GB2029174A

GB2029174A - Pilot regulation of repeatered line systems

Info

Publication number: GB2029174A
Application number: GB7835139A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-08-31
Filing date: 1978-08-31
Publication date: 1980-03-12
Also published as: GB2029174B

Abstract

In a repeatered analogue line system, using pilot tones for gain control, undesirable seasonal changes occur in the mean traffic level due to temperature dependent cable losses. The invention provides means 7 for altering the level of the transmitted pilot tone relative to the mean traffic level, with complementary adjustment of the level of the received pilot tone by means 3' at the receiver. The mean traffic level can thereby be adjusted to take account of the seasonal changes in cable loss. <IMAGE>

Description

SPECIFICATION Pilot regulation of repeatered line systems This invention relates to an arrangement for pilot tone regulation of repeatered analogue line systems.

It is common in such systems to use a pilot tone which is compared, at the repeater, with a reference signal to produce a gain control signal for that repeater. However, cable losses are temperature dependent and thus cause undesirable seasonal fluctuations in the system mean traffic level.

According to the present invention there is provided an arrangement for pilot tone regulation of a repeatered analogue line system comprising means at the transmitting terminal of the system for altering the level of the transmitted pilot tone relative to the mean traffic level and means at the receiving terminal for altering the level of the received pilot tone, the two altering means being operated in complementary fashion.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates a prior art form of line repeater, Figure 2 illustrates the operation of a pilot tone regulation scheme, and Figure 3 illustrates the pilot tone regulation arrangement of the present inventton.

Fig. 1 shows the most common form of gain control applied to analogue repeatered line systems. The repeater amplifier 1 amplifies a band of analogue signals together with a pilot tone. A filter 2 is connected to the output of the amplifier 1 and selects the pilot tone from the other traffic. The pilot tone is then amplified by amplifier 3 and rectified by rectifier 4 to produce a dc voltage proportional to the level of the pilot. A further amplifier 5 amplifies the difference between the rectified dc and a reference voltage Vref. If the rectified pilot differs from the reference the amplifier 5 provides a controlling signal to change the gain of amplifier 1. The polarity of the control signal is chosen so that the gain of 1 changes to make the rectified pilot level equal to the reference voltage.Since the gain through elements 2, 3 and 4 is fixed, the effect is to fix the level of pilot at the output of the repeater amplifier despite variation in its input level.

The manner in which the regulation scheme is normally operated is shown in Fig. 2. The pilot is sent at reduced signal level relative to the traffic. Under normal conditions the singal levels along the system are represented by the horizontal axis. In cold conditions the cable loss is lower than normal so that the level at the input to regulated amplifier 1 is higher than normal as shown by the full ramp. The regulating action restores the levels to nominal at the output of each regulator. The deviation of the pilot level from nominal in winter therefore follows a sawtooth characteristic above the nominal level. The level of a test signal in the traffic is normally fixed 10 db (10 X power) stronger than the pilot, as shown in Fig. 2 by the vertical separation between Traffic an Pilot sawtooth graphs.In summer the deviations from nominal are opposite and result in sawtooth characteristics that are below the nominal axis (horizontals).

The change in mean traffic level from winter to summer is undesirable. In winter the increased mean level increases distortion and reduces the margin of safety against overload.

In summer the reduced mean traffic level is no longer as far above background noise as it should be.

Fig. 2 shows how the mean level of traffic can be lowered in winter to the dotted sawtooth characterstic. This is symmetrical about the nominal level and has only half the deviation from nominal so that performance remains much closer to nominal in winter.

This change in traffic level is achieved in the present invention by altering the relative level of the pilot from 10 db below traffic in normal conditions to 8 db (say) in winter. The regulators in the line (Fig. 1) act on the pilot as shown before in the lower characteristic of Fig. 2. In winter the pilot deviation is restored to nominal at the amplifier output, but the traffic is restored to a level below nominal by the change in relative pilot and traffic levels.

In summer the opposite correction may be applied by increasing the level difference to 1 2 db instead of decreasing it to 8 db.

In practical systems the cable response varies most at the higher frequencies between winter and summer, and so the regulators of Fig. 1 are designed to vary their gain more at higher frequencies. It will be noted that such a regulator will apply a shift in the level of traffic that is similarly frequency dependent if the level of pilot entering is controlled. This is the desired type of precompensation of traffic level. Fig. 3 shows a preferred method of effecting the control of traffic level.

It operates as follows: The pilot tone is generated in an oscillator 6 and injected into the traffic by an amplifier 7 whose gain may be adjusted by a traffic level control. Control may be switched resistors over a + 3 db range for example. The level of traffic entering the regulator 8 will be close to nominal, but the pilot is under external control. The regulator 8 corrects the pilot to nominal and in so doing it introduces a frequency dependent pre-compensation of the traffic level. The regulator 8 may be situated in the transmit terminal equipment or as a regulated repeater amplifier preferably near the terminal. On the receive side of the terminal it is necessary to remove the precompensation of traffic level.

This may be done by a regulator 9 in which the pilot pick-off amplifier 3' is not fixed as in Fig. 1, but is controlled in a similar manner to 7. In this way the level to which regulator 9 restores the pilot may be controlled, and in effecting this control, the traffic is also varied.

By making the gain changes of pilot amplifiers 3' and 7 equal but opposite the precompensation is removed. The control of precompensation of traffic level may be manual or automatic. (There will generally be some means for making seasonal adjustments to frequency response of the system which should also be suitable for controlling precompensation of traffic level).

It is of special note that the control of pilot level at regulators is not under control at the terminal, but is automatically corrected to nominal level at every regulated repeater output. The new feature is to provide a control of traffic level relative to the pilot. This control affects the traffic levels at all repeaters and would normally be provided at a single point in the system.

CLAIMS (31 Aug 1978) 1. An arrangement for pilot tone regulation of a repeatered analogue line system comprising means at the transmitting terminal of the system for altering the level of the transmitted pilot tone relative to the mean traffic level of the received pilot tone, the two altering means being operated in complementary fashion.

2. An arrangement for pilot tone regulation substantially as described with reference to Fig. 3 of the accompanying drawings.

CLAIMS (18 Jan 1979) 1. An arrangement for pilot tone regulation of a repeatered analogue line system comprising means at the transmitting terminal of the system for altering the level of the transmitted pilot tone relative to the mean traffic level and means at the receiving terminal for altering the level of the received pilot tone, the two altering means being operated in complementary fashion.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

the pilot pick-off amplifier 3' is not fixed as in Fig. 1, but is controlled in a similar manner to 7. In this way the level to which regulator 9 restores the pilot may be controlled, and in effecting this control, the traffic is also varied.

By making the gain changes of pilot amplifiers 3' and 7 equal but opposite the precompensation is removed. The control of precompensation of traffic level may be manual or automatic. (There will generally be some means for making seasonal adjustments to frequency response of the system which should also be suitable for controlling precompensation of traffic level).

It is of special note that the control of pilot level at regulators is not under control at the terminal, but is automatically corrected to nominal level at every regulated repeater output. The new feature is to provide a control of traffic level relative to the pilot. This control affects the traffic levels at all repeaters and would normally be provided at a single point in the system.

CLAIMS (31 Aug 1978)

1. An arrangement for pilot tone regulation of a repeatered analogue line system comprising means at the transmitting terminal of the system for altering the level of the transmitted pilot tone relative to the mean traffic level of the received pilot tone, the two altering means being operated in complementary fashion.

2. An arrangement for pilot tone regulation substantially as described with reference to Fig. 3 of the accompanying drawings.

CLAIMS (18 Jan 1979)

1. An arrangement for pilot tone regulation of a repeatered analogue line system comprising means at the transmitting terminal of the system for altering the level of the transmitted pilot tone relative to the mean traffic level and means at the receiving terminal for altering the level of the received pilot tone, the two altering means being operated in complementary fashion.