All Bits are Significant; Some are More Significant Than Others

Neil Young told MTV News that listeners of MP3 audio hear only "5 percent" of the data in an original recording. He continued: "We're in the 21st century and we have the worst sound that we've ever had. It's worse than a 78 [rpm record]."

The math is solid. Looking at raw numbers, 24-bit/96KHz LPCM sound has 18x the bit rate of a 256K MP3. Invert that, and you get an MP3 worth roughly 5% of the original.

[Image credit: ~4ntigravity.]



But is bit rate alone a good relative measure? I'd like to convince you that it is not. Sample size is key.

In any discussion of bit rates, a chart like this is employed at some point. I'm guilty of using it myself. The trouble is, if you're trying to be more specific in making format comparisons than simply 'more is better,' reducing the argument to numeric multipliers is simplistic. All bits are significant; some are more significant than others.

Information increases exponentially with bit width.

Information is a measure of decrease in uncertainty. Saying that a sound sample can be encoded in N bits implies that N yes/no questions must be answered to resolve the uncertainty of its actual value. The maximal uncertainty to resolve (and hence potential information content) grows exponentially as sample size N increases.

As Carl Sagan said, not all bits have equal value. The greatest uncertainty is  removed by question 1, or the most significant bit (MSB), so this bit has the highest information value. The smallest uncertainty is removed by question N, or the least significant bit (LSB), so this bit has the lowest information value. Paradoxically, answering the questions becomes increasingly harder as you progress from MSB to LSB due to greater detail being supplied, i.e. the more sample bits you want, the more difficult they become to obtain.

In contrast, information content grows linearly with sampling rate. Doubling your rate produces twice as much information. Tripling the rate triples information, and so on.

Thus information contributed by sample size and sampling rate increase on different scales. While 24 16-bit samples and 16 24-bit samples have the same bit total, the larger samples took more effort to obtain and are more valuable bit-for-bit. I previously said you should seek and preserve maximum bits in the vinyl-to-digital transfer process. What I really meant was, seek and preserve information value.

Choose bigger samples over higher sampling rate if you can't maximize both.

              Vinyl-to-Digital Restoration #12              

Title: Live Rust
Artist: Neil Young & Crazy Horse
Genre: Rock
Year: 1979



Music buyers who only know an industry dominated by iTunes can't imagine a time when you needed to buy an entire double LP set just to get the one or two tracks you really wanted. The most significant bits for me on this title are from "Powderfinger" and "Like a Hurricane." Well worth the effort to obtain.

© 2012 Thomas G. Dennehy. All rights reserved.

A Xerox of a Poloroid of a Photo of a Painting

What format and bit rate should you choose for ephemeralizing your LPs?

While it may be popular, lossy compressed audio has its detractors. In an interview, Grammy-winning producer T. Bone Burnett likened its fidelity to the sonic equivalent of "a xerox of a poloroid of a photo of a painting."

[Image: low-res Beatles Sgt. Pepper album cover.]


Nonetheless, the 256 KBit/sec MP3 has become the de facto standard for purchasing and streaming music in the cloud.

Like WMA and AAC, MP3 is a lossy compressed format. (The three formats are interchangeable for the purpose of this discussion.) You can't uncompress an MP3 and get back the original audio. Some information is thrown away in the compression process to gain additional compaction over lossless compressed formats.

Lossless uncompressed formats incorporating linear pulse-code modulation (LPCM) capture a direct digital representation of an analog wave. "CD-quality" uses 16-bit samples taken at 44.1 KHz. The equivalent bit rate of 1411.2 KBit/sec transmits more than 5x the information in MP3 audio, with no loss due to compression. (See figure at right.)

Even a CD-quality copy introduces downsampling from the original. Most digital studio recordings are made with 24-bit samples taken at 96 KHz. The equivalent bit rate of 4608 KBit/sec transmits 18x the information in MP3 audio and more than 3x the information in CD audio.

It is possible to make 24-bit/96KHz recordings at home, 18x richer than a "good" MP3. Gigahertz computer clocks facilitate high sampling rates. Near-zero cost of storage makes compression unnecessary.  But is it worth generating and storing all those bits? Can anyone really hear the difference?

My Harman colleague Dr. Sean Olive, Director of Acoustic Research, is actively seeking a scientific answer to that latter question. My answer is simple: I don't care.

When it comes to information, more is always better. The digital transfer pipeline is software-driven. You may or may not be able to "hear" the difference, but your software tools can "see" the difference and work better when they have more to chew on. I'll use use the next couple posts to try to convince you to seek and preserve as many bits as possible when recording and processing, even if you ultimately choose a compressed format in which to store and enjoy your end products.


              Analog-to-Digital Restoration #8             

Title: Diamonds and Pearls
Artist: Prince & the New Power Generation
Genre: Soul and R&B
Year: 1991



Everything we've said regarding digital transfer of LPs applies identically to other analog source material. If you still have the equipment to play something, you can usher it into the digital future. I recently bought a like-new Denon DRS-810 at an estate sale for $25—a real find—to add to my recording station. My wife's equally like-new cassette copy of this Prince title is now ephemeralized.


© 2012 Thomas G. Dennehy. All rights reserved.