- Published on 27 September 2011
Channel D Pure Music High Resolution Music Server Software Design and Setup
Apple's iTunes software is the de-facto leader in the computer audio player world, and for good reason. Its interface is, in my opinion, clearly the best from a usability point of view. The fact that most other players try to imitate the iTunes interface is evidence for this conclusion. iTunes has many issues, though, when it comes to audio playback. It is most definitely not an audiophile product. It cannot play FLAC files and can perform bit depth and sample rate conversion without the user's knowledge. In addition, its audio playback engine is a black box. In theory, iTunes on a Mac is bit perfect as long as all the audio processing options are turned off and the volume control is set to 100% (meaning the PCM output is bit for bit identical to what's in the audio file), but its up to you to be sure every option is configured correctly.
Channel D has written Pure Music to retain the interface of iTunes, with all of its usability advantages, while replacing the playback engine with custom, high performance code written specifically for the best possible audio quality. Pure music is a software tool that simultaneously launches with iTunes, and "docks" to the iTunes window. The iTunes window is used to browse music and manage the library. When the user presses the "play" button, Pure Music takes over the job to play the file. The Pure Music window docks around the top and left of the iTunes window. It provides level meters, file info, level control and other control tools. Unlike the Apple ideal of providing a minimal number of configuration options, Pure Music allows the user to configure virtually every detail of the playback software. This is great for geeks like me, but could result in a steep learning curve for those who are less computer-inclined. Mercifully, the standard options work pretty well.
Pure Music does have one serious disadvantage that should be made clear up-front: It is for Mac OSX only. There is no Windows version of Pure Music, nor will there be any time soon. The audio engines under the hood of OSX and Windows are so different that there's no way to "port" the software. It would have to be re-written completely from scratch. Rob Robinson of ChannelD recommends that any mac-less audiophile that wants to use Pure Music just get one of Apple's $599 Mac Minis. It has plenty of computing power, memory and hard drive space to use as a dedicated music server, all for a cost that's a lot less than many of us have paid for other audio components. Hook this up to your home theater display via HDMI, attach a USB to SPDIF bridge of some sort and get a wireless mouse and keyboard, and you'll be all set. I used Pure Music on my 2006 vintage Macbook Pro 15" running OSX 10.6.8. The machine has an Intel Core 2 Duo 2.16 GHz processor and 3 GB of 667 MHz DDR2 RAM. My iTunes library of ~6000 songs was stored on a 750 GB Western Digital USB2 external hard drive, and consists mostly of 16-bit, 44.1 kHz Apple Lossless files I ripped myself from my own CDs using Max 0.9.1 ripping software. I also have some 24-bit, 96 kHz files also ripped from PCM DVD discs and DVD-A discs, also stored in Apple Lossless format. I began the review with Pure Music 1.74a, and upgraded to 1.8 during the review. Version 1.8 added the ability to play FLAC and DSD files while still using iTunes for file organization. DSD files are converted to PCM for playback in real time. The new software also allows other computer audio sources (say, internet radio) to play through Pure Music to take advantage of the software's volume control and re-sampling tools. Interface to my Bryston BDA-1 DAC was via the Halide Design USB to SPDIF bridge, both of which I reviewed last year. I liked them so much I bought them.
The real magic of Pure Music is not the interface, but the highly configurable playback engine under the hood. It offers pretty much all the features and performance an audiophile could want in computer playback software. It can play every file iTunes can play natively, plus FLAC and DSD files. It has gapless playback, and will support any output device that is supported by OSX. It has several features that are of great use to audio enthusiasts. First and most obviously useful to me is automatic sample rate switching. With normal iTunes, the output bit depth and sample rate will be set to whatever is chosen in the operating system Audio Settings, regardless of what properties the file has. This means users had to manually use the "Audio MIDI Setup" utility to switch the sample rate to match the file before playing, or iTunes will perform sample rate conversion to match the specified output configuration. Pure Music does this automatically, configuring the output sample rate and bit depth to match the file being played. Pure Music does all of its internal math with 64-bit precision, rather than the usual 24-bit or 32-bit used in hardware. The software has digital, dithered volume control, a digital crossover function and real time upsampling and bit depth conversion to sample rates as high as 384 kHz (including downsampling high sample rate formats to lower sample rates). Given the 64-bit precision and the ability to implement complex algorithms in software, all of these features should theoretically work better than in hardware. Of course, all of these features are completely configurable, and can be turned off.
I set the software up using two features that should result in the best possible audio quality: memory play and Hog mode. In memory play, an audio file is loaded into RAM in advance, and played out of memory rather than off the hard drive. This will eliminate any errors or issues caused by the data flow from the hard drive being interrupted. You can choose normal memory play, where the ENTIRE file is loaded before playback begins, or Hybrid Memory Play, where the file is buffered into memory and playback begins as the file continues to load into memory. I used the latter to speed things up. With hybrid memory play engaged there is a short delay between selecting a file to play and playback beginning. In addition, the iTunes scrubbing control does not work as expected. If you want to be able to use the scrubbing control, you have to turn off memory play. Memory play does require a considerable amount of memory, but that's cheap these days. I upgraded my system from 1.5 GB of RAM to 3 GB of RAM for about $50. Hog mode allows you to select a different audio device than is currently being used by the OS, and "lock" that device to Pure Music alone. This prevents system sounds or audio from other programs from being mixed into the music output by the OS. I set my system to use the Halide bridge as the Pure Music output, and left the OS set to the built-in speakers (turned off, of course).
My Bryston BDA-1 upsamples to 192 kHz. I chose to turn the upsampling in Pure Music off. My choices were to upsample in software to 96 kHz (the maximum bit rate of my Halide bridge), and turn the upsampling of the BDA-1 off, upsample in two steps, in software to 96 kHz and in the BDA-1 to 192 kHz, or upsample to 192 kHz just in the BDA-1. I thought the latter would be the best option. If I had a 192 kHz interface, I would have used the 64-bit upsampling in Pure Music. I did not test the crossover functions or any Audio Unit plugins, and only briefly tested FLAC playback. All my files are Apple Lossless already, so I had to go find some FLAC files to test. I had no DSD files, so I didn't test that functionality either.
Pure Music is available for download from ChannelD's website. A 15-day fully featured trial license is available to try the software out. Since it uses your normal iTunes library, it is very easy to install or remove the software without making any changes to your library. It already is compatible with Apple's latest OSX release 10.7 (Lion).