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.

Windows/Boot Camp Experience Dissection
POST A COMMENT

289 Comments

View All Comments

  • Constructor - Tuesday, September 27, 2011 - link

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


    No. The former is an interface protocol, the latter a component.

    But Thunderbolt can work on a completely different level with GPUs than USB can:

    Internal GPUs are usually connected to the CPU through 16 PCIe Lanes, which adds up to 64Gb/s in each direction with PCIe 2. These GPUs also have the necessary power and cooling available for advanced 3D performance.

    And at their outputs, they're pumping out substantially more than 5Gb/s for a high-resolution video signal.

    USB3 cannot supply an external GPU with enough data throughput to come even close to an internal GPU. The outright horrible inefficiency of the USB protocol may well eat up half of the nominal bit rate again as it has with earlier incarnations. It also has too little bandwidth to tunnel the video output signal from an internal GPU, let alone on top of full-speed data transfers at the same time.

    Thunderbolt on the one hand provides at least double the data bandwidth of USB to drive an external GPU if so desired (which might actually be enough for mid-range 3D applications) and with Thunderbolt that is actually the net throughput with next to no overhead, and on the other it piggybacks not just one but even two complete high-resolution video streams onto the same cable at the same time if so desired!

    The two are playing in completely different leagues – USB falls far short of what Thunderbolt can do, not just theoretically but in actual practice.

    You will never see anything like the Thunderbolt Display with USB3, at least not remotely with the same performance since it is simply not possible.

    I have no perceivable latency on my USB mouse, and I'm sensitive to it.


    Sorry, but I think you don't understand what I am talking about. That you may not be able to perceive the latency of your USB mouse with the naked eye is as it should be, but extremely far removed from the micro- or even nanosecond-range latency requirements of many more advanced interfaces (human perception begins only to notice differences starting in the millisecond range, usually even just at tens or even hundreds of milliseconds).

    There are good reasons for advanced audio and video interfaces insisting on using FireWire so far and now starting to switch to Thunderbolt. USB has always had extremely bad response latencies which is inherent in its protocol specification. Critics even suspected that Intel had screwed up the USB protocol that badly because they wanted to keep their CPUs busy so they could keep selling faster ones, even though that is probably just a myth... 8-)

    Theoretically USB3 could improve on that to some degree (which is a completely different issue from sheer data throughput), but we'll have to wait and see whether that will actually come about.

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

    USB has so far wasted about than half of its nominal bandwidth idling in its extremely inefficient and wasteful protocol. When have you ever had true 480Mb/s (60MB/s) transfer rates on USB2? Right: Never.

    USB3 may again improve over the older variants, but it's to be seen how much of these problems will actually get solved in practice.

    With Thunderbolt the 10Gb/s are effective throughput, right out of the gate. And that's even just half the total throughput, with the other half tunneling Display Port.

    Thunderbolt plays in a completely different league from USB, even USB3.

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


    Maybe. Maybe not... ;-)
    Reply
  • Constructor - Tuesday, September 27, 2011 - link

    Sorry for the screwed-up boldfacing above. Post editing would be really handy there...! B-) Reply
  • AnnonymousCoward - Tuesday, September 27, 2011 - link

    You're mixing the topics of GPU processing and the interface to the monitor.

    Some facts:
    -There's no question that USB3 is not suitable for driving high resolution raw video. Throughput maxes out around 3.2Gbps. 1080p60 24-bit requires about 3.6Gbps.
    -Thunderbolt just combines the display and data connectors. When comparing this to a USB solution, compare it with USB3+DisplayPort.
    -No external connection can saturate 3.2Gbps. No hard drive, no web cam, no sound card, no peripheral. So Thunderbolt provides no benefit in its 10Gbps vs 3.2Gbps ability.
    -Thunderbolt gives you few ports and forces you to daisy-chain. And some Thunderbolt devices only have 1 port, meaning they stop the chain. Having 4 USB3 ports in 1 place is more centralized/convenient.
    -Intel probably charges $15/chip. Versus $2/chip for USB3 hosts. The cables are also far more expensive. And for what benefit? To combine data and video cables and force you to daisy-chain.
    Reply
  • repoman27 - Tuesday, September 27, 2011 - link

    Some corrections to your "facts":

    -Plenty of external devices can saturate USB 3.0. For instance: single current generation SSD's, SSD or HDD RAIDs, 10 Gigabit Ethernet adapters, Fibre Channel adapters, pro video gear, any number of medical or research lab devices.

    -No Gigabit Ethernet adapters exist for USB 3.0 yet, so you would be limited to < 320 Mb Ethernet connections for a while.

    -No USB to FireWire adapters exist. USB does not support peer to peer transfers the way FireWire or Thunderbolt do, and thus performance drops off dramatically when copying between two devices connected to the same root hub.

    -Some USB devices also block a port... actually most do.

    -Thunderbolt doesn't actually force you to daisy chain. Although the silicon doesn't exist for them yet, in theory Multi-port Thunderbolt switches are quite possible.
    Reply
  • AnnonymousCoward - Tuesday, September 27, 2011 - link

    Your points don't apply to 99% of users.
    -500MB/s SSDs are internal drives. All external drives I'm aware of are platter-based. RAID-away with platters, and you'll still have a hard time hitting 400MB/s.
    -10Gb ethernet? Most people don't care about LAN traffic, and hard drives are often the bottleneck anyway at 90MB/s.
    -Fiber equipment and pro video gear--1% of users?

    You said it doesn't force you to daisy chain; when you buy a CrapBook Pro, you get 1 port. That means you have to daisy chain.
    Reply
  • repoman27 - Wednesday, September 28, 2011 - link

    "Facts" aren't facts if they're wrong even 1% of the time though. While all of the examples I cited may be corner cases, they are all valid and contradict points which you presented earlier as factual statements.

    Also note that among the 1% of users that can benefit greatly from technologies like Thunderbolt are quite a few of the folks that create the music you listen to, produce the movies and TV shows you watch, make the products you purchase and use every day, develop treatments for illness, and help us to better understand the world we live in. Even if you resist adoption of new technologies personally, you can still benefit greatly from them.

    Apple has long had the philosophy that if you put advanced technologies into consumer devices that the users will find creative and unforeseen ways to exploit these new capabilities. I don't think they're going to give up on this just because you happen to disapprove.
    Reply
  • AnnonymousCoward - Wednesday, September 28, 2011 - link

    Ok, you are technically right that "No external connection can saturate 3.2Gbps" isn't an accurate blanket statement. It was intended to be effectively true for almost all users. Sure you can make a $5000 RAID array that reaches that. Many people seem to think Thunderbolt offers a speed advantage over USB3 in applications where it doesn't (nearly all applications).

    You may be overstating Thunderbolt's benefit for that 1%. So it lets you dock a laptop without a normal docking station, or it saves you from plugging in wired ethernet (yet it doesn't save you from plugging in wired analog audio). That isn't going to help any artist do their work. The 1 unique capability I see that it offers, is the bandwidth to download raw video to a PC. But PCIe cards probably already exist that receive HDMI or DP for a computer.

    And I have a feeling that if James Cameron had a Thunderbolt-enabled computer and associated equipment, Avatar would have sucked just as badly.
    Reply
  • repoman27 - Thursday, September 29, 2011 - link

    Believe it or not, there are actually people who can use (and are excited about) the little known already shipping Thunderbolt devices such as: http://store.apple.com/us/product/H7293LL/A/promis...

    And I'd be willing to venture that your feeling about Avatar constitutes an actual fact.
    Reply
  • Mystermask - Sunday, October 09, 2011 - link

    Unbelievable how stupid so called "tech enthusiasts" behave when they read the Name "Apple".

    1. We've been reading the same technophobic "you don't need that" comments in the past about almost any switch Apple made: the replacement of proprietary ports with USB, 5 1/4" floppys with 3", 3" floppys with CD, CDs with DVDs, cell phones with physical keyboards with Multi Touch devices, etc
    2. Let the market decide what is needed. E. g. I'm always astonished how software / solutions make creative use of given hardware capabilities. This kind of evolution can never happen when HW vendors only stick to "what 99% of users need today".
    3. By your standards, we should stick to old HW (CPUs, GPUs, ATA, PS2, ..) because 99% of all users don't need newer stuff: They surf, write e-mails, watch photos or use bloated Word from time to time. No
    Reply
  • Jamezrp - Monday, September 26, 2011 - link

    The video's really well done. I'd recommend using some of the YouTube features for links at the end, though it's clear you did this yourself even though the production value is clearly high up there (good camera, good audio, but minimal editing and control). Great stuff.

    That said, I've been pondering the Bluemic for some time and the quality on this video just sold me. Completely wrong product...I'd love the Thunderbolt Display to go along with my 13" MBA, but I've already got a Dell 2408 and Acer G24 for my PC which takes up my entire deskspace, and don't need the MBA on a bigger display anywhere else.
    Reply

Log in

Don't have an account? Sign up now