Challenges Of CD-RW/DVD Combo Drive Design - Technology Information

Computer Technology Review, Nov, 2000 by Dan Salmonsen

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.