Portable Hard Disk Recorder How-To

Use an old laptop to build a multitrack hard disk recorder.

Podcasting is great. More and more people have discovered Audacity to record their podcasts, and in many ways it's ideal. It's like a toe in the shallow end of the recording and producing pool. With a podcast, you rarely need more than a mono channel—maybe a stereo setup if you're recording with someone else in the studio. But, when you get your feet wet with basic dialogue podcasting, it's hard to stop.

After all, there are so many things in the world worth recording and podcasting. There are podcast novels, such as those put out by the folks at podiobooks.com, and a good production can run into dozens of tracks between sound effects, guest voices, music and layering.

If you've gone that far, why not go the rest of the way with a full-cast radio drama? Even if you aren't a writer, plenty of plays in the public domain work very well for audio. If you're of a more musical bent, you could set up a recording session for your kid's garage band. However, once you get into doing complex live audio, such as recording full-cast radio dramas, concerts or stage plays, you're going to need more than Audacity. You're going to need a full-fledged portable recording studio.

Selecting the Hardware

To record a moderate-sized stage play, for example, your recording rig must capture multiple tracks simultaneously and independently while keeping the separate streams in sync. There's nothing quite so stressful as coming home from an excellent performance riding the mixing board to discover that, despite what you were hearing in your headphones, you've inadvertently created phase cancellation in your mixdown with your microphone placement. The data you don't have on your tape or hard drive is data lost forever—data that wouldn't have been lost if the eight-mic zone system had run to a multitrack recorder instead of a stereo mixdown. If you had the individual tracks, you could control for phase and positioning problems, sweeten the mix and even create a surround-sound environment. But, with all of the mics mixed down to stereo, what you hear is probably the best you'll ever get.

Moving from simple stereo recordings on Audacity to doing sophistocated multitrack recordings and mixes is, thankfully, a natural next step on Linux, but it requires some initial cash outlay. First, it's off to the great electronics mall to grab the appropriate hardware to build your studio. Resist your urge to indulge in one of the latest gamer's sound cards; they look sexy, but you're never going to get them to record in anything better than stereo. No, for our purposes, you'll want a good Pro Audio card that's well supported. It's not a cheap move, but for about $500 you can choose from a good range of interfaces that are exceptionally well supported. Of course, you won't want to chuck the sound card your computer already has; a Pro Audio card is designed to interface with mixing boards, digital audio devices and public-address systems—it may not even include a plug for your speakers. For our purposes here, its central feature is that it pipes multiple simultaneous nonmultiplexed streams of audio to your computer, often including MIDI.

It's one thing to build a multitrack hard disk recorder out of your desktop computer—the choice of hardware for your interfaces is embarrassingly extensive, the price range quite broad, and there is far more Linux support than you'd expect in terms of user community. But, unless your home recording studio is both uncommonly large and very well soundproofed, you may have some problems with a desktop HDR. Who wants to lug a desktop computer and monitor around in order to do field recording? If you had a spare $1,500 you could pick up a multitrack appliance, such as the ones made by Alesis and available at fashionable Guitar Centers everywhere, but why bother? You'd still need the Pro Audio card in your desktop to pipe the data back into the computer for mixing, so the total cash outlay is more than $2,000—much too large for even a dedicated hobbyist.

But have faith, all is not lost! In my case, I needed a multitrack portable recording rig to do client site recording of stage shows and panel discussions, and because of the variety of voices and the prevalence of shoddy mic techniques in the world, I needed to preserve the option of postproduction in each individual audio track. I looked into the commercial HDRs, particularly the Alesis models, as I have previously used Alesis' ADAT tape-based system extensively. The advantage of these devices is that you turn them on and they just work—they're firmware-controlled and they operate in hard real time, which is an absolute must for multitrack recording. Even so, in the end, they were not worth the money. Their capabilities are too limited for my needs—most are limited to 48KHz sample rates, which is fine for dialogue, but far too low for recording sound effects, complex movement in a space or music—the 48KHz sample rate simply can't capture the fine high-end detail and phase information that make such sounds distinct (explaining the audiological reasons behind this is beyond the scope of this article, but a quick read on the basics of acoustical sampling will give you the background you need). I needed a better solution.

Fortunately, I have an old laptop lying around, and Linux—unlike some other popular operating systems I could name—has real-time hardware preemption, which is essential if one wants to build a hard disk recorder. A laptop, of course, will not accept PCI or PCI Express cards, so the choice of Pro Audio interfaces is limited to the external—something that can plug in either to the CardBus slot or the USB or FireWire port.

The list of compatible external audio interfaces is far shorter than the list of internal cards available for desktops, but it's still long enough to require a lot of research. I actually do have an old Swissonics USB Studio D on my rack, which is nicely supported by ALSA and normally sits connected to my desktop HDR station, but it's too big and draws too much power to lug around to remote locations where I might have to run off my car battery.

For my purposes, I needed something around $600, with enough inputs that I could mic a stage play—eight analog inputs, minimum. This automatically culled out most of the FireWire-based interfaces. Even though the FireWire boxes from Presonus can draw all their power from the FireWire bus and let your interface run off the laptop battery—a big plus—the boxes in my price range tend to be limited to four or six tracks. So, on the advice of Ardour Project maintainer Paul Davis (www.ardour.org), I checked out the RME Hammerfall HDSP Multiface 2, which is a CardBus-based device with a very nice external breakout box. It helps that its Linux drivers are written and maintained by Davis, who is no mean slouch when it comes to writing tight code. It also helps that I've got one of RME's cards in my desktop HDR, so I knew they were likely to work handsomely. RME's mixing and control panel software for the HDSP is every inch as professional and easy to use as its Windows and Macintosh analogs.

After plugging this interface in to my laptop and configuring it properly, I have a multitrack hard disk recorder that can simultaneously record 24 tracks at a maximum sample rate of 96KHz, well above the maximum available sample rate on far more expensive commercial HDRs, with more available input tracks.



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24 channels @ 96KHz?

Josja's picture

Do you record 24 channels @ 96KHz with one HDSP II and some 'adaptor cables', costing around $600? Amazing, I really like to know how!
As an recording engineer I love your idea to swap my 24 tracks Alesis HD24 for a RME HDSP... But as the HDSP you are mentioning only has 8 line-inputs I can't figure how to connect 24 line-level signals to it. What is exactly the trick with the 'adaptor' cables?

As for the higher than 48KHz samplerate, I personally love the higher definition when it comes to recording classical, the HD24 will do that on 12 tracks with an optional AD converter, (pop/rock recordings don't benefit noticeably imho), but I don't see any reason for recording higher than 48KHz as my 'audience' (the consumer market) doesn't hear the difference and they are very happy with CD recordings @ 44.1KHz and most of them even love MP3s(!).

About that RME gear

David L's picture


Good article, but...

I went looking for the RME Hammerfall HDSP Multiface II and it says that in order to use the break out (which only seems to support balanced 1/4" jacks rather than the standard XLR that most mics come with - is that a phantom power issue I wonder? Do you use XLR to 1/4 successfully?) I need the PCMCIA CardBus which you don't seem to mention, pushing the price of the gear closer to $1000. Did you use the Hammerfall without the cardbus? If so was it over Firewire because I don't see a USB interface on the back side of the Multiface II.

While not technically a Linux issue, it is key to making the whole thing work successfully and maybe a little more detail would make it a little more clear.


dsawyer's picture

David -

One of the things I keep around are a collection of adapter cables, a couple batter-powered preamps and, now, a portable mixer - that deals with the problems related to the XLR inputs. But indeed, if you're needing XLR inputs right in the box, one of the Presonus interfaces is probably more up your alley than the RME.

I actually did buy the cardbus card in addition to the Multiface, only to discover that the Multiface shipped in a case with one of the cardbus cards. It wasn't as nice as the cardbus cards that sell separately, but it does the job perfectly well. So, unless I got lucky and they accidentally popped a cardbus card into the wrong box, then the multiface II really does ship with a bare-bones cardbus card. I wound up returning the Cardbus HDSP that I bought separately and buying a really nice portable Yamaha mixer with the cash back.

Since I wrote the article, I've been using it to record my podcasts at http://www.reprobateshour.com and http://sculptgod.jdsawyer.net.

Hope that helps
-Dan Sawyer

Thanks - that's a start!

kg4giy's picture


That does help, yes. It seems that there are some package cardbus deals out there which did not seem to keep the price in line but I will take a look at the other suggestions you made since I don't keep the spares around that you do, although I do have a portable mixer that would do the trick in a pinch.

I will be working on this over the Christmas holidays and with any luck I will report out my findings for those that are following along at home.