The 16th bit
We should feel sorry for the 16th bit. You know, the last little guy in the digital word of a CD, the ending of the ubiquitous 44.1/16.
The technical term for the 16th bit in the digital word is “least significant”. Imagine if you were the last child in the family and your parents referred to you as the least significant child. (Of course, if you had the devil in you as I did you’d likely have gotten away with plenty more as no one would notice you).
If it weren’t bad enough to be the least significant in the family, my friends Robert Becker, Dave Fletcher, “Prof.” Keith O. Johnson and Michael “Pflash” Pflaumer, were guilty of doing away with number 16 altogether when they launched HDCD. Number 16 was ruthlessly stripped of even its insignificant role as tiny musical data supplier and relegated to that of a mere toggle for greater bit depth in downstream gear. Instead of adding its tiny contribution to actual information, HDCD now condemned it to a mere switch: sitting idly by or just dithering around.
And if you’ve not yet mustered any pity for the 16th bit, just think of how badly it must feel when it lost its last-place least significant status to higher bit rate formats of 20 and 24 bits. At least last place had some distinctive merit.
No, it’s really quite the shame.
The 16th bit just can’t win for losing.
We went to see Andy McKee last night and talk about dynamics!!
Dynamic range describes the difference between soft and loud. What’s not defined is how soft or how loud—only the magnitude between the two—which is a problem if the softest sounds fall below the threshold of audibility. When that happens the range of dynamics is truncated, a fact not apparent when quoting dynamic range figures.
I’ll give you an example. The threshold for hearing is defined as the minimum amplitude the average person can detect sound. This level happens to be around 15dB for a middle-aged male. If we want to be able to listen to the full range of a CD, which is 96dB, we have to adjust the system’s loudest peaks to be 111dB (15dB + 96dB). That’s pretty loud and beyond the capability of most speakers to comfortably hit those levels. Which means we aren’t going to get the full dynamic range possible out of a CD.
But, wait a minute. A CD is 16 bits limited in dynamics (96dB). Higher bit depth, like 24 bits, can theoretically go as high as 144dB, though noise and other factors set the practical limit at about 123dB. How then can this greater dynamic range from greater bit depth matter, if we’re already losing dynamic range to the threshold of audibility?
It can’t, at least not by much.
And vinyl? Heck, we’re lucky to get 70dB from well-pressed vinyl, forcing the mastering engineer to compress the presentation into that smaller space. That said, vinyl’s the only medium that actually scales pretty easily. You get it all at most volume levels, though the price is the compression.
So, the bottom line in this ramble is simple. Even though a CD is limited to 96dB of dynamic range, by making sure to turn the level up so the softest passages can just pass the threshold of audibility, we can get close to the dynamics of live music. Deeper bit depth makes it easier, but not by a lot.