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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital video signal processing methods and
apparatus. Embodiments of the invention are particularly, but not
exclusively, intended for use in broadcasting studios.
2. Description of the Prior Art
A digital slow motion processor comprising a large random access memory
capable of storing several seconds of a digital video signal and operating
cyclically has been proposed by us for use in broadcasting studios. Thus,
by placing the slow motion processor in parallel with a transmission or
recording path, that is between a video camera and a transmitter or a
recorder, the last few seconds of the transmitted or recorded signal is
always available in the memory. In the case, for example, of a sports
broadcast, the input to the memory can be cut at the end of a particular
bit of action, and the slow motion processor can then read out the stored
few seconds of the digital video signal including the bit of action for
repeated transmission or recording. The read-out will usually be at a
non-standard speed, most usually in slow motion, and the slow motion
processor includes adaptive interpolation filters of known form to do the
necessary field rate conversion to provide an appropriate sequence of
fields at the system field frequency for transmission or recording.
The present invention is concerned with an adaptation of the form and use
of apparatus similar to such a slow motion processor to permit cut
editing; that is, to permit omission from transmission or recording of
small unwanted portions of a digital video signal being received from a
live source such as a video camera.
OBJECTS AND SUMMARY OF THE INVENTION
One object of the present invention is to provide a method of processing a
digital video signal whereby a small unwanted portion of the signal can be
omitted without the omission being perceptible to a viewer.
Another object of the present invention is to provide a digital video
signal processing apparatus which can omit a small unwanted portion of a
video signal without the omission being perceptible to a viewer.
According to the present invention there is provided a method of processing
a digital video signal, the method comprising:
storing a segment of an input digital video signal in a random access
memory;
writing said input digital signal in said memory at a standard speed,
writing proceeding cyclically from the start of said memory to the finish
of said memory, then returning to the start of said memory, and so on;
reading said digital video signal from said memory at said standard speed,
reading normally lagging behind writing by an amount substantially equal
to the capacity of said memory;
cut editing the read digital video signal by step reducing said amount by
which said reading lags behind said writing; and
subsequent to a cut edit reducing the reading speed below said standard
speed until the amount by which reading lags behind writing is restored to
said normal amount.
According to the present invention there is also provided a digital video
signal processing apparatus comprising:
a random access memory capable of storing a segment of an input digital
video signal;
means to write said digital video signal into said memory at a standard
speed, writing proceeding cyclically from the start of said memory to the
finish of said memory, then returning to the start of said memory, and so
on;
means to read said digital video signal from said memory at said standard
speed, reading normally lagging behind writing by an amount substantially
equal to the capacity of said memory;
means to cut edit the read digital video signal by step reducing said said
amount by which reading lags behind writing; and
means operative subsequent to a cut edit to reduce the reading speed below
said standard speed until the amount by which reading lags behind writing
is restored to said normal amount.
The above, and other objects, features and advantages of this invention
will be apparent from the following detailed description of illustrative
embodiments which is to be read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the
accompanying drawing, in which:
FIG. 1 shows diagrammatically the writing of a digital video signal into a
memory;
FIG. 2 shows diagrammatically the writing of a digital video signal into a
memory and the reading of the digital video signal from the memory; and
FIG. 3 shows in block diagrammatic form an embodiment of digital video
signal processing apparatus according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing the embodiment, the operation of a memory forming part of
the embodiment will be described with reference to FIGS. 1 and 2. The
memory is a large random access memory (RAM) capable of storing a segment
equal to several seconds, say five seconds, of an input digital video
signal. That is, in a 50 Hertz television system, the RAM can store 125
frames, and in a 60 Hertz television system, the RAM can store 150 frames.
In the embodiment to be described below, an input digital video signal
derived from a live source such as a video camera is continuously written
into the RAM. The writing proceeds cyclically from the RAM start to the
RAM finish, whereupon the writing immediately returns to the RAM start,
and so on. At all times therefore the RAM is storing the last five seconds
of the digital video signal.
This is represented in FIG. 1 where the RAM 1 is indicated as a circular
store and the write position is represented as a vector W rotating with an
angular velocity .omega..sub.W.
Due to the nature of the RAM 1, reading of the digital video signal can be
effected independently of the writing.
This is represented in FIG. 2 where additionally the reading position is
represented by a vector R rotating with an angular velocity .omega..sub.R.
The relative angular velocity .omega..sub..DELTA. between writing and
reading is given by:
.omega..sub.W =.omega..sub.R +.OMEGA..sub..DELTA. (1)
If .omega..sub.R =.omega..sub.W and reading is following writing at an
angular distance .theta., then the delay in seconds in between writing and
reading will be:
.theta.x n/360.degree. (2)
where n is the store length in seconds, five seconds in the present
example. .theta. can be selected to give a delay of any value up to the
store length in seconds, and .theta. is preferably selected to give a
delay of an integral number of frames, although it may be set to give a
delay of an integral number of fields or even samples.
In the embodiment to be described, the RAM 1 forms part of a digital video
signal processing apparatus which is interposed in the signal path between
a video camera and a transmitter or a recorder, and which is operable to
perform live cut editing. That is to say, small unwanted portions of the
digital video signal can be cut from transmission or recording, the cut
being made at least substantially imperceptible to a viewer.
To accomplish this, the RAM 1 is operated with .omega..sub.R =.omega..sub.W
and with .theta. equal to just less than 360.degree.; that is, the maximum
delay, five seconds in the present example, is used. In other words, at
any given instant the oldest frame in the RAM 1 is being read. When the
cut mode is triggered, .theta. is instantaneously changed to just more
than 0.degree. if a cut duration, in the present example, of five seconds
is required, or to the appropriate angle between 360.degree. and 0.degree.
if a cut of some lesser duration is required. Immediately thereafter
.omega..sub.R is changed to .omega..sub.R -.omega..sub..DELTA., where
.omega..sub..DELTA. is a predetermined relative angular velocity, the
value of which is selected to cause .theta. to increase back to just under
360.degree. over a suitable time interval, which may, for example, be
selected to be two minutes. .omega..sub.R remains unchanged throughout.
During the time interval while .omega..sub.R is equal to .omega..sub.R
-.omega..sub..DELTA., the digital video signal will be read from the RAM 1
at a non-standard speed equal to:
(.omega..sub.R -.omega..sub..DELTA.) normal speed/.omega..sub.R (3)
and to lock the read digital video signal to the system synchronizing
signals it must be frame rate converted by adaptive interpolation filters
of known form.
When .theta. again becomes equal to just less than 360.degree., the reading
speed reverts to .omega..sub.R, so as again to become equal to the writing
speed .omega..sub.W. If a further cut edit is required before this state
is reached, then the duration of the cut is limited to that proportion of
the store length represented by the current value of .theta..
Referring now to FIG. 3, the embodiment of digital video signal processing
apparatus comprises the RAM 1, which receives the input digital video
signal in the form of write data over a write bus 2. The write data is
assumed to have been derived from a live source by a video camera and an
analog-to-digital converter generally identified in FIG. 3 as a source 3.
Writing in the RAM 1 is controlled by write addresses supplied over a
write address bus 4 from a write address counter 5. The write address
counter 5, like the source 3, is locked to system synchronizing signals,
and writing in the RAM 1 proceeds continuously, and cyclically, at a real
time constant speed which is determined by the system synchronizing
signals.
Read out of the digital video signal is under the control of read addresses
supplied to the RAM 1 from a read address counter 6, over a read address
bus 7. The read digital video signal is supplied over a read data bus 8 to
adaptive interpolation filters 9. The read addresses supplied by the read
address counter 6 are field-based, that is to say, they cause reading by
the RAM 1 of the data relating to the field nearest to the field
addressed. In normal operation, when reading is proceeding at the same
speed as writing, this means that a complete sequence of fields is read in
the correct order from the RAM 1, and these fields are passed unchanged by
the interpolation filters 9 to an output 10 which may be a
digital-to-analog converter and a transmitter, or a recorder.
The apparatus further comprises a cut and field select controller 11 which
supplies read address controls to the read address counter 6, and field
polarity information to the interpolation filters 9. Finally, an output
speed selector 12 is provided to control the cut and field select
controller 11.
The operation is as follows. When a cut edit is to be effected, the cut and
field select controller 11 is controlled to cause the read address counter
6 to jump the read addresses by the required amount, that is, in the
terminology of FIGS. 1 and 2, to change .theta., usually, but not
necessarily, to just more than 0.degree.. Thus, the unwanted segment is
cut from the digital video signal read out over the read data bus 8 and
supplied to the interpolation filters 9. Immediately thereafter the output
speed selector 12 controls the cut and field select controller 11, which
in turn controls the read address counter 6 to slow the reading speed
until reading again lags behind writing by the required amount, normally
the full capacity of the RAM 1, or, in the terminology of FIGS. 1 and 2,
until .theta. is again equal to just less than 360.degree..
The rate at which this is done may be fixed by the output speed selector
12, or the output speed selector 12 may include a variable control
permitting the rate to be varied; but the rate will normally be selected
to be such that the effect is imperceptible to a viewer, and it may, for
example, be arranged that reading from the RAM 1 is restored to the normal
condition over a time interval of two minutes. During this interval, the
read data bus 8 will be supplying fields of the digital video signal to
the interpolation filters 9 at a non-standard rate, while the cut and
field select controller 11 will be supplying field polarity information to
the interpolation filters 9. During this interval, therefore, the
interpolation filters 9 are operative to interpolate fields as necessary
to provide a continuous sequence of frames, at the frequency determined by
the system synchronizing signals, to the output 10.
The times and frequencies above are, of course, given purely by way of
example.
Although illustrative embodiments of the invention have been described in
detail herein with reference to the accompanying drawings, it is to be
understood that the invention is not limited to those precise embodiments,
and that various changes and modifications can be effected therein by one
skilled in the art without departing from the scope and spirit of the
invention as defined by the appended claims.
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