luca@alberta.UUCP (Luca Vanzella) (02/25/86)
I've been reading this newsgroup for a while and would like to know if anyone has posted some guidelines on the evaluation of CD players. I'm looking for a CD player and I have read and heard various opinions on things like: over-sampling, digital vs analog filtering, single vs three beam laser tracking, etc. If anyone has posted such an article in the recent past I would appreciate it if they would e-mail to me at the address below. One thing in particular - a local dealer (carries JVC, Sanyo, and Sony) suggested to me that 3-beam laser units would not last as long as 1-beam laser units due to "the power configuration". He suggested 6-8 years life expectancy vs 8-10 years. Any comments? Please e-mail them. I will summarize to the net, if possible. I've read with interest many of the articles on this group. Thanks to all those who post reviews of good/bad quality CD's. Thanks in advance. Luca Vanzella (...!ihnp4!alberta!luca)
ben@catnip.UUCP (Bennett Broder) (03/04/86)
In article <817@alberta.UUCP>, luca@alberta.UUCP (Luca Vanzella) writes: > One thing in particular - a local dealer (carries JVC, Sanyo, and Sony) > suggested to me that 3-beam laser units would not last as long as 1-beam > laser units due to "the power configuration". He suggested 6-8 years > life expectancy vs 8-10 years. This is just not true. This salesman seems to suffer from the common misconception that 3 beams means 3 lasers. This just isn't the case. The three beam units use half silvered mirrors to split the beam of a single laser into three individual beams. The center beam is used for reading audio information -- it is equivilent to the single beam used in other machines. The other two beams are aimed just to the left and right of the track. The outputs from the photodetectors from the side beams are compared. If the unit is tracking properly, the difference should be zero. But if the pickup is off slightly, one of the auxiliary beams will be dimmer (because it will be catching the edge of the pits). The sign and magnitude of the difference between the two auxiliary beams tells the player how much of a correction it needs to make and in what direction. This mechanism gives a three beam player a big advantage in tracking eccentric discs. -- Ben Broder {ihnp4,decvax} !hjuxa!catnip!ben {houxm,topaz}/
atoy@watnot.UUCP (Andy Toy) (03/05/86)
In article <253@catnip.UUCP> Ben Broder writes: >This is just not true. This salesman seems to suffer from the common >misconception that 3 beams means 3 lasers. This just isn't the case. >The three beam units use half silvered mirrors to split the beam of a >single laser into three individual beams. The 3 beam players do indeed have only one laser which is split, but I think it is done using a diffraction grating and not half silvered mirrors. >The center beam is used for >reading audio information -- it is equivilent to the single beam used >in other machines. The other two beams are aimed just to the left and >right of the track. The outputs from the photodetectors from the side >beams are compared. If the unit is tracking properly, the difference >should be zero. But if the pickup is off slightly, one of the auxiliary >beams will be dimmer (because it will be catching the edge of the pits). >The sign and magnitude of the difference between the two auxiliary beams >tells the player how much of a correction it needs to make and in what >direction. This mechanism gives a three beam player a big advantage in >tracking eccentric discs. Just to elaborate a bit on this, the beams on either side of the central beam are half on and half off of the adjacent pit tracks. If, for example the beams start drifting to the left, the left beam will get darker since it will be in the pits more and the right beam will be brighter since it will be out of the pits more. The 3 beams will not be right beside each other, but one will be ahead of the centre beam and the other will be behind. This also aids in tracking since only one of the beams will be in a radial scratch at any one time. -- Andy Toy, Mapping Analysis and Design Group (MAD), Faculty of Environmental Studies, University of Waterloo, Waterloo, Ontario, CANADA N2L 3G1 (519) 885-1211 x6592 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= | UUCP: ...!watmath!watdcsu!atoy BITNET: atoy at watdcsu | # CSNET: atoy%watdcsu@waterloo.csnet CDN: atoy@dcsu.waterloo.cdn # | ARPA: atoy%watdcsu%waterloo.csnet@csnet-relay.arpa | =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
sjc@mips.UUCP (Steve Correll) (03/07/86)
> The three beam units use half silvered mirrors to split the beam of a > single laser into three individual beams. The center beam is used for > reading audio information -- it is equivilent to the single beam used > in other machines. The other two beams are aimed just to the left and > right of the track. The outputs from the photodetectors from the side > beams are compared. If the unit is tracking properly, the difference > should be zero. But if the pickup is off slightly, one of the auxiliary > beams will be dimmer (because it will be catching the edge of the pits). > The sign and magnitude of the difference between the two auxiliary beams > tells the player how much of a correction it needs to make and in what > direction. This mechanism gives a three beam player a big advantage in > tracking eccentric discs. The argument above would seem to apply equally to one-beam or three-beam units: it's the number of detectors, not the number of beams, that matters. (For reference, the papers on the CD system in the Philips Technical Review volume 40 number 6 describe a mechanism wherein one wide beam impinges on four photodiode detectors; it too compares the levels from the diodes to the left and right of the track to decide whether to servo the optical mechanism to the right or left.) I doubt that any CD player could do away with such servoing, since there's nothing else to keep the beam aligned with the track. A servoless system accurate enough to advance the optics at just the right speed would cost a lot more than a servo system! Does a three-beam unit with three detectors really offer advantages over a one-beam unit with four detectors? If so, why? -- ...decwrl!mips!sjc Steve Correll
schley@mmm.UUCP (Steve Schley) (03/13/86)
In article <253@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes: >In article <817@alberta.UUCP>, luca@alberta.UUCP (Luca Vanzella) writes: >> One thing in particular - a local dealer (carries JVC, Sanyo, and Sony) >> suggested to me that 3-beam laser units would not last as long as 1-beam >> laser units due to "the power configuration". He suggested 6-8 years >> life expectancy vs 8-10 years. > >This is just not true. This is a pretty strong statement, without any hard facts to back it up. It _is_ possible that three-beam pickups might die young. See below. >This salesman seems to suffer from the common >misconception that 3 beams means 3 lasers. This just isn't the case. >The three beam units use half silvered mirrors to split the beam of a >single laser into three individual beams. The center beam is used for >reading audio information -- it is equivilent to the single beam used >in other machines. The other two beams are aimed just to the left and >right of the track. The outputs from the photodetectors from the side >beams are compared. If the unit is tracking properly, the difference >should be zero. But if the pickup is off slightly, one of the auxiliary >beams will be dimmer (because it will be catching the edge of the pits). >The sign and magnitude of the difference between the two auxiliary beams >tells the player how much of a correction it needs to make and in what >direction. This is, to the best of my knowledge, an accurate description of how three-beam pickups work. Now, let's try a thought experiment, comparing the two types of pickups. To get the same raw data S/N from the disc, I suggest that the intensity of the three-beam pickup's center beam must equal the total intensity of the single-beam pickup's laser. Since the three-beam pickup is also generating two side beams, its total output must be greater than that of the single beam pickup, which means higher power dissipation in the diode. This generally reduces life expectancy of an electronic component. I do not know of any studies that discuss actual failure rates of CD player laser diodes. >This mechanism gives a three beam player a big advantage in >tracking eccentric discs. This statement, while possibly true in specific comparisons, is not true in the general case. A while back, reviewers were finding that certain Philips single-beam players would out-track certain Japanese three-beam players on defective discs. -- Steve Schley ihnp4!mmm!schley
ben@catnip.UUCP (Bennett Broder) (03/16/86)
In article <601@mmm.UUCP>, schley@mmm.UUCP (Steve Schley) writes: > In article <253@catnip.UUCP> ben@catnip.UUCP (Bennett Broder) writes: > >In article <817@alberta.UUCP>, luca@alberta.UUCP (Luca Vanzella) writes: > >> One thing in particular - a local dealer (carries JVC, Sanyo, and Sony) > >> suggested to me that 3-beam laser units would not last as long as 1-beam > >> laser units due to "the power configuration". He suggested 6-8 years > >> life expectancy vs 8-10 years. > > > >This is just not true. > > This is a pretty strong statement, without any hard facts to back it up. > It _is_ possible that three-beam pickups might die young. See below. > > >This salesman seems to suffer from the common > >misconception that 3 beams means 3 lasers. This just isn't the case. > > {description of 3 beam technology} > > This is, to the best of my knowledge, an accurate description of how > three-beam pickups work. Now, let's try a thought experiment, > comparing the two types of pickups. To get the same raw data S/N from > the disc, I suggest that the intensity of the three-beam pickup's > center beam must equal the total intensity of the single-beam pickup's > laser. Since the three-beam pickup is also generating two side beams, > its total output must be greater than that of the single beam pickup, > which means higher power dissipation in the diode. This generally > reduces life expectancy of an electronic component. > Single beam lasers have a much wider beam. Tracking information must still be provided for the servo circuit. > I do not know of any studies that discuss actual failure rates of CD > player laser diodes. Neither do I, though the one case I know of where a laser diode failed was an old top loading Magnavox, a single beam design. > >This mechanism gives a three beam player a big advantage in > >tracking eccentric discs. > > This statement, while possibly true in specific comparisons, is not > true in the general case. A while back, reviewers were finding that > certain Philips single-beam players would out-track certain Japanese > three-beam players on defective discs. By eccentric discs, I mean those with the hole punched off-center, not with other types of defects. According to the service technician I spoke with at Studer/Revox (a company that makes single beam units), the configuration of the three beam machines is much more resistant to this type of mistracking. -- Ben Broder {ihnp4,decvax} !hjuxa!catnip!ben {houxm,topaz}/
sjc@mips.UUCP (Steve Correll) (03/18/86)
> > This statement, while possibly true in specific comparisons, is not > > true in the general case. A while back, reviewers were finding that > > certain Philips single-beam players would out-track certain Japanese > > three-beam players on defective discs. > > By eccentric discs, I mean those with the hole punched off-center, not > with other types of defects. According to the service technician I spoke > with at Studer/Revox (a company that makes single beam units), the > configuration of the three beam machines is much more resistant to this > type of mistracking. Why are they more resistant to eccentricity? As I said in an earlier posting, the original Philips Technical Review papers explain that their single-beam mechanism shines one beam onto four detectors, and uses the imbalance between the detectors to the right of the track and those to the left of it to generate a correction signal that servos the optical mechanism. This is quite similar to Mr. Broder's description of the three-beam mechanism. Can somebody explain why one wide beam impinging on four detectors is inferior to three beams impinging on three detectors? -- ...decwrl!mips!sjc Steve Correll