Converters and clocks are hotly debated topics, and it’s been my opinion for a while now that the reason they’re such sore topics is that people are often trying to establish objective and definitive statements about them. Clocks, power supplies, converter chips, and analog components get discussed in terms of jitter, stability, accuracy, transparency, distortion, and color. How all of these aspects really work is just mysterious enough to inspire countless debates, a number of ponderous shootouts, and a great deal of confusion. As I’ve said in my reviews of the Antelope 10M clock ( Tape Op #68) and the Burl B2 converter (#79), my personal opinion is that because different converters have different sounds, the best way to evaluate them is to put aside the quest for objectivity and to simply use your ears to subjectively evaluate the sound, just as you would evaluate any other piece of audio gear. So, for this review, I am not even going to discuss the components of the HD I/O or its measured specs. I’m just going to tell you what I hear, because in the end, that’s all that really matters. This review is also not a shoot out with the rest of the converters on the market, but a focused comparison with the Digidesign 192 I/O, which the HD I/O is replacing.

First of all, the HD I/O has been reconfigured from its prior inception as the 192 I/O. The HD I/O can be configured in three ways. The 8x8x8 has eight analog ins, eight analog outs, and eight AES digital I/O. The 16x16 Analog has sixteen analog ins and sixteen analog outs on DB25 connectors, and the 16x16 Digital has exactly that many AES digital ins and outs. Each configuration also includes two channels of AES/EBU I/O (supporting 192 kHz single-wire), two channels of S/PDIF I/O, and eight channels of ADAT I/O (supporting S/MUX II and IV). While the 16x16 Analog’s rear bays are full, there is one free bay on the 8x8x8 and two free on the 16x16 Digital for further expansion and customization. Also different from the 192 I/O is the elimination of the second set of trim pots. Apparently, this feature wasn’t so necessary, and Avid decided to remove the second set on all new converters moving forward. While this is perhaps a simpler configuration, I can imagine some studios will miss them. Aside from these differences, the HD I/O is physically nearly identical to the 192 I/O.

I was using an HD I/O configured as a 16x16 Analog. In order to judge the sound of the unit, I did a direct comparison tracking drums at Mavericks Studio in NYC, a room where I have been tracking drums through the 192 I/Os for over five years straight. It’s a sound I know very well, and I know what the 192 I/Os do with that sound. My recording setup was a pair of Neumann U 87s as overheads being fed through API 7600 channel strips (Tape Op #43), an AKG D 112 on the kick, and a Royer R-121 (#19) as a room mic being fed through API 512 preamps. All channels were first sent directly to the 192 I/O, and I tracked a beat at 96 kHz. Then I switched the converter over to the new HD I/O, and I played the same beat. The only difference in the signal path was the converter. I didn’t bother trying to track the same performance through each converter because I would have been introducing the clocking issue. I wanted to hear each unit on its own, clocked internally.

In a nutshell, the new HD I/O gets rid of everything I never liked about the sound of the 192 I/O and leaves a far more open and “natural” sound. To my ears, the 192 I/O has always had a slightly crispy-crunchy top end. Not a horrible sound, but lacking a certain nuance and smoothness that I always noticed, especially with hi-hats, strummed acoustic guitars, and vocal esses. Similarly, the bottom end on the 192 I/O always seemed to lack a certain 3D quality that I heard when using analog tape, or even when using other converters like the Crane Song HEDD 192 ( Tape Op #26) or the new Burl B2. Perhaps because of these qualities, the 192 I/O often sounded hyped in the midrange — just a touch. Again, these aren’t huge complaints as much as observations made over nearly a decade working very happily with the 192 I/Os day-in and day-out.

With the new HD I/O, I immediately enjoyed the more open and smooth top end. For these drum tracks I did, that meant that the hi-hats were more nuanced and not as harsh on my ears, the snare seemed to sit in the kit very nicely, and the crash cymbals were just a touch smoother and complex. Also, the smoother and more detailed top end allows the great room sound at Mavericks to ring just a bit more deeply. If you’ve ever clocked a 192 I/O to an Antelope, Big Ben, HEDD or other external clock, you may have noticed that you get a more 3D soundstage. I heard a similar 3D quality when listening to the HD I/O over the 192 I/O. Again, not enormous, but apparent.

The bottom end on the HD I/O was similarly more open and extended. I have a particular interest in making the low end of my recordings sound as 3D as possible. I love the big, expansive low end Daniel Lanois and T Bone Burnett are able to achieve on records like Rockets and Raising Sand, respectively. These are kick drum sounds that you can see; they’re like enormous, weightless boulders drifting through the night. With the HD I/O, that kind of low end was rendered more vividly than with the 192 I/O, and I think this quality will be important in any production style, from live orchestral performances to programmed TR-808s.

So if you own a 192 I/O, I wouldn’t tell you to run out and buy an HD I/O. If you’re looking to upgrade your 192 I/O, I might suggest you try out some external clocks and use your ears to determine if that’s a sound you like. (Again, I suggest bypassing all theoretical and specs-based arguments.) Perhaps a clock will work for you and save you a few bucks. However, if you are in the market for a new converter, the HD I/O should definitely be on your list of options, and if you do get an HD I/O to complement your 192 I/O rig, I’d bet money that you’ll be routing to the HD I/O’s conversion channels first.

At $4995 street for the 16x16 Analog, this is not a small investment. With the number of less-expensive third-party converters out there that will work with Pro Tools, I think the decision of which converter to buy is only going to become even more confusing and complex in the next few years. I encourage everyone to ignore the specs, theories, debates, and confusion as much as possible and to make a thoroughly subjective decision based on what you hear and can afford. The new HD I/O may not be cheap, but I’m sure it’s going to win a lot of listening tests. (Check out Allen’s audio samples at (I/O Digital $2495 street, 8x8x8 $3995, 16x16 $4995;

–Allen Farmelo,

Tape Op is a bi-monthly magazine devoted to the art of record making.

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