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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a novel method and apparatus for the
simultaneous transmission of audio and video information over a medium
with restricted bandwidth, and more particularly, it relates to the
transmission of digitized video images in part of the band of such media,
while voice or other audio information is simultaneously transmitted
through another portion of the band.
It is well known that standard NTSC video images can be sent on coaxial
cable, optical fiber, earth satellite, and microwave links. Such links
have pass bands in the megahertz to gigahertz range, and so are capable of
transmitting a 4.5- or 6-MHz signal required for NTSC video (or even
greater bandwidths for SECAM and HDTV) and/or audio and video transmission
in real time.
Such transmission is relatively costly because of the bandwidth
requirements and the concomitant need for expensive cabling. For various
historical and practical reasons, the conventional telephone "loop" or
connection from a local subscriber to a central office is by means of a
twisted-wire pair. The telephone system deliberately utilizes an overall
band width of about 4 kHz, and in practice the pass band utilized
encompasses from about 100 to about 3500 hertz. This is considered
adequate, not only to convey the meaning of speech, but also to provide at
least some of the particular timbre or character of a speaker's voice so
that individual voices can frequently be recognized.
Amateur radio has available a relatively limited bandwidth, but over the
years radio amateurs have successfully transmitted video images with a
technique called "slow-scan television". A number of these techniques are
described, for example, in Ingram, "The Complete Handbook of Slow-Scan
TV", Tab Books, 1977. These techniques have enabled the transmission of
images over distances of three-quarters of the earth's circumference,
without satellite or forward-scatter techniques. Moreover, the
simultaneous transmission of video and audio signals through ordinary
telephone lines has been shown in U.S. Pat. No. 3,873,771. The method of
the patent utilizes an FM slow-scan video signal and an AM single side
band signal occupying the bandwidth of 100 to 1000 hertz. This narrowed
audio signal can sacrifice some of the clarity of the voice and some of
the distinctive character and timbre otherwise perceived at the receiver.
An improvement of this patent in U.S. Pat. No. 3,851,095 describes the use
of an image storage tube. Such apparatus is available, but the cost is
relatively high, so that it cannot readily be furnished to a large number
of consumers.
U.S. Pat. No. 4,503,468 shows a pair of linked terminals which are capable
of generating images and altering such images at local and remote input
devices. U.S. Pat. No. 4,360,827 shows a method and system for
establishing interactive audio and video conferences. The audio signal is
sent over ordinary telephone lines, while the video signal is transmitted
via satellite. U.S. Pat. No. 4,349,701 relates to a television receiver
which has telephone network access means. The receiver provides for
viewing a television picture while utilizing a talk channel for carrying
out a two-way telephone conversation. U.S. Pat. No. 4,338,492 also shows a
similar system.
U.S. Pat. No. 4,494,144 shows a reduced bandwidth video transmission system
which is designed to provide a good video signal of selected portions of a
bandwidth. U.S. Pat. No. 4,485,400 shows apparatus and a method for
transmitting video data over telephone lines, and is said to be
particularly useful for transmitting and receiving images of persons
speaking to each other. The method utilizes a television camera with slow
scanning and image retaining means for displaying the image of a person
having a similar unit.
U.S. Pat. No. 4,388,489 shows a so-called video conversational data
communication network to permit conversational video textual data
transmission. U.S. Pat. No. 4,386,365 shows a wired broadcast system for
television, so as to provide visual communication through a frequency
changer.
THE INVENTION
The present invention provides a method and apparatus for simultaneously
sending an audio and a video signal over an ordinary telephone line. The
invention provides the capability of carrying out a two-way telephone
conversation, while at the same time a video image is being sent over the
line. It provides for transmission of selected or time-selected images of
the speakers during a telephone conversation and it likewise permits the
transmission of graphic, printed, or other visual information at the same
time that the matter is being discussed.
Briefly, the invention comprises a method for the simultaneous transmission
of voice and video information over a standard telephone line, having a
limited bandwidth, for example, of from 300 to 3500 hertz, which method
comprises obtaining one frame of a video image, converting the video image
to a binary number or other digitized form, optionally reading at least a
portion of the digitized video image to a random access memory, using the
digitized image from the memory to modulate a signal so that marks and
spaces are used to represent the zeros and ones of the digitized image,
the marks and spaces being in the range, for example, of from about 2400
to less than 4000 hertz, and preferably less than about 3500 hertz,
filtering voice signals with a low pass filter to limit the range of such
voice signals to a frequency range outside of the digitized image signals,
and in certain preferred embodiments, a frequency range lower than the
video signal band, combining the modulated image signals and the filtered
voice signals, and transmitting such signals over the standard telephone
line.
The invention also contemplates receiving such transmitted signals, passing
the signals through a filter to separate and recover the audio and image
signals, as for instance, passing the signals through a high pass filter
to remove the audio signals and produce a video signal comprising marks
and spaces, feeding the audio signal to a receiving means, and feeding the
visual signal to a means which restores the picture as sent.
The invention further contemplates apparatus for the simultaneous
transmission of audio and video information over a standard telephone line
having a limited bandwidth which apparatus comprises means to receive an
audio signal, means for providing one frame of a video image, means for
converting the video image to a binary number or other digitized form,
means for modulating a signal with the digitized image to modulate a
signal to provide marks and spaces to represent one's and zero's of the
digitized image, the marks and spaces being in one range of the bandwidth,
filter means to limit the frequency range of the audio signals to a
portion of the bandwidth other than that used by the the modulation of the
marks and spaces, means to combine the modulated image signals and the
filtered audio signals, and means to transmit the combined signals over
the standard telephone line. Also contemplated is apparatus for receiving
and reconstituting the audio signal and the image produced by the
foregoing apparatus which reconstituting apparatus comprises means for
receiving the combined signals from the standard telephone line, filter
means to separate the frequency range of the audio signals and the
frequency range of the modulated image signals, transducer means to
produce audio output from the audio signals, demodulation means to convert
the marks and spaces of the frequency range containing the marks and
spaces to an image bit stream, and means to produce an image from the
image bit stream.
In one preferred embodiment, the invention also provides a novel apparatus
comprising a "frame grabber" or image capture board to obtain a single
frame of an image, a modem, a high pass filter for the modem to produce
video signals, a telephone transmitter or microphone for audio signals, a
low pass filter for the audio signals, and means for placing the video and
audio signals onto a standard telephone line.
The present invention is further described with respect to the accompanying
drawings, wherein:
FIG. 1 is a schematic of an embodiment of a system for obtaining an image
of a subject;
FIG. 2 is a schematic of an embodiment of a system for sending the images;
and
FIG. 3 is a schematic of a high pass filter reversing switch.
Referring to FIG. 1, camera 10 is used to obtain an image of a person, a
group of persons, an object such as a chart or drawing, or other subject
of which it is desired to transmit an image. One camera suitable for
monochrome use is Micron-Eye, produced by Micron Technology, Inc. An NTSC
composite video signal is sent to "frame grabber" 12 through line 11.
Frame grabber 12 is an AT&T Targa 16 standard image capture board, with
others being available from a number of manufacturers, such as Idetix by
General Imaging Corporation. It will be understood that while the
transmission of the image to image capture board 12 is presently described
in terms of an NTSC composite video signal, the image could equally well
be a PAL, SECAM, HDTV (high definition television), or RGB signal.
Image capture board 12 comprises decoder 20 which decodes the NTSC signal
to its RGB (red-green-blue) components and feeds each of the separate
colors to its respective analog-to-digital converter 22, 24, and 26. The
converters feed the digitized signals to switchable multiplexer 26 which
can feed the digitized signals either to memory 54 or directly to monitor
32. Multiplexer 26 is shown in the latter mode. In this latter, or "live",
mode, multiplexer 26 feeds the signals to display bus 28 and thence to
digital-to-analog converters 29, 30, and 31 which respectively provide
red, green, and blue signals to RGB color monitor 32. It will be
understood that the image produced by camera 10 could equally well be a
monochrome image or it could be converted to a monochrome image, and that
monitor 32 could be an NTSC monitor coupled to camera 10 either directly
or through display bus 28.
The function of monitor 32 is to provide the opportunity to view the image
from camera 10 and to select a particular frame which it is desired to
transmit over the ordinary telephone line according to this invention.
Multiplexer 26 can also be set to select the frame of interest through
shift register 40. To this end, image capture board 12 contains video
timing controller 42, display registers 44, and control registers 46.
Control registers 46 communicate with bus interface 52 through conduit 48
and with display RAM (random access memory) 54 through line 50. Display
RAM 54 communicates in turn with shift register 40 via conduit 55.
Bus interface 52 communicates with central processing unit (CPU) 58 through
bus 56. The CPU used can be for example an IBM PC AT provided with monitor
59 and keyboard 60. Thus, image capture board 12 functions as described to
provide an image which can be stored in memory and retained for
transmitting. As shown in FIG. 1, the multiplexer is in the live/capture
mode which displays a "live" image on monitor 32. When the desired image
is seen on monitor 32, multiplexer 26 transfers or "grabs" that image to
display RAM 54. The image can be transmitted immediately in one aspect of
the invention, or alternatively it can be fed to mass storage such as a
floppy diskette, bubble memory, tape or a hard disk or disk pack for later
transmission.
FIG. 2 shows camera 10 and monitor 32 connected to image capture board 12
and CPU 58, shown as computer unit 70. The digital signals from computer
unit 70 communicate with modem 74 via channel 72. Modem 74 is a standard
Bell 212A protocol modem which normally operates at 2400 Hz. It will be
appreciated from the present disclosure that various commercial modems can
be modified for use in the practice of this invention. Thus, an Avatex
1200 bps modem made by E+E DataComm Corp, Sunnyvale, Calif., can readily
be utilized. In order to accomplish transmission according to the present
invention, the 22 MHz crystal of the standard Bell 212A protocol modem is
replaced with a 29 MHz crystal. This changes the operation of the modem
from 2400 Hz to 3200 Hz and provides a broader channel for the audio
signals. It will be understood that in this loopback mode of operation
there are no originate and answer modes possible, so that duplex operation
is not used in this embodiment. When an image is to be received from
another site, modem 74 is not itself sending an image. A modem can also be
constructed to use non-Bell 212A protocol originate mark-and-space and
answer mark-and-space frequencies, so full duplex operation is available.
Modem 74 converts the digital signals it receives from CPU 70 to DPSK
(dibit-encoded differential phase-shift keying) signals which modem 74
sends out through line 76. Line 76 conveys the DPSK analog signals though
line 76 to high-pass filter 78. High pass filter 78 which is optional for
the transmission of DPSK to local telephone line 80 has a sharp cut-off at
3000 Hz. Filter 78 functions importantly in the receive mode to keep lower
frequency audio signals out of the circuit to modem 74 when the modem is
receiving signals.
Simultaneously with the transmission of the DPSK signals from modem 74
through local line 80 and thence to the ordinary telephone company
subscriber "loop" or line 82, voice signals are transmitted to line 80 and
thence to loop 82. Appropriate coupling means (not shown) can be
incorporated at the local loop portal, if desired or if required by
telephone or PTT rules or tariffs.
In the embodiment shown in FIG. 2, speaker phone 90 is modified by
connecting it to microphone 96 through low pass filter 92 and by
connecting it to speaker 98 through low-pass filter 94. Each of filters 92
and 94 sharply attenuates frequencies above about 2800 Hz in the FIG. 2
embodiment of the invention. This has the effect of keeping the 3 kHz-plus
video signals out of the audio circuits and also keeps the audio signals
above 3 khz out of the video circuits. Modified speaker phone 90 and its
associated components are thus capable of receiving and of transmitting
voice signals in the audio duplex mode through line 80 and from and to
subscriber loop 82. It will be apparent to those skilled in the art that
once the signals transit the local loop, they are treated at the local
exchange just as any regular telephone signals and they can either be
switched locally or through a long lines network. Similarly, signals such
as those produced by apparatus like that of FIG. 2 can also be fed into
subscriber loop 82 to local line 80.
From this description, it will be apparent to those skilled in the art that
signals transmitted to loop 82 from local line 80 contain audio
frequencies in the range of, say, 300 Hz to about 3500 Hz. This is safely
within the standard telephone line bandpass. The actual telephone lines
may not carry frequencies down toward zero Hertz, but they do generally
carry the frequency range of 300-3500 Hz. Line 80 will contain audio
frequencies of 300 to about 2800 Hz. It has been found that this bandpass
for the voice frequencies is more than ample to give good recognition of
voices and good intelligibility of speech. Simultaneously, while the voice
information is being carried below 3000 Hz, video is simultaneously being
carried simplex over the same telephone lines.
In the system described in FIG. 2, the speed of video or picture
information transmission is restricted. At the resolution of 200 lines by
256 pixels on the horizontal lines and using 2 bytes per pixel, it is
necessary to transmit 102,418 bytes. At the rate of 120 bytes/second, it
requires two minutes to transmit one full frame. In actual use,
compression techniques known in the art are used so that as little as 16 k
Bytes can be used to transmit an acceptable image. Under these conditions,
a frame can be transmitted in a little over two minutes. It will be
appreciated that the transmission time can be reduced to one-half with a
2400 bps modem or to one-eighth of this time with a 9600 bps modem.
The compression techniques referred to herein are well-known in the art.
For example, ".ICB" or ".VDA" files can be created from images such as
those obtained by camera 10. It should also be noted that the present
methods permit the transmission of computer graphics and images generated
by methods other than direct TV photography, while at the same time
permitting the transmission of voice information.
While the present invention is described in terms of compressing both voice
and images into standard telephone lines, it will be appreciated that
these methods can also be used simultaneously to transmit two different
channels of information on a single conduit by filtering the two channels
and keeping them separate during transmission on a single information
channel. If for instance a high-quality telephone line were used, it is
possible to transmit high-quality audio and images. Moreover, the image
channels can themselves be broadened to permit the simultaneous
transmission of two or more images or the more rapid transmission of each
frame of an image.
Assuming that the picture is monochrome and that each bit represents one
pixel, it would require 29,406 bytes to encode one frame. In this
embodiment, the modem encodes the digital image utilizing frequencies
which are above 3 kHz and below 4 kHz, or below some other figure which is
less than the upper bandwidth limit of the telephone line. It has been
found useful to utilize a frequency of 3025 Hz for the "mark" and 3225 Hz
for the "space".
It has been found desirable that the cut-off should be sharp and a high
roll-off rate is also desirable in certain preferred embodiments.
In order to obtain the audio portion of the signal, the voice is picked up
through a transducer, such as microphone 96. The signal is then sent
through a low pass filter having a sharp cut-off at or below 3 kHz.
The audio output of the low pass filter is combined with the video output
of the high pass filter and the composite signal is fed through line 80 to
a standard telephone loop. Such loops are generally capable of handling a
band pass of 300 Hz to 3500 Hz, so that the combined audio and video
signal is converted to be within the band which an ordinary unconditioned
telephone loop can handle.
In order to receive a signal, the same equipment is utilized. The incoming
signal contains audio information in the band of 300 Hz to 3000 Hz. The
incoming signal is passed through a low pass filter having a cut-off at
3000 Hz. This filter generally has the same cut-off characteristics as the
low pass filter used in conjunction with the outgoing audio signal. The
signal from the low pass filter is then passed through an amplifier to
speaker 98.
In one embodiment a speaker phone can be used, in which case the amplifier
is unneeded and the audio signal from the speaker phone is passed through
the low pass filter before reaching the speaker. The video signals are
passed through the same high pass filter shown for the outgoing video
signal.
FIG. 3 shows reversing switch 110 which is a National Semiconductor
LF13333, a 16-pin DIP. In effect, it functions as a double-pole,
double-throw device to switch high pass filter 112, for instance, a
National Semiconductor MF10. This provides falloff at the low end of the
frequency range of 24 db per octave. Two of these National Semiconductor
MF10 filters are gauged to provide roll-off with a rate of 48 db per
octave.
FIG. 3 shows reversing switch 110 in the logic "0" state for the transmit
mode. The logic input from a switch is input via line 114 which in turn
connects the modem through line 116 and telephone line 118.
Those skilled in the art will appreciate that the presently described
method and apparatus has broad application to simultaneous transmission of
various kinds of information, and the invention is to be construed
according to the accompanying claims. It will be appreciated for instance
that the method can be used with communications channels which are not as
band-limited as conventional telephone lines. Thus, a "high quality"
telephone line can be used to provide a higher bit rate for the
transmission of the video information or to provide a broader frequency
response for the audio portion of the transmitted information. If very
broadband transmission channels are available, real-time pictures can be
transmitted with simultaneous voice communication.
The invention is thus capable of compressing audio and video information
into a narrower bandwidth than heretofore customary or possible. Broader
bandwidth can also be utilized to provide two sets of mark and space
frequencies so that duplex video exchange can take place.
It will accordingly be appreciated from the present disclosure that the
portion of the limited bandwidth of a channel used by the audio signal and
the portion used by the video signal can be selected depending upon the
total bandwidth available in the channel, the audio fidelity, resolution
of the video image, speed of transmission of the video image, nature of
the video image protocol, and the like. The bit rate of the modem can also
influence this. It has been found in practice that acceptable audio
fidelity is obtained by using a sharp audio roll-off beginning at about
2400 Hz and by rolling-off the video at frequencies below about 3000 Hz.
The audio signal is filtered so that it will not interfere with the video
portion of the channel and the video signal can be filtered to ensure that
more of that signal is combined with the audio. At the receiver, at least
the video is filtered to remove it from the audio, and both signals can be
filtered separately.
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