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Claims  |
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What is claimed is:
1. In a video system for processing compressed video data into composite
video data, the composite video dam corresponding to a standard composite
video signal type selected from a plurality of standard types, the
plurality of standard types including at least one of an NTSC composite
video signal type and a PAL video signal type, the video system including
a processor and a standard video decompressor to process the compressed
video data into decompressed video data and user data, the user data
having VBI data encoded therein, the processor culling the VBI data from
the user dam, a video interface to process the decompressed video data and
the VBI dam into the composite video data, the composite video data having
the VBI data encoded therein, the video interface comprising:
a VBI data generator to process the VBI data into VBI signal data;
a mixer coupled to the video decompressor to receive the decompressed video
dam, the mixer being coupled to the VBI data generator to receive the VBI
signal data, the mixer providing output Y, U and V data during an active
video portion of a scan line; and
circuitry to process the Y, U and V data into the composite video data.
2. The video interface of claim 1, wherein the VBI data generator includes
circuitry to receive the VBI data from the processor and generate the VBI
signal data according to the VBI data, the VBI data including at least one
of packet ID data, line number data, odd/even field data, high value data,
low value data, symbol width data, number of symbols data and symbol data.
3. The video interface of claim 2, wherein the circuitry of the VBI data
generator comprises:
a filter to process the symbol data into the filtered VBI data; and
circuitry to amplitude scale the filtered VBI data to become the VBI signal
data based on the high value data and the low value data.
4. The video interface of claim 2, wherein the circuitry of the VBI data
generator comprises circuitry to time scale a symbol rate of the VBI
signal data based on the symbol width data.
5. The video interface of claim 2, wherein the VBI signal dam includes a
sequence of digitally represented sampled data corresponding to an analog
line signal to be regenerated in an active portion of a scanning line in a
vertical blanking interval of the standard composite video signal.
6. The video interface of claim 5, wherein the analog line signal is a
closed captioning signal.
7. The video interface of claim 5, wherein the analog line signal is a
station ID signal.
8. The video interface of claim 5, wherein the analog line signal is a time
code signal.
9. The video interface of claim 5, wherein the analog line signal is an
undersampled VITS signal.
10. The video interface of claim 9, wherein the undersampled VITS signal is
one undersampled signal of a plurality of undersampled VITS signals, the
analog line signal being reconstructable from the plurality of
undersampled VITS signals, each undersampled signal of the plurality of
undersampled VITS signals corresponding to VITS signal data, the plurality
of undersampled VITS signals corresponding to a set of VITS signal data,
each VITS signal date of the set of VITS signal data being associated with
a respective video frame, the video interface further including a line
store memory to save the set of VITS signal data and circuitry to
reconstruct the analog line signal from the set of VITS signal data.
11. In a video system for processing compressed video data into composite
video data, the composite video data corresponding to a standard composite
video signal type selected from a plurality of standard types, the
plurality of standard types including at least one of an NTSC composite
video signal type and a PAL video signal type, the video system including
a processor and a standard video decompressor to process the compressed
video data into decompressed video data and user data, the user data
having color burst parameters encoded therein, the processor culling the
color burst parameters from the user data, a video interface to process
the decompressed video data and the color burst data into the composite
video data, the video interface comprising:
a subcarrier data generator to generate sine data and cosine data;
a first multiplier to process sine data into scaled sine data based on a
function of a first parameter of the color burst parameters from the
processor;
a second multiplier to process cosine data into scaled cosine data based on
a function of a second parameter of the color burst parameters from the
processor;
a mixer to combine the scaled sine data and the scaled cosine data, the
mixer generating color burst portion data of the composite video data; and
means for combining the color burst portion data with the decompressed
video data to form the composite video data.
12. In a video system for processing compressed video data into composite
video data, the composite video data corresponding to a standard composite
video signal type selected from a plurality of standard types, the
plurality of standard types including at least one of an NTSC composite
video signal type and a PAL video signal type, the video system including
a processor and a standard video decompressor to process the compressed
video data into decompressed video data and user data, the user data
having pan-scan data encoded therein, the pan-scan data defining a
sub-portion of an active video portion of the decompressed video data, the
processor culling the pan-scan data from the user data, a video interface
to process the decompressed video data and the pan-Scan data into the
composite video data, the video interface comprising:
a mixer coupled to the video decompressor to receive the decompressed video
data, the mixer providing output Y, U and V data during the sub-portion of
the active video portion of a scan line according to the pan-scan data;
and
circuitry to process the Y, U and V data into the composite video data.
13. The video interface of claim 11, wherein:
the user data further includes VBI (vertical blanking interval) data;
the processor culls the VBI data from the user data;
the means for combining includes a VBI data generator to process the VBI
data into VBI signal data;
the means for combining further includes a VBI mixer coupled to the video
decompressor to receive the decompressed video data, the VBI mixer being
coupled to the VBI data generator to receive the VBI signal data, the VBI
mixer providing output Y, U and V data during an active video portion of a
scan line and providing the output U and V data as the first and second
parameters during a non-active video portion of the scan line.
14. The video interface of claim 13, wherein the VBI data generator
includes circuitry to receive the VBI data from the processor and generate
the VBI signal data according to the VBI data, the VBI data including at
least one of packet ID data, line number data, odd/even field data, high
value data, low value data, symbol width data, number of symbols data and
symbol data.
15. The video interface of claim 14, wherein the circuitry of the VBI data
generator comprises:
a filter to process the symbol data into the filtered VBI data; and
circuitry to amplitude scale the filtered VBI data to become the VBI signal
data based on the high value data and the low value data.
16. The video interface of claim 14, wherein the circuitry of the VBI data
generator comprises circuitry to time scale a symbol rate of the VBI
signal data based on the symbol width data.
17. The video interface of claim 14, wherein the VBI signal data includes a
sequence of digitally represented sampled data corresponding to an analog
line signal to be regenerated in an active portion of a scanning line in a
vertical blanking interval of the standard composite video signal.
18. The video interface of claim 12, wherein:
the user data further includes VBI (vertical blanking interval) data;
the processor culls the VBI data from the user data; the video interface
further comprises a VBI data generator to process the VBI data into VBI
signal data; and
the mixer is coupled to the VBI data generator to receive the VBI signal
data, the mixer providing the VBI signal dam as the output Y, U and V data
during an active video portion of a scan line during the vertical blanking
interval.
19. The video interface of claim 18, wherein the VBI data generator
includes circuitry to receive the VBI data from the processor and generate
the VBI signal data according to the VBI data, the VBI data including at
least one of packet ID data, line number data, odd/even field data, high
value data, low value data, symbol width data, number of symbols data and
symbol
20. The video interface of claim 19, wherein the circuitry of the VBI data
generator comprises:
a filter to process the symbol data into the filtered VBI data; and
circuitry to amplitude scale the filtered VBI data to become the VBI signal
data based on the high value data and the low value data. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to digital signal transmission, and
more particularly, to a system and method for multiplexing a plurality of
digital services, including compressed imaging services and ancillary data
services such as closed captioning for the deaf associated with the
imaging services for transmission to a plurality of remote locations.
2. Description of Related Art
The background of the present invention is described herein in the context
of pay television systems, such as cable television and direct broadcast
satellite (DBS) systems, that distribute a variety of program services to
subscribers, but the invention is by no means limited thereto except as
expressly set forth in the accompanying claims.
In the pay television industry, programmers produce programs for
distribution to various remote locations. A "program" may consist of
video, audio and other related services, such as closed-captioning and
teletext services. A single programmer may wish to supply many programs
and services. Typically, a programmer will supply these services via
satellite to individual subscribers (i.e., DBS subscribers) and/or cable
television operators. In the case of cable television operators, the
services transmitted via satellite are received at the operator's cable
head-end installations. A cable operator typically receives programs and
other services from many programmers and then selects the
programs/services it wishes to distribute to its subscribers. In addition,
a cable operator may insert locally produced services at the cable-head
end. The selected services and locally produced services are then
transmitted to the individual subscribers via a coaxial cable distribution
network. In the case of DBS subscribers, each subscriber is capable of
receiving a satellite down-link from the programmers directly.
In the past, pay television systems, including cable and DBS systems, have
operated in the analog domain. Recently, however, the pay television
industry has begun to move toward all digital systems wherein, prior to
transmission, all analog signals are converted to digital signals. Digital
signal transmission offers the advantage that digital data can be
processed at both the transmission and reception ends to improve picture
quality. Further, digital data compression techniques have been developed
that achieve high signal compression ratios. Digital compression allows a
larger number of individual services to be transmitted within a fixed
bandwidth. Bandwidth limitations are imposed by both satellite
transponders and coaxial cable distribution networks, and therefore
digital compression is extremely advantageous.
Further background can be found in U.S. patent application Ser. No.
968,846, filed Oct. 30, 1992, titled System and Method For Transmitting a
Plurality of Digital Services. This application is hereby incorporated by
reference as if fully set forth herein.
With the growing trend toward a merger of the previously separate
technologies of telecommunications including voice and data
telecommunications and television including satellite, broadcast and cable
television, there has emerged an increased interest in developing
adaptable transmission systems capable of handling any one or more of a
collection or plurality of such services. The primary media investigated
for providing such services to date comprise, for example, coaxial cable,
land-based microwave, so-called cellular radio, broadcast FM, broadcast
satellite and optical fiber, to name a few.
Each media has its own characteristics. For example, comparing cable and
satellite for digital data transmission, cable tends to have a medium
error rate, but, when errors appear, the errors come in long bursts.
Satellite as a media has a pretty poor error rate, primarily due to the
requisite weak signal power, and hence, low signal to noise ratio. In
satellite, then, the poor error rate is specially corrected utilizing such
techniques as convolutional error correctors, not required in a cable
environment.
In copending U.S. application Ser. No. 07/968,846 filed Oct. 30, 1992 and
entitled "System and Method for Transmitting a Plurality of Digital
Services", the disclosure of which is incorporated herein by reference,
there is described an encoder for generating a multiplexer data stream
carrying services to remote locations via, for example, a satellite or a
cable distribution network. The generated data stream comprises a
continuous sequence of frames, each frame comprising two fields, and each
field comprising a plurality of lines. A first group of lines of a field
defines a transport layer and a second group of lines defines a service
data region. A feature of the disclosed scheme is the ability to
dynamically vary the multiplexed data stream from field to field. A
further feature of the disclosed scheme is that the data transmission rate
of the multiplexed data stream is related to the frequency of known analog
video formats, i.e. frame, field and horizontal line rates.
In copending U.S. application Ser. No. 07/970,918 filed Nov. 2, 1992,
entitled "System and Method for Multiplexing a Plurality of Digital
Program Services for Transmission to Remote Locations", the disclosure of
which is incorporated herein by reference there is described another
system, this for multiplexing a plurality of digital program services
comprising a collection of, for example, video, audio, teletext,
closed-captioning and "other data" services. According to the disclosed
scheme, a plurality of subframe data streams are generated, each having a
transport layer region and a program data region. These subframe data
streams are then multiplexed together into superframes having a transport
layer region and a subframe data region.
A decoder within a subscriber's home or at any other receiving terminal
separates the multiplexed data stream into the various services. One such
service is conventional television video images. In order to efficiently
transfer video images, redundant information in the image is preferably
removed, a process referred to as video compression. A number of video
compression techniques are proposed and some techniques have been adopted,
for example, International Standards Organization, ISO-11172 and 13818,
generally referred to MPEG standards (including MPEG1 and MPEG2) where
MPEG refers to Moving Picture Expert Group. Several manufacturers have
developed integrated circuits to decompress MPEG 1 and MPEG2 compressed
video data, for example Thompson-CSF, C-Cube and LSI Logic Corporation
have all developed such decompression integrated circuits.
A standard analog NTSC composite video signal is already a compressed
format. Diagonal luminance resolution has been given up in order to
accommodate the color subcarrier within the luminance bandwidth.
Unfortunately, the existence of the color subcarrier decreases the
correlation between adjacent samples and adjacent frames, making it
difficult to apply further stages of compression. For this reason,
efficient compression algorithms are applied, not to the NTSC signal, but
to the original separate components: brightness Y, first color difference,
and second color difference or Y, U, V (luminarice and color differences).
Video compression is based on eliminating redundancy from the signal. There
are two principal types of redundancy in video signals: psychovisual
redundancy and mathematical redundancy. Efficient transmission systems
attempt to eliminate both types of redundancy.
Psychovisual redundancy in the signal occurs as a result of transmitting
information which the human eye and brain does not use, and cannot
interpret. The most obvious example is the excess chrominance bandwidth in
R,G,B signals. When a video signal is represented as R,G,B components,
none of these components can be reduced in bandwidth without perceptible
decrease in picture quality. However, when a full bandwidth luminarice
signal is present, the eye cannot detect significant bandwidth reduction
of the color information. NTSC makes use of this human psychovisual
characteristic by converting the original R,G,B signal to Y,U,V components
through a linear matrix. The U,V components can then be reduced in
bandwidth to 25% of the luminance component without a loss of apparent
picture quality. However, NTSC does not fully exploit color difference
redundancy. It reduces bandwidth only in the horizontal dimension, whereas
the eye is equally insensitive to chrominance detail in the vertical
dimension. Other forms of psychovisual redundancy include: luminance
detail on fast-moving objects, luminance diagonal resolution, and
luminarice detail close to high contrast ratio transitions (edge masking).
Elimination of psychovisual redundancy proceeds by transforming the signal
into a domain in which the redundant information can be isolated and
discarded.
Mathematical redundancy occurs when any sample of the signal has a nonzero
correlation coefficient with any other sample of the same signal. This
implies that some underlying information is represented more than once,
and can be eliminated leading to data compression. In a minimally sampled
television luminance signal of a typical image, adjacent samples are
normally 90 to 95% correlated. Adjacent frames are 100% correlated in
stationary areas, and more than 90% correlated on average.
In video compression, redundant information in a moving image is removed on
a pixel-to-pixel basis, a line-to-line basis and a frame-to-frame basis.
The compressed moving video image is transferred efficiently as one part
of the services in the multiplexed data stream.
In present day equipment, moving video images are typically encoded in
standard signal types such as analog NSTC composite video signals and PAL
signals. These signals include periodic repetition of frames of
information, each frame including periodic repetition of lines of
information (the lines in a frame being organized into first and second
fields), and each line including a synchronization information portion and
an active video portion. A first subset of the lines of a frame is used
for vertical synchronization. A second subset of the lines of a frame is
used to transfer ancillary data, such as closed captioning for the deaf on
line 21 among the many types of data, during a vertical blanking interval
(hereinafter VBI), the data transferred during the vertical blanking
interval being referred to as VBI data. The remaining lines, constituting
a third subset of the lines of a frame, contain the video image. It is
this video image which is most capable of being compressed in the standard
video compression techniques.
In video compression techniques, the ancillary data or VBI data is removed
from the image before compression. Thus, users who have come to rely on
the ancillary VBI data transmitted during the vertical blanking interval
of, for example, an NTSC signal, will be unable to enjoy these services
when receiving compressed moving video image services transmitted through
the multiplex data stream.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the shortcomings in
the prior art. It is a further object of the present invention to provide
an apparatus conforming to industry-wide standards for transmitting all
types of VBI dam. It yet another object of the present invention to
provide an apparatus for decoding VBI data encoded in standard MPEG format
so as to flexibly provide for control of the symbol width, symbol high
data level and symbol low data level by encoding appropriate data at the
encoder end.
These and other objects are achieved in a video system for processing
compressed video data into composite video data, the composite video data
corresponding to a standard composite video signal type selected from a
plurality of standard types, the plurality of standard types including at
least one of an NTSC composite video signal type and a PAL video signal
type, the video system including a processor, a standard video
decompressor and a video interface. The standard video decompressor
processes the compressed video data into decompressed video data and user
data, the user data having VBI data encoded therein. The processor culls
the VBI data from the user data. The video interface processes the
decompressed video data and the VBI data into the composite video data
with the VBI data encoded therein. The video interface includes a VBI data
generator to process the VBI data into VBI signal data, a mixer coupled to
the video decompressor to receive the decompressed video data, the mixer
being coupled to the VBI data generator to receive the VBI signal data the
mixer providing output Y, U and V data during an active portion of a scan
line, and circuitry to process the Y, U and V data into the composite
video data.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in detail in the following description of
preferred embodiments with reference to the following figures wherein:
FIG. 1 is a partial block diagram of a decoder portion according to the
present invention;
FIG. 2 is a block diagram showing the video processor of the present
invention;
FIG. 3 is a block diagram showing the video interface of the present
invention;
FIG. 4 is an exemplary block diagram showing the VBI data generator of the
present invention;
FIG. 5 is a schematic diagram showing a simple combiner exemplary of the
present invention;
FIG. 6 is a partial block diagram of a system for multiplexing a plurality
of digital services for transmission to a plurality of remote locations,
as described in U.S. Patent application Ser. No. 968,846;
FIG. 7 is a graphical illustration of the multiplex data stream generated
by an encoder;
FIG. 8 shows in detail the general arrangement and contents of a related an
frame of the multiplex data stream for transmitting NTSC video services;
FIG. 9 shows in detail the data and services that can be carried in an
exemplary first field of a related art frame of the multiplex data stream;
FIG. 10 shows in detail the data and services that can be carried in an
exemplary second field of a related art frame of the multiplex data
stream;
FIG. 11 is a block diagram illustrating, in a simplistic manner, an
exemplary environment for the present invention;
FIG. 12 is a block diagram of a decoder 216;
FIG. 13 depicts an exemplary line 21 waveform in accordance with the
present invention;
FIG. 14 is a block diagram of the line 21 former circuit 242 of FIG. 12;
FIG. 15 is an alternative frame format according to the present invention;
and
FIG. 16 is another alternative frame format according to the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
A portion of the disclosure in application Ser. No. 968,846 is summarized
and amended hereinbelow with respect to FIGS. 6-10 to provide further
information useful in explaining a specific application of the present
invention.
FIG. 6 shows a partial block diagram of a system 100 for multiplexing a
plurality of digital services for transmission to a plurality of remote
locations (not shown). In the pay television context, the system 100
comprises a plurality of service encoders 112 each of which is operated by
a "programmer." As illustrated, any number N of programmers may be present
in the system 100. As mentioned in the background, programmers are
entities that provide "programs" for distribution to various subscribers.
For example, as shown in FIG. 6, programmer 1 may provide programs 1
through N. Each program comprises a set of related services, such as
video, audio and closed-captioning services. By way of example, FIG. 6
shows that programmer 1 is providing program 1 which comprises a video
service 114 and two related audio services 116, 118 such as stereo sound.
A given program can comprise a collection of related services, and a
programmer may provide any number of programs.
Typically, the individual services of each program are produced in an
analog format. According to the system and method of the present
invention, each encoder 112 has a plurality of analog-to-digital
converters 120 for converting services in analog form to digital services.
In addition, video and audio services may be compressed by video and audio
compression devices 122, however, compression is not required. As those
skilled in the art know, there are many video and audio compression
techniques available. For example, the Moving Picture Expert Group (MPEG)
has developed a video compression algorithm that is widely used in the
digital video services industry. Vector quantization is another, more
recent compression technique for digital video. According to the present
invention, any compression algorithm may be employed by the video and
audio compression devices 122, and the devices 122 are by no means limited
to any one compression method. Furthermore, as mentioned above,
compression of audio and video services is not required. Compression
merely serves to increase the amount of data that can be transmitted
within a given bandwidth.
Each encoder further comprises a service multiplexer 124. As described
hereinafter in greater detail, the service multiplexers 124 functions in
accordance with the method of the present invention to multiplex the
individual digital services for transmission to remote locations, such as
a cable head-end installation or DBS subscriber. The service multiplexer
124 in each encoder 112 generates a multiplex data stream 126 which is fed
to a transmitter 128 for transmission to the remote locations via a
satellite 130. As illustrated in FIG. 6, each programmer (e.g., programmer
1 . . . programmer N) provides its own multiplex data stream 126. As
described hereinafter in greater detail, the multiplex data streams may be
received at various remote locations, such as a cable head-end, a DBS
subscriber or a cable subscriber. Each remote location employs a service
demultiplexer which extracts selected services from the multiplex stream
in accordance with the method of the present invention. Further details of
the service demultiplexer will be provided hereinafter.
FIG. 7 is a graphical illustration of the multiplex data stream 126
generated by each service multiplexer 124 in each encoder 112. According
to the present invention, the multiplex data stream 126 comprises a
continuous sequence of "frames." Each frame includes two "fields" as
shown. As described hereinafter in greater detail, each field contains
multiplexed service data and a "transport layer" that contains certain
"system data" necessary for operating the system of the present invention.
Because certain types of system data are too numerous to transmit in a
single field, these types of data are transmitted over a series of fields
referred to herein as "Groups of Fields." For example, a group cycle may
comprise eight (8) fields; however, a group cycle can be defined by any
number of fields. Essentially, group cycles define boundaries in the
multiplex data stream 126 within which a complete set of system and
encryption related data is transmitted. These group cycle boundaries may
be either fixed or dynamically varying. As described hereinafter, the
service demultiplexer at each remote location needs the system data in a
given group cycle in order to extract selected services from the service
data contained in the next group cycle.
As explained above in connection with FIG. 6, the video services carried in
a multiplex data stream typically originate as analog video signals
(except for HDTV signals), and as shown in FIG. 6, the analog video
signals are "digitized" by analog-to-digital converters 120 and thus
become "digital services." As described hereinafter in greater detail, at
subscriber locations, selected digital video services are extracted from
the multiplex data stream for viewing on a display device, such as a
television, for example. Prior to viewing, however, the digital video
services must be converted back to their analog form. As those skilled in
the art know, there are several analog video signal formats widely used in
the television industries. The NTSC format is widely used in the U.S.,
whereas the PAL format is used in most of Europe.
In one embodiment of this invention and in order to simplify hardware
design and frequency generation throughout the system 100, the overall
frame structure and transmission rate of the multiplex data stream 126 are
preferably related to, and dependant upon, the particular analog video
format of the video services being carried in the multiplex. The frame
structure and digital transmission rate of the multiplex differ depending
upon whether the video services carried in the multiplex are PAL video
signals or NTSC video signals. Providing digital multiplex data rates and
clocks that are related to key analog video frequencies simplifies
hardware design throughout the system. In particular, the regeneration of
analog video (as well as audio) signals at subscriber locations is greatly
simplified.
FIG. 8 shows the general arrangement and contents of an exemplary frame of
the multiplex data stream of FIG. 7 when the video services carded in the
multiplex are based on the NTSC video signal format. The frame structure
and transmission rate of the multiplex data stream are preferably related
to their analog NTSC counterparts. As described below in greater detail,
for example, the overall data rate of the multiplex data stream is related
to the analog television line frequency F.sub.h which in the case of NTSC
video signals is 15.734 kHz (i.e., F.sub.h =15.734 kHz). As illustrated in
FIG. 8, a frame preferably comprises a plurality of lines each of which
are 171 bytes long (i.e., 1368 bits), wherein when the video services
carried are NTSC format signals, the frame has 525 lines. For example, a
digital service may include frames of 525 lines, each line having 171
bytes and transmitted at a rate of 15,734 lines per second to correspond
to a respective analog service. As those skilled in the art will
recognize, the 525 lines of the frame correspond to the number of lines in
an analog NTSC picture. Additionally, the lines in each frame may be
organized to include two "fields," each of which contains 262 lines. A
test line 140 is added to the second field to achieve the 525 line total.
As those skilled in the art will further recognize, this two-field
structure is analogous to the two-field format of NTSC signals.
To achieve correspondence between the multiplex dam rate and analog NTSC
frequencies, each line of the frame is transmitted at a frequency equal to
F.sub.h, the horizontal line frequency. In the case of NTSC video, F.sub.h
is 15.734 kHz. Thus, when NTSC video services are carried in the
multiplex, the multiplex data rate is:
##EQU1##
As expected with 525 lines, the overall frame rate is 29.97 Hz which is
equal to the analog frame rate of NTSC video signals. As those skilled in
the art will recognize, the multiplex rate of 1368 F.sub.h does not
exactly match the NTSC regeneration rate. The NTSC regeneration rate is
actually 1365 F.sub.h, and therefore, decoders at subscriber locations
must perform a rate conversion in order to accurately regenerate the
analog NTSC video signals. A single 21.5 Mbps multiplex data stream may be
modulated and transmitted within a 6 Mhz cable channel, and two 21.5 Mbps
multiplex data streams can be interleaved and transmitted over a single
C-Band satellite transponder.
Referring still to FIG. 8, each field of the frame begins with a VSYNC word
142, and each line begins with an HSYNC byte 146. As described
hereinafter, a service demultiplexer in a decoder at each subscriber
location uses the HSYNC and VSYNC patterns to establish frame and field
synchronization after receiving a multiplex data stream. The VSYNC word
142 is generated similarly for each field, and may be bit-inverted every
other field. The HSYNC byte 146 is preferably the same for each line. The
VSYNC word 142 in each field is preferably followed by a "transport layer"
144. In general, the transport layer 144 in each field contains "system
data" needed for operation of the system of the present invention, and
more importantly, specifies the contents and structure of the "system
data" and service data that follow in the field. As described hereinafter
in greater detail, an important part of the transport layer 144 is the
"multiplex map" which follows directly after the VSYNC word 142 in each
field. The multiplex map specifies the number and location of transport
layer packets that follow in the field and is dynamically adjustable on a
per field basis to achieve great flexibility.
As shown in FIG. 8, the transport layer 1.44 of each field is followed by a
service data space 148 which contains the audio and video service data
carded in the multiplex data stream. As explained hereinafter in greater
detail the plurality of video services and audio services carried in each
field are variably partitioned within the field so that the system can
accommodate multiple service data rates. The data rate for a service can
vary from the HDTV rate (approx. 17 Mbps) to the telecommunications
standard T1 data rate of 1.544 Mbps. The amount of data assigned to video,
audio and other services can be adjusted among the services. Portions of
the service data space not used for audio services may be reassigned as
video or other service data. Audio services are not tied to video services
within the field, and therefore, the system can provide "radio" services.
Because of the dynamic allocation of service data within a field, the
individual video services are not required to have the same data rate. The
possible combinations of services that a programmer can provide in one
multiplex data stream are limited only by the maximum data rate of the
multiplex data stream (i.e., 21.5 Mbps) and the variable partitioning
increment size. With the flexible method any future digital services with
data rates as low as the telecommunications standard T1 rate can be
accommodated. As further shown, the transport layer 144 and service data
portion 148 of each frame are error coded using a 20 byte Reed-Solomon
error correcting code 150. Those persons sk | | |
