supplied to another divider 64, which divides the carrier wave frequency fc
This is a frequency dividing circuit that divides i by an integer. Both dividers 54
The outputs θB and 68 of 84 and 68 are input to the phase comparator 58. Phase comparator 58 produces an output on control line 80 that compares the circular frequencies and adjusts the frequency of oscillator 56 so that they are equal.

More specifically, the phase comparator 58 detects frequencies fp/a and fc.
i/b are compared, fp/a= fci/b (
? ) The oscillation frequency of the oscillator 56 is adjusted so as to provide the following effect.

Here, fp/a= fB
(8), that is, in Japan and the United States, 8M
If it is Hz, then fci = b 11fB from equation (7)
(!3). This means that all carrier waves are harmonics of fH.

On the other hand, let fp/a be smaller than this, for example 0.25.
If MHz, fci = b @fp/a
(10), so it can also be placed on a harmonic of 0.25 MHz. This means that it can be tuned to the frequencies of many existing television channels. However, it goes without saying that in this case, the condition of equation 1 (la) must be satisfied.

This is satisfied by the following conditions: That is, let the integer C be c = fB/fp/a (
fl) and d is an integer, fo= d 11fp/a
(12) fci = fo+ i * fB = d reference fp/a+i*c@fp/a = (d+ f IIc) fp/a
(t3) Therefore, from equation (10), b=d+i*c (14) It is clear that it is not necessary to use a series of channels consecutively for channel i, and it is sufficient to use them at appropriate intervals. Interference from adjacent channels can be avoided by preventing transmission between adjacent channels.The present invention is particularly effectively applied to a system that can secure a wide transmission band, such as a dual cable system.

肱-1 According to the present invention, - Allows greater cross-modulation distortion in the upstream direction than before. In other words, when applied to a bidirectional cable transmission video signal system, visual beat disturbance is greatly reduced. . Therefore, large variations in signal levels are allowed in the upstream direction, and a large transmission channel capacity can be achieved.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an embodiment in which a bidirectional cable transmission system according to the invention is applied to a single cable type bidirectional CATV. FIG. 2 is a block diagram showing a configuration example. Lightning 24..., Transmitter 54.84. , divider ・si+, , oscillator - 5B, , phase, phase comparator

Claims (1)

[Claims] 1. In a bidirectional cable transmission system in which a plurality of taps are connected to a head end via a bidirectional cable transmission path, the taps are transmitted from the head end in the downstream direction of the transmission path. receiving the first carrier wave of a predetermined frequency, and generating a second carrier wave phase-synchronized with the first carrier wave;
A bidirectional cable transmission system characterized in that a second carrier wave is modulated to form an upstream transmission channel of the transmission path. 2. The transmission method according to claim 1, wherein the phase synchronization is performed by a phase-locked loop. 3. The transmission method according to claim 1, wherein the second carrier wave has a predetermined frequency interval with respect to adjacent transmission channels. 4. In the system according to claim 3, the second carrier wave is modulated by a television signal, and the frequency interval is an integral multiple of a predetermined frequency bandwidth that is greater than or equal to the occupied bandwidth of the television signal. A transmission method characterized by: 5. The transmission method according to claim 4, wherein the first carrier wave has a frequency that is an integral multiple of the predetermined frequency bandwidth. 6. The transmission method according to claim 5, wherein the second carrier wave has a frequency that is an integral multiple of the predetermined frequency bandwidth.