Duplication vs Replication

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Re: Duplication vs Replication

Post by Eddy Deegan »

I am now clear on the difference between duplication and replication. I've no idea why I wouldn't have known but it's clear I've missed the distinction over the years and I'm happy to have been educated on it :blush:

Some CD players I've owned have had trouble with burned CDs. My assumption was that this was down to a weaker bounce-back of the laser with a CDR as opposed to the physical pits of a pressed CD.

CDRs are less robust than pressed discs, especially if exposed to sunshine for a bit. The first album I released back in the early '90s was distributed on CDRs but the physical offering of my next will certainly be on silver pressed CDs!
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Re: Duplication vs Replication

Post by Hugh Robjohns »

Eddy Deegan wrote: Thu Aug 18, 2022 11:52 pmSome CD players I've owned have had trouble with burned CDs. My assumption was that this was down to a weaker bounce-back of the laser with a CDR as opposed to the physical pits of a pressed CD.

Your assumption is correct Eddy.

In a pressed CD the laser is focused to a beam slightly wider than the width of the bump (the stamped pits are sputtered with the reflective layer then sealed with a lacquer and the label printed on top, but the laser shines through from the other side so sees bumps, not pits).

If there's no bump, lots of light gets reflected, so the sensor sees bright. Where there is a bump, its roughly a 1/4 wavelength high, so light reflected from the top of the bump is phase shifted by 1/2 wavelength compared to light reflected from the flat surrounding area. This only works, of course, because the wavelength of the laser light can be determined very precisely, and all the light is phase coherent — all in the same phase to start with.

Consequently, these two reflections from the bump and surrounding land interfere and largely cancel, and so the sensor sees dark. The changing light (no bump) and dark (bump present) reflections as the disc spins and bumps pass under the laser convey the encoded data.

With 'burned' CDs, the data is encoded within a dye layer which the laser light has to get through to reach the reflecting layer, and then get back through a second time to reach the sensor. Consequently, the bright bits aren't anywhere near as bright as on a pressed disc, and so some (especially elderly) players struggle to recognise the difference between bright and dark.... because its more like dim and dark!

And often the problem is compounded in that not only is the encoded data difficult to see, but the servo tracking and focusing mechanisms struggle to follow the track properly too because its like trying to drive down a bendy road in thick fog!

CDRs are less robust than pressed discs, especially if exposed to sunshine for a bit.

Yep, heat and light degrade the dye layer, making everything even darker. It also degrades over time (years or tens of years) too, although some dye chemistries are better than others in that respect.
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Re: Duplication vs Replication

Post by Eddy Deegan »

Hugh Robjohns wrote: Fri Aug 19, 2022 1:22 am If there's no bump, lots of light gets reflected, so the sensor sees bright. Where there is a bump, its roughly a 1/4 wavelength high, so light reflected from the top of the bump is phase shifted by 1/2 wavelength compared to light reflected from the flat surrounding area. This only works, of course, because the wavelength of the laser light can be determined very precisely, and all the light is phase coherent — all in the same phase to start with.

Consequently, these two reflections from the bump and surrounding land interfere and largely cancel, and so the sensor sees dark. The changing light (no bump) and dark (bump present) reflections as the disc spins and bumps pass under the laser convey the encoded data.

I had to read that a couple of times to really get it but now I do. Very enlightening and thank you for putting it terms that make sense Hugh :thumbup:
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Re: Duplication vs Replication

Post by giddyap »

Hugh Robjohns wrote: Fri Aug 19, 2022 1:22 am
Eddy Deegan wrote: Thu Aug 18, 2022 11:52 pmSome CD players I've owned have had trouble with burned CDs. My assumption was that this was down to a weaker bounce-back of the laser with a CDR as opposed to the physical pits of a pressed CD.

Your assumption is correct Eddy.

In a pressed CD the laser is focused to a beam slightly wider than the width of the bump (the stamped pits are sputtered with the reflective layer then sealed with a lacquer and the label printed on top, but the laser shines through from the other side so sees bumps, not pits).

If there's no bump, lots of light gets reflected, so the sensor sees bright. Where there is a bump, its roughly a 1/4 wavelength high, so light reflected from the top of the bump is phase shifted by 1/2 wavelength compared to light reflected from the flat surrounding area. This only works, of course, because the wavelength of the laser light can be determined very precisely, and all the light is phase coherent — all in the same phase to start with.

Consequently, these two reflections from the bump and surrounding land interfere and largely cancel, and so the sensor sees dark. The changing light (no bump) and dark (bump present) reflections as the disc spins and bumps pass under the laser convey the encoded data.

With 'burned' CDs, the data is encoded within a dye layer which the laser light has to get through to reach the reflecting layer, and then get back through a second time to reach the sensor. Consequently, the bright bits aren't anywhere near as bright as on a pressed disc, and so some (especially elderly) players struggle to recognise the difference between bright and dark.... because its more like dim and dark!

And often the problem is compounded in that not only is the encoded data difficult to see, but the servo tracking and focusing mechanisms struggle to follow the track properly too because its like trying to drive down a bendy road in thick fog!

CDRs are less robust than pressed discs, especially if exposed to sunshine for a bit.

Yep, heat and light degrade the dye layer, making everything even darker. It also degrades over time (years or tens of years) too, although some dye chemistries are better than others in that respect.

Let's assume there has been no damage to a CDR. Do you think that a CDR that couldn't be read by someone's 2001 Ford Explorer would still be unreadable in someone's new Tesla? (I\what I'm getting at is car audio technology)

Also, if I have a that 2001 Ford Explorer and it DOES read an undamaged CDR then is it safe to assume that it would also be read in the new Tesla?
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Re: Duplication vs Replication

Post by Eddy Deegan »

giddyap wrote: Fri Aug 19, 2022 1:55 am Let's assume there has been no damage to a CDR. Do you think that a CDR that couldn't be read by someone's 2001 Ford Explorer would still be unreadable in someone's new Tesla? (I\what I'm getting at is car audio technology)

Based on my own experiences albeit without first-hand knowledge of either car systems I would consider that a distinct possibility.

giddyap wrote: Also, if I have a that 2001 Ford Explorer and it DOES read an undamaged CDR then is it safe to assume that it would also be read in the new Tesla?

I'd say no, it isn't. Each system will have its own quirks when it comes to that sort of thing. There are old systems that will handle CDRs well, there are new ones that won't, there are new systems that will and there are old ones that won't.

When offering a new release on physical CDs, replicated is the way to go because that will work with pretty much everything whereas compatibility with CDRs/duplicated vary more from system to system.
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Re: Duplication vs Replication

Post by Mike Stranks »

giddyap wrote: Fri Aug 19, 2022 1:55 am
Let's assume there has been no damage to a CDR. Do you think that a CDR that couldn't be read by someone's 2001 Ford Explorer would still be unreadable in someone's new Tesla? (I\what I'm getting at is car audio technology)

Also, if I have a that 2001 Ford Explorer and it DOES read an undamaged CDR then is it safe to assume that it would also be read in the new Tesla?

Does the Tesla have a CD audio player?

I'm only asking because my last three cars - since 2016 - have not had any means of playing CDs and, as an avid viewer of new car reviews, I haven't seen one for several years. These days it's all Apple Car Play and Android Auto.

Of course, I speak from a UK perspective. Things may be different elsewhere.

(BTW: the high cars/years ratio is because of force of circumstance - two 'duff' cars in succession... :( ... :) )
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Re: Duplication vs Replication

Post by Hugh Robjohns »

Eddy Deegan wrote: Fri Aug 19, 2022 1:44 amI had to read that a couple of times to really get it but now I do.

Sorry, my inner sub-editor had already gone to sleep when I wrote that, so it probably wasn't as clear as it could have been!

The important points are that the depth of the pressed pits (which become bumps to the reading laser) are design to equal 1/4 wavelength of the infrared laser's frequency.

And a laser is used because it only produces light at one frequency, (and thus one wavelength) with all the light being emitted in the same phase.

So, where there is no bump all the focused spot of laser light reflects from the 'land' of the disc, giving a bright reflection.

Where there is a bump, some of that focused spot covers the bump, and some covers the land around the bump. So some laser light reflects from the top, while some light travels 1/4 wavelength further to the land, and then 1/4 wavelength back before its level again with the top of the bump.

The result is that this 'land light' has travelled a total of 1/2 wavelength further than the 'bump light' putting it 180 degrees out of phase.

Consequently, these two elements combine and cancel out to produce a very dim overall reflection.

The dimensions and precision involved in CDs — and even more so for DVDs and Blurays — are mind boggling small!

I have in my head an analogy that i used when I taught this stuff to BBC engineers, that the tracking servo for a CD has to perform the equivalent of keeping the nose wheel of a jumbo jet aligned directly above the central barrier of a motorway while flying at 700 mph!

Compared to a vinyl record where a bit of rock glued to a telegraph pole is dragged along a ditch by an old tractor, CDs, DVDs and BRs are truly awesome technologies.
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Re: Duplication vs Replication

Post by Folderol »

CDRs are plain weird. I've had ones that won't play back on the same drive that they were recorded with, yet are fine on a different drive :?

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Re: Duplication vs Replication

Post by MOF »

I’ve had cdr versions in my cars for well over ten years in all sorts of weather, only one developed some clicks on the last track and that hasn’t got worse over the years.
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Re: Duplication vs Replication

Post by FrankF »

Hugh Robjohns wrote: Fri Aug 19, 2022 11:19 am Compared to a vinyl record where a bit of rock glued to a telegraph pole is dragged along a ditch by an old tractor...


Ha! Nice one, centurion! :lol:

Or, occasionally, an old tractor with a copper kettle or two balanced on the seat. :D
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Re: Duplication vs Replication

Post by ConcertinaChap »

If you go the duplication route make sure the company providing the service can handle CD Text. This happened to me once with a run of 100 CDs which, when I got them, were missing all the text (ISRC codes and the like) I'd lovingly set up. The company claimed their duplication software didn't handle CD text and why did I want it anyway? Had to have a hell of a row there.

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Re: Duplication vs Replication

Post by Mike Stranks »

As is often the case, a topic gets 'legs' and takes off...

Of course, the OP can do what he wants to do; he knows his audience and what they want to buy...

But I recall that on previous occasions forumites have been quick to observe that CDs - however produced - are yesterday's technology, that (with some exceptions - eg Hugh) they're not buying them, and observing that car-makers abandoned the technology several years ago.

So... if you still want to run with this...

* Do you still buy CDs?
* Do you play CDs in the car?
* If you make music, do you still produce CDs? How many do you sell? Do you make your music available via other platforms? Ratio of download:CD sales?
* If you gig, do you have a merch table that includes CDs? Do they sell?
* If you're a facilitator - eg James P - are you still asked to provide CDs? Frequently? Occasionally?

Personally, I've not bought a CD for several years now; I have no means of playing them in the car and no-one asks me if I can produce a CD for them - it's all 'files for download'. (I'm waiting to hear from a charity about their CD-library of talks etc they hold. They asked for my advice/assistance. They're keen to transfer the most-requested stuff to MP3 and work out a selling mechanism... The aim is to close the library as it's moribund... Similarly, the charity for whom I was audio-producer decided 8 years ago to stop producing audio material on CD and make it available exclusively via web-based downloads/players.)

I'm genuinely interested in what the music-buying public is buying in terms of product...
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