Great thanks goes out to Reinhart for his professional evaluation of the Thomson laser in the following article! (Only one word of "correction" so to speak, Thomson drives are marginally capable of reading CD-R media and it is largely dependent on the media, I just didn't want any one to read this and take it for gospel that a Thomson will never read a CD-R...  They are not good at it but they can succeed consistently with the right media.)

-Llamma

The Following Article by Reinhart-

This article discusses the various design features of the optical pickup used in the Thomson TGM-600 DVD-ROM drive, OEMed specifically for use in the Microsoft X-Box.

The pickup is built on a die-cast chassis, implementing a single gallium-aluminum-arsenic (GaAlAs) semiconductor laser operating at a wavelength of 650 nanometers for use with DVD-Video and DVD-ROM media.  It is also capable of reading redbook Compact Disc and CD-ROM media, provided that the media is press-replicated and not recorded on CD-R discs.  Due to the use of a red laser, pythalocyanine (short strategy) and cyanine (long strategy) CD-R media cannot be read by this disc drive.  Bluish and greenish colors typical of the recording dye media used in CD-R discs can intercept red light, inhibiting the laser from scanning the disc properly.

The voice coil shield for the objective lens focus drive is metallic and glued onto place.  Removal of the shield reveals the voice coil actuator elements wrapped around the objective lens holding frame.  The objective lens frame is suspended by four firm wire supports which also double as power feeds to the voice coils.  Fixed permanent magnets for reactance to the electromagnetic fields generated by the voice coils for lens movement are positioned parallel to each other and are placed perpendicular with the chassis.  Removal of the objective lens and the voice coil elements as a whole is accomplished by removing a single torx screw on the top of the assembly and lifting upward and away. 

The lens and voice coil elements rest atop a removable sheet metal platform with a hole cut in the middle to allow the beam to travel to and from a fixed lens.  Underneath the lens is a fixed mirror, which is exposed and glued into place.

Inside the optical pickup with the shielding removed and the pickup ribbon connector facing away from you, to the top on the angled side of the pickup is the GaAlAs laser diode.  Behind the diode is the power circuit.  In front is the grating, which splits the beam emitted from the diode into three.  The disc drive uses a traditional three beam tracking system, with the center beam carrying the signal while the adjacent pilot beams work to keep the signal beam centered onto the track.  After passing the grating, the beams strike a prism which directs the beams towards the fixed mirror to the right of the prism.  The beams reflect off the mirror and upwards through fixed lens to the objective lens.  The objective lens handles beam focus, tangent, and tracking on the disc.  After the beams strike the disc, it is reflected back down through the objective lens and fixed lens and reflected back inside the pickup by the fixed mirror.  The fixed mirror reflects the beams back to the prism, which directs the beams towards a cylindrical lens.  The beams pass the cylindrical lens into the photosensor array, which senses the signal beam and turns it into an electrical signal that is demodulated and error corrected to retrieve the information contents encoded in the disc.  The photosensor array also provides tracking values from the pilot beams and focus values from the signal beam to the servo control.

Servo control for three beam tracking works through the use of two pilot beams.  The pilot beams scan both sides of the main pit and land track.  There are two photosensors in the array which senses the pilot beams, providing electrical values used as feedback by the servo control.  If any of the pilot beams increase in value as sensed by the photosensors, the servo control knows that the main signal beam is not within track and applies a correction automatically and instantaneously to the objective lens.

Servo control for the focus control works through the use of four photosensors arranged in a square divided into quadrants and uses the main signal beam.  Perfect focus is achieved when the beam strike on the photosensors is circular, striking all quadrants equally.  A deviation in focus causes the beam strike to become ovular, which strikes half the quadrants more than the other half.  This feedback informs the servo control that the focus needs to be corrected and applies it automatically and instantaneously to the objective lens.  The photosensor quadrants also convert the light sensed into electrical impulses that are decoded into data usable by the X-Box.

Servo control based on feedback provided by the photosensor for both focus and tracking is constant during normal reading.  This means that discs with imperfections that can affect the read are usually compensated for by the servo control instantly without the user knowing about it.

Simply, this is one of the worst optical pickups I’ve ever seen.  It is apparent right from the initial observation that this pickup was made with corner cutting in mind.  For one thing, there are numerous exposed or inadequately sealed parts, which is an unacceptable design practice.  One of the parts that are exposed to the outside is the sensor side of the photosensor array.   This all adds up to a pickup with its vital innards vulnerable to dust accumulation and smoke contamination during normal operation and at rest.

Microsoft likely chose the TGM-600 for the X-Box because Thomson was able to mass produce required quantities of the drive at their Chinese facilities at an attractive price point.  Unfortunately, like many products from Thomson (such as their RCA and GE branded TV sets and DVD players), the drive is problematic from design shortcomings, mainly in the optical pickup.  The rest of the drive is a fairly good design, however.  A simple rack and pinion configuration is used for the pickup kicker mechanism while a brushless DC motor is used for the disc spindle.  The drawer and traverse elevation mechanism is also simple and fairly reliable.  Unfortunately, these benefits are undermined by a lousy pickup.

Comments on the side, you can’t expect the French to develop decent electronic components.  Thomson Electronics is owned and operated by the French government.  Just like Renault cars, a lot of Thomson products are garbage in comparison to the competition.  Microsoft would have likely avoided customer relations headaches by simply commissioning a reputable Japanese company to make DVD drives for their first batch of X-Box consoles instead of hiring Thomson in the first place.  Although, to be fair, Microsoft obviously couldn’t hire Sony or Panasonic to make drives for their X-Box as Sony is a competitor with their PS-2 and Panasonic was heavily involved in drive and disc manufacturing for the Nintendo GameCube.  Pioneer, however, was likely available and could have produced a vastly superior drive in comparison to the beleaguered Thomson product.  Pioneer might have also produced a drive that likely would’ve been superior to the Philips and maybe even the Samsung product.  Toshiba was another possibility, but they developed the Emotion Engine architecture in the PS-2 along with Sony.)