Two-dimensional magnetic recording

General concept for TDMR using multiple read elements

Two-dimensional magnetic recording (TDMR) is a technology introduced in 2017 in hard disk drives (HDD) used for computer data storage. Most of the world's data is recorded on HDDs, and there is continuous pressure on manufacturers to create greater data storage capacity in a given HDD form-factor and for a given cost. In an HDD, data is stored using magnetic recording on a rotating magnetic disk and is accessed through a write-head and read-head (or read-element). TDMR allows greater storage capacity by advantageously combining signals simultaneously from multiple read-back heads to enhance the recovery of one or more data-tracks. In this manner, data can be stored with higher areal-density on the disks thus providing higher capacity in each HDD.[1][2][3] TDMR is a read-back technology and thus applies equally well to future recording (writing) technologies such as heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR).[4]

## Overview

The TDMR approach arose from a working group set up under INSIC to explore alternative future storage technologies.[5][6] In the initial concept, the data-tracks were assumed to be very narrow tracks created by shingled recording and subject to considerable mutual interference. The read-heads were assumed to be each centered over a corresponding data-track and a joint detector would optimally recover data from several tracks simultaneously.[7][8] The technique was viewed as akin to PRML in providing gains similar to and in addition to the gains from PRML but operating across the tracks rather than down the track. A relatively large body of subsequent work has explored this configuration primarily from the perspective of signal processing.[9][10][11][12][13] However, the technical challenge of creating an array of closely spaced read-heads and the complexity of jointly detecting data simultaneously on several tracks are both considerable.

## Implementations

First implementation of TDMR in a product (2017)

In 2017, M. Fatih Erden announced at the TMRC conference that Seagate had been shipping HDDs with TDMR since earlier that year.[14][15] This was followed by Western Digital in 2018[2][16] and Toshiba in 2019.[17][18] These actual first implementations of TDMR are much simpler and very different to the scenario originally envisioned above. Current implementations recover only a single track using a read head with just two read-elements stacked one above the other (i.e. downtrack) and rely on the skew arising from the use of a rotary actuator to create some cross-track separation between the sensors.[19] This TDMR approach is being applied to both Shingled (SMR) and conventional PMR HDDs.[20] The gains achieved are quite modest (6 to 12%) but are expected to increase going forward as more complex schemes are implemented.[21]
In concept, there is little change to the read electronics except that the equalizer that shapes the signal prior to detection now has two inputs and must be appropriately optimized.[22] However, in practice, there is significant added complexity in the read electronics and in the setup process during manufacturing. This complexity is associated with optimization of the equalization (waveform shaping) and timing recovery for the dynamically varying offtrack conditions – further complicated by the cross-track offset between readers that varies with radius.[23][24]

The HDD servo system also utilizes the position error signals from the two readers. Doing so reduces the repeatable runout, especially when the readers have a wider separation.[25]

## References

1. ↑ A. Shilov speaking with Mark Re, Seagate CTO, "The evolution of hdds in the near future", AnandTech: 6 July, 2016
2. ↑ 2.0 2.1 T. Coughlin, "Two-dimensional Magnetic Recording and other HDD news", Forbes: Enterprise Tech., Apr 29, 2018
3. ↑ R. LuTchessi, "Two-Dimensional Magnetic Recording (TDMR)", Silverton Consulting blog, Mar. 5th, 2014
4. ↑ C. Mellor, "MAMR Mia! Western Digital's 18TB and 20TB microwave-energy hard drives out soon", The Register, Sept 4, 1019
5. ↑ R. Wood, "Shingled Writing and Two-Dimensional Magnetic Recording", INSIC Annual Meeting, Alternative Storage Technologies 2009: You Want to Build What?, 5 Aug. 2009
6. ↑ R. Wood, "Shingled Magnetic Recording (SMR) and Two-Dimensional Magnetic Recording (TDMR)", JMMM, Vol. 561, article 169670, Nov. 2022
7. ↑ R. Wood, M. Williams, A. Kavcic, J. Miles, "The Feasibility of Magnetic Recording at 10 Terabits per Square Inch on Conventional Media", IEEE Trans. Magn., vol. MAG-45, No. 2, pp. 917-923, Feb. 2009
8. ↑ R. Wood, "Shingled Magnetic Recording and Two-Dimensional Magnetic Recording", presented at IEEE Magnetics Society, Santa Clara Valley Chapter, Oct. 19th, 2010
9. ↑ Krishnan, Radhakrishnan, Vasic, Kavcic, Ryan and Erden. Two-Dimensional Magnetic Recording: Read Channel Modeling and Detection. IEEE. Trans on Magnetics, 45:3830-3836, 22 Sept., 2009. 2009 (Best Student Paper” award for 2010 by IEEE Data Storage Committee)
10. ↑ K. Chan, R. Radhakrishnan, K. Eason, M. Elidrissi, J. Miles, B. Vasic, A. Krishnan, "Channel Models and Detectors for Two-Dimensional Magnetic Recording", IEEE Trans. Magn., Vol. 46, No. 3, pp. 804 - 811, Mar. 2010
11. ↑ A. Kavcic, X. Huang, B. Vasic, W. Ryan, F. Erden, "Channel Modeling and Capacity Bounds for Two-Dimensional Magnetic Recording", IEEE Trans. Magn., Vol. 46, No. 3, pp. 812-818, 17 Feb., 2010
12. ↑ R. Victora, S. Morgan, K. Momsen, E. Cho, F. Erden, "Two-Dimensional Magnetic Recording at 10Tbits/in2", IEEE Trans. Magn., Vol., 48 No. 5, pp. 1697 - 1703, 19 April 2012
13. ↑ S. Garani, L. Dolecek, J. Barry ; F. Sala ; B. Vasić, "Signal Processing and Coding Techniques for 2-D Magnetic Recording: An Overview", IEEE Proceedings, Vol. 106, No. 2, pp. 286-318, Feb. 2018
14. ↑ M. Kief, I. Tagawa, "The 28th Magnetic Recording Conference (TMRC 2017)", IEEE Trans. Magn., Vol. 54, No. 2, Feb. 2018 0200101
15. ↑ Tom's Hardware: P. Alcorn, "Seagate Announces 14TB Barracuda Pro, IronWolf, and IronWolf Pro", Sept. 10th, 2018
16. ↑ Western Digital Ultrastar HC530 data sheet
17. ↑ AnandTech: Toshiba at CES2019: Worlds first 16TB TDMR HDD Debuts
18. ↑ M. Abe & T. Hara, "Nearline TDMR HDDs with Industry’s Largest Capacity of 16 Tbytes", Toshiba Technology Review, Vol. 74., No. 6, pp. 8-11, Nov. 2019
19. ↑ J. Coker, "Opportunities and Challenges for Two-Dimensional Magnetic Recording", IEEE Distinguished Lecture, May 7th, 2015
20. ↑ "Non-shingled and ready to mingle: WDC catches up with 14TB disk rivals", C. Mellor, The Register, 18 Apr, 2018
21. ↑ S. Dahandeh, F. Erden, R. Wood, "Areal-Density gains and Technology Roadmap for Two-Dimensional Magnetic Recording", TMRC 2015 Digest Book, paper F1, 17-19 Aug., 2015
22. ↑ R. Wood, R. Galbraith, J. Coker, "Two-Dimensional Magnetic Recording (TDMR): Progress and Evolution", IEEE Trans. Magn., Vol. 51, No. 4, pp. 3100607, April 2015
23. ↑ M. Oberg and N. Nagare, "Two Dimensional Magnetic Recording System, Devices, and Methods", US Patent 9728221, Aug. 8, 2017
24. ↑ G. Mathew, S. Dziak ; K. Worrell, J. Singleton, B. Wilson, H. Xia, "2-D Equalization With Location Diversity and Pre-Adaptation to Handle Off-Track Variation in Array-Reader-Based Hard Disk Drives", IEEE Trans. Magn., Vol 54, No. 2, p. 3000307
25. ↑ G. Guo and J. Yu, "Data storage device comprising dual read sensors and dual servo channels to improve servo demodulation", US Patent 901382, Apr. 21, 2015.

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