Challenges Of CD-RW/DVD Combo Drive Design.

What's required to reduce costs and boost write speeds

The first part of this article appeared in the October issue (Q3 2000) of Storage Inc. CTR had planned to run the second part in this issue, but, because of the complexity of the article, we have decided to run the article in its entirety.

With the burgeoning popularity of both CD-RW and DVD drives, the market is eagerly awaiting an affordable combo drive with performance comparable to today's state-of-the-art individual CD-RW and DVD drives. However, early combo drives appearing in the market have been too expensive for widespread acceptance and their write speeds lag a generation or more behind separate CD-RW drives.

Part of the problem is a perception by some vendors that adding a DVD read function is enough to make a combo drive competitive with a stand alone CD-RW drive. Early lessons of the CD-RW market have shown that users are far more interested in write speed and quality than in read speeds.

This article will discuss the challenges involved in creating combo CD-RW/DVD drives and how these challenges can be addressed to boost performance and reduce cost.

Market Overview

According to a recent consumer storage research report from IDC (Sept. 2000), nearly half (47%) of retail PCs are now equipped with CD-RW drives, and approximately 20% ship with dual optical drive configurations, either CD-RW and DVD combinations or CD-RW and CDROM. DVD is also well on its way to broad acceptance in the retail PC market, shipping in more than 20% of new retail systems.

It's a reasonable prediction that many users would prefer a combo CD-RW/DVD drive rather than having to choose one or the other, or to specify a second drive, as long as the performance and price are comparable. To date, however, the high price of combo drives, approaching $300 with only a 4X write speed, has been a barrier, particularly when standalone CD-RW drives offer higher write speeds.

What are the reasons behind the lag in combo drive performance? DVD and CD-RW technologies began taking off in similar time frame, led by different companies. The early leaders in DVD development didn't have strong CD-RW capabilities and got into the recordable market slightly late. Conversely, the early leaders in CD-RW technology saw a greater opportunity in the recordable market, and put greater emphasis on CD-RW development than on DVD. As a result, the two technologies developed in parallel, without plans from the outset to integrate them into a single drive. Some of the challenges in creating a cost-effective, high performance combo drive are discussed below.

Technical Challenges

The technical challenges in creating n cost-effective combo drive involve all three major elements of an optical disk drive control system: the Optical Pick-Up Unit (OPU), the drive controller, and the analog front end (AFE) signal processor. Figure 1 represents a typical block diagram for CD-RW drive control circuitry, and Figure 2 a typical block diagram of DVD-ROM control circuitry. The major challenges in creating a solution integrating all the functionality of both these devices involve the lasers in the OPU, as well as the controller and AFE circuitry.

Overview Of CD-RW And DVD Circuitry

The OPU is a specialized module with a read and write laser that reads the signals from the CD or DVD media. Its major components are a laser diode, a photo detector diode array, beam shaping and splitting optics, and a voice coil controlled lens that can be moved up and down for focus and sideways for fine tracking control. The OPU is mounted to a sled that provides for a rough movement between the inner and outer diameter of the disc. All movements of the OPU lens and sled are accomplished by the drive controller IC.

The drive controller manages read and write functions, servo control, and host interface operations. It is a mixed signal device containing analog I/O and a digital servo processor for controlling beam focus, lens and sled tracking and seeking, and disc spin rate. During reading, the controller is also responsible for slicing and digitizing the analog RF data signal from the OPU after it is processed by the AFE and then performing error correction on the digital data stream while buffering the data for host transfer. The host interface can be Ultra ATA, SCSI, or USB depending on the system requirements. During the write process the controller receives data from the host authoring application and calculates Reed-Solomon error correction codes for the data before it is written to the media. These codes will be used later during read back in order to ensure error free data expected of CD media. The faint ATIP track of a blank disc is then read to determine the disc characteristics and is also used to guide the laser during the write process much like the faint lines on school paper assist during handwriting. The data is then formatted with the proper timing marks and modulated into a digital serial stream. The edge timings of the digital stream are then adjusted by the controller with sub nanosecond accuracy to ensure that the resulting pits and spaces written on the media are of the exact size required for accurate read back.

The AFE provides for power level control during writing and is responsible for the condition of the data and servo feedback signals from the OPU. The AFE contains various circuitry, including RF-amplifier, sample and hold module, and write digital to analog converter (DAC), analog peripheral controller DAC & 3:1 analog multiplexer. For both CD-RW and DVD drives, highly integrated front-end signal processors are now available on the market. However, without careful system level design between the AFE and the controller, the actual drive level implementation may require more than 100 discrete passive and active components, such as DACs, resistors, capacitors, inductors and active filters, signal amplifiers, and comparator circuits to effectively couple an AFE and controller chip.

Write Performance

To date, the limitation in combo drive write speeds has been due to both the Optical Pick-Up Unit and the write controller chips used in Combo drives. Leading edge write speeds are not currently available in a Combo OPU. Additionally, the CD-RW chip sets currently used in Combo drives are at the limit of their capabilities even with 8X and 12X write speeds. Since a 16X write speed is fast becoming a reality for CD-RW drives, the challenge for a Combo OPU and IC controller manufacturer will be to provide at least a 16X write capability in an integrated controller. This level of write performance in a combo device is most likely to be achieved in the short term by suppleirs with experience in CD-RW controllers.

Additionally, the system level engineering work required to achieve high quality writing at 12X or 16X is extremely difficult to repeat on a completely different platform. For this reason, drive makers will look to preserve their investment :in drive level write technology by choosing a combo controller that is compatible with their current CDRW solution.

Optical Pick-Up Unit

A CD-RW OPU requires many enhancements over a read-only pickup. In addition to a more powerful laser and laser driver, the photodiode array has additional diodes for receiving the faint pre-groove signals used in tracking and addressing when the media is blank. A DVD-ROM pickup unit requires two laser diodes, one for DVD and another for CD since reading CD-R media is a requirement for a combo drive. The reason for the second laser is that the die used for CD-R media is not visible to the 630-650nm wavelength of a DVD laser, therefore a CD laser with 780nm wavelength is required in order to read that media.

In a Combo OPU, this second laser diode needs to be a higher power device to support writing, and the laser driver and photo diode array must be redesigned to support the write function. This means the OPU maker must have significant experience in both DVD and CD-RW technology. Initially, different OPU manufacturers led the market in DVD and CDRW. For this reason, the number of suppliers coming to market early with combo OPUs is a smaller subset of current CD-RW and DVD OPU makers. Suppliers who were dominant in DVD have high DVD read speeds but are one or two generations behind in write speeds. Conversely, suppliers who were dominant in CD-RW are able to offer the latest write performance but may be one or two generations behind in DVD read speed. Ultimately it will be the write performance that determines acceptance of a combo drive since there is an easily understood benefit to writing data faster on CD, whereas higher DVD read speeds do not result in any significant user benefit, since a IX read speed is all that is required for current DVD movie applications.

Analog Front End

For a CD-RW drive, the AFE component is very complex because of the need to obtain servo feedback and control the laser power levels while writing. The challenges for a DVD front end are very different and manufacturers must weigh the tradeoffs between combining these functions in a single device or resolving the design challenges of getting two separate AFEs to work together. One problem area is whether the combo OPU has a single or dual photo diode array for receiving the DVD and CD signals. (Figure 3). In the case of a dual photo diode, the combo OPU can almost be treated as a separate DVD and CD-RW OPU with separate AFE chips. In the case of a single photo diode array, an integrated DVD/CD AFE must be used.

Optical Storage Controller

An integrated combo controller requires expertise in both a stand-alone CD-RW controller with leading edge performance and a DVD-ROM controller capability.

Controller vendors that already have both technologies will have an advantage since either of these technologies can take years to develop and perfect. The primary challenges for current CD-RW chip suppliers would be to develop the DVD servo control, RF data slicing, and DVD ECC (error correction code) logic required for DVD-ROM and DVD-RAM read functions. Only the media format and ECC logic for DVD is adequately described by standards. The other technologies and system expertise such as digital servo control and data recovery techniques for DVD must be developed. While these technologies are not easily developed in a short period of time, the task of developing recordable technologies can be even greater.

The main challenge for current DVD chip suppliers that wish to add a CD recording function is the development of CD encoding logic to add error correction codes to the data, ATIP decoding logic to read the faint guiding signal on blank media, write strategy logic for fine tuning the pit and space sizes as they are being written to the media, and CDRW AFE technology including servo sampling to allow for focus, tracking and power level detection, and control during the write process. It is safe to say that developing these technologies at a competitive performance and cost level is an extremely difficult task

A look Ahead

Development efforts are underway by drive makers, OPU manufacturers, and semiconductor controller specialists to address the issues outlined above, in order to reduce combo drive costs and increase performance to match current CD-RW and DVD technology. In the case of the OPUs, further development and maturity of processes for the combo OPU will ultimately lead to better cost and performance metrics. For the drive controllers and AFE, reducing the chip count through further integration of the controller IC with' the analog front end. Functions required to support a Combo OPU will lend to further drivelevel cost reductions.

The result of development efforts already underway by the OPU makers, semiconductor suppliers, and drive manufacturers, faster, more affordable combo drives that address the issues outlined here can be expected in the market beginning in early 2001. Prices will continue to decrease while performance increases throughout 2001 as volume production' ramps.

Dan Salmonsen is the director of strategic marketing in the Optical Storage Group at Oak Technology (Sunnyvale, CA).