Twin Laser Pick-ups and their purpose in this world.

[Seth=L]

This was prompted from another thread when I just happened to become more curious concerning the mystery (mystery to me) of older DVD players with twin/dual laser pick-up mechanisms. The Toshiba SD-9000 for example has two lasers diodes, one for DVD and another for CD. Sony apparently employed this method as well in their first batches of players.

From Sony's website-

Twin Laser Optical Pick-Up
Twin Laser pick-up allows you to enjoy a variety of media through single player. Twin laser pick-up uses separate lasers for CD and DVD format. This allows accurate data transfer from virtually any disc.
Twin Laser Pick-Up
Sony uses a set of twin lasers to read discs. One laser is used for reading DVDs while the other laser is used for reading CDs. This gives aids in greater read back quality and durability as a specific dedicated laser is used for each defined function.

This makes sense to a degree. Just like most technologies that are just starting out like refrigerators, where initially designed to be somewhat reliable. Then after a few years they go cheap and start making disposable products. Of course the gap closes more and more as each product is released to were the initial product is almost as unreliable as the disposables they push on consumers. This is largely the consumer's fault of course, but I won't get into that.

As far as the quality difference in sound, even with a different bandwidth it seems unlikely, unless some of the first lasers just had big problems with tracking. I don't recall having any problems with tracking or SQ issues in regards to older DVD players.

From Panasonic-

Twin Laser Pickup for DVD/CD Playback
All Panasonic DVD player models feature a newly developed Twin Laser Pickup. Using one objective lens with two laser units having different wavelengths optimized for DVD and CD reproduction, this advanced pickup can also play CD-R/RW discs. The integrated design of the Twin Laser Pickup DVD unit reduces noise pickup by using an OSIC (optical electric IC) with built-in I/V amp. This highly sensitive state-of-the-art optical pickup is made possible by Panasonic’s advanced optical device technology.

Apparently this technology is still being used by Panasonic and likely other manufacturers as well. According to this, it reduces noise. I think that "OSIC" is supposed to be OEIC.

And here is a slice of what that is about here-

Papers are solicited in the general area of optoelectronic integrated circuits (OEIC). Optical and/or electronic devices are integrated to address the issues of cost, reliability and performance. Demands for greater bandwidths have driven the telecommunication research communities to realize complex optoelectronic integrated circuits such as transceivers, switching systems, low chirp optical sources and multichannel optical distribution systems. As bandwidth need increases in the multimode data link arena, integration of vertical cavity surface emitting lasers arrays with multiple wavelengths, monitoring photodiodes, and drivers are on the horizon. Other applications such as OEIC neural systems, smart pixel arrays, projection displays, integrated read/write heads for DVD, and printing are emerging OEIC technologies.

Anyway, I had just been curious about the purpose of twin/dual laser pick-ups. If anyone would like to add anything it would be appreciated.

 

[mtrycrafts]

This was prompted from another thread when I just happened to become more curious concerning the mystery (mystery to me) of older DVD players with twin/dual laser pick-up mechanisms. The Toshiba SD-9000 for example has two lasers diodes, one for DVD and another for CD. Sony apparently employed this method as well in their first batches of players.

From Sony's website-

Twin Laser Optical Pick-Up
Twin Laser pick-up allows you to enjoy a variety of media through single player. Twin laser pick-up uses separate lasers for CD and DVD format. This allows accurate data transfer from virtually any disc.
Twin Laser Pick-Up
Sony uses a set of twin lasers to read discs. One laser is used for reading DVDs while the other laser is used for reading CDs. This gives aids in greater read back quality and durability as a specific dedicated laser is used for each defined function.

This makes sense to a degree. Just like most technologies that are just starting out like refrigerators, where initially designed to be somewhat reliable. Then after a few years they go cheap and start making disposable products. Of course the gap closes more and more as each product is released to were the initial product is almost as unreliable as the disposables they push on consumers. This is largely the consumer's fault of course, but I won't get into that.

As far as the quality difference in sound, even with a different bandwidth it seems unlikely, unless some of the first lasers just had big problems with tracking. I don't recall having any problems with tracking or SQ issues in regards to older DVD players.

From Panasonic-

Twin Laser Pickup for DVD/CD Playback
All Panasonic DVD player models feature a newly developed Twin Laser Pickup. Using one objective lens with two laser units having different wavelengths optimized for DVD and CD reproduction, this advanced pickup can also play CD-R/RW discs. The integrated design of the Twin Laser Pickup DVD unit reduces noise pickup by using an OSIC (optical electric IC) with built-in I/V amp. This highly sensitive state-of-the-art optical pickup is made possible by Panasonic’s advanced optical device technology.

Apparently this technology is still being used by Panasonic and likely other manufacturers as well. According to this, it reduces noise. I think that "OSIC" is supposed to be OEIC.

And here is a slice of what that is about here-

Papers are solicited in the general area of optoelectronic integrated circuits (OEIC). Optical and/or electronic devices are integrated to address the issues of cost, reliability and performance. Demands for greater bandwidths have driven the telecommunication research communities to realize complex optoelectronic integrated circuits such as transceivers, switching systems, low chirp optical sources and multichannel optical distribution systems. As bandwidth need increases in the multimode data link arena, integration of vertical cavity surface emitting lasers arrays with multiple wavelengths, monitoring photodiodes, and drivers are on the horizon. Other applications such as OEIC neural systems, smart pixel arrays, projection displays, integrated read/write heads for DVD, and printing are emerging OEIC technologies.

Anyway, I had just been curious about the purpose of twin/dual laser pick-ups. If anyone would like to add anything it would be appreciated.

Here is an interesting read on CDs. It shows how the zeros and ones are determined, that I briefly mentioned in another post, by length of the mountain or valley. The leading and trailing edges are the transition from one to the other.

http://www.deluxemedia.com/images/cdintroduction.pdf

Now, to your post, the above link misses a crucial point, what happens at these leading and trailing edges. I reviewed my book 'Principles of Digital Audio,' and forgot parts of this, that the pit height is 1/4 wavelength(not 1/2 as I posted in that other message) of the laser frequency. When the light hits this, the reflected light from the sides compared to the pit, it is 1/2 wave length apart in the return trip and causes a 'destructive interference' over the pit, reducing intensity for that pit duration.

So, since the pits are different height for the DVD and CD based on that 1/4 wavelength of the frequency, that is why the two lasers. Right now I don't see how it would be done with only one laser unless the laser frequency can be changed.