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Description  |
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The present invention relates generally to cable television systems that
enable users of the system's televisions to view, on demand, any one of a
large number of recorded video programs in addition to a set of standard
broadcast television stations, and particularly to a "view on-demand"
cable television system that carries television signals in two or more
signal formats, such as the NTSC, PAL and SECAM television signal formats.
BACKGROUND OF THE INVENTION
The present invention is applicable to video systems for hospitals,
schools, condominiums as well as hotels and other systems having a large
number of independently controlled televisions or video monitors. For the
purposes of illustrating and explaining the invention, however, the prior
art and the preferred embodiment of the invention will be described in the
context of a video system for a hotel.
Hotels often provide video program services beyond those available on
publicly available television channels and publicly accessible cable
television. The main types of prior art systems are video tape rental
systems, fixed schedule movie systems that broadcast pay-per-view
programming on predefined channels on a fixed schedule as well as free
programming on other channels, and on-demand movie systems that broadcast
free programming on one set of channels and use other channels to transmit
pay-per-view programming to users upon request by the users.
The present invention provides a number of improvements on the on-demand
type of cable television systems disclosed in U.S. Pat. No. 4,947,244,
issued Aug. 7, 1990, entitled "Video Selection and Distribution System."
In particular, the present invention is designed to work in settings where
the cable television system must carry television signals in two or more
formats. For instance, in some places such as Singapore, some television
channels are broadcast in PAL format while other television channels are
broadcast in NTSC format. In other places television signals are broadcast
both in SECAM and PAL formats. Furthermore, movies and other programming
from various sources are recorded on cassettes in a variety of video
signal formats, and each such cassette must be played using a video
cassette player that outputs television signals in the corresponding
signal format.
Another problem addressed by the present invention is that in some
situations it is unacceptable to place a television converter box on top
of the user's television set or anywhere else in the user's visual field,
and yet the user must still be able to make program or channel selections
using a remote control device pointed at the user's television.
Furthermore, in these situations it is generally not possible (or not
economically feasible) to modify the control circuitry in the television
sets due to cost considerations and the age of the television sets. As a
result, the cable converter box must be hidden from the user's view and
cannot be connected to the television set's internal controller. Thus the
only connection between the cable converter box and the television is a
cable connected to the antenna or cable input port on the back of the
television.
This presents two sets of problems: (A) the controller cannot turn the
television on and off and cannot set the television to a particular
channel, and (B) the controller cannot obtain signals directly from the
television's controller for determining whether the television is on and
similarly cannot obtain signals directly from the television's controller
for determining the channel to which the television is tuned. Thus, unless
some countervailing measures are taken, random use of the remote control
wand while the user's television is off might result in the selection and
playing of a pay-per-movie and a subsequent charge to the user despite the
fact that the user's television is off.
SUMMARY OF THE INVENTION
In summary, the present invention is a cable television system in which a
multiplicity of television program sources provide television signals in
various predefined television signal formats. The program sources include
continuous television sources provided to all users free of charge as well
as pay-per-view program sources transmitted to users only upon request.
Each television in the system has a multi-standard receiver for receiving
television signals in those predefined television signal formats, and is
coupled to a transmission medium such as a coaxial cable. A television
signal switch transmits the television signals from various ones of the
program sources onto the transmission media in assigned respective
frequency channels.
Hidden behind each television, or elsewhere out of view of the user, is a
converter coupling the respective television to the transmission media.
The only connection from the converter to the television is a cable
connected to the television's cable or antenna input port. Each converter
includes a controller for selecting a channel in accordance with user
commands, a channel map memory for storing data assigning to each
accessible channel in a frequency band and a predefined television signal
formats, and a frequency converter for switching a television signal
received from transmission media on the selected channel to a predefined
output channel to which the television is tuned.
Each respective controller also includes an upstream transmitter for
transmitting user program selection commands onto the transmission medium.
The user program selection commands are received at the head end of the
system by a switch that decodes those commands and transmits corresponding
setup signals to the television signal switch.
Each television includes an on/off switch for turning the television on and
off a tuner coupled to an input signal port, a wireless signal sensor and
decoder for receiving and decoding a first defined set of wireless command
signals having a first predefined set of coding sequences. The
television's wireless signal decoder ignores wireless command signals not
in the first defined set. The television also includes a controller that
responds to wireless on/off commands by toggling its on/off switch, and
responds to TV channel selection commands by sending corresponding channel
selection signals to the tuner.
The converter box includes a wireless signal sensor and detector for
receiving and decoding a second defined set of wireless command signals
having a second predefined set of coding sequences. The converter box's
wireless sensor is preferably located near or on the housing of the
television. The television's wireless signal decoder ignores wireless
command signals not in the second defined set. The converter's controller
responds to wireless channel selection commands by tuning the frequency
converter to a respective television channel and/or by transmitting
pay-per-view requests to the controller at the head end of the system.
A remote controller signal transmitter is used by a user of each
television. The remote controller includes a user interface for selecting
user commands, a memory table, and a transmitter. For each wireless
command the memory table stores data representing the command and the
carrier frequency to use when transmitting the wireless command. Some user
commands are transmitted as wireless commands to the television, other
user commands are transmitted as wireless commands to the converter and at
least one user command is transmitted as wireless commands to both the
television and the converter.
Each television further includes a raster scan CRT and CRT control
circuitry that controls the raster scan sweep pattern of the electron
beams generated by the CRT. The CRT control circuitry includes a
horizontal scan synchronization circuit that synchronizes each horizontal
sweep of the CRT electron beams with a horizontal synchronization signal
in the received television signal, and generates a distinctive transient
voltage signal at the beginning of each horizonal sweep of the CRT
electron beams.
External to the converter box is a horizontal sweep pickup antenna suitable
for positioning proximate the monitored television. Internal to the
converter box is a filter that extracts the horizontal synchronization
component of the television signal on the channel selected by the user. A
synchronization signal comparator compares the extracted horizontal
synchronization component of the selected television signal with the
signals, if any, received from the antenna, and outputs a result signal
indicating whether the television is both turned on and tuned so as to
receive the selected television signal.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional objects and features of the invention will be more readily
apparent from the following detailed description and appended claims when
taken in conjunction with the drawings, in which:
FIG. 1 is a block diagram of a preferred embodiment of a cable television
system incorporating the present invention.
FIG. 2 is a conceptual representation of the remote control scheme used
with each television in the preferred embodiment of the present invention.
FIG. 3 is a block diagram of the converter used in conjunction with each
television in the preferred embodiment of the present invention.
FIG. 4 is a circuit schematic of a multi-standard IF signal filter used in
the preferred embodiment of the television signal converter of the present
invention.
FIG. 5 is a circuit schematic of the audio/video signal splitter circuit
used in the preferred embodiment of the television signal converter of the
present invention.
FIG. 6 is a circuit schematic of the message generator circuit, for
superimposing messages on the video signal of the selected television
channel, used in the preferred embodiment of the television signal
converter of the present invention.
FIG. 7 is a circuit schematic of the audio/video remodulation circuit used
in the preferred embodiment of the television signal converter of the
present invention.
FIG. 8 is a circuit schematic of the television status monitoring circuit,
for determining if a television is on and tuned to a particular channel,
used in the preferred embodiment of the television signal converter of the
present invention.
FIG. 9 is a block diagram representation of the data map stored in the
television signal converter in the preferred embodiment of the present
invention.
FIG. 10 is a block diagram representation of the components of the remote
control device used in the preferred embodiment of the present invention.
FIG. 11 is a schematic representation of the user interface of a television
and converter remote control device used in the preferred embodiment of
the present invention.
FIG. 12 is a block diagram representation of the wireless command map
stored in the memory of the remote control device in the preferred
embodiment of the present invention.
FIG. 13 is a block diagram of the controller at the head end of the
preferred embodiment of a cable television system.
FIG. 14 is a flow chart showing how the interactions between the head end
controller and one converter box's controller when a user selects a
pay-per-view program.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown a cable television system 100 having a
number of cable distribution lines 102-1 to 102-N on which free television
programs and pay-per-view programming are transmitted to the users of the
system. For each user's television set, a converter 104 is provided for
the purposes of (A) receiving user commands to select a television channel
or pay-per-view program and for (B) shifting the selected program from its
broadcast channel to a pre-selected channel, such as channel 2 or 3, to
which the user's television is tuned.
The preferred embodiment of the cable television system 100 will be
described in the context of an installation of the system in a hotel.
However, the present invention could be used in a much broader spectrum of
applications, including cable television systems for institutions and
cable television systems for residential neighborhoods. The primary
difference between such various applications of the present invention will
be the manner in which the user's are charged for various services. The
underlying hardware, program distribution methodologies and the like would
remain substantially the same as described herein.
Overview of Head End of Cable Television System
The programming sources used by the system include continuous broadcast
television program receivers 106 that receive programming on various
preassigned television channel frequencies. In most cases the continuous
broadcast television programs are retransmitted to all users of the system
free of charge. In many cities there are continuous broadcast television
programs available in more than one standardized broadcast format. For
instance, in some locations both PAL (the European television signal
format) and NTSC (the American television signal format) television signal
are used, while in other locations both PAL and SECAM (the French
television signal format) television signals are used. While
"multi-standard" television sets are available for use in such locations,
television cable systems have generally used a single television signal
standard for all programs transmitted on a particular cable system.
Other programming sources used by the system 100 include non-continuous
broadcast sources, herein called "pay-per view" sources, such as video
cassette players and video disk players 108. In the preferred embodiment,
both PAL and NTSC video cassette players can be used. In other
embodiments, both continuous broadcast television program sources and
non-continuous television program sources using other combinations of
television signal formats could be used.
One additional source of "programming" for the system is a set of screen
buffers 110, also called frame buffers, for storing video images to be
transmitted to users of the system. The screen buffers 110 store images
representing menus of currently available pay-per view movies, images
explaining to users how to use the system, as well as additional screen
buffers for interactive services such as displaying a hotel guest's bill
as part of an express check-out service. The images stored in the screen
buffers 104 are generated and updated as necessary by a controller 112,
typically a microcomputer such as an "IBM compatible" computer using a
803086 Intel microprocessor.
The audio-video signals from the programming sources 106, 108 and 110 are
routed, frequency shifted to occupy selected television channels and
retransmitted onto the various distribution cables 102 by program routine
and transmission circuitry 114. Circuitry 114 includes a video crossbar
switch for each cable line 102 and a set of re-transmission modulators
that enables simultaneous connection of any selected combination of the
programming sources 106, 108 and 110 to the corresponding cable 102.
A more detailed description of the program routing and transmission
circuitry 114 can be found in U.S. Pat. No. 4,947,244, issued Aug. 7,
1990, entitled "Video Selection and Distribution System," which is hereby
incorporated by reference. U.S. Pat. No. 4,947,244 also contains a
detailed description of how the controller 112 generates menus of
available pay-per programs and also how the controller 112 responds to
user requests for menus and pay-per view (PPV) programming by sending
appropriate "PPV source control signals" to the video cassette players 108
and appropriate switch setup signal to the program routing and
transmission circuitry 114.
It should be noted that the video program sources 106, 108, 110, the
controller 112 and the program routing and transmission circuitry 114 are
all located at the "head end" of the cable system. Signals and programming
transmitted from the head end of the cable system to the users are said to
travel "downstream". Request signals transmitted by the cable boxes 104 in
user's rooms are said to travel "upstream".
Each of the cable lines 102 is coupled back to the controller 112 by a
communication line 116 and a bi-directional low-pass filter 118 that
couples the communication line 116 to each respective cable line 102.
Digitally encoded messages between the controller 116 and individual cable
boxes 104 are sent using modems, which operate a frequencies much lower
than television signals, nominally 7 MHz in the preferred embodiment.
Television channel signals typically start at frequencies above 50 MHz. In
the preferred embodiment the low pass filters 118 has a cutoff frequency
of 8 MHz.
In most respects, the head end of the cable television 100 of the present
invention is configured and operates in the same manner as described in
U.S. Pat. No. 4,947,244. The few aspects of the head end's operation that
differ from U.S. Pat. No. 4,947,244 are described below with reference to
FIG. 13.
Cable Box Multi-Standard Converter
Referring to FIG. 2, one aspect of the present invention is the physical
configuration of equipment in each user's hotel room. Generally, the
user's television 120 includes an infrared (IR) receiver 122 and decoder
124 for receiving control commands from the user via a remote control wand
126. The normal remote control wand for the television is replaced by a
programmable remote control wand 126 that is programmed to transmit
signals to both the television 120 and the cable box 104, as will be
described in more detail below.
The cable converter box 104 can be hidden behind the television 120, or on
the floor behind the piece of furniture on which the television is
mounted, with the only connection from the converter box 104 to the
television 120 being a standard cable television connection to the
television's antenna port or cable input port 130 on the back of the
television. Tethered to the converter box by an extended cable 131 are an
infrared (IR) receiver 132 and digitizer 134, as well as a pickup plate
136 that acts as an antenna for detecting the large voltage horizontal
synchronization pulses output by the television's CRT controller 138. The
digitizer 134 generates a binary pulse stream that tracks the wireless
commands received by the IR receiver 132, although wireless commands that
are not transmitted on the carrier frequency to which the receiver 132 is
tuned are ignored.
The raster scan cathode ray tube (CRT) 140 in the television 120 requires a
large voltage transient to reset the CRT's beam back to the left edge of
the screen at the beginning of each horizontal sweep of the CRT's beams.
The CRT controller 138 produces the requisite horizontal reset and
vertical reset control voltages in synchronization with the horizontal and
vertical synchronization signals in the television signal output by the
television's tuner 142.
The converter box's IR receiver 132 and pickup plate 136 are typically
positioned proximate the television 120, such as under or on the side of
the television, with the IR receiver 132 being positioned so that all IR
encoded signals output by the remote control wand 126 are received by both
the television's IR receiver 122 and the converter box's IR receiver 132.
Referring to FIG. 3, the converter's CPU 156, which is an 8032
microcontroller made by Intel in the preferred embodiment, receives the
binary pulse train produced by the IR sensor 132 and digitizer 134 and
decodes the pulse train to determine the command, if any, that is being
sent to it. Note that the CPU's wireless command decoder software 150
ignores wireless commands that do not match a predefined signal or coding
format and/or command prefix associated with all wireless commands
directed to the converter box 104.
The cable signal received by the converter box 104 passes through a
splitter 151, which is essentially a low pass filter for passing digitally
encoded signals from the head end controller to a modem 152 in the
converter box, and a high pass filter for passing video signals to a tuner
154.
The cable 102 in the preferred embodiment carries PAL format television
signals on some channels and NTSC format television signals on other
channels. NTSC tuners typically output an intermediate frequency (IF)
signal having a carrier frequency of 45.75 MHz and PAL tuners typically
output an IF signal having a carrier frequency of 38.9 MHz. The
converter's tuner 154 receives a channel selection signal from the
converter's CPU 156. The tuner 154 is a special multistandard tuner
capable of outputting 45.75 MHz video for NTSC signals and 38.9 MHz video
for PAL signals. This is accomplished by tuning the local oscillator of
the tuner to produce an IF of either 45.75 or 38.9 MHz.
The output of the tuner is filtered by an IF SAW filter 158, shown in more
detail in FIG. 4, so as to pass only the selected channel. Based on the
channel map and the user selected channel, the CPU 156 generates a binary
signal, labeled NTSC/PAL that indicates whether the currently selected
channel is an NTSC or PAL channel. The NTSC/PAL control signal is used to
select the path for the IF signals (NTSC IF or PAL IF).
The output of the IF filter 158 is next processed by a video signal
processing circuit 160 (the MC44301 made by Motorola) that includes an
audio intercarrier signal separator circuit, a video signal demodulator
and video signal inverter. The video signal demodulator portion of circuit
160 shifts the entire IF signal down to baseband and also inverts the
video signal if the Invert signal produced by the CPU 156 is enabled. The
audio separator portion of circuit 160 automatically separates the audio
component of the IF signal and outputs it on line 161 (labeled "Aural").
In the preferred embodiment, the video baseband signals of pay-per-view
programs are inverted at the head end before transmission over the cable
102 so that the pay-per view programs are not viewable by an ordinary
television connected to the cable line 102. In alternate embodiments of
the invention other signal protection schemes could be used for pay-per
view programming.
The baseband video signal generated by video demodulator 160 still contains
an audio component, which is offset by 4.5 MHz from the video signal
carrier in NTSC format signals and is offset by 5.5 MHz from the video
signal carrier in PAL format signals.
FIG. 5 shows the audio intercarrierfilter 162 used in the preferred
embodiment to filter out the radio frequency component of the baseband
video signal upon which the audio signal is modulated. In particular,
circuit 162 receives a NTSC/PAL signal from the CPU 156. If the NTSC/PAL
signal is equal to "1", indicating that the video signal on the selected
channel is a PAL format signal, then the PAL sound trap portion of the
circuit is enabled so as to remove the intercarrier audio signal centered
at about 5.5 MHz. If the NTSC/PAL signal is equal to "0", indicating that
the video signal on the selected channel is an NTSC format signal, then
the NTSC sound trap portion of the circuit is enabled so as to remove the
intercarrier audio signal centered at about 4.5 MHz. In both cases, the
resulting filtered signal is further filtered by a 5.0 MHz low pass
filter, and then the video signal is output on line 163. The audio signal
on line 161 is not demodulated, but rather is the high-pass filtered
portion of the television program signal, and thus will have a carrier
frequency of 4.5 or 5.5 MHz for NTSC or PAL television signals,
respectively.
Referring to FIGS. 3 and 6, a synchronization separator circuit 164 (such
as the LM1881N made by National Semiconductor) filters the video signal
output by audio filter 162 to generate horizontal and vertical
synchronization signals CSync (which is actually a composite
synchronization signal) and VSync/(which is a negative logic signal that
goes low whenever a vertical synchronization pulse occurs in the video
signal). Instead of generating a separate horizontal synchronization
signal, the synchronization separator circuit 164 generates a "combined
synchronization" signal CSync that includes both the horizontal and
vertical synchronization pulses. Since vertical synchronization pulses
occur much less often than horizontal synchronization pulses, the CSync
signal is usable as a horizontal synchronization signal for the purposes
of the present invention, as will be described below.
The converter box's CPU 156 is programmed to generate commands that specify
sequences of characters to be superimposed on the video image shown on the
user's television, such as temporarily displaying a channel identifier for
the channel selected by the user. A character generator 165 (such as the
M888303-P made by Fujitsu) receives the commands from the CPU 156
specifying the on-screen characters to be displayed, as well as the
horizontal and vertical synchronization signals from the synchronization
separator circuit 164, and generates a corresponding video signal that is
synchronized with the video signal on line 163.
More specifically, the character generator 165 provides a "box" signal VDE
which provides a black area on the screen where white characters are
overlaid by the VOW signal. Both the VDB/ and VOW signals are black-low.
In order to add the box and characters to the video signal, they must all
have a common reference level. The most convenient and stable reference is
black. To reference the video signal, synchronous clamp circuit 166
references the video signal's back porch to the DC voltage produced by
voltage divider 167-168. The FET series switch 169 stores the back porch
voltage on capacitor 170. Amplifier 1 71 applies the DC error signal
through resistor 172 to assure that a constant DC level corresponding to
black is maintained at the base of transistor 173. The purpose of
transistors 173, 174 and 175 is to "punch out" a black box with white
characters from the video background. The singular polarity (PNP) of these
devices provides first-order temperature compensation with a simplified
switching function. When transistor 174's base is pulled low by VDB,
voltage divider 176-177 causes the same black level to be asserted as
established by voltage divider 167-168. Simultaneously, transistor 173 is
reverse biased, disconnecting the video signal. When transistor 175 is
turned off by VOW going high, the black box video signal from transistor
174 is passed through transistor 178. VDB/ is always low in a defined
character region or window. When transistor 175 is turned on by VOX going
low, a white signal level forming the body of a character is determined by
voltage divider 179, 180, 181. Thus, video summing circuit 182 combines
the input video signal from line 163 with the character generator's video
signal to produce a composite video signal on line 183.
Modulator Circuit for Recombining Audio and Video Signals
Referring to FIGS. 3 and 7, remodulator circuit 184 adds the intercarrier
audio signal (Aural) on line 161 back into the combined video signal from
line 183. The remodulation circuit 184 has separate bandpass filtering
paths for NTSC audio signals and PAL audio signals because of their
distinct carrier frequency positions. The resulting filtered intercarrier
audio signal on line 185 is combined with the video signal from line 183
and remodulated onto a specified one of two channel frequencies by
television remodulator circuit 186, which in the preferred embodiment is a
TDA5664 made by Siemens. The channel frequency on which the television is
output is selected by a command signal from the converter box's CPU. In
one preferred embodiment the modulator circuit outputs the remodulated
television signal on VHF channel 2 or 3, while in another preferred
embodiment the remodulator circuit 184 outputs the television signal on
UHF channel 31 or 32. The modulated signal output of the remodulator
circuit 186 is passed through an isolation transformer 187 and then a
coaxial cable 188 and connector 189 that can be coupled to a corresponding
connector on the user's television.
Television On/Off and Channel Monitoring
Referring to FIGS. 2, 3 and 8, the converter box 104 monitors whether the
television 120 to which it is (or should be) connected to determine (A) if
the television 120 is on, and (B) if the television 120 is tuned to the
channel on which the cable box is generating its output signal. It is an
underlying premise of the present invention that no direct connections
other than the output cable 188 exist between the cable box 104 and the
television set.
This television monitoring aspect of the present invention is based on the
following observation. While the horizontal sweep rate of all NTSC
television channels is approximately the same, and the horizontal sweep
rate of all PAL television channels is approximately the same, the exact
timing of the horizontal synchronization signals on the various channels
are not synchronized. That is, the horizontal sweep frequency can vary
from one channel to the next by slight amount, and there is no
coordination whatsoever of the phases of the horizontal synchronization
signals of the various channels.
The primary purpose of the synchronization processing circuit 190 is to
compare the (low frequency) horizontal synchronization signals from the
baseband version of the television channel that is being output by the
cable box 104 with the horizontal sweep control signals generated by the
CRT controller 138 in the television set 120. The synchronization
processing circuit 190 also generates a clean version of the vertical sync
signal and a signal indicating whether the television set is on, both for
use by the CPU 156. The CRT controller 138 includes a horizontal scan
synchronization circuit that synchronizes each horizontal sweep of the CRT
electron beams with a horizontal synchronization signal in the received
television signal, and generates a distinctive transient voltage signal
(sometimes called the "horizontal flyback" signal) at the beginning of
each horizonal sweep of the CRT electron beams.
The pickup antenna 136 is positioned proximate the monitored television 120
so as to pick up low frequency (e.g., less than 100 KHz) electromagnetic
signals produced by the CRT controller 138, the dominant component of
which is the 15.734 KHz horizontal flyback signal. The transient signals
picked up by antenna 136 are digitized by comparator 191 and fed to the
input for a retriggerable monostable multivibrator (also known as a
one-shot circuit) 192 such as a 74LS123. The multivibrator 192 is set up
to generate a signal with a pulse width of 540 microseconds, which is
considerably longer than the 63.5 microsecond period of the horizontal
flyback signal. Therefore, as long as horizontal pulses from the
television set 120 are detected at most 540 microseconds apart (i.e., as
long as at least one of every eight horizontal flyback signals are
detected), the output of the multivibrator 192 on the TVPwrOn line 194
will always stay high. In summary, the TVPwrOn signal on line 194 is kept
continuously high if and only if the monitored television 120 set is
turned on, regardless of the channel to which the television is tuned.
Circuit 190 also detects whether the channel selected for output by the
converter's CPU actually contains a video signal. In particular, the VSync
signal produced by the synchronization separator circuit 162 is used to
trigger a one-shot circuit 196. The one-shot circuit 196 produces a stream
of VideoDetect signals on line 197 if and only if the selected channel is
carrying a video signal.
Finally, subcircuit 200 is used to determine if the horizontal
synchronization signal CSync of the selected channel matches the
HorizPickup signals on line 193. Two one-shot circuits 202 and 204 process
the CSync signal so as to produce a negative logic clock signal on line
205 that matches the phase and frequency of the horizontal synchronization
signal on the selected channel. D flip-flop 206 reads the HorizPickup
signal on line 193 at the rising edge of each clock signal on line 205.
The value stored by the flip-flop 206 is output in negative logic form as
the SyncLock/ signal on line 207. In particular, if the television set is
tuned to the channel output by the converter box, the HorizPickup signal
will be high (i.e., equal to a logic value of "1") when the signal output
by one shot 202 on line 205 has a rising edge. Thus the SyncLock signal
will be low when the television is tuned to the channel output by the
converter box.
The control software 210 for the converter box 104 is programmed to respond
to the 'l'VPwrOn, VideoDetect and SyncLock signals as follows. Table 1
contains a pseudocode representation of the control software routines
executed by the CPU in the converter box to monitoring the VideoDetect,
TVPwrOn and SyncLock/ signals. Those routines will now be described in the
same order as they are found in Table 1.
An interrupt routine is triggered by the VideoDetect signal, which should
happen 60 times per second, and computes a value called VsyncCnt that
indicates if the vertical sync pulse (i.e., the VideoDetect signal) has
been consistently detected. If no vertical sync pulse is detected for a
period in excess of two seconds, or if vertical sync pulses are detected
less than fifty percent of the time, then the control software sends a
"NoVid" message to the head end controller and switches the tuner in the
converter box to a free-to-guest channel.
The SyncLock/ monitoring routine is triggered by the expiration of the
VsyncWait timer, which is a 4 millisecond software timer that is triggered
by each occurrence of the VideoDetect signal. The routine computes a value
called SyncLockCnt that indicates if the SyncLock SyncLock has been
consistently detected. However, this routine is used only when the
television is tuned to a special services channel for viewing a movie,
menu or the like. If the SyncLock/ signal is not detected for a period in
excess of one second, or if SyncLock/ is consistently detected less than
fifty percent of the time, that indicates that the program on the special
services channel is either not being received or that the quality of the
received video signal is poor. Alternately, the television set may not be
tuned to the correct channel (e.g., channel 2 or 3) for receiving video
signals from the converter box. If the television is tuned to the wrong
channel, the SyncLock signal will typically be invalid (i.e., high) more
than 90% of the time and that will cause a "no sync lock" condition to be
detected very quickly, typically in about one second. Whenever a "no sync
lock" condition is detected, the control software sends a "NoVid" message
to the head end controller and switches the tuner in the converter box to
a free-to-guest channel. In an alternate embodiment, the control software
sends a "TV on Wrong Channel" message to the head end controller when the
VideoDetect signals are being consistently received but the SyncLock
signal is not valid. Either way, the controller at the head end of the
system will receive a message indicating that the converter box no longer
needs to receive a menu. However, unlike the "NoVid" message, a "TV on
Wrong Channel" message does not indicate a problem with a video source
(such as a screen buffer) or with the cable lines 102.
Note that if the television is tuned to the wrong channel, the user will be
unable to view the menus of movies and other special services sent by the
head end of the system, and thus the user will be unable to select a movie
for viewing.
TABLE 1
__________________________________________________________________________
Control Software Pseudocode for Vertical Sync and SyncLock
__________________________________________________________________________
Monitoring
Definitio | | |
