July 1, 2008

24-bit/176.4kHz Computer Audio: Can’t Get There from Here

I’m taking a break from my "Answering the Questions No One’s Asking" series to tell you about a recent and frustrating audio misadventure.

I’m a fairly early adopter of a hard-drive-based music server using an Apple MacBook and various USB D/A converters, and so far I’ve been thoroughly impressed. The sound quality is better than with any CD player I’ve ever used, and the user interface is something that, once experienced, I didn’t want to give up. Now, I wouldn’t think of going back to a disc spinner.

Using iTunes to import my CD collection in Apple Lossless (yes, with error-correction on, a must) has been a snap. I’ve had no problems with any 16-bit/44.1kHz "Red Book" CDs, nor have I had any problem reading the CD layers of many hybrid SACDs. And I’ve heard no clicks or pops from my MacBook’s USB output. So what’s the problem?

Well, there wasn’t one -- until I received a Reference Recordings HRx DVD-R with 24/176.4 audio, containing a file that RR calls a "digital master." The disc can’t be played by any disc player, but must be loaded onto a computer, then played as a WAV file. I had no problem saving the HRx recording in iTunes or playing it back through my Stello DA220 Mk.II DAC. And yes, the sound was better than from the CD of the same performance that RR had sent along for comparison purposes. But I’m not exactly sure why that was the case, because I wasn’t playing the file in its native resolution.

And here’s the problem. Because it says so right on the faceplate, I’d assumed that my Stello DA220 Mk.II DAC was a 24-bit/196kHz D/A converter. But it isn’t. It upsamples "Red Book" CDs to that resolution, but it won’t accept that resolution natively through any of its digital inputs, and its USB input accepts nothing higher than 16/44.1.

I read everything I could find on the Internet, and came to the conclusion that there are so many opinions out there, and so much flux in the industry regarding computer audio, particularly in terms of high-resolution music, that no one seems to know exactly what’s going on. Conflicting opinions? Yes. Misinformation? You bet. Outright lies? Maybe not, but misinformation is close enough.

To save you some headaches, here’s what I think I know, as of today. Just don’t take it as gospel until you’ve verified it for yourself.

1. As far as I can tell, there is no DAC currently for sale that will handle a 24-bit/176.4kHz signal via USB.
   
   
2. The Apple MacBook is capable of sending out a 24/176.4 signal, possibly via USB, but definitely through its FireWire output.
   
   
3. The USB inputs of most DACs on the market today are limited to 16/44.1. An exception is the newest model from Benchmark, the DAC1 USB, but even that is only 24/96.
   
   
4. The only DAC I could find capable of 24/196 via its FireWire input is the new Weiss Minerva, a sample of which will, I hope, have arrived by the time you read this.
   
   
5. One of the newest and most talked-about DACs, the Berkeley Audio Designs Alpha DAC, has no USB or FireWire inputs. It can accept a 24/196 signal via AES/EBU, but that leaves no interface available for Mac folks. Oh well . . .
   
   
6. iTunes will output up to 24/196 audio, as stated before, but it won’t switch automatically based on the native resolution of the file. You must go into the Audio Midi settings and do it each time you choose an audio file with a different resolution.
   
   
7. If your DAC won’t accept a hi-rez audio signal, iTunes will automatically sample-rate-convert the file to 16/44.1. You might think you’re outputting a full-resolution signal, but you aren’t.
   
   
8. In Apple’s Audio Midi settings, the available audio resolution displayed on the screen is what your DAC is capable of. Remember that USB is a bidirectional format: your Apple senses what resolution your Audio DAC is capable of, and that’s what it will let you choose as the highest output resolution.

Although the Weiss Minerva is my best shot at hearing Reference Recordings’ HRx series of DVD-Rs, I’m still looking for other options. If you’re an expert in computer playback of hi-rez sound, please let me know what I’ve missed or gotten wrong, and I’ll pass it along to our readers.

. . . Jeff Fritz
jeff@ultraaudio.com

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