Two dimensional magnetic recording (TDMR)

A head assembly on a Seagate disk drive by Robert Scoble (cc) (from flickr)
A head assembly on a Seagate disk drive by Robert Scoble (cc) (from flickr)

I attended a Rocky Mountain IEEE Magnetics Society meeting a couple of weeks ago where Jonathan Coker, HGST’s Chief Architect and an IEEE Magnetics Society Distinguished Lecturer was discussing HGST’s research into TDMR heads.

It seems that disk track density is getting so high, track pitch is becoming so small, that the magnetic read heads have become wider than the actual data track width.  Because of this, read heads are starting to pick up more inter-track noise and it’s getting more difficult to obtain a decent signal to noise ratio (SNR) off of a high-density disk platter with a single read head.

TDMR read heads can be used to counteract this extraneous noise by using multiple read heads per data track and as such, help to create a better signal to noise ratio during read back.

What are TDMR heads?

TDMR heads are any configuration of multiple read heads used in reading a single data track. There seemed to be two popular configurations of HGST’s TDMR heads:

  • In-series, where one head is directly behind another head. This provides double the signal for the same (relative) amount of random (electronic) noise.
  • In-parallel (side by side), where three heads were configured in-parallel across the data track and the two inter-track bands. That is, one head was configured directly over the data track with portions spanning the inter-track gap to each side, one head was half way across the data track and the next higher track, and a third head was placed half way across the data track and the next lower track.

At first, the in-series configuration seemed to make the most sense to me. You could conceivably average the two signals coming off the heads and be able to filter out the random noise.  However, the “random noise” seemed to be mostly coming from the inter-track zone and this wasn’t as much random electronics noise as random magnetic noise, coming off of the disk platter, between the data tracks.

In-parallel wins the SNR race

So, much of the discussion was on the in-parallel configuration. The researcher had a number of simulated magnetic recordings which were then read by simulated, in parallel, tripartite read heads.  The idea here was that the information read from each of the side band heads that included inter-track noise could be used as noise information to filter the middle head’s data track reading. In this way they could effectively increase the SNR across the three signals, and thus, get a better data signal from the data track.

Originally, TDMR was going to be the technology that was needed to get the disk industry to 100Tb/sqin. But, what they are finding at HGST and elsewhere, is even today, at “only” ~5Tb/sqin (HGST helium drives), there seems to be an increasing need to help reduce noise coming from read heads.

Disk density increase has been slowing lately but is still on a march to double density every 2 years or so. As such,  1TB platter today will be a 2TB platter in 2 years and a4TB platter in 4 years, etc. TDMR heads may be just the thing that gets the industry to that 4TB platter (20Tb/sqin) in 4 years.

The only problem is what’s going to get them to 100Tb/sqin now?



4 Replies to “Two dimensional magnetic recording (TDMR)”

  1. Good detailed explanation. On the business side, I can certainly see an increase on mfg costs and consequent higher end-user price from now on. TDMR, HAMR and so on add quite weight and cost which will only be balanced by the increased capacity these techniques will allow.

    1. Roberto, The disk industry continues to increase density and performance the only way they know how, but TDMR represents just one of the possible paths ahead. I thought it was an interesting approach inasmuch as they use more (2X) signal to reduce the (more) noise coming off the media. But why stop there if 2 is good, 3's better. The problem in disk density growth is that the easy engineering tricks to get more data on/off magnetic media seem to be going away. Each new trick seems to last less time than the one before. GMR heads seemed to last almost 2 decades. But Helium, HAMR, TDMR, & SMR seem to be shorter term fixes and seem likely to only last 1/4th as long… Which makes R&D more expensive which makes disk capacity more expensive. We seem to hurdling towards a marginal utilization wall. I just don't know where/when it will go vertical. Probably one reason we are down to 3 rotating disk suppliers…

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