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Posted: February 21st, 2012, 7:37 pm
QuoteOriginally posted by: outrunI would however get rid of the speaker microphone bit, replace it with a cable.I can't even find an HF radio with a jack like that. So there is no chance anyone with a cool antenna to test will have one.
Posted: February 24th, 2012, 10:37 am
Am wrong, or is this completely wrongheaded?QuoteIntilop debuts TCP Offload Engine offering low latency using Altera Stratix IV FPGASANTA CLARA, Calif., 20 Feb. 2012. Intilop Inc., maker of complex ultra-low-latency networking Mega IP building blocks and systems, is introducing its 4th Gen. SX-Series 10G ultra-low-latency TCP Offload Engine (TOE) + EMAC + Altera PHY Mega IP cores running on a Stratix IV field-programmable gate array (FPGA). The TOE?s architecture is scalable, customizable, and adaptable. Intilop's product-line solutions are available in flexible FPGA/ASIC/SoC technologies, which can accommodate a diverse set of technical design specifications.Doesn't TCP just do a CRC check or something, and discard the entire packet and do a resend if it doesn't match up? You could have one bad byte and throw the whole thing away. Or worse, you could be using some crap hardware signal processing to resolve the bytes, when a more ambitious software algorithm could extract them accurately from the line signal. I know software is not as fast as hardware. But anything is faster than a resend. At the very least it could pass on the raw signal to the software, in hopes it could flip the ambiguous byte and get a correct CRC matchup in less time than it takes to resend.Again, I don't really know what I am talking about, so I post it to see if anyone else does.
Posted: February 24th, 2012, 10:53 am
Anyway, in the interest of open source, how can I build the ultimate 20Mhz directional antenna using vertical posts? I am getting an amateur radio operator license so I don't have to look over my shoulder building it.1) How tall do the posts have to be? Do they need a minimum length - some multiple of the wavelength - or an exact length?2) How far apart do I space the live post from the focusing posts? How many can I use?3) What do I make these focusing posts out of?4) What exactly is the hardware design of the live post? What is inside that thing, a single continuous wire, wound wires that are not actually connected, what?
Posted: February 24th, 2012, 11:09 am
QuoteOriginally posted by: outrunthere are multiple bit flips possible to fix the CRC, but you won't know which CRC fix is the correct one.If you have the raw signal, you are going to see which one does not look right, which one is not clean. Even if you do not have the raw signal, if you have sent uncompressed data (or even compressed data?), there is some natural CRC in the data. Like if you are sending an order to have a pizza delivered, you will know that "flub" is not a correct topping, or $600 just does not seem right for Apple stock. Takes time, but it may be finished before the resend gets there.
Posted: February 24th, 2012, 12:19 pm
Well anyway, the basic goal of TCP is to turn bad packets into late packets. Spending energy to manufacture late packets seems to be inconsistent with the general spirit of a low-latency communication.
Posted: February 24th, 2012, 2:27 pm
You might just want to use a 56k modem for your first demos or some DSL units if you want to demonstrate higher bit rates. Those boxes will have all the modulation circuits to handle all the digital-to-analog-to-digital magic.I'm not sure CRC is the right way to go given that the goal is low latency. CRC has the disadvantage that the receiver doesn't know if the first bit is valid until the last bit of the cycle arrives. If low latency is the goal, then you'll really want to design this from the ground up. That means creating parallel error-correction in the signal (i.e., the shape of the signal at time t provides all the data you need for robust reconstruction of the transmitted bit at time t) rather than serial error-correction (like CRC where you wait for error correction data).Similarly, TCP is entirely the wrong technology because it's designed for a packet-switched network and you have a circuit-switched one. IP technologies are great for connecting many users to many users in a mesh of routed links, but they suck suck suck for latency. The old telco engineers knew how to do circuit-switched networks, which create an end-to-end connection between origin and destination with none of the routing/buffering crap of internet-based networks.As for your antenna, the first issue is picking the exact frequency. You don't want to use 20 MHz because that's what the Feds use for broadcasting the atomic clock time signal. The FCC will tear you a new one if you interfere with that. This document lists frequency allocations and applications for the U.S. cross referenced to FCC rule parts. I don't know what band you can operate in, but you'll need to do a bit of studying.As for the gist of the antenna design, you might end up with this high-gain 7-element YAGI for 21-21.4 MHz. It's big, 23 ft x 55 ft, and probably should be mounted on a 20-60 foot mast but that's what it takes at these low frequencies.
Posted: February 24th, 2012, 6:22 pm
You should have seen the old freaks at the ham radio convention. I must be getting old, because that 60x55x23 foot antenna turns me on! My innate sense of radio propagation and ionized particles tells me from deep in my primitive brain that thing can make the transmission in one hop! I will be giving airline stewardesses ovarian cancer in some kind of freaky ham-radio eugenics!