Watermarking
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From: D Yuniskis on 18 May 2010 16:36 Hi George, George Neuner wrote: > On Mon, 17 May 2010 14:32:03 -0700, D Yuniskis > <not.going.to.be(a)seen.com> wrote: > >>> shallow water pass over a modern era wreck that isn't a danger to >>> surface navigation, there is usually enough steel around to cause a >>> significant amount of compass deviation. I've had a complete 360 deg. >>> turn caused by such a wreck and innumerable S wiggles. With a little >>> local knowledge you soon learn to avoid the more troublesome ones. >> Ah, that's possible! > > I've seen significant compass deviation passing under bridges or over > tunnels, pipes, electrical cables and even over shallow net anchorages > (steel nets placed to snag anchors where the bottom is too soft to > hold). I think the water is too deep at this point -- we're out in the Atlantic (SE of P-town). >> It is also possible that the anomalies >> in the recorded track happened when we were in "manual" >> control (fishing for Blues). It could also have been an >> anomaly in the LORAN receiver. <shrug> >> >> It's been 30+ years. I haven't heard of any *deaths* so I >> don't lose too much sleep over it! :> (though I really would >> have liked a resolution "back then") > > My Koden unit (forget which model but it was the high end one at the > time) had 3 precision modes - trading accuracy for speed - but I > almost always used it at highest precision because there are a lot of > reefs and shoals in my area. I found that in the fast mode, the unit > considered a waypoint to be roughly a 300ft circle. I had to be able Depending on where you are operating on the chains, the geometries can conspire to give you really crappy -- or really *good* -- data. E.g., there are areas where the ambiguities inhgerent in the geometry can cause you to be "here" -- or "over there" :> If "here" and "there" are too close... <shrug> > to steer compass courses through channels as narrow as 30ft (lotsa fun > at night in pea soup fog). At high precision my unit could repeatedly > find a station set waypoint within 5ft up to about 10kt. In the prototype run, I set all the waypoints to be marked buoys so I could verify we were where we should be (i.e., there are no street corners on the open ocean :> ). I can recall coming close enough to actually fear we were going to hit the buoys (we'd run at about 25kt's to cover as much ground as possible -- the entire trip was several hundred Nmiles). At one point, the "next leg" was almost "back the way we came" (i.e., turn *really* hard to starboard). We passed just to the left of the buoy (narrowly missing another craft that was sitting nearby), then heard the "groan" that comes from trying to make too sharp a turn too quickly as the boat turned around and appeared ready to make another run at the other craft sitting nearby :> (we ended up with the buoy on our left again -- i.e., neatly going *around* it). Much of the rest of the trip was a blur. I had pulled an all-nighter to get the prototype built and coded in time for the trip. So, I was pretty tired. And, none of use thought to bring any *food* (though we all managed to bring BEER!). Thankfully, the boss's wife was a bit more practical and sent along some food with him! :-/ (at least *I* had an excuse for my thoughtlessness -- I think I was 18 at the time so *my* priorities didn't focus on *food*! :>) > If your unit had different precision modes and the mode was changed > unexpectedly that might have affected your plot. I just took in TD's (.01uS, IIRC), did the TD-lat/lon conversion, looked at where I wanted to be, looked at where I had *been*, determined the cross-track error (to try to gauge local effects of drift) then "turned" the autopilot into the "current" appropriately such that the vector representing the boat's *steered* course and the vector representing the "local drift" would "sum" to the desired destination. I.e., the goal was to act as a "smart helmsman" and not as a "stupid autopilot". If I had notes to consult (I *probably* have the sources for the device here somewhere -- no doubt on 17" wide tractor feed "green stripe" paper), I could see which chains we were using and check to see if we were in a region of high GDOP, or maybe some of the trig was running in nasty areas (e.g., tan 90). <shrug> As I said, I doubt anyone has *died* from this so I won't lose sleep over it. But, I don't like things that can't be explained (I relentlessly pursue "intermittents" for this reason). Unfortunately, I wasn't in shape to follow up on this at the time ;-)
From: D Yuniskis on 18 May 2010 16:41 Hi Walter, Walter Banks wrote: > D Yuniskis wrote: > >>> D Yuniskis wrote: >>>> <grin> I am haunted by an autopilot (marine) I designed some >>>> 30+ years ago. After returning from our test run, an examination >>>> of the actual course taken showed an "S" in the plot at one >>>> particular place. Did my software "divide by zero" (or >>>> something similar)? Or, was this the spot where we stopped >>>> to fish (which requires constantly readjusting the boat's >>>> direction to keep it pointed into the swells)? >>>> >>>> My boss wasn't worried about it (since the rest of the >>>> trip -- I think 7 legs? -- went uneventfully) but the image >>>> of that "S" is burned into my memory... :-/ >>> Not uncommon if your autopilot uses a fluxgate or other magnetic compass >>> sensor as its primary heading reference. If you are in relatively >> Onto this (*my* "claim to fame") was a software servo loop >> that took LORAN-C coordinates of "destination" and kept >> tweeking the "motor drive" to update the "new" course >> (i.e., instead of conventional autopilot that seeks to >> maintain a constant heading, my goal was to reach a desired >> *destination*) > > Fly or float pointed directly to the destination has a problem > (Not a big one but a significant one) > > Current or wind drift forces the craft off track and the control > system then corrects by turning to the destination that is good, > however it is not the shortest track to the destination. It results > in a track shaped like a "?". Correct. I looked at where we had been, where we we going and where we currently were. From this, estimated "drift" and compensated accordingly. I.e., worked to keep cross track error at "0" Nowadays this would be a lot easier with GPS (LORAN was slow and had a hard time giving you good accuracy). Of course, you've got a lot more MIPS available nowadays to *do* the processing! With a 3MHz 8085 and a hundred bytes of RAM, it takes the better part of a GRI to *compute* the lat/lon let alone *act* on it! :>
From: Dave Nadler on 18 May 2010 20:56 On May 18, 4:36 pm, D Yuniskis <not.going.to...(a)seen.com> wrote: > Hi George, > > George Neuner wrote: > > On Mon, 17 May 2010 14:32:03 -0700, D Yuniskis > > <not.going.to...(a)seen.com> wrote: > > >>> shallow water pass over a modern era wreck that isn't a danger to > >>> surface navigation, there is usually enough steel around to cause a > >>> significant amount of compass deviation. I've had a complete 360 deg. > >>> turn caused by such a wreck and innumerable S wiggles. With a little > >>> local knowledge you soon learn to avoid the more troublesome ones. > >> Ah, that's possible! > > > I've seen significant compass deviation passing under bridges or over > > tunnels, pipes, electrical cables and even over shallow net anchorages > > (steel nets placed to snag anchors where the bottom is too soft to > > hold). > > I think the water is too deep at this point -- we're out in > the Atlantic (SE of P-town). > > >> It is also possible that the anomalies > >> in the recorded track happened when we were in "manual" > >> control (fishing for Blues). It could also have been an > >> anomaly in the LORAN receiver. <shrug> > > >> It's been 30+ years. I haven't heard of any *deaths* so I > >> don't lose too much sleep over it! :> (though I really would > >> have liked a resolution "back then") > > > My Koden unit (forget which model but it was the high end one at the > > time) had 3 precision modes - trading accuracy for speed - but I > > almost always used it at highest precision because there are a lot of > > reefs and shoals in my area. I found that in the fast mode, the unit > > considered a waypoint to be roughly a 300ft circle. I had to be able > > Depending on where you are operating on the chains, the geometries > can conspire to give you really crappy -- or really *good* -- data. > E.g., there are areas where the ambiguities inhgerent in the > geometry can cause you to be "here" -- or "over there" :> > If "here" and "there" are too close... <shrug> > > > to steer compass courses through channels as narrow as 30ft (lotsa fun > > at night in pea soup fog). At high precision my unit could repeatedly > > find a station set waypoint within 5ft up to about 10kt. > > In the prototype run, I set all the waypoints to be marked buoys > so I could verify we were where we should be (i.e., there are no > street corners on the open ocean :> ). I can recall coming close > enough to actually fear we were going to hit the buoys (we'd run > at about 25kt's to cover as much ground as possible -- the entire > trip was several hundred Nmiles). At one point, the "next leg" > was almost "back the way we came" (i.e., turn *really* hard to > starboard). We passed just to the left of the buoy (narrowly > missing another craft that was sitting nearby), then heard the > "groan" that comes from trying to make too sharp a turn too > quickly as the boat turned around and appeared ready to make > another run at the other craft sitting nearby :> (we ended > up with the buoy on our left again -- i.e., neatly going *around* > it). > > Much of the rest of the trip was a blur. I had pulled an > all-nighter to get the prototype built and coded in time for > the trip. So, I was pretty tired. And, none of use thought > to bring any *food* (though we all managed to bring BEER!). > Thankfully, the boss's wife was a bit more practical and sent > along some food with him! :-/ (at least *I* had an excuse > for my thoughtlessness -- I think I was 18 at the time so *my* > priorities didn't focus on *food*! :>) > > > If your unit had different precision modes and the mode was changed > > unexpectedly that might have affected your plot. > > I just took in TD's (.01uS, IIRC), did the TD-lat/lon conversion, > looked at where I wanted to be, looked at where I had *been*, > determined the cross-track error (to try to gauge local effects > of drift) then "turned" the autopilot into the "current" > appropriately such that the vector representing the > boat's *steered* course and the vector representing the > "local drift" would "sum" to the desired destination. > > I.e., the goal was to act as a "smart helmsman" and not > as a "stupid autopilot". > > If I had notes to consult (I *probably* have the sources > for the device here somewhere -- no doubt on 17" wide > tractor feed "green stripe" paper), I could see which chains > we were using and check to see if we were in a region of > high GDOP, or maybe some of the trig was running in > nasty areas (e.g., tan 90). > > <shrug> As I said, I doubt anyone has *died* from this so > I won't lose sleep over it. But, I don't like things that > can't be explained (I relentlessly pursue "intermittents" > for this reason). Unfortunately, I wasn't in shape to follow up > on this at the time ;-) From this discussion I think I've now understood what "watermarking" is... See ya, Dave
From: George Neuner on 19 May 2010 12:07 On Tue, 18 May 2010 13:36:39 -0700, D Yuniskis <not.going.to.be(a)seen.com> wrote: >George Neuner wrote: > >> to steer compass courses through channels as narrow as 30ft (lotsa fun >> at night in pea soup fog). At high precision my unit could repeatedly >> find a station set waypoint within 5ft up to about 10kt. > >In the prototype run, I set all the waypoints to be marked buoys >so I could verify we were where we should be (i.e., there are no >street corners on the open ocean :> ). I can recall coming close >enough to actually fear we were going to hit the buoys (we'd run >at about 25kt's to cover as much ground as possible -- the entire >trip was several hundred Nmiles). At one point, the "next leg" >was almost "back the way we came" (i.e., turn *really* hard to >starboard). We passed just to the left of the buoy (narrowly >missing another craft that was sitting nearby), then heard the >"groan" that comes from trying to make too sharp a turn too >quickly as the boat turned around and appeared ready to make >another run at the other craft sitting nearby :> (we ended >up with the buoy on our left again -- i.e., neatly going *around* >it). With my unit I found a significant difference between the finding of hand entered coordinates (charting a course) vs recording the current coordinates while on station. Most likely the coordinate registers had more precision than the display and what could be hand entered. Cruising to a new destination, I'd would get within 100 or so feet of the waypoint buoy (or whatever) - easy visual range if not in bad weather. But if I maneuvered right next to the buoy and replaced the hand entered waypoint with a recorded one, even if the displayed coordinates were the same, on the way back the unit would steer me right over it. George
From: George Neuner on 19 May 2010 16:11
On Tue, 18 May 2010 13:12:19 -0700, D Yuniskis <not.going.to.be(a)seen.com> wrote: >Hi George, > >George Neuner wrote: > > On Mon, 17 May 2010 09:52:20 -0700, D Yuniskis > > <not.going.to.be(a)seen.com> wrote: > >> Is there an easy/high-speed way to verify prerecorded > >> media is "playable"? E.g., discs that see lots of > >> circulation (e.g., "Blockbusters", public library, etc) > >> that need to be verified as "undamaged" before being > >> reintroduced into circulation? > > > > There's no way for you to check the integrity of the stamped aluminum > > cookie other than to try to play it ... on the production line where > >Why is that? Can't you just "read" it in a DVD-R >(or whatever)? Verify no read errors, etc.? No. First, music CDs (stamped or burned) have no error correction. Data CDs and all DVD formats do have block error correction ... but being playable means only that error correction or overscanning allowed the particular drive used to get a clean or repeatable read. Another drive may not be so lucky. Overscanning is rereading the block[*] with the head offset. Consumer disc players now typically overscan (up to) 4..16 times with varying head positions. The ability of the drive to overscan makes a big difference in how well a damaged disc will play. [*] since CDs and DVDs are recorded using spiral tracks, rereading the same area is a complex maneuver. Consumer units have a few seconds of delay in playback to allow for read-ahead overscanning. Since music lacks error correction, music players read the same "block" multiple times and play the results if they get a repeatable read - they skip if all the read tries return different results. VCD and DVD players skip if they exhaust overscan retries without getting a (possibly corrected) clean read. However, most computer disc drives lack overscan (the mechanism is the same as the consumer unit but the firmware is different). Computer drives generally rely solely on ECC and on the operating system to retry failed reads. The expectation (realistic or not) is that people are more careful handling data discs. Good copiers such as Nero are able to make the drive overscan, but it won't do so normally. [I don't know if you've personally experienced this, but often a scratched disc will play just fine in a consumer player but have fits or not play at all on a computer. It is most likely to occur with music CDs but can happen with any disc.] On the flip side, with recordable discs there is the issue of +R vs -R recording formats and media reflectivity. +R players can read -R discs, but the reverse is not true. Even if the player can read the recording format, it may have problems dealing with different brands of discs. All recordable media is less reflective as compared with stamped discs and the reflectivity varies by manufacturer (what mix of dyes is used). With darker discs, the player can perceive phantom pits which it interprets as corrupt data. Players that read recordables have a variable reflectivity threshold which is determined by trial and error when the disc is inserted and identified ... but not all players have enough range to reliably read all the different brands of discs. > > Probably you were looking for an answer like: "stick it in the player > > and run this <obscure> software" ... Sorry. > >Yeah, something the functional equivalent of "playing it and >'watching' it" -- except at high speed. (e.g., a library >probably loans thousands of DVDs daily. It's just not >practical to "watch" every one as it is returned... You can take a look at Nero DiscSpeed (www.nero.com) and/or K-Probe (www.k-probe.com). They purport to determine disc quality and are meant for periodic testing of archive recordables. They can be used on any disc, but IMO they really can tell you only how well your own equipment is working. I would not go so far as to say a disc which passes their tests is widely circulatable. George |