PRML Read Channels: Bringing Higher Densities and Performance to New-Generation Hard Drives
- Advanced PRML (Partial Response Maximum Likelihood) read channels are enabling hard drive manufacturers to
meet computer OEMs' most important requirements for new drive products: higher data densities, improved drive
performance, and high data integrity. PRML read channels represent a major technological advance over
traditional peak detector read channels, where increased data densities or transfer rates may adversely affect
- Quantum Corporation's patented PRML read channel technology incorporates an efficient data encoding scheme in
addition to advanced digital filtering and data-detection techniques. As a result, the company's Quantum Empire
drives and new Quantum Grand Prix drives feature areal density increases of 30-40 percent when compared with most
competitive offerings, as well as fast media data transfer rates (64 megabits (Mb)/second for Empire and 78
Mb/second for Grand Prix). The resulting performance boost leads to very high sustained data transfer rates,
making the drives particularly suitable for disk-intensive applications such as multimedia and graphics.
- The PRML read channel designed by Quantum is compatible with new magneto-resistive read/write heads as well as with thin-film inductive heads, a proven head technology that is widely available today. It also works with other state-of-the-art drive technologies, such as multiple zone recording and embedded servo, to enable further increases in data densities. Thus a one gigabyte, one-disk drive will become a reality in the not-too-distant future.
This issue of Quantum Technical Information Papers (TIPS) explains in detail PRML read channel technology and its ability to meet system OEMs' requirements for aggressive density improvements and high- performance levels in new drive products, as well as high data integrity.
PRML read channels deliver many benefits by permitting and compensating for a phenomenon called inter-symbol interference (ISI), which can occur during data read operations in hard drives with high bit densities (i.e., with data bits packed together very tightly on the drive's disks).
As a drive's bit density is increased, the analog signal "peaks" that are detected during read operations tend to stream through a drive's read/write head, and into its read channel, at higher rates. At such high rates, the possibility arises that the signal peaks will significantly overlap - i.e., create ISI - which, in turn, can lead to data bit errors.
PRML read channels prevent ISI-associated bit errors in high-density drives by using digital filtering techniques that "shape" the readback signal so that it exhibits desired frequency and timing characteristics (thus achieving the "partial response" characteristic of PRML). They then employ digital processing and powerful maximum-likelihood digital data detection to determine the most likely sequence of data bits that was written to the disk (hence the "maximum likelihood" component of PRML). This highly accurate sequence-detection method is implemented using the "Viterbi" algorithm, named for Andrew Viterbi, the man who invented the algorithm.
PRML read channels thus ensure high data integrity in hard drives while permitting significant increases in bit density. Such increases, in turn, lead to faster internal data rates, since the higher the bit density (measured in bits per inch, or bpi), the more data bits that can be transferred from a drive's disk to its read channel in a given time period. Higher data density also means fewer disks are needed to achieve a desired drive capacity.
As a result, PRML read channel drives provide computer OEMs with the high-performance, high-capacity storage solution needed for multimedia, graphics, and other data-intensive applications, while also reducing drive costs and boosting drive reliability.
The PRML approach differs markedly from traditional peak detector read channels, which do not compensate for ISI. Instead, peak detector read channels reduce the possibility of significant ISI by employing a data-encoding scheme during write operations that effectively separates the analog signal peaks during read operations. The read channel's analog detection circuitry then can accurately detect each signal peak - one at a time.
The problem with peak detection is that the data-encoding scheme typically reduces the amount of user data that can be stored on a disk in relation to the total amount of user data and encoded data stored. This reduction in the user data to stored data ratio has had two effects of consequence for hard drives: more disks have been needed to achieve desired capacities, and the decrease in user data density has tended to slow data transfer rates.
Now, advanced PRML read channels like Quantum's are rectifying this situation by achieving the same low bit error rate as peak detection in drives using the same heads and media, while enabling higher drive densities, capacities, and performance.
PRML technology itself is not new: it has served for nearly two decades in the general field of data communications, where one of its applications is to improve modem performance. Nor is PRML completely new to the hard drive industry: IBM introduced a first-generation PRML read channel in 1990.
What is new are more advanced forms of PRML technology, and Quantum Corporation is at the forefront of this trend with its patented PRML read channel (see Figure 1). Because of its in-house digital signal processing (DSP) expertise and ASIC design capabilities, Quantum was the first non-captive hard drive manufacturer to design such a read channel. The channel was initially incorporated into its new-generation Empire drives in 1993, and a second-generation channel is used on the Grand Prix drives.
Significantly, Quantum intends to implement its new PRML read channel design across all of its product lines. Quantum's OEM customers, and their end users, thus can expect to benefit from improved levels of capacity and performance in hard disk drives aimed not only at workstations and other high-end systems, but at desktop and notebook computers as well.
To understand PRML read channels and their associated benefits, one must first consider the basic workings of traditional read channel designs.
Essentially, the read channel performs the data encoding and conversions needed to "write" computer-generated information on a magnetic medium (the disk), and then read back that information with a high degree of accuracy. During a write operation, the encoded data is converted into an analog signal, which the drive's read/write head uses to cause magnetic flux changes, or changes in the magnetic patterns, on the drive's disks, hence "writing" data to the disk.
During a read operation, the read/write head "senses" the magnetic flux changes on the disk sector(s) to be read, then generates an analog readback signal that is transmitted to the read channel. The read channel detects the signal's peaks (each of which represents one bit of data), converts the data back into digital information, and finally decodes the data into its original sequence of 1s and 0s (see Figure 2).
Central to a discussion of PRML read channels versus peak detection read channels is the particular data encoding scheme they employ during write operations to facilitate data readback. Both types of read channels use a scheme called RLL, which stands for "Run Length Limited."
RLL encoding imposes constraints on the user data that will be written to the disk. For peak detection, these constraints are typically denoted as (1,7). Here, "1" means there must be at least one digital 0 between every digital 1 in the sequence of bits in a data stream. This translates into at least two encoded-bit time periods between every magnetic flux change. It also means that the peaks in the analog readback signal will occur at least two encoded time periods apart.
The "7" in the (1,7) constraints means there can be no more than eight of these encoded-bit time periods between magnetic flux changes. The peaks in the readback signal thus will occur no later than eight encoded time periods apart.
The RLL 1,7 constraints, then, are what separate the peaks in a peak detector read channel and reduce ISI during readback of user data. These constraints ward against bit errors very effectively; the downside is that the requirement for two encoded time periods between flux changes eats up disk space. The result: for hard drives that use peak detection, the ratio of user data to stored data on the drives' disks is only 2-to-3 (as in Quantum drives that use peak detection), which leads to lower areal density and data transfer rates.
In today's most advanced PRML read channels, such as those designed by Quantum for its Empire 1440/2160 and Grand Prix 2130/4270 drives, (0,4,4) constraints replace the (1,7) constraints during data write operations. Here, the "0" means that the digital 1s in a data stream can occur right next to each other. Hence the magnetic flux changes on the disk can occur with just one encoded-bit time period between them, and the peaks in the analog readback signal are not separated.
The first "4" in the encoding constraints means there can be no more than four digital 0s between digital 1s in a data stream, or no more than six encoded-bit periods between flux changes. (The latter "4" refers to the maximum number of 0s between 1s that can occur in certain data subsequences, a constraint that simplifies implementation of digital data-sequence detection.)
The result of this more efficient PRML encoding scheme is that the user data to stored data ratio for the Empire and Grand Prix drives is increased to 8-to-9. This ratio, in turn, contributes to a bit density of 80,000 bpi for the Empire products, or 26 percent higher than the 63,600 bpi density achieved by Quantum's Empire 540 and 1080 drives featuring peak detection and using similar heads and media. The internal data rate also increases to 64 Mb/second for the Empire products and 78 Mb/second for the Grand Prix products, significantly higher than the 48 Mb/second rate achieved by the Empire 540 and 1080 drives.
Now the question becomes: how can a PRML read channel deliver the same low bit error rate as a peak detector read channel if its encoding scheme does not separate the peaks in the analog readback signal? The answer rests with the PRML read channel's advanced digital filtering, data processing, and data feedback techniques, which "shape" the readback signal so that the data written to the disk can be accurately detected by the read channel's Viterbi detection circuitry.
Quantum's PRML read channel operations (simplified) include the following key steps, which occur prior to Viterbi detection:
- Ongoing sampling of the analog readback signal to extract various points along the waveform (peak detection,
by contrast, looks only for the peaks) (see Figure 3).
With Quantum's PRML read channel, analog signals are sampled at various points and then converted to digital data.
- Conversion of these analog values into digital data samples.
- Filter equalization of the digital data samples. The filter modifies the data samples so that they cluster around three target values: zero, a
negative value, and a positive value. The filter also modifies the sine wave components of the analog readback signal in order to produce
a particular signal shape. (This step is analogous to the functions of a stereo system's equalizer, which also modifies sine wave components - bass and treble - to produce a particular "shape" or desired sound.)
- Ongoing processing and feedback of the filter's output to several of the read channel's front-end analog components, while analog-to- digital conversion continues to take place. These operations ensure that the analog signal will exhibit the needed characteristics for analog-to-digital conversion, and that sampling of the signal will take place during a specified timing window.
As a result of these steps, the digital data samples will exhibit the frequency domain and time domain characteristics required by the Viterbi detector to determine the original data sequence that produced the analog signal.
The essential difference between peak detection and Viterbi detection is that peak detector circuitry detects one data bit at a time, while the Viterbi algorithm detects an entire
sequence of bits at a time. This difference is analogous to the way good readers and average readers read.
Take the word "laughs." The average reader will see one letter at a time, determine what each letter is, and then put those letters together to come up with the correct word. The good reader might register only some of the letters, then instantly determine from that information that the word is "laughs," based on his or her familiarity with written English.
In much the same way, when the digital circuitry that performs Viterbi detection receives a data sample, it doesn't have to decide immediately whether the sample represents a "1" or a "0"in the original write data. Instead, it compares sequences of samples with sequences of possible samples. It then uses the compar ison to determine what an entire sequence of data bits must be.
After Viterbi detection, the PRML read channel decodes the data into the original user data sequence that the host system had transferred to the hard drive.
Quantum's PRML read channel technology, incorporating an efficient RLL encoding scheme, produces both improved areal density (30 percent to 40 percent higher) and faster internal data transfer rates (64 Mb/second for Empire and 78 Mb/second for Grand Prix) compared with peak detection, while delivering the same high data integrity. In addition to boosting both drive capacity and performance, Quantum's PRML read channel design provides its drive products with a number of advantages that include the following:
- Use with a proven head technology. Quantum's PRML read channel
performs both sophisticated signal filtering and write precompensation (i.e., it modifies the magnetic flux transition times in the write waveform current to make the time between adjacent transitions longer). These two functions enable Quantum's PRML read channel to be used with widely available thin-film inductive read/write heads, as well as with newer magneto-resistive heads.
- Potential for better drive diagnostics and extendibility to higher
performance schemes in future Quantum drive products. These advantages derive from use of a digital data-detection technique in contrast with peak detection, which employs analog detection methods.
- Potential for a one gigabyte, one-disk drive. Quantum's PRML read channel can work in tandem with the company's multiple zone recording and embedded servo technologies to push drive densities to unprecedented heights. Thus system OEMs can look to the introduction of a single-platter, gigabyte-capacity drive in just a few years.