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! :>
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.
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.
You can take a look at Nero DiscSpeed
formatting link
and/or K-Probe
formatting link
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.
"Each track on an Audio CD consists of sectors with each sector containing
1.75 seconds of audio. Also, each sector has error detection and correction codes to prevent or correct any possible data error or damage, as well as the control that tells exact location of the audio."
and
"Audio CD (Red Book, CD-DA) First CDs were for audio, so the CD-ROM of today is divided into time and track. Sector is the basic unit that constitutes each track. Sectors contain error correction code (ECC), as well as audio data. As a result, A little scratches on disc surface do not translate into any sound troubles."
Huh? I thought CD's had a CRC and that the CRC was physically distributed around the track to take advantage of the fact that "scratches" are "local". Though this is just a remnant of a memory from years ago -- maybe it pertained to MO drives?
OK, you've confused me (too early in the morning to be thinking :>). "... have no error correction" "do have block error correction" ??
OK. But isn;t that true of any removable medium? I.e., just because I can read a floppy, doesn't mean *you* can read it (?)
Offsetting the head hopes to get a better read than where it
*should* be reading the track? (I was under the impression the head was servoed to *follow* the track's location)
OK, so that's just a matter of how "good" the user's "media player" is. But, is there some "norm" which defines the criteria that a media player *should* be able to accommodate? I.e., how do you ensure media from the publisher is "acceptable" (since "I" may have a crappy player, etc.)
Meaning, their (video) buffers run dry before they can "recover" so they just "move on". I am trying to recall how my LVD's handled problems...
Ah, so the "difference" lies in "data" vs "music"?
I will have to try this. The only time I have seen problems with (audio) CD's is when trying to play copied discs in the car (though I attributed that to using "non-audio" media and the laser seeing the wrong color ink, etc.)
[I also had a store bought disc which was apparently defective but that's an exception]
Understood.
My experience with recorded DVD's (not data) has been *better* in the computer than in the player! E.g., I put slide shows, home movies, etc. on DVDs and can almost always play them on a PC. Yet, often have problems playing them on a player. (I discarded one player simply because it had so many problems with these media that I figured the player was defective!)
Ahhhh...
So, the approach the public library uses -- regardless of *what* it actually is -- is flawed. Even if they have a consumer player "in the back room" and sit and watch every disc reported as "defective", there is no guarantee that they will *see* it as defective! I.e., all they can hope to do is verify something is GROSSLY defective (my point being: this is a collosal waste of their time!!)
Yes, I understand that. In the context of my original question, all you *need* is a CRC to tell you if your read *was* correct. (that was my stated goal).
ECC just makes it easier to *get* a "good read" in the presence of (some number of) errors.
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