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Old 2008-02-17, 03:02 PM
Re: Vinyl records vs. Cds.

0.1.1 Why 24 bits? Isn’t 16 bits enough?

A: 16-bit technology is over 20 years old, but that’s not simply a reason in and of itself. The Compact Disc format and specification that promised superior quality audio was derived from a compromise between playback time and sonic quality. It is this compromise that facilitated putting 16-bit audio within reach of everyone’s home, car, and person. However, it is this same compromise that has left audiophiles to grasp tightly onto their old vinyl LP’s and their audiophile-grade turntables in hopes that something better would come along. Today, 24-bit audio technologies promise to deliver the same sonic experience once promised by CD’s. The notable difference here is, 24-bit technology actually delivers the level of performance most audiophiles expected long ago. Sure, nothing beats the live performance experience, and 30 i.p.s. 2” analog reel tape is second to none in its ability to reproduce that experience. But we can surely say that the edge 24-bit recording has over their 16-bit counterparts puts a big smile on the audiophiles face. Why? What’s missing on my 16-recording?

Simply, the answer is detail. The PCM format provides its optimal resolution when signal levels are at their very highest. As signal levels decrease to lower levels, resolution deteriorates, leaving quiet cymbals and string instruments sounding typically sterile, dry, harsh, and lifeless. The more bits you have available to you in the process of quantizing the amplitude of a waveform at any given sampling, the more accurately a lower level signal can be represented. If an instrument is very loud while standing next to it, but is recorded at a low level, there are less numbers that can be used to represent just exactly how loud it is at any given moment. We know that a wave modulates between silence and its maximum amplitude or volume, while the number of times per second this modulation occurs gives us the pitch of the wave.

For example, if there are only 4 discrete numbers that can be used to represent the volume level of a particular recording, 1 would be silent, 2 would be very audible, 3 would be louder, and 4 would represent the loudest level. Can you imagine what all your audio would sound like if these were the only choices for representing amplitude at any given moment? Definitely horrible, and it would sound like square wave distortion and noise. This example would be a 2-bit recording.

In order to make this sound better, we need to be able to have discrete values in between these values. A fading piece of music can’t just go from very audible to silent, or it wouldn’t be a smooth fade. A 4-bit recording would have 16 discrete possible amplitude levels. Can you again imagine what this would sound like? Definitely better, but its still a totally horrible representation of the sound. We can deduce from this that the more discrete values available to us, the better it will sound. Is there a limit to the human ear’s ability to perceive these inaccuracies? Definitely, but it unfortunately does not stop at the 65536 discrete values afforded to us by 16-bit technology.

The overriding concept here is called dynamic range, and is measured in dB. The dynamic range of a recording is the difference between its loudest point and its quietest point.

To elaborate further, each bit gives us the ability to represent about 6dB of dynamic range. A passage that is 6dB louder than another passage is said to be twice as loud as the other passage. In the 4-bit example, we theoretically have 24dB of dynamic range that can be used. But what if recording doesn’t take advantage of all that dynamic range? What if the recording never peaks beyond 6dB of its maximum possible limit? In this case, the recording would only take advantage of 3 of what we call the least significant (or left-most) bits, meaning 18dB of dynamic range. 16-bit recordings are capable of a theoretical maximum limit of 96dB of dynamic range. This means that a single wave could have up to 65536 discrete values that can be used to represent it. But if the same wave recorded at 16-bit peaks at 48dB below its maximum possible limit, then there would only be 256 discrete values that can be used to represent it, taking advantage of only 8 of the least significant bits. The 8 most significant bits would contain no information whatsoever, and would remain unused. In the case of 24-bit recording, you’d have a maximum of 16,777,216 values to choose from, and in the case of a wave peaking at 48dB below its maximum possible limit, the wave would still have 65536 possible discrete amplitude values that could be used to represent it.
The more discrete values available in the digitization stage, the better, until the limits of human hearing to perceive inaccuracies are reached. The DA converter has to reconstruct the waveform from the discrete values. The more available values there are digitially, the more accurate the conversion can be to analog.

2_______________4__________________________12 dB
3_______________8__________________________18 dB
4_______________16_________________________24 dB
8________________256_______________________48 dB
12_______________4,096______________________72 dB
16________________65,536____________________96 dB
24____________16,777,216____________________144 dB
32____________4,294,967,296_________________192 dB
48___________281,474,976,711,000_____________288 dB
56_________7.20575940379 E16 (add 16 0s)______336 dB
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