Schiit Yggdrasil -- 21 bit?


Schiit says that Yggdrasil is a 21 bit DAC. But the DAC chips that they put in the device ( Analog Devices AD5791BRUZ, 2 per channel) are 20 bit with the error of plus-minus 0.5 LSB.

How can the DAC be 21 bit if the chips are 20 bit? Using two chips per channel does reduce the RMS voltage of the noise by  a square root of 2. But how can you get to 21 bit from there?

Can someone please explain.
defiantboomerang
@gbmcleod   Looking forward to hearing about the Yggdrasil and where it falls for you. I have found it responds well to isolation, so your use of the Townshend platforms will help stretch it's performance. If you are using the USB input, make sure you get the Gen 5 board installed, should yours be an older unit. Holiday Cheer!
@gbmcleod 

Best of luck with your new Yggy. I am a relatively recent owner of one myself, and I'm very pleased with it. I'll be interested to read your impressions of it.

To David's point, if you recently placed an order, it will ship with Gen 5. They all do. You will know this by a sticker on the back panel above the USB input and from your computer when it is connected via the USB (the device name will indicate Gen 5).

As to your statements (paraphrasing) about listening with ones ears instead of relying on specifications, I believe both are very important and useful. All available information should (IMO) be considered when evaluating a component, and that certainly includes your own ears, but should not be limited to your own ears. 
I remember reading Martin Colloms’ testing of the Jadis JA-200. Tested TERRIBLE on the bench, but magical in my system. I DO agree that maybe it helps those with technical knowledge, although I've seen people sneer about things they've never heard. Psychologists used to sneer about EMDR as an effective treatment for trauma: now it's a completely valid treatment. But the fact that they behaved that way without first-hand knowledge was something that disgusted me, as I KNEW what it did, and they did not.
In the same vein,  I hear live music weekly, so, a spec is not going to influence my buying choices. My ears have rarely led me wrong - although my expectations of life experiences, have.  Specs benefit those who can understand them (and I go cross-eyed reading test bench results such as Atkinson’s - and I should have made that clear - so I can understand how it would matter to others, but as I said, the Jadis JA-200 tested terribly, but sounded magnificent.
I confess to being astounded Atkinson didn’t mention even listening to the unit to hear what it sounded like, particularly given his being unimpressed with the technical results. That’s when I would have sat down and listened.  To MY mind, that’s like having possession of a Ferrari, looking under the hood, testing it - but never driving it. Blows my mind.
David and dhal:
I think it will be at least a week before I can discern how closely the Yggdrasil approaches the sound of live music. Mine arrived earlier than I expected: when I checked the tracking, it had already been routed to the local Fed Ex office, so I picked it up early this AM, and, of course, plugged it in immediately, and played it. I did this even though I KNOW I won’t hear anything new, because I just bought the Transparent SPDIF cable yesterday afternoon, and it, too, will have to break in (no other choice: haven’t had a DAC since the Bryston BDA-2 and sold the last coaxial cable long ago). No matter. I will still enjoy hearing the sound blossom. The only thing that was surprising to me is receiving an email from someone at Schiit, saying that they did not believe in "burn-in" time. This, especially given that Robert Harley, in his TAS review, wrote "Although Moffatt warned me that the Yggy wouldn’t sound good right out of the box, I gave it a quick listen anyway after an hour of warm-up. He was right; the Yggy was hard, bright, forward, and flat. I checked in with it a couple of times over the next week and heard it improving somewhat, but it was still disappointing." So I’m unclear if there was more to the conversation, but that Moffatt only meant that until the unit’s thermal whatever-it-is reached optimum temperature, that it wouldn’t sound good. Harley clearly didn’t like it even after a week.
And this is why I only use my own ears and why I don’t compare components (or use measurements): for me, only the sound of a live instrument has a reality that is indisputable. And, as I said, I’ll look at the specs, but I’m always going to go with "how close does this sound to a live cello/guitar/harmonica" or whatever, despite the fact that these instrument are recorded, and very little that is recorded sounds like ’the real thing.’ either. But heck, anyone who listens to a stereo knows it isn’t going to sound like the real thing. The only thing for me is, again: ’How Close’?

The Boston Pops RCA CD under Fiedler sounded the same as it did without the Yggy, as did Yma Sumac, the Rolling Stones, and Chesky Records System Setup and Surround Sound CD, and so far, it’s been 5 hours. I got to listen for 10 minutes this morning before I had to leave the house, and while It didn’t sound "bright," I had just plugged it into my NAD integrated, which I like quite a bit. I was going to involve the CJ setup, but decided against it, since I’d been using the NAD the past few weeks for checking room acoustics and the Townshend Seismic platform and pods, and don’t want to introduce other variables into the listening the first week. The ONLY thing I could tell is, that, despite the newness of the Transparent cable and the Yggy, the organ on the Chesky CD was distinctly more powerful than before the Yggy. I guess I AM going to do ONE thing additionally, in a few days: plug in the Nola Thunderbolt subwoofers to the second preamp-out of the NAD, just so I can get an idea of what the deep bass is like. I expect that THAT should be quite the thing to play at midnight tomorrow night! Either that or the 1812 Overture on Mercury Living Presence just to hear the (actual) cannon they used!