Archive for Hifi Posts

“Bitperfect” – huh?

After a rather long hiatus from audio technology, i was re-immersing myself in the technology — especially with regard to the evolution of digital formats and streaming.  An odd word kept coming up – “bitperfect” – commonly used, almost never defined, what the heck? Of course bits are perfect. The problems are all analog.

I’ll not go down this rat-hole today, but suffice it to say that digital audio signals have analog characteristics to them that have direct impact on the reconstruction of the analog wave.  More on THAT later.

So what is “biperfect” and what’s imperfect about much digital (computer) audio?

I’ll oversimplify.  Most of this has to do with how volume is controlled in computer audio.  One would think that once in the digital domain, manipulation – for example turning down the volume – would be easy and without distortion.  In theory, it can, but in reality, one would be wrong. the vast majority of music is coded initially as “RedBook” – CD format with a resolution of 16 bits – or “65,000 shades of gray” which is pretty darned good – and IMNSHO, NOT where the problems in CD audio lie.  But if we simply do volume-control multiplication (like make it half as big) on the 16-but words, we slowly lose resolution (think through the math its true). If this doesn’t make sense – think about an extreme example – we digitally “turn down” the volume  99.99something% of the way and are left with only three digital levels – zero, one and two.  This is two-bit resolution and will sound like absolute crap. That’s a technical term.  For a comparison, if you can, turn your monitor to 4 or 8 bits of color and look at the screen.  Yuk.  ‘nuf said I hope.

You can see the numbers in a presentation by ESS Technology here:

http://esstech.com/files/3014/4095/4308/digital-vs-analog-volume-control.pdf

To get it right we need to do two things:

  1. do our math at higher resolution, for example 24 or 32 bits, so we can maintain full 16 bit resolution,
  2. **AND** directly convert these higher-resolution words to analog (meaning a 32-buit conversion process) so we don’t truncate that resolution.

Simply doing the math in 32 bits is pretty simple. Yes, we’d need some code to convert it and do floating point math, and we’d need to temporarily save the much bigger buffered file, but that’s easy for machines that edit in photoshop.  The problem comes next; we need to convert this 32 bit word into a squiggly AC analog music voltage.  Problem: we have a 16-bit D/A chip.  And all the interfaces (SPDIF, AES/EBU, carrier pigeons).

You can read more about it, but the bottom line is this: in 99% of all cases, and 100% of all PC/MAC/LINUX cases, you should never use the digital volume control – that convenient little slider.  Just say no.  Set the volume to full and send the output to your DAC or networked digital player – and let ALL the bits get there, to be converted to a nice, clean, high-res music signal.  Then you can attenuate it with good, old-fashioned resistors.

(note: if you are just playing MP3 files through the sound card to your earbuds, none of this really matters)

So “bitperfect” is a word that we should never have had to invent nor explain. It comes of shortcuts made in commercial music players.

Fortunately, most high res players with real aspirations know this, and take care of it. JRiver, Roon, etc. are all bitperfect. Sorry to leave out many others.

I’ll add that for Macs there is a surprisingly good app that simply takes your iTunes library and hijacks the signal, delivering it without manipulation – in other words, “biperfect” and costs $10.  Its called….. Bitperfect for Mac.

Enjoy!

 

Grant

CEO, Sonogy Research, LLC