Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1997-09-16
2001-04-17
Olms, Douglas (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S385000, C370S465000
Reexamination Certificate
active
06219359
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the transmission of digital signals such as digitally encoded video signals and, more particularly, to the simultaneous transmission of a number of different signals, from different sources, via a single transmission medium.
BACKGROUND OF THE INVENTION
It is well known to multiplex a plurality of signals for transmission via a single transmission medium. In particular for digital systems, this is done by assigning a predetermined proportion of the total bitrate available in the transmission medium to each of the signals that it is desired to transmit. The proportion of the available bitrate assigned to each signal is fixed in each system.
These types of systems are often referred to as Variable Quality Fixed Bitrate systems;
FIG. 2
presents an illustration of such a system. The
FIG. 2
system includes a number of encoders
20
,
20
1
,
20
2
. . .
20
n
connected to a Multiplexer (MUX)
22
. A video signal is transmitted through each of the encoders such that a plurality of fixed bitrate signals
24
,
24
1
,
24
2
. . .
24
n
are transmitted to the MUX
22
. Each video signal, by way of typical example, constitutes a different television channel. A Multiplex Control Computer (MCC)
26
controls the output from the MUX so that a satellite signal
28
is generated which includes all of the television channels. The satellite signal is then transmitted, via a satellite (not shown), to a receiver station (not shown) which receives the satellite signal and decodes it back into the plural different television channels. The full details of the decoding operations do not form a part of the present invention and are accordingly not described in detail herein.
One problem with such systems is associated with the fact that, in practice, some video pictures require a much higher bitrate than others to achieve acceptable picture quality. In the above-mentioned arrangement, for example, it may be that the signals from some picture sources or television channels cannot be properly encoded using the available bitrate, while others do not require all of the bitrate capacity that is available to them.
In the conventional Fixed Bitrate, Variable Quality system, encoders are allocated a bitrate by the MCC as described above. This bitrate is then fixed for the duration of the MCC schedule. Thus, an encoder in the fixed bitrate system cannot respond to changes in criticality of the source video, as a result of which the picture quality of the decoded output of a fixed bitrate encoder varies with the criticality of the picture material. As the picture material becomes more demanding (i.e. contains more motion and complexity), the picture quality will decrease and the decoded output will contain more encoding artifacts. This is clearly undesirable.
One object of the present invention is to therefore provide a system or arrangement which overcomes at least some of the limitations of known systems.
SUMMARY OF THE INVENTION
In general terms, the present invention provides a method of transmitting a plurality of digital signals through a plurality of channels, the channels having a predetermined total bitrate allocated to them, and the method including the steps of encoding each digital signal, multiplexing the encoded signals, adjusting the distribution of the bitrate allocation between the channels in response to a control system, providing an indication of a target quality for each channel, providing an indication of the actual quality of each channel, and repeatedly adjusting the distribution of the bitrate allocation in response to differences between the indicated actual quality and the indicated target quality for each channel so as to tend to equalize the difference between the actual and target quality across some or all of the channels.
Preferably, the invention further includes the steps of grouping the channels into groups, assigning a bitrate to the group, and varying the individual bitrates of each channel in the group such that the total bitrate of the group approximates the assigned bitrate.
Preferably, and advantageously, the encoding step comprises encoding the digital signals with an encoder having an adjustable bitrate.
Advantageously, the digital signals are digital television signals provided as a data stream representing a plurality of frames, the adjustment of the bitrate is carried out as often as possible (but preferably once every frame), and the actual and target qualities are the actual and target picture qualities.
The present invention thus aims to provide an improved multiplex arrangement, overcoming or at least effectively minimizing the problem in prior art systems as described hereinabove. In principle, by grouping encoders together in what will be termed herein “statistical multiplex groups”, and making real time decisions about the bitrate requirements for those encoders, bitrate can be dynamically allocated to maximize picture quality for the group.
The theory is that for a variety of different picture sources in a statistical multiplex grouping, to achieve a target picture quality the bitrate requirements of each will vary with coding difficulty. Thus, a channel within the statistical multiplex group that is experiencing little difficulty in encoding its picture can free bits to channels that are having greater difficulty. The effect is to smooth the picture quality and to subjectively improve it.
In a preferred arrangement, the adjustment of the bitrate of each encoder is determined in relation to an achievable picture quality, the ratio of the achievable picture quality to the target picture quality being the same for each channel.
In accordance with the present invention, there is also provided a transmitter for transmitting a plurality of digital signals through a plurality of channels, the channels having a predetermined total bitrate allocated to them, and including a plurality of encoders (one associated with each channel), a multiplexer for receiving the encoded digital signals and transmitting them as a stream of data and capable of adjusting the distribution of the bitrate allocation between the encoded signals, and processing means for providing an indication of a target quality and an actual quality for each channel and causing the multiplexer to repeatedly adjust the distribution of the bitrate allocation in response to differences between the indicated actual quality and the indicated target quality for each channel so as to tend to equalize the difference between the actual and target quality across some or all of the channels.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
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patent: 4864562 (1989-09-01), Murakami et al.
patent: 5123015 (1992-06-01), Brady, Jr. et al.
patent: 5136587 (1992-08-01), Obana et al.
patent: 5216503 (1993-06-01), Paik et al.
patent: 5506844 (1996-04-01), Rao
patent: 5619337 (1997-04-01), Naimpally
patent: 5708664 (1998-01-01), Budge et al.
patent: 5959677 (1999-09-01), Date et al.
patent: 5963256 (1999-10-01), Tahara
Budge Michael Richard Joseph
Jordan Jonathan Paul
Stedman Robert Julian
Ladas and Parry
Olms Douglas
Tanberg Television ASA
Vanderpuye Ken
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