WO1985002743A1

WO1985002743A1 - Two-way catv system with aml commands

Info

Publication number: WO1985002743A1
Application number: PCT/US1984/001996
Authority: WO
Inventors: Richard W. Citta
Original assignee: Zenith Electronics Corporation
Priority date: 1983-12-09
Filing date: 1984-12-07
Publication date: 1985-06-20
Also published as: EP0163734A1; CA1209687A; US4554579A

Abstract

In a two-way cable television (CATV) system, multiple subscriber address codes are provided during designated vertical blanking interval (VBI) lines of the video signal transmitted from the CATV headend to a plurality of system subscriber terminals. Each subscriber terminal is provided with a decoder in which is stored subscriber unique address code for comparison with the received subscriber address codes. If there is an address match, an address match latch (AML) signal is generated for each VBI line address match. The address match signals thus generated from a multibit pulse coded downstream command signal for initiating various functions in the CATV system, e.g., acknowledging subscriber upstream requests, subscriber interrogation, power adjustment, etc.

Description

TWO-WAY CATV SYSTEM WITH AML COMMANDS

This invention relates generally to two-way cable television (CATV) systems and is particularly di¬ rected to the transmission of data from the headend to a plurality of^'~subscribers in a two-way CATV system. CATV systems- ypically include a central master source of television programming information termed a headen^'d which imparts programs, or groups of programs at different assigned frequencies, to a net¬ work of cable connections which, through various dis- tribution and selection network components, ultimately terminate in a branched out plurality of subscriber terminals typically in residence or commercial es¬ tablishments. CATV program signals are broadcast from the headend to^" "individual subscribers in a "downstream" direction over different assigned carrier frequencies typically from about 50 to 450 MHz. The CATV headend not only transmits television programming information downstream, but may also transmit data in the form of, for example, subscriber address and authorization in- formation, which allows the stored program decode author¬ ization information, which allows the stored program decode authorization status of each subscriber to be individually controlled from the headend. Frequently, such data is encoded in the vertical blanking interval (VBI) of a transmitted CATV program signal. The pro¬ gram decode authorization status stored in each sub¬ scriber' s terminal is compared with a program code also encoded in the VBI of the received CATV signal to estab¬ lish whether or not the subscriber is authorized for de- coding the received program signal. In a two-way CATV system each subscriber is able to transmit signals back to the CATV head¬ end. These subscriber-originated signals may in¬ clude program purchasing requests, opinion poll re- sponses, and CATV converter status information. These upstream signals typically make use of out of band frequencies in the 5-32.5 MHz band. The up¬ stream signal distribution network is in the form of a "merging tree topology" in which the signals gen- erated by many sources, or subscribers, converge and are transmitted on a single transmission line back to the CATV headend. Such techniques as signal multiplexing and frequency diversity have been pro¬ posed and are utilized to accommodate large numbers of subscribers in two-way CATV systems.

Proper response to the subscriber-originated signals normally requires the exercise of headend control over the various remotely located subscriber terminals coupled to the CATV system. The exercise of such control requires the downstream transmission of various command signals from the headend con¬ troller to the subscriber terminals.

The object of the present invention is to provide a facility for enabling headend control of a plurality of subscriber terminals in a two-way CATV system.

Accordingly, the present invention provides a system for controlling a selected subscriber terminal from the headend of a two-way CATV system wherein video program signals are transmitted down¬ stream from said headend to a plurality of subscriber terminals each uniquely identified by a respective stored muitibit subscriber address code and wherein a plurality of muitibit subscriber address codes may be selectively transmitted during a vertical blank¬ ing interval of said video program signals, said system being characterized by means associated with said headend for transmitting a muitibit command code during the VBI of a selected field of said program signal for controlling said selected sub¬ scriber terminal, each bit of said command code being represented by a transmission or non-trans¬ mission during a respective time interval of the ad¬ dress code corresponding to the stored address code identifying said selected subscriber terminal; means coupled to said cable for receiving the trans¬ mitted command code and for generating a match sig¬ nal in response to each received address code that corresponds to the stored address code identifying said selected subscriber terminal; and means for decoding the match signals generated during each vertical blanking interval for selectively controlling said selected subscriber terminals.

The system of the present invention pro¬ vides a facility whereby such headend control of subscriber terminals may be conviently realized within the environment of a prior art CATV system employing VBI data transmission techniques. In particular, a headend command signal is transmitted during a single VBI in the form of a muitibit code, each bit of which is represented by the transmission or non- transmission of a subscriber address code during a respective predetermined time interval of the VBI.

At the subscriber's terminal, the received address codes are processed to reconstruct the muitibit code which is subsequently decoded for use in controlling various aspects of the terminal.

The system of the present invention is ex¬ tremely convenient and cost-effective.

The invention itself, as well as further features and advantages thereof, will best be un¬ derstood by reference to the following detailed de¬ scription of the preferred embodiment taken in con¬ junction with the accompanying drawings, in which: FIG. 1 shows in block and schematic diagram form a two-way CATV system in accordance with the present invention; and

FIG. 2 is a waveform diagram useful in explaining the operation of the system shown in Figure 1.

Referring to FIG. 1, there is shown in com¬ bined schematic and block diagram form a subscriber terminal 10 for a two-way CATV system in accordance with the present invention. The system also includes a cable headend 12 coupled to each subscriber terminal 10 via a cable 15 and which typically includes a computer, multiple terminals, disc drives and a tape backup. These elements of the cable headend 12 are not shown in FIG. 1 as they do not form a part of the present invention. Downstream signals from the cable headend 12 to individual subscribers may in¬ clude such . information as subscriber address, pro¬ gram autorization, and market codes as well as pro- gram identification data which is transmitted in television channels having frequencies between about 50 and 450 MHz. This data may be provided during the vertical blanking interval (VBI) of the down¬ stream video programming signals, such as in lines 10 through 13 of theVBI _as shown in Table I. Al¬ though the present invention is described herein as making use of VBI lines 10-13 of the video programming signals for the downstream transmission of subscriber terminal commands, it is not limited to the use of these VBI lines and may utilize any portion of the vertical retrace signal including any combination of VBI lines, consecutive or nonconsecutive.

- ^' TABLE I

n-BIT

VBI Line 10 m-BIT ADDRESS CODE AUTHORIZATION

CODE n-BIT

VBI Line 11 m-BIT ADDRESS CODE AUTHORIZATION

CODE n-BIT

VBI Line 12 m-BIT ADDRESS CODE AUTHORIZATION

CODE

VBI Line 13 X-BIT MARKET CODE y-BIT PROGRAM TAG

The VBI occurs during vertical retrace of the raster scanned cathode ray tube (CRT) of the subscriber's television receiver and comprises about 21 horizontal scanning lines. Referring to Table I above, in a known one-way addressable CATV system an m-bit subscriber address code and a related n-bit program authorization code are selectively trans¬ mitted during each of lines 10 , and 12 of the VBI. Each address code may comprise 20 bits with each authorization code comprising 5 bits. During line 13 of the VBI there is continuously transmitted an x-bit- (e.g. 11 bits) market code which defines the geographic area in which the system is operating as well as a y-bit (e.g. 7 bits) program identification code, or tag, defining the accompanying television program. These downstream signals are received by each subscriber terminal 10 and coupled by a diplex filter 14 to a decoder 18 which includes a suitable tuner tuned to an encoded CATV channel. Each re¬ ceived m-bit address code is compared with a unique subscriber address code stored in a programmable read only memory (PROM) 34 of decoder 18 in each sub¬ scriber terminal 10. If the received subscriber ad- dress code matches the unique subscriber address code stored in the decoder's PROM 34, the program autoriza- tion code of the same VBI line is stored in a random access memory (RAM) 78 in decoder 18 which defines the program decode authorization status of the sub- scriber. A match between the received and stored address codes will also result in the generation of an address match latch (AML) pulse by decoder 18, which pulse is provided on an output line 36 of de¬ coder 18. The x-bit market code received during line 13 of each VBI is compared with a subscriber terminal market code also stored in the decoder's PROM 34. A match between the transmitted and stored market codes will enable the decoder 18 and also result in the generation of an address match latch (AML) pulse which is similarly provided on line 36. A match between the received y-bit program tag code and the stored program decode authorization status permits the decoder to descrabmle the received CATV video programming signal which may then be viewed on the subscriber's television receiver (not shown) . The absence of a match between the received program tag code and the stored program authoriza¬ tion status results in the video programming signal being passed on by the subscriber terminal 10 in a scrambled form. Thereafter, decoder 18 continuously monitors incoming data for detecting each subsequent address- or market code match.

As previously mentioned, each match between a received subscriber address code and the unique subscriber address code stored in PROM 34 of de¬ coder 18 results in an AML pulse on line 36. Ac¬ cording- to the present invention, the selective trans¬ mission of subscriber address codes on horizontal lines 10-13 of the VBI is used to provide a 4-bit headend command control code on line 36, each bit being represented by the presence or absence on an AML pulse. As will be explained in further detail hereinafter, this 4-bit AML control code is decoded by counting the number of AML pulses developed dur¬ ing each VBI and by determining the mode of opera- tion of terminal 10 such that various two-way related functions of the terminal can be controlled from the headend.

With further reference to Figure 1, the AML pulses developed on line 36 by decoder 18 are couled to a binary counter 22 and to a one-shot monostable multivibrator 24. Various other signals are also output by decoder 18. These signals include key¬ stroke serial data provided via line 38 to a micro¬ computer 20. This keystroke serial data may include such information as channel number selection, cable selection, or program authorization requests as provided to decoder 18 via line 23 by a subscriber input device 21. This data may also be provided directly from subscriber input device 21 to micro- computer 20 by means of line 55 (shown in dotted line form) in another embodiment. The decoder 18 is coupled to ground potential via line 40. Fin¬ ally, decoder 18 provides a +VDC output to the var¬ ious components of the CATV subscriber terminal 10 as required. .

The AML pulses provided by decoder 18 to counter 22 and monostable multivibrator 24 are in the form of 8 microsecond pulses as shown in FIG. 2. As previously mentioned, according to the present invention, various combinations of the AML pulses are formed during each VBI by selectively transmitting appropriate address codes for selectively commanding each terminal from the headend. For example, the pulse diagram of FIG. 2 shows three AML pulses P , ^• P₂ ^{a d P}3* ^AML P^{u ses p}ι ^{and p}2 represent subscrib^¬ er address code matches during VBI lines 10 and 11 while AML pulse P, represents a market code match during VBI line 13. It will be appreciated that the market code match AML pulse P., cannot be used to represent any data since it is transmitted during every VBI by the headend. During the subsequent field, an AML pulse P₄ represents an address code match during VBI line 10 and an AML pulse P₅ re¬ presents a market code match during- VBI line 13. With each AML pulse width equal to 8 microseconds, a 64 microsecond interval is provided between adjacent AML pulses. " The AML pulses produced on line 36 are Pro¬ vided to the A input of an OR gate 26, which in combination with multivibrator circuit 28 forms one- shot monostable multivibrator 24. The output of monostable multivibrator 24 is provided, in turn to a D-type flip-flop circuit 30 which provides a timed output to the INT input pin of microcomputer 20 for initiating an interrupt therein. Following receipt from fli-flop 30 of an interrupt signal provided to its INT input pin, microcomputer 20, which in a preferred embodiment is an 8048 microcomputer with a 2K ROM and a 64 byte RAM, finishes its current op¬ eration, or task, and executes another subroutine. In a preferred embodiment of the present invention the subroutine executed by microcomputer 20 fol- lowing receipt of the interrup signal involves in¬ terpreting the headend command represented by the AML pulses developed during the current VBI and, if necessary the upstream transmission of data from the subscriber terminal 10 to the CATV headend 12 in response thereto. The upstream data transmission from microcomputer 20 is effected via output line 50, transmitter 16 and diplex filter 14.

In greater detail, the AML pulses on line 36 are provided to the A input of OR gate 26 of one- shot monostable multivibrator 24. The inverted B pin of OR gate 26 is coupled to a +V, source. Each AML pulse provided to OR gate 26 triσgers multivibra- tor 28 for initiating a negative-going pulse at its Q output which is coupled to the .CLK input of D- type flip-flop circuit 30. The RC time constant of the circuit coupled to multivibrator 28 is selected such that the width of this negative-going pulse is about 250 microseconds in a preferred embodiment. D-type flip-flop 30 is triggered by the positive going edge of the Q output of multivibrator 28. It will be recalled that an AML pulse is produced by decoder 18 during horizontal line 13 of each field in response to a market code- match. The 250 micro¬ second duration of the output of monostable multi¬ vibrator 28 is of sufficient length to insure triggering of flip-flop 30 only 250 microseconds after this market code induced horizontal line 13

AML pulse so as to provide a precise timing reference signal at its Q output* ⁿ particular, multivibra¬ tor 28 will be triggered or retriggered by the horizontal line 13 AML pulse regardless of the num- ber of AML pulses preceding it in a given field and before producing a positive-going output transition such that the Q- output of flip-flop 30 will go low at a fixed time each field, i.e., 250 microseconds after the horizontal line 13 AML pulse. The low Q output from flip-flop circuit 30 is provided to the INT input pin of microcomputer 20 for initiating an interrup routine for sampling the output of counter 22 provided on pins ₂ι_* ^P22 ^and P₂~. Based on the sampled count and on the mode of the subscriber's terminal 10, appropriate responsive action is taken. Following a predetermined time interval as established by the operating program in the microcomputer's ROM 23, microcomputer 20 pro¬ vides a reset pulse from its ₂₄ output pin to the RESET input of counter 22 and, via inverter 32, to the CLEAR (C- and RESET (R) inputs of multivibrator 28 and flip-flop 30, respectively. The reset output from microcomputer 20 prepares counter 22 and the combination of one-shot monostable multivibrator 24 and flip-flop 30 for receipt of the next series of AML pulses from decoder 18 during the VBI of a sub- sequent video field.

Counter 22 is a binary counter, to the CLK input of which is provided the AML pulses from de¬ coder 18. Counter 22 provides a binary output sig¬ nal via lines 44, 46 and 48 to the P 1' ^P ₂₂ ^an<^ ^P23 input pins, -respectively, of microcomputer 20 repre¬ senting the number of AML pulses developed by decoder 18 during each field of the received signal. As will be explained in further detail below, according to the present invention this binary coded signal which, during each field, may take a value between decimal 1 and decimal 4, is used to provide head¬ end initiated command signals to microcomputer 20 which may include system status information requests or headend initiated command signals for controlling each remote subscriber terminal.

Referring to Table II, there is shown an exemplary definition of the headend commands repre¬ sented by the muitibit AML control signals which may be developed on line 36 during the VBI's of the re- ceived signal and the corresponding output of counter 22. In this Table, a "1" represents the development of an AML pulse in a particular VBI line by decoder 18 with the count of such SML pulses during each VBI by counter 22 being given in decimal format^". Also, for purposes of convenience the horizontal line 13 AML pulse is not considered since it is always pres¬ ent. TABLE II

MODE 1 MODE 2 COUNTER 22

MESSAGE PENDING STANDBY

VBI DATA LINES 10, 11, 12

(XXX)

(100)

POSITIVE ACKNOWLEDGE ⁽oio⁾ IGNORED

(001)

(011) IGNORED (101) INTERROGATE (110)

NEGATIVE ACKNOWEDGE (111) POWER ADJUSTMENT

MPL MESSAGE PENDING LIGHT (YELLOW LED)

EPL ERROR LIGHT (RED LED) ACL ACKNOWLEDGE LIGHT (GREEN LED)

In accordance with the present invention, each subscriber terminal 10 is capable of operating in two different modes. A first mode is termed the "message pending" mode, while the second mode of op¬ eration is termed "standby". In the message pending mode, the subscriber terminal 10 has transmitted a ^■ message upstream to the CATV headend 12, such as a program authorization request, and is waiting for a response thereto from the headend. The upstream mes¬ sage may be generated in response to subscriber en¬ tries via subscriber input device 21 which, in turn, are provided to microcomputer 20. The headend res¬ ponse may be in the form of a POSITIVE ACKNOWLEDGE command (indicating that the requested program author¬ ization will be granted) or a NEGATIVE ACKNOWLEDGE command (indicating that the requested program auth¬ orization will not be granted) .

In a second standby mode of operation the subscriber terminal 10 is in a ready state for re- . ceipt of downstream data from the CATV headend. In this mode, receipt of downstream data by the sub¬ scriber terminal 10 results in the performance of a specific operation by the subscriber terminal or the providing of information from subscriber terminal 10 to the CATV headend 12.

From TABLE II, it can be seen that the sub¬ scriber terminal interprets downstream data from the CATV headend 12 in accordance with its operating mode. For example, a single AML pulse in any of VBI lines 10, 11 or 12 (represented by counter 22 ac¬ cumulating a "1" count during the complete VBI) rep¬ resents a POSITIVE ACKNOWLEDGE signal from the CATV headend when the subscriber terminal ^^s in the message pending mode of pending mode of operation and is ignored when the terminal is in the standby mode of operation. Two AML pulses received in any of VBI lines 10, 11 and 12 (represented by counter 22 ac¬ cumulating a "2" count during the VBI) is interpreted by the terminal when in the standby mode as an INTER- ROGATE command and is ignored in the message pending mode. Finally, an AML pulse received in each of VBI lines 10, 11 and 12 (represented by counter 22 ac¬ cumulating a "3" count during the VBI) represents a NEGATIVE ACKNOWLEDGE command from the CATV headend 12 to subscriber terminal 10 in the message pending mode and as a POWER ADJUSTMENT command in the standby mode.

As mentioned previously the POSITIVE or NEGATIVE ACKNOWLEDGE commands may be used by the headend to provide an answer or response to a program authorization request by a subscriber. For example, receipt of a POSITIVE ACKNOWLEDGE command by a sub¬ scriber terminal^", may indicate that an upstream re- quest for access to a CATV program is acknowledged and that decode authorization for the requested program will be granted by the CATV headend. On the other hand, a NEGATIVE ACKNOWLEDGE may indicate that the request has been denied. The INTERROGATE com¬ mand may, for example, represent a request by the CATV headend for the subscriber terminal to provide information relating to the channel to which the sub¬ scriber terminal is tuned. Various other subscriber terminal operating parameters may be requested by means of the INTERROGATE command signal from the CATV headend.

With the CATV terminal in the standby mode, receipt of an AML pulse in each of VBI lines 10, 11 and 12 (represented by the accumulation of a "3" count by counter 22 during the VBI) represents a POWER AD¬ JUSTMENT command for adjusting the upstream signal power output of the subscriber terminal. The di¬ rection of power adjustment by the subscriber ter- minal is determined by the state of a power memory 35 within the random access memory (RAM) 25 of micro¬ computer 20. The power memory 35 in a preferred embodiment is a power status memory bit within the microcomputer's RAM 25. The power memory 35 operates as a binary switch which changes its state in re¬ sponse to receipt of an INTERROGATE command from the CATV headend 12 when the subscriber terminal 10 is in the standby mode of operation. Each time the sub- scriber terminal. Ϊ0 receives an INTERROGATE command from the CATV headend, the power memory 35 changes state. In addition, a response is provided by the subscriber terminal 10 to the CATV headend 12 indicat¬ ing whether power will be increased or decreased following receipt of the next POWER ADJUSTMENT signal from the CATV headend. Thus, after the CATV headend determines whether the subscriber terminal power is to be increased or decreased and using the aforemen¬ tioned power memory status information, the CATV headend then decides whether its next output should be an INTERROGATE command or a POWER ADJUSTMENT com¬ mand. If the power memory.35 is set in the desired direction, the CATV headend will next provide a POWER ADJUSTMENT command to the subscriber terminal. If, however, the CATV headend determines that a power adjustment executed by the subscriber terminal will be in the wrong direction, it provides an INTERROGATE command to the subscriber terminal which changes the state of the power memory 35 to permit subscriber terminal power to be adjusted in the opposite- di¬ rection following receipt of the next POWER ADJUSTMENT command from the CATV headend. The INTERROGATE com¬ mand thus acts to toggle the power memory 35 and re- suits in a response from the subscriber terminal 10 to the CATV headend 12 with a status message. The POWER ADJUSTMENT command causes either an increment¬ ing or a decrementing of the subscriber terminal out¬ put power and also results in a power signal being provided from the subcriber terminal 10 to the CATV headend 12.

In conjunction with the various commands provided from the CATV headend to a subscriber ter¬ minal, visual indications may be provided to a sub- scriber for indicating system status. For example, various light emitting diodes (LED's) 60, 62 and 64 may be coupled to the microcomputer 20 by means of plurality of LED drivers 66 for providing a visual indication of the command or status signal received from the ACATV headend 12. For example, in a prefer¬ red embodiment LED 60 (MPL) is a Message Pending Light used to indicate that a subscriber initiated message is pending, that the CATV headned 12 is currently busy executing another task, and that a response to the subscriber's request will be provided after the current task has been performed. LED 62 (ERL) is used to indicate the entry by a subscriber of an in- correct password or that tl.2 subscriber is not elig¬ ible for the program requested (i.e. a NEGATIVE ACKNOWLEDGE command) . Finally, illumination of LED 64 (ACL) indicates to the subscriber an acknowledgement of

_5 receipt by CATV headend 12 of its upstream message and that the subscriber's request has been authorized (i.e. a POSITIVE ACKNOWLEDGE command). These LED's are of different colors to facilitate visual identification of these various status and command signals by the sub- 0 scriber. For example, the Message Pending Light 60 may be yellow, the Error Light 62 may be red, and the Acknowledge Light 64 may be green.

There has thus b n shown .a two-way CATV system wherein headend initiated commands are trans- ⁵ mitted during the veritcal blanking interval of the downstream video program signals along with subscriber address and program authorization data. The command signals are in the form of a muitibit code, each bit of which is represented by the transmission or non- ^ transmission of a subscriber address code. Each transmitted address code is compared to an address code stored in the subscriber terminal for generating the muitibit command code, the bits of which are subsequently counted for decoding in ac¬ 5 cordance with the mode of operation of the teminal so as to control a selected function thereof.

While particular embodiments of the pres¬ ent invention have been shown and described, it will be obvious to those skilled in the art that changes 0 and modification may be made without departing from the invention in its broader aspects.

* VZEA

Claims

1. A system for controlling a selected subscriber terminal from the headend of a two-way CATV system wherein video program signals are trans¬ mitted downstream from said headend to a plurality of subscriber terminals each uniquely identified by a respective stored muitibit subscriber address code and wherein a plurality of muitibit subscriber address codes may be selectively transmitted during a vertical blanking interval of said video program signals, said system being characterized by means associated with said headend for transmitting a muitibit command code during the VBI of a selected field of said program signal for controlling said selected subscriber ter¬ minal, each bit of said command code being represented by a transmission- or non-transmission during a re¬ spective time interval of the address code correspond¬ ing to the stored address code identifying said selected subscriber terminal; means coupled to said cable for receiving the transmitted command code and for generating a match signal in response to each received address code that corresponds to the stored address code identifying said selected subscriber terminal; and means for decoding the match signals generated during each vertical blanking interval for selectively controlling said selected subscriber terminal.

2. The system as in claim 1 character¬ ized in that each of said muitibit address codes is transmitted on a different horizontal line of the vertical blanking interval of said selected field.

3. The system as in claim 2, character¬ ized in that said decoding means comprises means for counting the number of said match signals developed during the vertical blanking interval, of said sel¬ ected field.

4. The system as in claim 3, characterized in that said subscriber terminal is operable in a first STANDBY mode wherein said subscriber terminal awaits receipt of a headend-initiated command code or, in response to subscriber inputs, is operable in a second MESSAGE PENDING mode wherein said subscriber terminal awaits acknowledgment for the headend of receipt of an upstream message from said subscriber terminal and wherein said decoding means comprises means responsive to the number of said match signals counted during the vertical blanking interval of said selected field and to the mode of operation of said subscriber terminal for decoding said trans¬ mitted command code.

5. The system as in claim 4 characterized by means for providing an observable indication to the subscriber of the mode of operation of the sub¬ scriber terminal and of the receipt by the terminal of selected headend command codes.

6. The system as in claim 4 characterized in that said subscriber terminal includes a power di¬ rection memory whose state in toggled in response to the receipt of a first predetermined headend com¬ mand code when the terminal is in the standby mode and further including means for adjusting the up¬ stream power transmission level of the terminal in accordance with the state of the power direction memory in response to the receipt of a second pre¬ determined headend command code when the terminal is in the standby mode.

7. A system for controlling a selected subscriber terminal from the headend of a two-way CATV system wherein video program signals are trans¬ mitted downstream from said headend to a plurality of subscriber terminals each uniquely identified by a respective stored muitibit subscriber address code and^' wherein a plurality of muitibit subscriber ad¬ dress codes may be selectively transmitted during respective lines of a vertical blanking interval of said video program signals, said system being char¬ acterized by means associated with said headend for. transmitting a muitibit command code during the VBI of a selected field of said program signal for controlling said selected subscriber terminal, each bit of said command code being represented by the transmission or non-transmission during a respective vertical interval line of the address code correspond¬ ing to the stored address code identifying said selected subscriber terminal; means coupled to said cable for receiving the transmitted command code and for generating a match signal in response to each received address code that corresponds to the stored address code identifying said selected subscriber terminal; and decoding means for counting the match signals generated during each vertical blanking in¬ terval for selectively controlling said selected subscriber terminal in accordance with the number of match signals counted.

8. A system for controlling a selected subscriber terminal from the headend of a two-way CATV system wherein video program signals are trans¬ mitted downstream from a headend to a plurality of subscriber terminals each uniquely identified by a respective stored muitibit subscriber address code and wherein a plurality of muitibit subscriber ad¬ dress codes may be selectively transmitted during a vertical blanking interval of said video program signals, said system being characterized by means associated with said headend for transmitting a multi- bit command code during the VBI of a selected field of said program signal for controlling said selected subscriber terminal, each bit of said command code being represented by the transmission or non-trans¬ mission during a respective time interval of the ad¬ dress code corresponding to the stored address code identifying said selected subscriber terminal; means coupled to said cable for receiving the transmitted command code and for generating a match signal in response to each received address code that cor¬ responds to the stored address code identifying said selected subscriber terminal; means for decoding the match signals generated during each vertical blanking interval; and a power direction memory whose state is toggled in response to a first predetermined decoded headend command code and further including means for incrementally adjusting the upstream power trans¬ mission level of the terminal in accordance with the state of the power direction memory in response to a second predetermined decoded headend command.

"