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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to multiplexed analog component
(MAC) television systems and, more particularly, to an improved interface
between a subscriber and an integrated receiver-decoder (IRD) in a MAC
television system.
2. Description of the Relevant Art
For the purposes of the following discussion and this invention, the term
"subscriber" means one who is receiving a television service. The
"subscriber" could thus be an individual consumer with a decoder in his
own home, or could be a system operator such as a local cable TV operator,
or a small network operator such as a hotel/motel operator with a central
decoder for all televisions in the hotel or motel. In addition, the
"subscriber" could be an industrial user, as described in U.S. Pat. No.
4,866,770 assigned to the same assignee as the present application and
incorporated herein by reference.
For the purposes of this invention, a network is defined as a program
source (such as a pay television provider), an encoder (sometimes called a
"head end"), a transmission means (satellite, cable, radio wave, etc.) and
a series of decoders used by the subscribers. A system is defined as a
program source, an encoder, a transmission means, and a single receiving
decoder. The system model is used to described how an individual decoder
in a network interacts with the encoder.
A MAC color television signal is illustrated in FIG. 1, which is an
amplitude-vs.-time diagram of a single video line of 63.56 microseconds
duration. The first 10.9 microseconds is the horizontal blanking interval
(HBI) 22, in which no picture information is transmitted. Following HBI 22
are chrominance signal 24 and luminance signal 26, either of which may be
time-compressed. Between chrominance signal 24 and luminance signal 26 is
a 0.28 microsecond guard band 28, to assist in preventing interference
between the two signals.
The MAC color television signal of FIG. 1 is obtained by generating
conventional luminance and chrominance signals (as would be done to obtain
a conventional NTSC or other composite color television signal) and then
sampling and storing them separately. Luminance is sampled at a luminance
sampling frequency and stored in a luminance store, while chrominance is
sampled at a chrominance sampling frequency and stored in a chrominance
store. The luminance or chrominance samples may then be compressed in time
by writing them into the store at their individual sampling frequency and
reading them from the store at a higher frequency. A multiplexer selects
either the luminance store or the chrominance store, at the appropriate
time during the active video line, for reading, thus creating the MAC
signal of FIG. 1. Audio samples may be transmitted during the HBI; these
are multiplexed (and may be compressed) in the same manner as the video
samples. The single rate at which all samples occur in the MAC signal is
called the MAC sampling frequency.
FIG. 2 shows a prior art conditional-access system for satellite
transmission. In encoder 101, the source program information 102 which
comprises video signals, audio signals, and data is scrambled in program
scrambler 103 using a key from key memory 104. The scrambling techniques
used may be any such techniques which are well known in the art. The key
can be a signal or code number used in the scrambling process which is
also required to "unlock" or descramble the program in program descrambler
108 in decoder 106. In practice, one key can be used (single layer
encryption) or more than one key (not shown). The key is usually changed
with time (i.e.--monthly) to discourage piracy. The scrambled programs and
the key are transmitted through satellite link 105, and received by
conditional-access decoder 106. Decoder 106 recovers the key from the
received signal, stores it in key memory 107 and applies it to program
descrambler 108 which descrambles the scrambled program received over
satellite link 105, and outputs unscrambled program 109.
FIG. 3 shows the overall transmission format of a MAC system. As is
conventional in television, 30 "frames" each comprising a still image are
transmitted per second as indicated. Each frame includes two "fields," as
also shown. In a preferred embodiment of the invention, the video encoding
scheme employed is that referred to generally as "B-MAC." This is an
acronym for type B format, Multiplexed Analog Component system. "Type B"
refers to the fact that data is carried integral to the video signal. See
generally Lowry, "B-MAC: An Optimum Format for Satellite Television
Transmission," SMPTE Journal, November 1984, pp. 1034-1043, incorporated
herein by reference, which discusses in detail the B-MAC format and
explains why it was chosen over various competing systems.
The vertical blanking interval (VBI) of each field contains certain "system
data" necessary for operation of a subscription television system as well
as addressed packets and teletext lines used to carry data needed for the
operation of individual decoders and for transmission of messages to
individual subscribers. Preferably, the vertical blanking intervals of 16
total fields are used for complete transmission of all system data
required, which includes an encryption key which is changed every 16
fields, that is, on the order of three times per second. As also shown in
FIG. 3, each line also includes a horizontal blanking interval (HBI).
During the HBI are transmitted six channels of high quality
digitally-encoded audio information, with error correction, such that the
decoder can also be used to supply a high quality audio signal. This can
be used to provide the audio component of the corresponding video signal
(or several versions thereof, in different languages) or an additional
audio signal, such that subscription audio is also made available
according to the system of the invention.
FIG. 4 shows the format of the horizontal blanking interval (HBI). The HBI
perferably consists of 78 total bits of pulse amplitude modulated data.
The HBI is interposed between vertical blanking interval or video
information from a previous line and that of the present line. A typical
horizontal blanking interval as shown begins with a two-bit guard band 30,
followed by 45 bits of audio and utility data 32, a second two-bit guard
band 34, twenty bits of color burst information 36, a further guard band
38, six more bits of data 40 and a final guard band 42, after which the
VBI or the video signal of the particular frame commences. The position of
the color burst 30 within the HBI varies, to provide signal scrambling.
Descrambling involves the use of a repetitively-transmitted key.
FIG. 5 shows some additional details of the horizontal blanking interval
data 32 and 40 shown in FIG. 4. In the example shown, fifty-one total bits
of data are provided in each line of the HBI, and each bit is pulse
amplitude modulated encoded, such that each bit period includes
transmission of two bits. One bit can be referred to as sign and the other
as magnitude as indicated on FIG. 5. As shown, the first seventy-eight
bits are digital audio. Thus each frame provides a thirteen-bit digital
representation of a sample of each of six audio channels. High quality
transmission of audio frequencies up to approximately 15 kHz is thus
provided. Following the audio information are six bits of stepsize and
bandwidth information. The stepsize bits indicate the size of the steps
numbered by the thirteen bits of information preceding, and the bandwidth
information relates to the amount of the amount of emphases or de-emphasis
of the signal employed. Alternate fields carry the stepsize and bandwidth
data. Both these terms are used as conventional in the Dolby delta
modulation scheme, which is employed in the preferred embodiment of this
invention for transmission of the audio. Following are twelve bits of
error correction code (ECC) for correction of the audio, indicated at 48.
Four utility bits follow at 50, and the last bit 52 of the data are a
parity check bits for checking the parity of the error correction bits 48.
FIG. 6 shows the arrangement of the lines which make up the vertical
blanking interval (VBI). The VBI includes 16 lines in the 525-line NTSC
version of this invention. A slightly different number of lines are used
in the 625-line PAL. The functions of the lines and their arrangement in
other respects are identical.
As indicated, the vertical blanking interval is 377 bits wide. These bits
are pulse amplitude modulated FSK scheme used in the HBI as discussed
above. Lines 1, 2 and 3 includes the transmission of clock recovery,
synchronization and system service, as indicated in FIG. 6.
For the purposes of the present invention, the significant data contained
in line 3 is a system key which is updated every sixteen fields, that is,
which changes with each complete system data transmission as indicated
above in connection with FIG. 3. The system key is common to all decoders.
The system key is contained in the service data of line 3, and is used for
decryption of video program material, audio and teletext.
Lines 4-8 of the VBI include the addressed packets, as indicated by
reference numeral 62. As noted, these each contain an address which is
then followed by data, concluding with error correction coding (ECC). The
addresses are those of the individual decoders. The addresses in the
address packets are transmitted in clear text, such that they can be
received without decryption by the receiver. The remainder of the message
is encrypted. In this way, addressed packet data, which is, very
significant to the proper functioning of the system because one of the
addressed packets includes one of the decrypting ciphers needed, is
provided with a high degree of security. Addressed packets addressed to
differing decoders may be transmitted in a single field.
As indicated at 64, lines 9-13 of the VBI are used to transmit teletext.
The first part of each teletext line is a teletext identification which
indicates that the line in fact is teletext. As shown, two types of
teletext lines are used. Teletext headers include a relatively larger
number of flags, and indicate which of the following teletext lines are
part of a particular "page" or message. The text lines themselves include
a somewhat lesser number of flags and text data. Typically, forty
ASCII-encoded bytes are sent per text line, and up to twenty lines can be
displayed on the user's screen at once.
FIG. 7 shows in some additional detail the make-up of line 3. It begins
with the seventy-eight symbols of HBI data indicated at 72, followed with
a bit which is not used, and a number of message bits, each of which is
immediately followed by a parity bit. The message bits shown in line 3 of
FIG. 7 are each repeated three times and are each protected by parity
bits, such that of some 378 total bits, only sixty-two bits of useful data
are provided. This data comprises the "system data" used by the
subscription television system of the invention to keep control of a wide
variety of system functions. Three different versions of line 3 are
required to transmit all the system data needed, and each is transmitted
in five successive fields, such that the total system data transmission
consumes fifteen total field transmissions. A sixteenth field is not used
for transmission of system data. The fact that the system data transmitted
in line 3 includes a service key which is changed every 16 frames, i.e.,
on the order of three times per second. This service key must, of course,
be accurately received for the decoder to work properly. Therefore, it is
transmitted redundantly and in combination with extensive parity-based
error correction to ensure correct reception of the service key, as well
as the other system data.
The key contained in line 3 is also used to unscramble the location of the
color burst signal occurring during the HBI, which varies from the
exemplary position shown in FIG. 4.
Teletext is transmitted in a bipartite format. Teletext is transmitted in
the form of a number of text lines or rows, making up a page of text. The
rows making up the page are preceded in transmission by a teletext header.
The header indicates the fact that a teletext page follows and indicates
its page number. A decoder looking for a particular page number, for
example, a template page, scans the teletext page numbers provided in the
teletext headers for the particular page of interest. When the page number
sought is detected, the decoder then selects the following page, that is,
selects for storage all the teletext lines which follow until the next
teletext header line is identified.
FIGS. 8 and 9 show respectively the formats of the teletext header and text
lines. In FIG. 8, the teletext header 90 is shown as comprising a
thirty-two bit teletext identifier 92. This field simply indicates that
this particular line of the vertical blanking interval is a teletext line,
as opposed to, for example, an addressed packet. The next thirty-two bit
area 94 contains various control flags, which are discussed in detail
below. The teletext header then contains a 128 bit area 94 contains
various control flags, which are discussed in detail below. The teletext
header then contains a 128 bit field 96 which identifies the number of the
page which is comprised by the following text lines. The page number is a
sixteen bit number, each bit of which is encoded as a eight bit byte. The
flags 94 are similarly encoded: that is, a flag which is either a "1" or a
"0" data value is nevertheless encoded as an eight bit byte for
transmission, so as to enable its correct detection more probable than if
it were simply a single bit flag. For the same reason, the page number is
a 128-bit word in which each eight byte indicates whether the
corresponding bit is a 1 or a 0, again for extremely reliable detection of
page numbers. Finally, the last 165 bits 98 of the teletext header 90 are
not used.
The flags 94 include a header flag 94a which indicates whether the teletext
line is a header or is not, a linked page flag 94b indicating whether the
subsequent page of teletext is one of a number "linked" or related to the
present page, an encrypted page flag 94c indicating whether the subsequent
page is encrypted or not, and a box page flag 94d indicating whether the
text shown in the subsequent page should be displayed against a video
background or a black background.
The significance of the flags is as follows. The header flag 94a simply
indicates whether a particular teletext line is a header or is a line of
text. The linked page flag 94b is used to signify to the decoder that a
subsequent page contains data needed to complete the message begun in the
present page. For example, if a teletext message is too long to fit into a
single page comprising twenty 40-character lines of text, the user
typically will desire to see the subsequent text page. The linked page
flag 94b is used to alert the decoder to this fact and to cause it to copy
the page of text having the next higher page number into a random access
memory, such that if the user then indicates that he wishes to see the
subsequent page of text, it already stored in the random access memory. In
this way, the entire message can be displayed more or less immediately, as
opposed to waiting for a subsequent transmission of succeeding pages,
which may not occur for on the order of several minutes in a very busy
system. The linked page flag 94b thus provides an opportunity to improve
the teletext service to the user. More particularly, any number of pages
can be linked to provide lengthy text messages, e.g., stock price
quotations or the like, which can efficiently be read in sequence.
The encrypted page flag 94c indicates whether the text found in the
subsequent text lines making up a page is encrypted or not. In many cases,
of course, there is no reason to encrypt the teletext, for example, the
message is not private, or if its loss will not be damaging to the system
integrity, as would be, for example, the loss of control over a first-run
motion picture or the like. Hence, many teletext lines are not in fact
encrypted.
Finally, the box flag page 94d indicates to the decoder that the teletext
in a subsequent page is to be superimposed over whatever video is on the
screen at the time, instead of being displayed against a plain background.
This flag is useful for several purposes. For example, closed-captioned
teletext, providing lines of dialogue and the like so that the
hearing-impaired can follow the text of a film, is clearly best provided
in this way, such that a viewer can simultaneously see the text and the
video. On the other hand, important system messages, such as warnings of
community dangers and the like, may be more dramatically or effectively
presented against a plain background. Hence, this option is provided and
is controlled by the box page flag 94d as noted.
FIG. 9 shows the structure of an individual text line 100 up to twenty of
which may make up a page of text. As in the case of the teletext header of
FIG. 8, the first thirty-two bits 102 of the text line 100 are a teletext
identifier. These are identical whether the teletext line is in fact a
header or is a text line. The next eight bits are a header flag 103, which
is identical to the header flag comprised by flags 94 of the header line
90, that is, it is an eight bite byte indicating that the teletext line is
in fact a text line 100 and not a teletext header 90. The following 320
bits are devoted to the transmission of forty bytes of textual data.
Typically, these are encoded according to the usual ASCII standards,
whereby each byte is seven bits of data plus a parity bit for error
detection. Thus, each text line transmits forty characters which may be
any alpha-numeric character found in the ASCII character set. The last
seventeen bits 108 are not used.
Thus, in practice, the broadcast transmitter transmits a sequence of
teletext lines in lines 9-13 of the vertical blanking interval (see FIG.
6). Up to twenty textlines 100 may follow each teletext header 90. The
teletext head 90 contains a page number 96 which identifies the following
text lines as, for example, belonging to a template useful in displaying
billing status, or as including, for example, information concerning the
current movie being run, that is, describing its title, its lead
characters, it length, and the price the subscriber will be charged for
viewing it, or the like. It will be appreciated, therefore, that the
teletext lines in any given vertical blanking interval may be all text
lines 100, since only five teletext lines can be transmitted in a vertical
blanking interval. (It will be appreciated by those skilled in the art
that this numerical limitation relates to a 525-line NTSC-type signal; the
actual numbers of the lines-in the VBI are different in the PAL type
625-line system.)
A 9600 k baud asynchronous data channel for use, for example, by a personal
computer is also transmitted over the MAC signal. Additionally, one or
more audio channels may be used for data transmission.
Thus, the MAC signal includes video and up to six audio channels, as well
as text and data. Typically, a MAC decoder includes one pair of audio
outputs. These outputs are generally dedicated for stereo audio output to
accompany the video for a transmitted program. However, since up to six
channels of audio output are available, the other four channels may, for
example, carry a second language to accompany the video for the
transmitted program, radio transmissions, or high speed reassigned data.
The availability of these additional audio channels, along with text and
data, provides flexibility to system operators. Thus, in prior systems, a
subscriber could, for example, listen to a radio station transmitted over
transponder channel 4. A transponder is a microwave repeater which
receives, amplifies, downconverts and retransmits signals at a
communication satellite. To listen to the radio station, the subscriber
tuned to channel 4 and actuated a key designated "RADIO" on his or her
handheld remote or on a keypad on a front panel of the decoder. A radio
menu offering one or more selections would then appear on the screen to
invite the subscriber to make a selection, thereby enabling the subscriber
to listen to the selected radio station. A text screen identifying the
radio station tuned was then displayed. While such an arrangement utilizes
the features of a MAC signal, the above-described procedure can be
confusing to a subscriber since the video of transponder channel 4 and the
audio associated with the radio station, even though transmitted over the
same transponder channel, are typically unrelated. Thus, when a subscriber
tunes to channel 4 prior to activating the "RADIO" key, video and audio
unrelated to the desired radio station are presented. Further, the
subscriber must first consult a program guide to find the appropriate
transponder channel and then either again refer to the program guide or to
a menu and possibly submenus to listen to the radio station.
Other specific details of a prior art conditional access television system
may be found in commonly assigned U.S. Pat. No. 4,890,319, incorporated
herein by reference.
As noted, the MAC signal may also be utilized to transmit text for display
on a subscriber's television. Text screens may, for example, provide
weather reports, sports updates, and stock market quotations. Typically,
such information is presented on several screens through which a
subscriber may page by using, for example, a "NEXT" key. However, if the
information is presented on a large number of different screens, a
subscriber will need to page through a number of screens to obtain the
information he or she is seeking, resulting in delay and frustration.
Thus, although prior systems have utilized the inherent features of a MAC
system, present interfaces between the system and a subscriber desiring to
use these features can lead to confusion and delay.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
improved interface between a television system and a subscriber to permit
better utilization of the system features.
It is another object of the present invention to provide an improved method
of searching for and displaying text pages to a subscriber.
In accordance with the present invention, a decoder for use in a television
system is provided. The decoder includes a receiver for receiving a
television signal having at least one channel. Each channel of the
television signal includes video and audio components. A channel map maps
the channel received by the receiver to a plurality of virtual channels. A
first virtual channel utilizes a first combination of video and audio
components of the received channel and a second virtual channel utilizes a
second combination of video and audio components of the same received
channel different than the first combination. A selector allows a
subscriber to select one of the virtual channels.
Also in accordance with the present invention, an encoder for use in a
television system such as a multiplexed analog component (MAC) television
system including a plurality of remotely located decoders is provided. The
encoder includes a transmitter for transmitting a television signal having
at least one channel. Each channel of the television signal includes a
video component and an audio component having a plurality of audio
channels. A channel map is generated for use by the decoders to map the
channel to a plurality of virtual channels. A first virtual channel
utilizes a first combination of video and audio components of the
transmitted channel and a second virtual channel utilizes a second
combination of video and audio components of the same transmitted channel
different than the first combination. The encoder includes a mechanism for
downloading the channel map to the remote decoders.
Also in accordance with the present invention, a decoder for use in a
television system is provided including a receiver for receiving a
television signal including a plurality of text pages wherein which are
linked such that a first text page is accompanied by information
identifying a second text page associated therewith. A channel map maps
the text pages to a plurality of virtual channels, each virtual channel
having linked text pages mapped thereto in accordance with root text pages
defining a first text page on each of the virtual channels and root page
spacing defining a number of text pages mapped to each of the virtual
channels. A final page of a first virtual channel may be linked to the
root page of a second virtual channel. A selector enables a subscriber to
select one of the virtual channels.
Also in accordance with the present invention, an encoder for use in a
television system including a plurality of remotely located decoders is
provided which includes a transmitter for transmitting a television signal
comprising a plurality of text pages therein. The text pages are linked
such that a first text page is accompanied by information identifying a
second text page associated therewith. A channel map is generated for
mapping the text pages to a plurality of virtual channels, each virtual
channel having linked text pages mapped thereto in accordance with root
text pages defining a first text page on each of the virtual channels and
root page spacings defining a number of text pages mapped to each of the
virtual channels. A final page of a first virtual channel may be linked to
the root page of a second virtual channel. The encoder includes a
mechanism for downloading the channel map and the root page spacing to the
remote decoders.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the present invention and many of the
attendant advantages thereof will be readily obtained as the invention
becomes better understood by reference to the following detailed
description when considered in connection with the accompanying drawings.
FIG. 1 is an amplitude-vs.-time diagram of a single video line of a typical
MAC color television signal.
FIG. 2 is block diagram of a prior art satellite television system.
FIG. 3 shows an overall view of the video format according to the
invention.
FIG. 4 shows in broad outline the format of the horizontal blanking
interval.
FIG. 5 shows additional details of the format of the horizontal blanking
interval.
FIG. 6 shows an overview of the material carried in the 16 lines of the
vertical blanking interval in a 525 line embodiment of the invention.
FIG. 7 shows the arrangement of the system data carried in line 3 of the
vertical blanking interval.
FIG. 8 shows the outline of a teletext header line which can be transmitted
in lines 4-8 of the vertical blanking interval.
FIG. 9 shows a text line, that is a line of teletext which may be
transmitted during any one of lines 9-13 of the vertical blanking
interval.
FIG. 10 illustrates a satellite television system in which the present
invention may be implemented.
FIG. 11 is a block diagram of the integrated receiver decoder of FIG. 10.
FIG. 12 is a block diagram of the descrambler shown in FIG. 11.
FIG. 13 illustrates the relationship between a plurality of virtual
channels and a plurality of transponder channels.
FIG. 14 illustrates a channel map in accordance with the present invention.
FIG. 15 illustrates an arrangement of text pages on a plurality of virtual
channels in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is described below in terms of a B-MAC satellite
television system. However, the invention is applicable to other MAC
systems such as C-MAC, D-MAC, and D/2-MAC. Further, the invention may also
be implemented in NTSC (National Television Standards Committee), PAL,
SECAM, or high definition television systems.
A B-MAC satellite television system in which the the present invention may
be implemented is shown in block form in FIG. 10. B-MAC encoder 201
encodes a source program 202 for transmission over a satellite link 205 to
an integrated receiver-decoder (IRD) 206. Program source 202 may include
video, audio, and data information. The source program information is
scrambled in a program scrambler of B-MAC encoder 201 using a key (as
discussed above). The scrambled programs and key are transmitted through
satellite link 205. IRD 206 receives the scrambled programs and key. The
key is recovered from the received signal, stored in a key memory and
applied to a program descrambler which descrambles the scrambled program
and outputs unscrambled program 209 for display on television 220.
IRD 206 is coupled to public switched telephone network 207. The telephone
network is coupled to a phone processor 208 for receiving calls initiated
by the IRDs in the network. The phone processor may comprise, for example,
a Scientific Atlanta Model 8554-001 Phone Processor, available from the
assignee of the present application. A current implementation utilizes
eight model 8554-001 processors to handle incoming calls. A phone manager
computer 209 such as a Compaq.RTM. SystemPro.TM. controls phone processor
208.
Phone manager computer 209 is coupled to business system computer 210 for
compiling and processing billing information to bill subscribers. Phone
manager 209 is also coupled to a subscriber authorization computer (SAC)
211 which controls, for example, the authorization of subscribers to
receive particular programming. Subscriber authorization computer 211
contains information such as program tiers for a current month, credit
limits, service tiers, call-in billing group, call-in time zone, call-in
phone number, and store and forward disable for decoders in the network.
Subscriber authorization computer 211 is coupled to MAC encoder 201 to
permit communication between computer 211 and the IRDs in the network over
satellite link 205. Finally, a system supervisory control computer 212
coupled to phone manager 209 and MAC encoder 201 controls the overall
operation of the system.
Data or commands are transmitted to decoders in the network over satellite
link 205 in at least two ways. In a first way, system data generated by
supervisory control computer 212 carries program specific data for the
channel currently tuned by a decoder. In a second way, addressed data
packets (ADPs) are used to deliver decoder specific information to a
single decoder. Each decoder in the network is assigned a unique user
address and a secret serial number (SSN). When an addressed packet with an
address matching the user address of a decoder is received, the packet is
decrypted with the SSN. The packets preferably include a checksum which is
used to verify both correct reception and decryption with a matching SSN.
Typically, system data originates from supervisory control computer 212 as
noted, while addressed data packets originate from subscriber
authorization computer 211, although the invention is not limited in this
respect.
FIG. 11 is a block diagram of B-MAC IRD 206 shown in FIG. 10. H/V switch
301 switches between the horizontal and vertical polarities of the
incoming transmission over satellite link 205. The incoming signal then
passes to block 302 including a downconverter, a tuner, and demodulator.
The downconverter and tuner select a channel from the incoming signal and
lower it to some intermediate frequency (IF). The tuner may, for example,
comprise a synthesized tuner. The demodulator demodulates the signal to
generate composite baseband video which is input into B-MAC decoder 303.
Front panel 305 includes an IR receiver 306, user keypad 307, and LED
display 308. IR receiver 306 is adapted to receive control signals from an
associated IR remote control (not shown). User keypad 307 includes a
plurality of keys 310 for permitting the subscriber to input, for example,
channel selections and volume control. LED display 308 displays the tuned
channel and may display other information such as time. Power supply 310
supplies power to IRD 206.
A tuning and front panel control processor 304 may comprise a MC68HC05C4
and tunes the transponder tuner in block 302, scans front panel keypad 307
and any remote keypads for keystrokes, drives LED display 308 and provides
volume control. Keystroke interpretation is generally performed by display
control processor (DCP) 405 (see FIG. 12), except for volume control,
which is internal to tuning processor 304. Volume control keystrokes are
passed to DCP 405, but function only to instruct DCP 405 to un-mute audio.
B-MAC decoder 303 decodes the composite baseband video input thereto and
outputs NTSC video and audio as shown. VHF modulator 311 modulates the
video and audio outputs of B-MAC decoder 303 for reception by television
receiver 220 (FIG. 10). Modem 313 allows IRD 206 to interface with the
public switched telephone network to permit communication between IRD 206
and a system operator. For example, billing information related to impulse
pay-per-view purchases may be transferred to the system operator.
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