lbg@gitpyr.UUCP (Lee B Grey) (12/07/85)
Does anyone out there know if it is possible to transmit a television signal over a standard telephone line? It seems to me that there is such a limited bandwidth that there is no way. However, there is so much that I don't know about this kind of thing, that I have to ask. Please reply to me through mail. Thanks a lot. Lee Grey
dsi@unccvax.UUCP (Dataspan Inc) (12/07/85)
> Does anyone out there know if it is possible to transmit a > television signal over a standard telephone line? It seems to > ..... It depends on what you mean by "television signal." If one means live, NTSC-encoded composite television, forget it. To meet all the pertinent parts of FCC Section 73 with respect to broadcast quality video, there is no way that a standard dial-up telephone circuit has even 1/1000th the bandwidth required. (NTSC television requires a luminance bandwidth of at least 4.2 mHz) However, there are many forms of compression which involve tradeoffs of either time or spatial/contrast resolution that permit "television pictures" to be transmitted via telephone lines. In the temporal compression camp, we have single frame transmission systems, exemplified by the kind my company makes (DataSpan) and several competitors, including Colorado Video. The objective here is to transmit just one still frame which, at the other end, would meet the FCC R&R for a "single frame of television." Our system requires ~130-180 seconds to transmit a full frame of NTSC television, meeting and/or exceeding all broadcasting standards. The product is primarily used in the medical image communications field. Other products which are available do trade off bits of digital data and/or are variations of amateur slow scan television techniques. There are manufacturers (such as Raytel) who are staunch supporters of data compression of television images. Most still frame TV systems using data compression use the discrete cosine transform, which I have found to completely destroy the image where the contrastural information is most subtle and highest. Nevertheless, there are some applications where data compression is acceptable. Another temporal compression scheme which uses "telephone lines" (in a sense) involves the 56/64kbps DDS now emerging in the United States. You can also get these kinds of circuits (I think) in SCPC form from some satellite time brokers and owners. Widcom Corporation is the largest proponent of live, temporo-spatial compressed NTSC television. Their equipment has the smarts to not refresh stationary parts of the image, and thus is "semi-live" in that only the parts of the image needed to convey understanding are refreshed when they move or change hue/saturation/luminance beyond certain limits. In still television these equipments are quite good, and even for certain teleconferencing applications, the image quality is acceptable. You wouldn't want to telecast "Live from Studio 8-H" or the "Hallmark Hall of Fame" with it, though. This system actually works, and has applications for people who can stomach the tradeoffs involved. Hopefully, the AI people and neuropsychiatric types will discover something about human vision which can be implemented in hardware. Until the biology and mathematics about human vision is fully understood, you'll be stuck with raster scan television that can't be transmitted live through a 3000 Hz C message circuit. York David Anthony Director of Engineering DataSpan, Inc
karn@petrus.UUCP (Phil R. Karn) (12/09/85)
> > Does anyone out there know if it is possible to transmit a [standard, > > full-motion color] > > television signal over a standard telephone line? It seems to Way back in high school, my guidance counselor (who was also the sponsor of the amateur radio club) bet me a six pack that within 6 years this would be possible. I tried to quote Shannon, but he wouldn't have any part of it. Six years later I collected, and he gladly renewed the bet for another six years. It's about time for me to collect again. By the way, the Shannon limit on a typical analog voice line is roughly 30 kilobit/s. Of course, if the channel is being digitized and sent at 56 or 64 kilobit/s, than that also sets an ultimate limit assuming all other noise sources could be removed. Phil
mike@cucca.UUCP (Mike Ardai) (12/10/85)
There was an article in Radio Electronics a few months back on constructing such a system. I can't remember if it was over phone lines or radio, but it should work on either. /Michael L. Ardai cucca!mike ...!seismo!columbia!< tom!ardai
larry@kitty.UUCP (Larry Lippman) (12/12/85)
> > > Does anyone out there know if it is possible to transmit a [standard, > > > full-motion color] > > > television signal over a standard telephone line? It seems to > > By the way, the Shannon limit on a typical analog voice line is roughly 30 > kilobit/s. Of course, if the channel is being digitized and sent at 56 or 64 > kilobit/s, than that also sets an ultimate limit assuming all other noise > sources could be removed. Transmission of television images over 3 KHz telephone lines has been possible for a number of years using what are known as `slow-scan' converters. Transmission of motion in real time is not possible, however. I have used equipment manufactured by Colorado Video Inc. of Boulder, Co. for this purpose; CVI has a number of off-the-shelf slow-scan products with resolutions as great as 1024 x 1024 pixels with 8 shades of grey. I have also used scan converters made by Hughes Aircraft Industrial Products Division (Carlsbad, Ca.) and by Tektronix for this purpose. All of the above products are monochrome only, and I am not aware of any off-the-shelf color scan converters for this purpose; however, there is no reason why a color scan converter cannot be built. === Larry Lippman @ Recognition Research Corp., Clarence, New York === === UUCP {decvax,dual,rocksanne,rocksvax,watmath}!sunybcs!kitty!larry === === VOICE 716/741-9185 {rice,shell}!baylor!/ === === FAX 716/741-9635 {AT&T 3510D} ihnp4!/ === === === === "Have you hugged your cat today?" ===