Trouble in Promise-land

What's the first thing you do when you've got a display that has tons of interfaces and bandwidth at its disposal? Try them all at once to see if anything breaks of course. Over the course of the past few days that's exactly what I did. Unfortunately I did find a situation where things broke.

For whatever reason, if you're doing a lot of writes to the Promise Pegasus while playing music (or any other constant audio) through the Thunderbolt Display's internal speakers the audio will eventually corrupt. You can hear exactly what I'm talking about below:

TB Pegasus Audio Issue by AnandTech

This is a recording taken of me listening to music on the Thunderbolt Display (via its internal speakers) while writing a couple hundred gigabytes to the Pegasus R6. Note the introduction of what can only be described as really bad noise at the 6 second marker.

If you stop music playback and quickly resume, the problem will still be there. You have to restart the application that's using the audio codec to recover from this point. From a hardware standpoint, the codec just needs to go through an off/on (sleep/wake?) cycle to return back to normal. If you do this however and haven't stopped the transfer, the problem will creep up again. Stopping the transfer while playing back music won't fix the issue either. You have to stop the transfer and restart the music playback application for it to go away.

The issue goes deeper than that. I went out and bought a Creative Labs X-Fi Go Pro USB sound card to see if the problem stopped at the internal audio codec or extended to all USB sound devices. Unfortunately, it does even happen if you're using an external USB sound card connected to the Thunderbolt Display. Connect the same sound card directly to your Mac or use your Mac's 1/8" stereo jack and the problem goes away.

I was worried that what may appear as noise through speakers could result in data corruption over USB transfers. I ran the Pegasus write test while copying a bunch of files to an SSD attached via USB to the Thunderbolt Display and never saw any corruption on the SSD. This appears to be limited entirely to audio playback.

What's truly bizarre is I can only get the issue to appear when writing to the Pegasus, hundreds of GBs of sequential reads don't seem to produce it. Short bursts of writes don't seem to cause it either. Sending tons of data across the monitor's Gigabit Ethernet, FireWire 800 and USB ports doesn't seem to trigger it either. It appears to be an issue with the Pegasus and the Thunderbolt Display. But which device is ultimately at fault? Is it a problem with the Thunderbolt Display or the Pegasus? Ideally I'd use another Thunderbolt storage device to see if the issue remained, but I couldn't get my hands on a LaCie Little Big Disk.

I thought of something else.

First I needed to test and see if perhaps the issue was related to ultra high speed transfers. As we've already shown, the Pegasus can push as much as 1GB/s over Thunderbolt whereas none of the other bandwidth eaters come even remotely close to that. To determine if the issue was data rate invariant I wrote to the Pegasus at different speeds ranging from 480Mbps all the way up to 7.2Gbps. I tried putting SSDs in the Pegasus as well as standard mechanical hard drives. The problem remained. I got audio corruption regardless of what drives were in the Pegasus or what speed I wrote to the drives. The problem wasn't related to transfer rates.

I also took apart the Thunderbolt Display to confirm there weren't any obvious issues on the controller board (E.g. putting the Thunderbolt IC far too close to the audio controller). Nothing obvious there either.

While I was doing all of this, Apple put forth a Thunderbolt firmware update the other day, however it didn't seem to address the issue either. So I went back to my testing.

Since the problem appeared regardless of how fast (or slow) I was transferring and all I needed was another Thunderbolt storage device to vindicate either the Pegasus or the Thunderbolt Display I turned to the trusty MacBook Air.

As I mentioned in our original Pegasus review, if you have two Thunderbolt equipped Macs and a Thunderbolt cable you can actually put one of the machines in target disk mode and access its drives via Thunderbolt on the remaining Mac. You don't get super high performance but you can get around 500Mbps. Since I had reproduced the audio corruption issue at an even slower data rate I decided to give this a try.

I booted the MacBook Air in target disk mode by holding down the 't' key after turning on the machine. My MacBook Pro was connected to the Apple Thunderbolt Display and a Thunderbolt cable connected the display to the MacBook Air. This was the same setup as the Pegasus, but with the MBA in place of the Pegasus.

I wrote to the MBA just like I did the Pegasus (from a file server connected over the Thunderbolt Display's GigE, transfer rates were capped at around 500Mbps from the file server). After a couple hundred gigabytes were transferred without any audio corruption I swapped out the MBA and connected the Pegasus. I copied the same files at the same rate from the same source. After no more than 7GBs were written to the Pegasus the audio stream started to corrupt.

Based on my testing I can only conclude that the Pegasus seems to be at fault here, not the Thunderbolt Display. Given that the Pegasus was introduced prior to Apple's Thunderbolt Display it's not all that surprising that this issue made it through to production. It's unclear what the root cause is but it's hopefully something Promise can address either through firmware or a driver revision.

Update: I'm still verifying that this is indeed a "fix" but it looks like if you use a USB sound card plugged into a USB hub which is then plugged into the Thunderbolt Display then the sound corruption doesn't happen. This seems to point at noisy power as being the cause with the USB hub acting as a crude filter. It's still not ideal but this may be a workaround for Pegasus users until Promise supplies a fix.

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  • repoman27 - Monday, September 26, 2011 - link

    Well, the display itself is driven by DisplayPort. The ATD does require Thunderbolt though, otherwise they couldn't have included all the other stuff. Apple is releasing this as an accessory for new Macs that have Thunderbolt ports. It does not make anything obsolete. You can continue to use pretty much any display on the market with either an older Mac that only supports DisplayPort or a new Thunderbolt equipped Mac although you might need an adapter or three. Most display manufacturers are probably not going to go the Thunderbolt route, and will stick with HDMI for 1920x1080 panels, DisplayPort for the higher resolution jobs, and DVI ports until the cows come home.

    Apple may soon cease production of discrete DisplayPort sink devices, but that in no way means that they've turned their back on the standard. Thunderbolt ports are indeed backward compatible with DisplayPort devices, but DP ports have no way of being forward compatible with Thunderbolt devices.

    I find it odd that there are a lot of folks asking where all the Thunderbolt devices are, and then when one is released, everyone complains that they would need to buy a new Mac in order to use it. Thunderbolt devices are designed to be used with Thunderbolt enabled PC's, you're either in the market for them or you're not, end of story.
    Reply
  • eureka_swe - Monday, September 26, 2011 - link

    If you have say 2-3 FireWire 800 disk connected to the Display, do i need to Eject this on the Macbook Pro evry time i will disconnect the Display or is it just to pull out the Thunderbolt cable and the disk is still good ?

    its a big question for me that have 7 FW Disks :)
    Reply
  • Constructor - Monday, September 26, 2011 - link

    That you need to unmount volumes which are about to be disconnected doesn't change.

    The external FireWire controller is basically indistinguishable from one on the motherboard for the OS. And the file system is still the same.

    So unmounting any external FireWire, USB and (directly) Thunderbolt device is a must and will remain so until the file system is fundamentally altered in that regard.
    Reply
  • iSayuSay - Monday, September 26, 2011 - link

    Agreed on some Anand's points. Hooking a Macbook Pro/Air with Thunderbolt onto that display may looks cool, it might instantly look like a desktop. But the real performance is never going to be excellent, it only become acceptable - very good range.

    If I going to cash in such amount of money (consider basic 11" MBA for $900 + 27" Apple Thunderbolt display for another $1000, and for elegance purpose .. don't forget TrackPad/MagicMouse + Wireless keyboard for another $140) ..

    In total you already spend same amount with hi-end iMac 27" which performs much better, yeah sure .. it's not portable, but I don't carry around my MBA too much either :p

    So I say .. while looks nice and cool, I'm not ready to follow world trend to go mobile with today's performance
    Reply
  • dgingeri - Monday, September 26, 2011 - link

    I flat out refuse to buy any Apple products. I have 3 reasons behind this now. At first, it was because they did so much business with Foxconn, and the horrible living conditions of the Foxconn employees who built Apple products. Then I found out about their excessive patent applications on a great many things that have been common habits of electronics manufacturers for over a decade. Now, I add on the excessive legal activity and flat out cheating in court trying to ban competing products.

    Apple is quite simply an evil company. Do not buy their stuff. Do not support the attempt at becoming a dictatorship of the world under the guise of business.
    Reply
  • Mystermask - Sunday, October 09, 2011 - link

    You're a hypocrite if you accuse Apple for what happens in chinese factories.
    1. Name me one brand that does not go for cheap as possible production. And why? Just have a look at those endless discussions where people try to prove that they can build a PC that has the same specs like e. g. a MacBook Pro but is even cheaper. How do you believe this is possible?Aren't you happy when you can buy a PC for €300 when all other cost 500€? Who do you believe is paying that bill? The "race to the bottom" has a long history that started in the PC industry when almost all HW vendors decided to go with DOS / Windows and vendors could only distinguish themselves in the market by being cheaper than others. And consumers gladly bought the cheapest - unable to distinguish value and price - effectively cutting into their own flesh because this has cost all production jobs in the PC industry in western countries.
    2. Have a look at yourself. Do you wear Nike, Addidas, Reebook, Lacost, you-name-it? Do you use Dell, Acer, HP, HTC, Samsung, .. How and where are they produced?
    ..
    All of a sudden, reality looks not as easy and religios blind Apple bashing is certainly neither a solution nor would that change anything for Foxconn or countless other factory workers in China, Vietnam or wherever people have to offer their work for cheap to survive because of western ignorance and greed.
    Reply
  • The_Countess - Monday, September 26, 2011 - link

    i cant help but notice that you could do all this with just 2 cables for far less money and without the expensive proprietary technology.
    1 display cable + 1 usb3.0 cable connected to a USB hub.

    it'll even be fully backward compatible with practically every laptop still in operation on the planet!

    you could even run the display over usb3.0 although I'm not sure how much bandwidth would remain after that.
    Reply
  • repoman27 - Monday, September 26, 2011 - link

    You can get more than 800 MB/s of throughput from a RAID connected to a USB 3.0 hub?... While getting over 780 Mbps over a USB 2.0 Gigabit Ethernet adapter?... (There are no USB 3.0 ethernet adapters in the wild yet.) While recording 720p video from a camera connected to the hub?... While playing back audio from a USB audio interface?... And how are you going to connect a FireWire device to your USB 3.0 hub? There's no adapters for that, and besides, you're out of ports on your hub because the only USB 3.0 hubs on the market have just 4 downstream ports. Looks like you'll need to get another hub and cable to connect a keyboard and mouse or USB disk.

    And yes, you could connect your version to any laptop and enjoy all of those devices sharing less than 40 MB/s of bandwidth, or connect it to a USB 3.0 enabled machine and get less than 400 MB/s. Thunderbolt gives you 2500 MB/s. A 2560x1440 display such as this requires 5.8 Gbps of bandwidth, USB 3.0 can't even hit 3.2 Gbps of real world throughput yet.
    Reply
  • Constructor - Monday, September 26, 2011 - link

    USB ist not remotely capable to drive a display at the same performance level as the built-in GPU in the computer can. Those USB graphics adapters are orders of magnitude slower than that.

    And both 10Gb/s input + 10Gb/s output at the same time as a high-res display (or even two of those with a small output performance hit) are completely out of range for even USB3.

    Add to that the horrible latency problems you've got with USB, which are completely absent with Thunderbolt. (Which is one reason why you couldn't even have a full-performance FireWire port through USB3 since proper FireWire has very low latencies as well, which USB simply can't emulate.)

    USB is a very complicated and not too fast peripheral interface.

    Thunderbolt is effectively a part of the motherboard channeled through a thin cable to pluggable external motherboard expansions (the "motherboard" in the Thunderbolt display is effectively made a part of the computer's motherboard once you plug it in to the Mac).

    Completely different deal.
    Reply
  • AnnonymousCoward - Tuesday, September 27, 2011 - link

    You're comparing USB to a GPU? ok...

    "orders of magnitude"? USB is 5Gbps; apparently the alternative is 500Gbps!! Sorry, it's not even 50Gbps, or even half of that.

    "horrible latency problems you've got with USB" - I have no perceivable latency on my USB mouse, and I'm sensitive to it.

    "USB is a...not too fast peripheral interface." - 5Gbps isn't fast? No external connection aside from raw video can utilize that!

    Face it, you don't know what you're talking about.
    Reply

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