How to get envelope from AM signal without phase shift
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From: glen herrmannsfeldt on 6 Apr 2010 14:49 Jerry Avins <jya(a)ieee.org> wrote: (snip on modems and bit rates, where someone wrote) >> I think one reason for that is over time what constitues a 'phone >> line' has also changed. 2400 bps might as well be the limit for a end >> to end analog line. > I don't think so. The unstated assumption behind that line of reasoning > is binary signaling (and a signal robust enough not to need error > correction). Keep that assumption valid, and the limit really is about 2400. Well, an important part of going to 9600 was allowing the bands of the two directions to overlap. I believe that through 2400 the available bandwidth was divided in half, one half for transmit and one for receive. (Well, originate and answer in modem terminology.) In addition, normal (long distance) phone lines use an echo surpressor that senses which end is talking and reduces any echo from the other end. That is turned off for modem connections. At higher rates, where the bands overlap, each has to do echo cancelation, detecting a delayed version of its own signal and subtracting it. But yes, and important part of the need for error correction is the interference of ones own transmission. All these are for modems with similar bit rates in each direction. There were some simplex (one direction only) and half duplex (one direction at a time) modems, and also some non-symmetric (people type slower than computers) modems. -- glen
From: steveu on 7 Apr 2010 02:22 >Eric Jacobsen <eric.jacobsen(a)ieee.org> wrote: > >>Most VCs have some awareness of what they don't know or what they can't >>verify on their own. The smart ones then get help in the technical due >>diligence area. Sometimes they get incompetent help for that, but that >>happens in any aspect of business. > >>Clearly there's a lot of "non-smart" money out there. My former >>employer, despite being a global technology leader, had a reputation for >>not being smart with capital investments. I saw first hand how that >>process worked, and was not able to stop a few really bad investments >>(although I did help ward off a few, and helped close some good ones as >>well). > >If they had VC level business intuition, they'd be VC's >rather than middle-managers at a huge corporation. > >I once worked for a Dow 30 company. I won't detail the many >mistakes they made, but one troubling aspect is they could >conduct a huge venture that would be a massive failure, then >for the next huge new venture, they would put the same >individuals in charge. Well, there is a theory that people learn from their mistakes, and those hardened by failure are better prepared for the next attempt. I've never seen any evidence to support this. In fact, when most people make exactly the same mistake a second time they can't even see the similarity to the previous occasion. Steve
From: Vladimir Vassilevsky on 7 Apr 2010 06:32 steveu wrote: > Well, there is a theory that people learn from their mistakes, and those > hardened by failure are better prepared for the next attempt. I've never > seen any evidence to support this. In fact, when most people make exactly > the same mistake a second time they can't even see the similarity to the > previous occasion. People learn from their mistakes only if they pay for their mistakes. Pain is a good teacher. However, this is not the case if they gamble on somebody else's money. VLV
From: Rune Allnor on 7 Apr 2010 06:42 On 5 apr, 19:36, spop...(a)speedymail.org (Steve Pope) wrote: > It is part of a series > of possibly related phenomena that might represent new energy-producing > physics -- sonoluminesence, Sonoluminescenese *was* quite puzzling for a while in the early '90s. The effect occured when subjecting a liquid that contained gas bubbles of microscopic sizes to a very strong ultrasound field. It turend out that light was emitted from the microbubbles, which explains the term 'sonoluminescence' as 'sound-induced light emissions'. At first the exact mechanism that caused this light to be emitted was not understood. It seems avery kook and their cell mates (is that the correct term for room mates in psycho ward?) offered an explanation, ranging from quantum fluctuations to microscopic black holes. One fascinating aspect of all this was that the light bursts were incredibly short: No matter how sensitive sensors one used, the measured transients were the impulse rsponses of the sensor. As I recall, times on the scale of femto seconds (1e-15 s) were mentioned as typical for the light emissions. Around 2000 the effect was explained in terms of the collapse of the microbubbles when subjected to the ultrasound field: The sound shock wave hits the near side of the bubble, accelerating the liquid in the boundary layer into and across the gas-filled void. The liquid smashes into the far wall of the bubble, violently compressing the gas volume in the process. I don't remember off the top of my head exactly what caused the coonversion of sonic energy to light, but I suspect that the almost instant compression of the gas inside the bubble caused such a high temperature that heat was emitted as visible light. So while puzzling at first, sonoluminescence does not *produce* energy, but rather *converts* energy from high-intensity sonic shock waves to visible light. I heard a talk given by one lead investigator on the subject, Larry Crum, some time in 1996 or 1997. During the talk he gave a quick overview of the phenomenon, and also indicated that the bubble-collapse hypothesis, which was investigated at the time, was very promising. He also commented that he had been 'scientific advisor' to that Hollywood movie "Chain reaction", where sonoluminescense played a key part, as a way to produce energy. As I recall, Crum made a remark along the lines that "The movie makers got the science of sonoluminecence all wrong, but it was great fun to be there and see how such a movie was made." Rune
From: Rune Allnor on 7 Apr 2010 07:02
On 1 apr, 22:56, dbd <d...(a)ieee.org> wrote: > Rune may have been too kind. I know - I'm getting old: My mild-mannered amenity is getting the better of me. Rune |