Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02392683 2002-05-24
WO 01/50757 PCT/US00/32268
RF BACK CHANNEL FOR DTV
Technical Field of the Invention
The present invention relates to the
transmission of digital VSB data in a downstream channel
from an upstream digital VSB source to a downstream
digital VSB device and to the transmission of information
in an upstream channel (i.e., a back channel) from the
downstream digital VSB device to the upstream digital VSB
source.
background of the Invention
In digital television communication systems,
video and audio are transmitted to a digital television
over respective 6 MHZ channels occupying a bandwidth of
about 50 MHZ to 1 GHz. The digital video and audio are
provided as symbols at a symbol rate fs and are
transmitted as digital vestigial sideband (VSB) data.
The digital television receives the digital VSB data,
tunes to a selected one of the channels, demodulates and
decodes the digital VSB data in the selected channel
using a symbol clock synchronized to the symbol rate fs
in order to recover the corresponding video and audio,
and processes the recovered video and audio for supply to
a monitor and one or more speakers.
Certain auxiliary receivers, such as VCRs and
cable and satellite converters, also receive transmitted
VSB data, tune to a first selected channel from among the
plurality of channels, demodulate the VSB data in the
first selected channel, remodulate the demodulated data
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to a second selected channel (such as channel 3 or 4),
and communicate the remodulated data over the second
selected channel to a digital receiver such as a digital
television. Such auxiliary receivers are also
synchronized to the symbol clock. An auxiliary receiver,
inter alia, may be referred to herein as an upstream
source.
When an upstream source communicates digital
VSB data over a downstream channel to a downstream
device, such as another VCR, a digital television, or
other downstream receiver, it may be necessary for the
downstream device to communicate information back to the
upstream source over an upstream channel. For example,
some digital VSB data may be copy protected so that only
an authorized downstream device can receive the digital
VSB data and display the video and/or audio contained
therein.
In the case of copy protection, and as
described below, an upstream source of copy protected
data transmits a message in a downstream channel to a
downstream device which is to receive the copy protected
data. The message requests the downstream device to
identify itself. The downstream device in an upstream
channel answers this request with its identification.
The upstream source then transmits, in the downstream
channel, a key to allow the downstream device to properly
process the material that is to follow. The downstream
device acknowledges receipt of the key by transmitting a
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suitable acknowledgment in the upstream channel.
Thereafter, the upstream source transmits the digital VSB
data in the downstream channel to the downstream device.
The carrier frequency defining the upstream
channel should preferably be different than the carrier
frequency defining the downstream channel. The present
invention is directed to an upstream channel defined by a
carrier frequency which is different than the carrier
frequency of the downstream channel and which is
synchronized to the symbol rate at which downstream
digital VSB data is transmitted. The present invention
accordingly reduces the complexity required in defining
the upstream channel.
Bummary of the Invention
In accordance with one aspect of the present
invention, a communication system comprises an upstream
digital VSB source and a downstream digital VSB device.
The upstream digital VSB source transmits digital VSB
data to the downstream digital VSB device in a downstream
channel at a symbol rate fs, and the downstream digital
VSB device transmits information to the upstream digital
VSB source modulated on a carrier which is synchronized
to (N/M)fs.
In accordance with another aspect of the
present invention, a digital VSB apparatus has a
transmitter and a receiver. The receiver receives
digital VSB data in a downstream channel at a symbol rate
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fs, and the transmitter transmits information in an
upstream channel corresponding to a carrier frequency
(N/M)fs.
In accordance with yet another aspect of the
present invention, a digital VSB apparatus comprises a
transmitter and a receiver. The transmitter transmits
digital VSB data in a downstream channel at a symbol rate
fs, and the receiver receives information in an upstream
channel corresponding to a carrier frequency (N/M)fs.
In accordance with still another aspect of the
present invention, a downstream digital VSB device has a
receiver and a transmitter. The receiver receives
digital data having a frame sync transmitted by an
upstream digital VSB source in a downstream digital VSB
channel. The transmitter transmits information in a
group of time slots to the upstream digital VSB source in
an upstream channel. The transmitter synchronizes the
group of time slots to the frame sync received by the
receiver.
In accordance with a further aspect of the
present invention, a downstream digital VSB device
comprises a receiver and a transmitter. The receiver
receives digital data transmitted by an upstream digital
VSB source in a downstream digital VSB channel according
to a symbol clock. The transmitter transmits upstream
data to the upstream digital VSB source in an upstream
channel. The transmitter synchronizes the upstream data
to the symbol clock.
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Brief Descr=~tion of the Drawing
These and other features and advantages of the
present invention will become more apparent from a
detailed consideration of the invention when taken in
conjunction with the drawings in which:
Figure 1 illustrates a communication system
having an upstream source and one or more downstream
devices wherein the upstream source communicates digital
VSB data to the downstream devices in a downstream
channel and wherein the downstream devices communicate
information to the upstream source in an upstream channel
in accordance with the present invention;
Figure 2 illustrates in additional detail a
portion of the upstream source of Figure 1 that is
relevant to the present invention;
Figure 3 illustrates in additional detail the
receiver of the upstream source shown in Figure 2;
Figure 4 illustrates an exemplary time slot
containing an upstream message; and,
Figure 5 illustrates a plurality of tirnc~ slots
in which the downstream devices may communicate; and,
Figure 6 illustrates in additional detail a
portion of a representative one of the downstream devices
of Figure 1 where the portion is relevant to the present
invention.
Detailed Description
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As shown a.n Figure 1, a communication system 10
includes an upstream source 12 and downstream devices 14,
16, and 18. The upstream source 12 and the downstream
devices 14, 16, and 18 are interconnected by a
communication medium 20 such that the upstream
transmission path is the same as the downstream
transmission path. Accordingly, the upstream source 12
communicates with the downstream devices 14, 16, and 18
downstream over the communication medium 20, and the
downstream devices 14, 16, and 18 communicate with the
upstream source 12 upstream over the communication medium
20. Although the communication system 10 is shown with
three downstream devices, it should be understood that
the communication system 10 may include any number of
downstream devices. Moreover, although the communication
medium 20 preferably comprises a cable interconnecting
the upstream source 12 and the downstream devices 14, 16,
and 18, the communication medium 20 alternatively may
comprise a pair of wires, a wireless transmission
channel, and/or the like.
The upstream source 12 and the downstream
devices 14, 16, and 18 are preferably digital devices
with the upstream source 12 transmitting digital VSB data
to the downstream devices 14, 16, and 18. For example,
in digital television, a transmitter transmits digital
VSB data to receivers such as televisions and set top
boxes using a standard symbol rate of about 10.76 MHZ.
Accordingly, the transmitter compresses data, encodes the
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compressed data as corresponding data symbols at the
symbol rate fS, and modulates the encoded symbols as a
digital VSB modulated signal for transmission over a
selected television channel to digital receivers. The
transmitter includes a clock operating at the symbol rate
fs of about 10.76 MHZ so that the digital VSB data is
transmitted at this rate. The receivers receive the
digital VSB data, demodulate the digital vSB data to
recover the symbol clock of about 10.76 MHZ and to
recover the data symbols, and decode and decompress the
data symbols in order to recover the original data for
supply to displays and/or speakers.
In an exemplary digital television environment
for the communication system 10, the upstream source 12
may be a digital VCR, a cable converter, or a satellite
converter, the downstream device 14 may be a digital VCR,
and the downstream devices 16 and 18 may be digital
televisions. However, other digital equipment may be
provided for the upstream source 12 and the downstream
devices 14, 16, and 18.
As shown in Figure 2, the upstream source 12
may include, for example, a processor 30, a remodulator
32, a diplexer 34, a switch 36, and a receiver 38. Thus,
in the case where the upstream source 12 is a digital
converter or a digital VCR, the upstream source 12
receives a plurality of channels occupying the normal
television bandwidth of about 50 MHZ to about 1 GHz and
tunes to a selected one of these channels. Digital VSB
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data in that selected channel are supplied downstream by
the upstream source 12, for example to the downstream
device 18, either in real time or for delayed viewing.
In either case, the remodulator 32 remodulates the
received digital VSB data in the selected tuned channel
to a preselected downstream channel, such as channel 3 or
channel 4, and supplies the remodulated digital VSB data
to the communication medium 20 through the diplexer 34
and the switch 36 for communication to one or more of the
downstream devices 14, 16, and 18. The remodulator 32
also recovers the symbol clock fs from the VSB data that
it remodulates.
In some cases, it is necessary for the upstream
source 12 to receive information from the downstream
devices 14, 16, and 18. Therefore, the upstream source
12 is provided with the receiver 38. The remodulator 32
supplies the recovered symbol clock f5 and an internally
generated frame sync signal FsYrrc to the receiver 38.
Also, the diplexer 34 couples the information that it
receives from one or more of the downstream devices 14,
16, and/or 18 to the receiver 38. The receiver 38
demodulates the received information coupled to it by the
diplexer 34 and uses the clock signal fs to synchronize
this demodulation function to the upstream channel.
The receiver 38 is shown in more detail in
Figure 3 and includes a demodulator 40 and a carrier
recovery module 42. The carrier recovery module 42
receives the clock signal fs from the remodulator 32 and
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applies a multiplier of N/M to it in order to recover the
carrier frequency which is used by the downstream devices
14, 16, and 18 to transmit data into the upstream
channel. The carrier recovery module 42 also receives
the upstream data in order to recover the phase of the
upstream carrier. Thus, the carrier recovery module 42
supplies a synchronizing clock signal to the demodulator
40 so that the demodulator 40 is synchronized to the
upstream carrier in frequency and phase.
As shown in Figure 5, the downstream devices
14, 16, and 18 transmit their information in
corresponding time slots, where the start of each group
of time slots is synchronized to the frame sync (FSYNC)
that is recovered from the downstream data, i.e., the
digital television data transmitted.downstream by the
upstream source 12. The number of time slots in a group
is dependent upon the amount of time allocated to each
time slot. Moreover, the time slots in a group may be
divided between fixed time slots and contention time
slots. The contention time slots may be used by
downstream devices in order to reserve a fixed time slot
in which to transmit a message to the upstream source 12.
Accordingly, when a downstream device has a message to
transmit, it randomly selects one of the contention time
slots. Each of the other downstream devices does the
same. If the reservation request is received by the
upstream source 12 without contention from reservation
requests from the other downstream devices, the upstream
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source 12 assigns a fixed time slot to the downstream
device which then transmits its message in the assigned
fixed time slot. If the downstream device is not
assigned a fixed time slot, it knows that its reservation
request was not received by the upstream source 12
without contention and it will, therefore, try again in a
randomly selected contention time slot. However, the
time slots need not be divided between fixed time slots
and contention slots in this manner and, instead, each
downstream device may be pre-assigned one or more time
slots from each group of time slots.
The data transmitted in a time slot by the
downstream devices 14, 16, and 18 is transmitted as a
data message having the form shown in Figure 4. Each
message has a clock portion (a series of logical "1" bits
which modulate the RF carrier to provide a constant CW
signal), a preamble portion, a data portion, and a CRC
error checking portion.
The data received by the demodulator 40 is in
analog form. A sampler 44 of the receiver 38 samples the
demodulated data at the output of the demodulator 40 in
order to convert the demodulated data to digital form.
The sampler 44 is supplied with a sampling clock signal
by a clock phasing circuit 46. The sampling clock signal
is divided by a divider 43 to provide a frequency that
matches the data clock that is used by the downstream
devices 14, 16, and 18 when transmitting data upstream.
A preamble correlator 48 uses the preamble in the
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upstream data message in order to synchronize the phase
of the sampling clock signal supplied by the clock
phasing circuit 46 to the data clock of the downstream
devices 14, 16, and 18. A time slot generator 45 is
responsive to the frame sync signal FSYrrc to generate a
signal representing the received time slots which are
used by the clock phasing circuit 46 as a window to
facilitate the identification of the preamble in each
received message.
Accordingly, the sampler 44 samples the data in
the upstream data message and supplies the sampled data
to the processor 30. The processor 30 is responsive to
the preamble in each received message for locating the
data in the respective message. Also, the sampler 44
supplies the sampled data to a CRC checker 50 which uses
the CRC portion of the upstream data message in order to
determine the presence of transmission errors.
A portion of a downstream device 60 is shown in
Figure 6. The downstream device 60 is representative of
each of the downstream devices 14, 16, and 18 such that
each of the downstream devices 14, 16, and 18 may be
constructed similarly to the downstream device 60. The
portion of the downstream device 60 shown in Figure 6 is
the portion involved in the upstream transmission of
information to the upstream source 12. The downstream
device 60 includes a diplexer 62, a tuner 64, a VSB
demodulator 66, a downstream processor 68, an upstream
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control processor 70, a phase locked loop 72, and a
transmitter 74.
The diplexer 62 receives digital VSB data that
is transmitted by the upstream source 12 in the
downstream channel. The diplexer 62 couples this digital
VSB data to the tuner 64 which is tuned to the downstream
channel and which supplies the digital VSB data at IF to
the VSB demodulator 66. The VSB demodulator 66 recovers
the symbol clock fs and frame sync FSyNC from the received
digital VSB data. The VSB demodulator 66 supplies the
symbol clock fs to the upstream control processor 70 and
to the phase locked loop 72, and supplies the frame sync
FSYNC to the upstream control processor 70. The VSB
demodulator 66 also demodulates the received digital VSB
data in order to recover data symbols, and supplies these
data symbols to the downstream processor 68.
The upstream control processor 70 decodes any
control information that might be contained in the
downstream data which is demodulated by the VSB
demodulator 66. Such control information, for example,
can be used to set the data rate at which the upstream
control processor 70 supplies data to the transmitter 74,
to set the time slot in which the downstream device 60 is
to transmit upstream data, if any, and/or to set the
value of N/M which is described below.
The upstream control processor 70 receives from
the downstream processor 68 the data that is to be
transmitted in the upstream channel and causes this
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upstream data to be transmitted in a selected time slot.
Accordingly, each of the downstream devices 14, 16, and
18 transmits message in a different time slot.
Alternately, the upstream messages may be transmitted in
contention slots. In order to determine the start of
each time slot group, the upstream control processor 70
receives FgyNC from the VSB demodulator 66 and
synchronizes the beginning of each group with FSYNC-
Moreover, the data clock that the upstream
control processor 70 uses to insert upstream data into a
time slot may be set equal to or may be a fraction of the
symbol clock fs. This fraction, for example, may be
3/50, 1/50 or 1/100 of the symbol clock fs. If a
fraction of the symbol clock fs is to be used, the symbol
clock fs is divided by a divider 71 and the divided
symbol clock is used by the upstream control processor 70
in order to generate the data clock for the upstream
messages.
The phase locked loop 72 produces a carrier
signal having a desired frequency (N/M)fs, where N/M is a
value that is supplied to the phase locked loop 72 by the
upstream control processor 70. Alternatively, one or
more values for N/M may be preset in the phase locked
loop 72 and may be selectively used to generate a carrier
frequency. The phase locked loop 72 supplies this
carrier signal to the transmitter 74 to be used as the
carrier for the upstream channel.
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When the upstream control processor 70
determines the presence of the time slot allocated to the
downstream device 60, the upstream control processor 70
supplies upstream data to the transmitter 74, and at the
same time turns on the transmitter 74. The transmitter
74 then modulates the upstream data onto the carrier
supplied by the phase locked loop 72 so that the upstream
data is transmitted in the upstream channel defined by
the carrier frequency (N/M)fs. The upstream control
processor 70 otherwise maintains the transmitter 74 off
so that the transmitter 74 does not produce babble on the
upstream channel.
As discussed above, the carrier frequency of
the upstream channel is defined by a value related to the
symbol clock fS. This value depends upon the ratio N/M,
where N and M are preferably integers. For example, the
ratio N/M may be 1/3, ~, 1, 3/2, 2, and the like. The
value of N/M may be chosen so that (N/M)fs is below the
normal television bandwidth of about 50 MHZ to about 1
GHz. In this manner the upstream channel is tied to the
symbol clock fs.
The processors 30 and 68 determine, at least in
part, the information that is transmitted between the
upstream source 12 and the downstream devices 14, 16, and
18. The processors 30 and 68, for example, may be
arranged to provide copy protection in accordance with
the so-called 5C copy protection protocol. According to
this protocol, the processor 30 in the upstream source 12
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causes the upstream source 12 to transmit to a downstream
device, such as the downstream device 16 which is to
receive copy protected data, a message requesting the
downstream device 16 to identify itself. The downstream
processor 68 of the downstream device 16 recognizes this
message and, accordingly, causes the transmitter 74 to
transmit the identification of the downstream device 16
in the upstream channel defined by the carrier frequency
(N/M)fs. The receiver 38 receives this message and
passes this message to the processor 30 of the upstream
source 12. If the processor 30 recognizes the
identification of the downstream device 16, the processor
30 causes the upstream source 12 to transmit in the
downstream channel a key which allows the downstream
device 16 to properly decrypt the copy protected material
that is to follow.
The downstream processor 68 of the downstream
device 16 acknowledges receipt of the key by causing the
transmitter 74 to transmit a suitable acknowledgment in
the upstream channel assigned to the downstream device
16. The acknowledgment is received by the receiver 38
which passes the acknowledgment to the processor 30.
Thereafter, the processor 30 causes the upstream source
12 to transmit the copy protected digital VSB data in the
downstream channel to the downstream device 16. The
downstream processor 68 decrypts the copy protected
digital VSB data and supplies this decrypted copy
protected digital VSB data to appropriate video and audio
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processing modules. Accordingly, only an authorized
downstream device may receive and properly display the
video and/or audio contained in the copy protected
digital VSB data.
As described above, the upstream source 12
transmits VSB modulated data to the downstream devices
14, 16, and 18. The downstream devices 14, 16, and 18
may use a different modulation type, however, when
transmitting information in the upstream channels to the
upstream source 12. For example, as indicated by the
drawings, the transmitters 74 of the downstream devices
14, 16, and 18 may be BPSK transmitters which use BPSK
modulation in the transmission of upstream information on
a carrier having a frequency equal to (N/M)fs. If the
transmitters 74 of the downstream devices 14, 16, and 18
are BPSK transmitters, then the receiver 38 of the
upstream source 12 is a BPSK receiver.
The switch 36 is provided so that an external
source, such as an antenna, may be connected directly to
the downstream devices 14, 16, and 18.
Certain modifications and alternatives of the
present invention have been discussed above. Other
modifications and alternatives will occur to those
practicing in the art of the present invention. For
example, the transmitters 74 of the downstream devices
14, 16, and 18 are described above as BPSK transmitters
and the receiver 38 of the upstream source 12 is
described above as a BPSK receiver. However, the
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transmitters 74 of the downstream devices 14, 16, and 18
and the receiver 38 of the upstream source 12 may
implement other types of modulation such as QAM, QPSK, or
PSK.
Accordingly, the description of the present
invention is to be construed as illustrative only and is
for the purpose of teaching those skilled in the art the
best mode of carrying out the invention. The details may
be varied substantially without departing from the spirit
of the invention, and the exclusive use of all
modifications which are within the scope of the appended
claims is reserved.
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