While sorting through some old stuff, I came upon the introductory comms book I used at college. This book was published in 1971. In the section on information theory it goes through Shannon and Hartley's work on channel capacity. It then describes one possible way to get close to that limit, which is pretty much what we call OFDM (though it doesn't call it that - when was the term OFDM coined?) and says: "We shall see that, to obtain errorless transmission for R<C requires and extremely complicated communication system. In physically realizable systems, therefore, we must accept a performance less than the optimum." In fact, the whole tone of the description it that the system is implausible to implement. Just 34 year later, I am sending this through a 54Mbps OFDM link, and then through a 6Mbps OFDM link. They are both tiny, consume little power, and cost just a few dollars. I guess I would call that progress. :-) Regards, Steve
How things change
Started by ●March 3, 2005
Reply by ●March 3, 20052005-03-03
Steve Underwood wrote:> While sorting through some old stuff, I came upon the introductory comms > book I used at college. This book was published in 1971. In the section > on information theory it goes through Shannon and Hartley's work on > channel capacity. It then describes one possible way to get close to > that limit, which is pretty much what we call OFDM (though it doesn't > call it that - when was the term OFDM coined?) and says: > > "We shall see that, to obtain errorless transmission for R<C requires > and extremely complicated communication system. In physically realizable > systems, therefore, we must accept a performance less than the optimum." > > In fact, the whole tone of the description it that the system is > implausible to implement. Just 34 year later, I am sending this through > a 54Mbps OFDM link, and then through a 6Mbps OFDM link. They are both > tiny, consume little power, and cost just a few dollars. I guess I would > call that progress. :-) > > Regards, > SteveMy $10 wristwatch (airport kiosk price) keeps better time than a ship's chronometer of that era. And how about those hearing aids small enough to swallow? And a TV camera and transmitter in a lapel pin? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20052005-03-04
Hi Jerry, Jerry Avins wrote:> Steve Underwood wrote: > >> While sorting through some old stuff, I came upon the introductory >> comms book I used at college. This book was published in 1971. In the >> section on information theory it goes through Shannon and Hartley's >> work on channel capacity. It then describes one possible way to get >> close to that limit, which is pretty much what we call OFDM (though >> it doesn't call it that - when was the term OFDM coined?) and says: >> >> "We shall see that, to obtain errorless transmission for R<C requires >> and extremely complicated communication system. In physically >> realizable systems, therefore, we must accept a performance less than >> the optimum." >> >> In fact, the whole tone of the description it that the system is >> implausible to implement. Just 34 year later, I am sending this >> through a 54Mbps OFDM link, and then through a 6Mbps OFDM link. They >> are both tiny, consume little power, and cost just a few dollars. I >> guess I would call that progress. :-) >> >> Regards, >> Steve > > > My $10 wristwatch (airport kiosk price) keeps better time than a > ship's chronometer of that era. And how about those hearing aids small > enough to swallow? And a TV camera and transmitter in a lapel pin? > > JerryWhen that book was published the first digital watches were appearing, so your watch was certainly not implausible. Also, the accuracy of your watch is not so great. It only works well on yout wrist, with your body stabilising its temperature. Change the temperature, and the accuracy drifts a lot. The chronometer is beautifully temperature corrected, and has been since the King of England paid a bounty to make it so. Nothing much has changed here since the 70's. A temperature corrected electronic watch is still beyond what we can achieve with a tiny battery lasting a reasonable time. Hearing aids fit behind the ear in 1971, and now the go inside. Nothing too spectacular there. Surveillance TV cameras moved to CCDs in the 1970s. I can't remember if they did that as early as 71, but it was close. Even early CCD cameras were pretty small. I think there are a number of examples of really spectacular change in the last 34 years. Pretty much anything that needs dense digital processing has seen a change as great as OFDM, and for same reasons. However, there are plenty of non-electronic examples too. I'm not impressed by your examples. Come on Jerry, you usually come from left field with a really good one. :-) Its interesting how the concepts and the implementation so often don't line up. A 1971 book described something close to OFDM, and something close to TCM, well ahead of serious application being viable. In the 1970s, when I worked on first generation DSP based radar (note the use of upper and lower case there :-) ), much of what we were implementing had been laid down mathematically in the 1930s, as the very first primitive radars were being built. Coming up with ideas in this way, before they are viable, it to be expected. On the other hand, chirping was initially done to avoid the breakdown of moist air in the transmission equipment in the early 1940s. It was a high profile defence issue, however it only received a proper mathematical treatment in the late 1950s (and from guys at Bell - not an obvious source of insight into radar technology). Regards, Steve
Reply by ●March 4, 20052005-03-04
Steve Underwood wrote: ...> When that book was published the first digital watches were appearing, > so your watch was certainly not implausible. Also, the accuracy of your > watch is not so great. It only works well on yout wrist, with your body > stabilising its temperature. Change the temperature, and the accuracy > drifts a lot. The chronometer is beautifully temperature corrected, and > has been since the King of England paid a bounty to make it so. Nothing > much has changed here since the 70's. A temperature corrected electronic > watch is still beyond what we can achieve with a tiny battery lasting a > reasonable time.In 1972, I wore a self-winding Omega Chronotron (or some such: it's in a drawer somewhere) It was spec'ed to keep time to within a 10 seconds month on its back, face, left, and right sides (but oddly, not on its ends) in a hot box, at room temperature, and at ice point. Whatever the spec, the airport El Cheapo keeps better time on my wrist and in the drawer. My Hamilton 25-year you-made-it prize is a little better, but not worth talking about. The only times I set either is when I put in a new battery, at the DST<->standard switch, or when I change time zones long enough to make resetting worth while. It can't be body temperature stabilization. Most but not all nights my watch is on the night stand. For kicks, I had it in the freezer for a week with no apparent effect. Not all electronic clocks are that good. My computer loses about two seconds a day, but it's consistent enough to qualify as a chronometer. A chronometer's important spec is not accuracy, but consistency. A chronometer is never reset at sea or even on board ship. Current time is computed from the time and date of setting, its indicated time, and the known drift rate. The calculated time is used to correct the ship's clock and, for celestial navigation, set comparing watches.> Hearing aids fit behind the ear in 1971, and now the go inside. Nothing > too spectacular there.The batteries now last longer than an hour. They were analog then and deficient by todays standards. Today's lower limit on size is the danger of actually falling in!> Surveillance TV cameras moved to CCDs in the 1970s. I can't remember if > they did that as early as 71, but it was close. Even early CCD cameras > were pretty small.Camera and transmitter concealed in a stick pin, with the pin part as antenna? No way! Here's a quote from a recent news article about the receiving end: "Israeli troops have been outfitted with tiny video screens on their wrists. The screens display video shot by unmanned airplanes, and help troops identify and strike targets. The technology has been used for about a year, but was kept secret until the company that developed it, Elisra Group�s Tadiran Electronic Systems and Tadiran Spectralink companies, spoke to reporters about it this week."> I think there are a number of examples of really spectacular change in > the last 34 years. Pretty much anything that needs dense digital > processing has seen a change as great as OFDM, and for same reasons. > However, there are plenty of non-electronic examples too. I'm not > impressed by your examples. Come on Jerry, you usually come from left > field with a really good one. :-)Thanks for the praise, but I'm off my feed at the moment. The information doctors get from MRIs, CAT scans, and th like is phenomenal, but that's all made possible by computer and display technology. Virtual colonoscopy: yay!> Its interesting how the concepts and the implementation so often don't > line up. A 1971 book described something close to OFDM, and something > close to TCM, well ahead of serious application being viable. In the > 1970s, when I worked on first generation DSP based radar (note the use > of upper and lower case there :-) ), much of what we were implementing > had been laid down mathematically in the 1930s, as the very first > primitive radars were being built. Coming up with ideas in this way, > before they are viable, it to be expected. On the other hand, chirping > was initially done to avoid the breakdown of moist air in the > transmission equipment in the early 1940s. It was a high profile defence > issue, however it only received a proper mathematical treatment in the > late 1950s (and from guys at Bell - not an obvious source of insight > into radar technology).Early implementations of high-resolution side-looking synthetic-aperture radar did the necessary 2D Fourier transforms optically. There was no hope of using a computer of the day to calculate them. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20052005-03-04
On Fri, 04 Mar 2005 12:50:01 -0500, Jerry Avins <jya@ieee.org> wrote:>> Its interesting how the concepts and the implementation so often don't >> line up. A 1971 book described something close to OFDM, and something >> close to TCM, well ahead of serious application being viable. In the >> 1970s, when I worked on first generation DSP based radar (note the use >> of upper and lower case there :-) ), much of what we were implementing >> had been laid down mathematically in the 1930s, as the very first >> primitive radars were being built. Coming up with ideas in this way, >> before they are viable, it to be expected. On the other hand, chirping >> was initially done to avoid the breakdown of moist air in the >> transmission equipment in the early 1940s. It was a high profile defence >> issue, however it only received a proper mathematical treatment in the >> late 1950s (and from guys at Bell - not an obvious source of insight >> into radar technology). > >Early implementations of high-resolution side-looking synthetic-aperture >radar did the necessary 2D Fourier transforms optically. There was no >hope of using a computer of the day to calculate them. > >JerryDang, Jerry, you beat me to it. ;) My first job as an engineer doing signal processing was for Goodyear Aerospace, the guys that pretty much invented Synthetic Aperture Radar. Although I was working on the digital stuff, there were still some optical processors floating around (and some still deployed in the field) and plenty of old salts in the lab that had helped design them. I found it really fascinating and being able to see, side-by-side, how the optical processors worked to do the exact same signal flow that we were doing digitally was very insightful. They even had some cool ways to move the lenses slightly during processing to compensate for the motion of the collecting airplane. Very cool stuff, and very helpful in learning the whys and wherefores of a lot of signal processing concepts. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by ●March 4, 20052005-03-04
Eric Jacobsen wrote: ...> My first job as an engineer doing signal processing was for Goodyear > Aerospace, the guys that pretty much invented Synthetic Aperture > Radar. Although I was working on the digital stuff, there were still > some optical processors floating around (and some still deployed in > the field) and plenty of old salts in the lab that had helped design > them. I found it really fascinating and being able to see, > side-by-side, how the optical processors worked to do the exact same > signal flow that we were doing digitally was very insightful. They > even had some cool ways to move the lenses slightly during processing > to compensate for the motion of the collecting airplane. Very cool > stuff, and very helpful in learning the whys and wherefores of a lot > of signal processing concepts.I guess that you can imagine my delight when I saw the conical lens, whose shape was derived with abstruse math too deep to be the whole story, as a cylinder lens whose focal length varies with height in exactly the way needed to project what would have been a vertical image with perspective distortion onto a horizontal plane without any. After that, I troubled to explain most of the optical parts in purely optical terms, instead of as some miasma rising out of a mathematical stew, a conformation "that passeth all understanding". It strikingly brought the unity home to me, and convinced me that if I couldn't explain something to an intelligent and interested high-schooler, I didn't understand it well myself. Richard Feynman confirmed my opinion about that. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20052005-03-04
Eric Jacobsen wrote:> On Fri, 04 Mar 2005 12:50:01 -0500, Jerry Avins <jya@ieee.org> wrote: > > >>>Its interesting how the concepts and the implementation so often don't >>>line up. A 1971 book described something close to OFDM, and something >>>close to TCM, well ahead of serious application being viable. In the >>>1970s, when I worked on first generation DSP based radar (note the use >>>of upper and lower case there :-) ), much of what we were implementing >>>had been laid down mathematically in the 1930s, as the very first >>>primitive radars were being built. Coming up with ideas in this way, >>>before they are viable, it to be expected. On the other hand, chirping >>>was initially done to avoid the breakdown of moist air in the >>>transmission equipment in the early 1940s. It was a high profile defence >>>issue, however it only received a proper mathematical treatment in the >>>late 1950s (and from guys at Bell - not an obvious source of insight >>>into radar technology). >> >>Early implementations of high-resolution side-looking synthetic-aperture >>radar did the necessary 2D Fourier transforms optically. There was no >>hope of using a computer of the day to calculate them. >> >>Jerry > > > Dang, Jerry, you beat me to it. ;) > > My first job as an engineer doing signal processing was for Goodyear > Aerospace, the guys that pretty much invented Synthetic Aperture > Radar. Although I was working on the digital stuff, there were still > some optical processors floating around (and some still deployed in > the field) and plenty of old salts in the lab that had helped design > them. I found it really fascinating and being able to see, > side-by-side, how the optical processors worked to do the exact same > signal flow that we were doing digitally was very insightful. They > even had some cool ways to move the lenses slightly during processing > to compensate for the motion of the collecting airplane. Very cool > stuff, and very helpful in learning the whys and wherefores of a lot > of signal processing concepts. > > > Eric Jacobsen > Minister of Algorithms, Intel Corp. > My opinions may not be Intel's opinions. > http://www.ericjacobsen.orgAfter reading Jerry's post, I started to ask a question. But read your post and realized I might ask a specialist. 1. Any WEB available references on how to do Fourier transforms optically? 1a. Can I assume the typical demo of interference effects from 2 slits converting incident plane wave to 2 sources is a Fourier transform of something? Can you tell I've not gone to school in decades ;] 2. Did it depend on knowing the incoming carrier frequency? IE Given an "optical Fourier transformer" as a 2-port black box. Could you feed it an audio signal rather than RF and get out the appropriate transform?
Reply by ●March 4, 20052005-03-04
Jerry Avins wrote:> [snip] It strikingly brought the > unity home to me, and convinced me that if I couldn't explain something > to an intelligent and interested high-schooler, I didn't understand it > well myself. Richard Feynman confirmed my opinion about that. >OK, just how did he do it. Did a Google search on his name and first several hits were on writing very very very small. That didn't seem promising. Am I missing something?
Reply by ●March 4, 20052005-03-04
Jerry Avins wrote to himself: nice pix! http://www.osa-opn.org/view_file.cfm?doc=%24)%2C7-K0%20%20%0A&id=%25(%2C%2B.J%3C4%20%0A -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20052005-03-04
Richard Owlett wrote:> Jerry Avins wrote: > >> [snip] It strikingly brought the >> unity home to me, and convinced me that if I couldn't explain >> something to an intelligent and interested high-schooler, I didn't >> understand it well myself. Richard Feynman confirmed my opinion about >> that. >> > > OK, just how did he do it. > > Did a Google search on his name and first several hits were on writing > very very very small. That didn't seem promising. Am I missing something?He was a Nobel laureate in physics, but he was more. Not only an outstanding teacher who could see to the core of things, but a bongo player, magician and ... Aw crap, He should be remembered for having said, "Physics is like sex. Sure, it may give some practical results, but that's not why we do it." See http://www.amasci.com/feynman.html. You would enjoy reading his autobiography, "Surely You're Joking, Mr. Feynman". At least one misguided bookstore had it in the humor section, so look sharp. Feynman diagrams illustrate parts of quantum mechanics so even I [think I] can understand them. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
