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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  • sportherz - Tuesday, September 27, 2011 - link

    if I understand correctly, the test was done with a MBP. I was excited about the new TB display together with a new MBA so I ordered both. Since my TB display arrived last week I am not so excited anymore. The display causes the MBA to heat up such that the fans of the MBA are constantly running at 6000rpm making a loud and annoying noise. If I use the MBA on my old 24 inch cinema display I don't have that problem. Could you please test the performance using a MBA and see if this is a problem about the particular monitor MBA I have, or if it is general

    Thanks
    Reply
  • MacTheSpoon - Tuesday, September 27, 2011 - link

    Is it really necessary to spend all that money just for a laptop dock? I guess if you're already going to buy an external display and it must be an Apple display it makes sense. But what about a cheaper solution like this third-party dock, the Henge Dock? I think it's $70. I'd be curious to see Anand test that out. Maybe there are other third-party docks out there he could test, too. Reply
  • Constructor - Tuesday, September 27, 2011 - link

    The Henge Docks only re-route existing ports, so you gain nothing, except being able to (dis)connect those multiple ports in one single motion, but you lose the use of the internal display, keyboard and touchpad of the MacBook Pro.

    For the MacBook Air that's not even an option since there is just one USB port to re-route anyway on the side of the MagSafe connector.

    With the Thunderbolt Display you primarily pay for a large 2560*1440 display with LED backlight in a high-grade casing and with relatively good environmental compatibility. You don't get any comparable display alone for much less.

    All the additional ports and the easy connection through MagSafe and Thunderbolt in a single thin cable are effectively a bonus, if a very welcome one, particularly for the MacBook Air.

    And you can still use the built-in display, keyboard and touch pad of the MacBook in additon to the other ports of the MacBook directly.

    Not a bad deal, all in all.
    Reply
  • sheh - Tuesday, September 27, 2011 - link

    that "DC brushless" thing.

    About the audio problem, wouldn't it be the fault of both the display and the TB device? I wouldn't expect a faulty USB device to cause problems to other things connected to the computer (at least if the computer didn't get stuck), why not expect the same with TB?
    Reply
  • Wskcondor - Monday, October 10, 2011 - link

    I read this review and also the 2011 iMac review. I have a couple of questions someone with the hardware in front of them or with more knowledge may be able to answer...

    My idea is to travel with the 2011 Macbook Air, with the 1.8G i7 upgrade and 256G SSD.

    -I like the Thunderbolt display idea of a dock, because when I am "at home" I do a lot on the computer at the desk and I store a lot externally from the Air (movies, music, photos) and also still use an optical drive regularly. In effect, with a T-Bolt display, all of these things would be hanging off the display.

    Then I thought: why couldn't I buy a 2011 iMac and use it as a "display/dock station" when at home?

    1) Could I boot the 11" Air into TBolt target drive mode and then BOOT the iMac from the Air's drive, so all the "local changes" would be on the drive I take away on travel with me? The processor would be an i7 also in the iMac. BUT the configuration would be different, of course.
    -will this jack up settings in the Air's hard drive when I disconnect and reboot the Air from it's internal processor?
    -will this be too slow a configuration because the TBolt target drive mode has such slow throughput that the iMac will crawl along?
    -Can I even boot an iMac off the Air's TBolt target SSD drive?

    2) Can the iMac become a "Thunderbolt monitor" with drives, ports, etc if I boot the iMac into TBolt target mode? Or would it just be a blue screen with drives and ports, extending my Air's ports, but not acting as an external monitor?

    Questions.

    thanks to anyone who knows.

    Wskcondor
    Reply
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