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Description  |
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FIELD OF THE INVENTION
The present invention relates to the field of video signals and more
particularly to a method and apparatus for encoding and decoding a video
signal having multi-language capabilities.
BACKGROUND OF THE INVENTION
In places where multiple languages are spoken, there is a need to provide
movies, prerecorded television programs, educational videos as well as
many other forms of video/audio programming in multiple languages to
accommodate the multi-lingual viewing audience. Most often it desired to
have the capability to play back the multiple languages simultaneously
with a single video signal. One technique used to achieve this result is
to use different viewing channels for the different channels. A separate
prerecorded video tape is generated for each language. Each tape contains
the same video information but necessarily has a different audio signal
for each desired language. The tapes are individually played and
broadcasted on different channels. The viewer selects the channel
according to the language he wishes to hear. This method is quite
obviously inefficient as it requires a number of video playback means and
available television channels or frequencies equal to the number of
languages in question.
Briggs, U.S. patent application Ser. No. 07/411,214 entitled Multi-Language
Recording and Reproducing System describes a multiple language system
wherein a plurality of audio tracks, each representing a different
language, are encoded into the video signal using frequency modulation
techniques. However, that method is disadvantageous because it employs
analog techniques which compromise video quality and produce an encoded
signal incompatible or marginally compatible with several classes of
important communications channels.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a video
system in which multiple languages can be encoded within the bandwidth of
a standard video signal.
It is another object of the present invention to provide a multiple
language system that can operate in conjunction with existing videotape
formats and existing videotape players without modification of either the
format or the player.
It is another object of the present invention to provide a multiple
language broadcast system which provides a selectable multi-lingual
capability without modification of the bandwidth of the broadcast signal.
It is further an object of the present invention to provide a mulitple
language broadcast system which provides selectable multi-lingual
capability with little or no degradation in video quality or sound
quality.
An audio/video system for encoding and decoding a video signal and a
plurality of corresponding audio signals is disclosed. The system includes
an encoder means for receiving an input video signal, a first audio
signal, and a plurality of second audio signals which correspond to the
input video signal. The encoder means combines the input video signal and
the plurality of second audio signals to produce an encoded video signal
but leaves the first audio signal substantially unchanged and then outputs
the first audio signal and the encoded video signal. An audio/video signal
communication means is connected to the encoder. The audio/video signal
communication means accepts the encoded video signal and the first audio
signal and transmits them to a decoder. The decoder is coupled to the
signal communication means and accepts the encoded video signal and the
first audio signal. The decoder means then separates the plurality of
second audio signals from the encoded video signal leaving an unencoded
video signal. The decoder then combines each of the plurality of second
audio signals with a reproduction of the firt audio signal so as to form a
plurality of combined audio signals. The decoder then outputs the
unencoded video signal and the plurality of combined audio signals. A
signal delivery means which is coupled to the decoder includes a selection
means. Selection means allows at least one of the plurality of the
combined audio signals to be played with the unencoded video signal in a
manner determined by the user.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the components of the present system, showing
their respective input/output signals.
FIG. 2 is a block diagram of the encoder element of the present invention.
FIG. 3 is a block diagram of the decoder element of the present invention.
FIG. 4 is a block diagram of the preferred embodiment of the delivery
system of the present invention in conjunction with the decoder element.
FIG. 5 is a block diagram of an alternative embodiment of the delivery
system.
DETAILED DESCRIPTION OF THE INVENTION
An audio/video system with multi-language capability for encoding a video
signal is described. In the following description, numerous details such
as sampling rates and signal levels are set forth in order to provide a
better understanding of the present invention. It will be apparent to
those skilled in the art that the invention may be practiced without these
specific details. In other instances, well known components such as the
phase locked loop circuit are shown in block form so as not to obscure the
present invention unnecessarily.
In order to better understand the operation of the present invention, it is
helpful to briefly describe the manner in which a video signal is
reproduced on a video monitor. The composition of the video signal is
governed by industry standards--e.g., those established by the National
Television System Committee. ("NTSC"). Under the NTSC standard, the video
screen is divided up into 2621/2 separate horizontal lines of picture
elements. The electron beam which is used to actually generate the picture
by exciting the picture elements which causes the picture elements to be
illuminated at a predetermined intensity scans the lines in a left to
right, top to bottom fashion. As the electron beam moves across the video
screen it is modulated in intensity so as to modulate the intensity of the
picture elements and produce the video picture that is visible to the
user.
Timing pulses are embedded within the NTSC video signal to control the
operation of the electron beam. This is best explained by an example.
Assume that the electron beam is horizontally scanning across one of the
525 lines on the video monitor. Just as the beam reaches the right edge of
the screen, a timing pulse in the video signal will indicate to the beam
that it is to stop scanning. This timing pulse is known as a "horizontal
retrace" pulse. The beam will briefly turn off and execute a horizontal
retrace function. During this function, the electron beam will drop down
to the next lower scan line and return to the left side of the video
screen. After the horizontal retrace, the beam will turn back on and
re-commence scanning. This process repeats until all 2621/2 lines of the
video screen have been scanned by the electron beam. When the complete
screen has been scanned, a "vertical retrace" timing pulse is inserted
into the video signal. The electron beam is again briefly turned off and
executes a vertical retrace function, when it returns to the beginning
point at the top scan line of the video screen. In this manner, the entire
screen is continuously updated so as to form the entire video picture.
This discussion is not intended as a complete description of the operation
of the video monitor. Instead, it is included only to the extent that it
assists in the understanding of the structure and operation of the present
invention. Also, the present invention is not limited to use with the NTSC
standard for the composition of video signals. It will be apparent to
those skilled in the art that the techniques of the present invention may
be adapted for use with any sequential video format. These may include the
formats which are known in the art as PAL, SECAM, BMAC and the like. In
any event, the video monitor is not an element of the present invention.
The invention consists of the system for encoding a video signal as
described.
Referring first to FIG. 1, a block diagram of the preferred embodiment of
the major components of the present invention and their corresponding
input and output signals is shown. The system includes an encoder element
10 which accepts a standard NTSC compatible signal. Any NTSC compatible
signal has two components--the video portion and the audio portion. The
encoder element 10 therefore accepts an input video signal 13, as well as
a first audio signal which is termed herein the as the "music and effects"
signal 14. The standard NTSC signal provides two dedicated, high
bandwidth, channels for audio information. These two channels typically
correspond to the left and right stereo channels of the complete audio
signal. The stereo channels are usually timed to be in synchronization
with the video portion of the NTSC signal. In this description, these two
channels collectively make up the music and effects signal 14.
It has been found that the language portion of any given audio signal does
not occupy a large bandwidth. For example, a complete audio signal will
typically occupy a signal range from 0-20,000 Hz. However, the language
portion will usually occupy only a 4 kHz portion of that total signal
bandwidth. The rest of the bandwidth is occupied by other sounds such as
music, background noises, and the like.
As noted above, the language portion of the audio signal has a relatively
narrow bandwidth. The audio/video system of the present invention takes
advantage of the narrow bandwidth of the language portion of the audio
signal to combine the audio information on the auxiliary language tracks
15 with the video signal. The music and effects channel 14 contains all of
the remaining audio information that would normally accompany the video
signal.
The language portion of the audio signal is input through the auxiliary
audio channels 15. In the preferred embodiment there are twelve such
channels. In the preferred embodiment, these signals correspond to the
left and right stereo channels of complete audio signal for six different
languages. While the preferred embodiment inputs multiple language tracks
in stereo through the auxiliary channels 15, it will be understood by
those skilled in the art that other information can be provided through
these channels without departing from the overall scope of the present
invention. For example, there may be twelve monaural language signals or
some combination of stereo and monaural signals. Alternatively, the
auxiliary channels may be used to transmit other types of information
through the video format of the present invention. All of the auxiliary
audio channels in the preferred embodiment are timed to be in
synchronization with the video portion of the NTSC signal. It will be
apparent to those skilled in the art that this synchronization is not
required. Depending on the type of information which is being transmitted,
the synchronization may or may not be desirable. In the preferred
embodiment, the synchronization is accomplished by ensuring that the input
video signal and the auxiliary channels are in synchronization as they are
being input to the encoder. It will be apparent to those skilled in the
art that various time delay features could be built into the encoder so
that the input signals do not need to be in exact synchronization when
they are input. The nature and structure of these time delay features will
be apparent to those skilled in the art from reading the following
description of the encoder element.
The encoder element 10 combines the input video signal 13 with the
auxiliary audio channels to form an encoded video signal 16 which is NTSC
compatible. (For commercial reasons it is desirable that all video signals
be compatible with a particular standard. This allows the signal to be
used in conjunction with existing video equipment. A standardized signal
obviates the need to purchase specialized equipment.) The combination of
the audio and video signals is accomplished by "overwriting" the language
tracks from the auxiliary channels 15 onto preselected portions of the
input video signal 13. The music and effects audio signal 14 is simply
passed through the encoder element 10 without any substantial change. All
of the signals are then output by the encoder element 10. The structure
and operation of the encoder element 10 is described in more detail below
with respect to FIG. 2.
The output of the encoder element 10 is passed to an audio video signal
communication means 11 as shown in FIG. 1. The function of the signal
communication means 11 is to transfer the encoded video signal 16 and the
music and effects audio signal 14 from the encoder element 10 to the
decoder element 12. Additionally, the signal communication means may store
the signals for use at a later time. In the preferred embodiment, the
signal communication means 11 is a standard video cassette recorder
("VCR"). The VCR allows the output of the encoder element 10 to be
recorded for playback at any desired time. It will be apparent to those
skilled in the art, however, that many other devices can be used with
equivalent results. For example, the signal communication means may simply
comprise a fiber optic transmission cable or microwave transmitter for
transmitting the signals in real time to a distant location.
Alternatively, the content of the audio and video signals may be digitized
and stored in a binary computer memory for reproduction at a later date.
These and other equivalent means may be substituted for the VCR, as will
be apparent to those skilled in the art.
Referring again to FIG. 1, the present invention also includes a decoder
element 12. The function of the decoder element is to transform the
encoded video signal 16 into a signal which allows the information
contained on any or all of the auxiliary audio channels 15 to be played in
conjunction with the input video signal 13. As noted above, the decoder
element 12 accepts the audio and video signals from the signal
communication means 11. The decoder element 12 then removes the language
tracks from the encoded video signal 16. The decoder element 12 duplicates
the music and effects signal 14. Each of the language tracks are combined
with a reproduction of the music and effects signal 14 so as to form a
number of composite audio signals 18. These combined audio signals 18 are
high bandwidth signals. They are output by the decoder element for
eventual use by a user. The structure and operation of the decoder element
is described in more detail below with respect to FIG. 3.
Referring next to FIG. 2, a block diagram of the encoder element 10 is
shown. As discussed above, the encoder element 10 accepts three separate
groups of input signals. These are the input video signal 13, the
auxiliary audio channels 15, and the music and effects channel 14. As
shown in FIG. 2, the music and effects signal is passed through the
encoder without any substantial change. The video signal 13 is divided
into two components. The first component passes through amplifier 31. The
second component enters element 32. Element 32 is termed a synchronization
stripper in the preferred embodiment because it separates the
syncrhonization (timing) pulses from the video signal. These timing
signals are used to direct the horizontal and vertical retrace functions
of the electron beam as was described above. The output of the
synchronization stripper 32 is used to generate a system clock 40 for the
encoder element 10. The system clock is made up of the phase locked loop
33 ("PLL") and the voltage controlled oscillator 34 ("VCO"). The operation
of the PLO/VCO combination is well-known to those skilled in the art and
the operation of the clock 40 will therefore not be described in detail
herein. It is only necessary that the clock generate a series of timing
pulses at a desired frequency rate. The output of the synchronization
stripper 32 is transferred via signal line 41 to the timing logic and
address generator means 35. The output of the clock 40 is also applied to
the timing means 35 over signal line 42. The timing logic and address
generator means 35 uses these two signals to control the operation of the
digitial memory 39 as will be described more fully below.
The timing logic and address generator means 35 controls the overall
operation of the encoder 10. It is chosen from any one of a number of
suitable devices which have the capability to control the operation of
several digitial components simultaneously. In the preferred embodiment,
the timing logic means 35 is a custom made programmable gate array
("PGA"). The PGA is chosen for the preferred embodiment because it allows
high speed operation with a low manufacturing cost. It will be apparent to
those skilled in the art that other, equivalent, devices can be used with
equal effectiveness. For example, a digital microcomputer could replace
the PGA and still effectively control the encoder 12.
The auxiliary audio channels 15 are input in a parallel fashion to the
multiplexer 37. The multiplexer 37 is essentially a digital switch. It
uses time division multiplexing techniques in order to sequentially sample
each of the auxiliary audio channels. It is well known to those skilled in
the art that a minimum sampling rate will allow each of the auxiliary
audio channels to be multiplexed onto a single line without the loss of
any of the information contained therein. This minimum sampling rate is
known as the "Nyquist" rate. For any given analog signal the Nyquist rate
is equal to twice the bandwidth of the signal. As noted above, the
auxiliary audio channels will--in the preferred embodiment--contain
language information. These signals typically have a bandwidth of
approximately 4 kHz. Therefore, the Nyquist sampling rate for these
signals is 8 kHz. It has been found that the construction of filters and
other electric components is simplified when the sampling frequency of the
signal which is being processed is increased. Therefore, in the preferred
embodiment of the present invention, the auxiliary audio channels 15 are
sampled by the multiplexer 37 at a frequency rate of 15.75 kHz. It will be
apparent to those skilled in the art that this particular frequency is an
arbitrary design choice and that other frequencies can be used with equal
effectiveness. The timing details of the sampling rate are controlled by
appropriate clock pulses generated by the timing logic and address
generator means 35.
After the audio channels are sampled, they are converted to a digital
format by the analog to digital converter 43. This type of analog to
digital converter is well known to those skilled in the art and will not
be described in detail herein.
The output of the analog to digital converter 43 is transferred over signal
line 44 and stored in digital memory 39. Digital memory 39 is partitioned
into a number of sections. (Those partitions are not necessarily physical
in nature, but instead refer to the fact that the memory stores several
discrete groups of data.) The number of partitions is equal to the number
of auxiliary audio channels 15 which are input into the encoder 10. Thus,
in the preferred embodiment of the invention, there are 12 separate
partitions of the memory 39. The input stream from each auxiliary audio
channel is sequentially stored within its corresponding partition of the
memory 39. The timing logic and address generator means 35 generates an
address signal over address lines 45 which instructs the memory where to
store each bit of information from analog to digital converter 43. Timing
is controlled by the timing signals generated by the system clock 40 over
signal line 42. It is the information in memory 39 which is combined with
the input video signal 13 so as to form the encoded video signal 16.
The present invention encodes the audio information onto the video signal
by selecting a portion of the video signal which corresponds to certain
scan lines on the video display and replacing the existing video
information with desired audio information. In this description, when
information is described as being written onto (or read from) a given scan
line, it is to be understood that the audio is being combined with (or
being removed form) that portion of the video component of the signal
formal which corresponds to the particular scan line in question. The
information is not actually "written onto" a scan line; it is simply
inserted into the video signal at the proper time.
In the preferred embodiment of the invention, a preselected number of scan
lines located at the top portion of the video pictures are used to store
audio information. These specific lines are chosen for a variety of
reasons. Most importantly, when the video signal is displayed, these lines
cannot be seen by a user. Thus, there will be no loss of picture quality
and the user will not notice that the audio information has been encoded
onto the video signal. Of course, any of the other scan lines can be used
to store the audio information. In particular, the bottom scan lines can
be used because they too are not seen by the user when the picture is
displayed.
In the preferred embodiment of the invention, one entire scan line is
dedicated to each separate auxiliary audio channel. Thus, in the preferred
embodiment of the invention, the portion of the video signal corresponding
to the top 12 lines of the video picture are encoded with audio
information.
The operation of the actual encoding process is as follows. After the
electron beam executes a vertical retrace it will begin to scan the first
line of the video signal. The synchronization stripper 32 separates the
vertical retrace signal from the overall video component and transmits it
to the timing logic means 35 over signal line 41. The timing means 35 will
recognize the vertical retrace signal and instruct the memory 39 to output
the desired information for the first auxiliary audio channel. To do so,
it sends a signal over read/write line 46 instructing the memory to write
data, and transmits the proper address for the initial location in the
first partition of the memory over address line 45. Memory 39 then outputs
(in proper sequential format) the information which had been previously
stored for a first auxiliary audio channel.
The digital information from memory 39 is transmitted to digital to analog
converter 38 where it is converted back into an analog signal. This analog
signal is then passed to combiner 36 which writes the audio information on
top of the existing video signal. In this way, the audio information
replaces the existing video signal.
While the memory 39 is writing the data for the first scan line, the
electron beam is travelling across the scan line. As the electron beam
reaches the end of the scan line, a timing pulse in the video signal 13
will cause it to execute a horizontal retrace function. The
synchronization stripper 32 separates the horizontal retrace pulse from
the video signal and transmits it to the timing logic means over signal
line 41. The timing logic means recognizes the horizontal retrace pulse
and instructs the memory to cease writing the information for the first
auxiliary channel and to start writing the information for the second
audio channel. As the electron beam drops down one line and begins to scan
the second horizontal scan line, the memory will output the information
stored in the partition of the memory corresponding to the second audio
channel. The writing of data is accomplished as described above. This
process repeats for each of the auxiliary audio channels. The timing logic
and address generation means 35 will count the number of horizontal
retraces that are executed. After the twelfth horizontal retrace (in the
preferred embodiment) the digital memory 39 will cease outputting data and
will simply record the information from analog to digital converter 43 in
the appropriate partition of the memory.
As noted above, the present invention allows the audio channels to be fully
synchronized with the video signal. This contemplates that a separate real
time audio signal will be input over each of the auxiliary audio channels
15. The input of this information will of course continue even though the
information contained in that audio signal is not being written to its
corresponding horizontal scan line on the video signal at any given time.
In order to achieve the synchronization capability, the data is
transferred out of memory at a rate much higher than the rate at which it
is stored. Thus, the audio information for a particular auxiliary audio
channel is stored into the memory while the electron beam is scanning the
remainder of the video screen. It is only during the time that the
electron beam traces the horizontal scan line corresponding to the
auxiliary audio channel that the information is read from the memory and
placed onto the video signal.
Because there a large number of horizontal scan lines on a standard video
picture (2621/2 in the preferred embodiment), the audio information is
transferred out of the memory in high speed bursts at a rate greater than
that at which it is read into the memory. This effectively establishes a
data compression scheme which allows all of the audio information for each
auxiliary audio channel to be placed onto the video signal without any
loss of information.
Referring next to FIG. 3, a block view of the components of the decoder
element 12 is shown. The decoder element 12 accepts the encoded video
signal 16 from the signal communication means 11 (not shown in FIG. 3).
The encoded video signal is split into 3 components. The first component
is applied to amplifier 61. The second component is applied to analog to
digital converter 50 and the third component is applied to element 51.
As with the encoder element 10, the element 51 is termed a synchronization
stripper because it separates the synchronization (timing) pulses from the
video signal. A PLL 52/VCO 53 combination acts as a system clock 60. This
system clock is similar in operation to the system clock of the encoder
element and a description of its operation will not be repeated here. The
system clock provides a series of timing pulses at the desired frequency
to the timing logic and address generator means 55.
As noted above, the first component of the encoded video signal is
transmitted to amplifier 61. The encoded signal passes through the
amplifier and reaches component 54. Component 54 is a line blanking means.
The line blanking means is used to insert a null signal into those
portions of the video signal where the audio information had been
overwritten by the encoder 10. When the video picture is displayed, this
portion of the signal will appear to be a blank (black) line. The line
blanking element only inserts a null signal to those particular scan lines
which correspond to the auxiliary audio tracks 15. The timing logic means
35 is used to control the operation of the line blanking means 54. The
timing and logic means 55 counts horizontal retrace signals in the encoded
video signal and outputs a control signal over signal line 63 at the
appropriate times.
The output of the line blanking means 54 is the unencoded video signal 14.
This video signal is output to a user and can be viewed by any compatible
video equipment. The unencoded video signal 14 is identical to the input
video signal 13 with the exception that the top twelve scan lines (in the
preferred embodiment) will now appear to be black to a viewer. As
explained above, however, these blank lines do not present a serious
problem, because they are normally "lost" in the display process.
The remaining structure of the decoder element 11 is directed towards
removing the audio information from the encoded video signal and combining
it with the music and effects signal 14 so as to form a complete audio
signal. As noted above, one component of the video signal is applied to
analog to digital converter 50. The function of the analog to digital
converter 50 is to convert the audio information which is encoded onto the
video signal into a digital format. Because the encoded audio information
is only present on the uppermost scan lines, this element does not operate
continuously. Analog to digital converter 50 is normally turned off and
does not transmit information. It is turned on by the timing means 55 only
when the portion of the video signal corresponding to the appropriate scan
lines is being input to the decoder.
The timing logic and address generator means 55 is similar in operation and
construction to the timing logic and address generator means 35 which is
present in the encoder element. Thus, in the preferred embodiment it is a
PGA. Of course, other equivalent devices can be substituted, as was
described with respect to the encoder. The programming of the timing means
55 will be slightly different for the decoder than for the encoder.
However, the specific details are well understood by those skilled in the
art. The timing means 55 counts horizontal and vertical retrace signals
which are present in the video signal and uses them as the basis of
operation for its controller functions. Thus, after the electron beam
executes a vertical retrace function, the timing means turns on the analog
to digital converter 50. The analog to digital converter remains on until
the electron beam has scanned twelve (in the preferred embodiment)
complete lines. At that time the timing means 55 will send a signal to the
analog to digital converter, instruction it to turn off.
The output of the digital to analog converter 50 is transferred over signal
line 64 to the digital memory 56. Like the memory 39 in the encoder,
memory 56 is divided into a number of partitions. The number of partitions
is equal to the number of auxiliary audio channels 15--in the preferred
embodiment twelve. As the digital audio information in input over signal
line 64 it is sequentially stored in the appropriate partition of the
memory 56. The operation of the read and write functions of the memory is
controlled by signals from the timing means transmitted over read/write
line 67. The timing logic and address generator means also transmits the
appropriate address locations for where the data is to be written to, or
read from, over data bus 65.
The audio information is written into the memory at a high rate of speed
and is read therefrom relatively slowly. This is, of course, directly
opposite to the operation of the encoder. Such symmetry would be expected
because the decoder 12 is unpacking the audio information which was
compressed by the encoder 10. The output of the memory is passed over
signal line 66 to digital to analog converter 57. Digital to analog
converter converts the audio information back into an analog format. From
there it is passed to the demultiplexer 58.
Demultiplexer 58 converts the serial output stream from the digital to
analog converter 57 and memory 56 onto a group of serial output signals.
These output signals are substantially identical to the auxiliary audio
channels 15 which were input to the encoder 15. Thus, at this point the
decoder element 12 has succeeded in separating the auxiliary audio
channels from the encoded video signal. Since the auxiliary audio signals
are not a complete audio signal, however, the reconstruction of the audio
signal is not complete. The digital to analog connector 58 therefore
transmits the auxiliary audio channels in parallel format over the audio
bus 68 to the audio matrix 59.
The function of the audio matrix 59 is to combine the auxiliary audio
channels with the music and effects signal 14 so as to form a plurality of
composite audio signals 18. Because these signals contain all of the
information in the audio signal, they are high bandwidth signals. A user
may then listen to any one of these complete audio signals while viewing
the output video signal 14. The operation of the audio matrix will first
be described in conjunction with the preferred embodiment.
In the preferred embodiment the twelve auxiliary audio channels correspond
to the left and right channels for six different languages. The music and
effects signal is therefore reproduced six times by the audio matrix
59--once for each language. The left channel of the first language is
combined with the left channel of the music and effects signal. The right
channel of the first language is combined with the right channel of the
music and effects signal. This forms a complete stereo signal for the
first language. The process is repeated for languages two through six. In
this way six pairs of composite audio programs 18 are formed. Each pair of
composite audio signal 18 in the preferred embodiment will necessarily be
slightly different from each other.
It will be apparent to those skilled in the art that various different
structures for the audio matrix 59 may be employed, depending on the type
of information encoded onto the video signal and the desired output
format. For example, auxiliary audio channels may not contain stereo
information, but may be recorded in monaural instead. Obviously in such a
case there will be no need to form complete stereo signals with the music
and effects signal 14. As noted above, the auxiliary audio channels may
input digital, numeric, information to be encoded onto the video signal.
In that case it may not be desirable to combine the music and effects
signal 14 with each of the auxiliary channels. The audio matrix 59 may
then be constructed to provide such a signal directly to the user. These
and other modifications to the audio matrix will be appreciated by those
skilled in the art.
The audio programs in the preferred embodiment are designed to be in
complete synchronization with the video portion of the signal. Therefore
all of the duplication and combination functions of the audio matrix are
performed in real time. The exact structure of the audio matrix is not
described in detail here. However its construction is relatively
straightforward and will be known to those skilled in the art. The
structure of the audio matrix may include a resistor network to duplicate
and combine the audio signals. Transistors may be used to amplify signal
levels as required. The audio matrix 59 may be constructed as some form of
custom integrated circuit.
The output of the decoder 12 is fed into the delivery system 70. Although
the delivery system 70 is not illustrated in FIG. 1, the structure of the
preferred embodiment is shown in FIG. 4. The function of the delivery
system is to allow a user to select any | | |
