Tarktones
05-02-2008, 03:57 PM
I was recently reading a cookbook. The author was emphasizing the importance of using good salt over cheaper, generic salt. He described the flavor differences and insisted it had an important impact on the final dish. I chuckled with skepticism. But, I happened to have two different brands of salt in the cupboard. One was a domestic brand, a sea salt of an unknown origin. Its container described how it was better because it didn't have chemical bleaching agents and retained the natural trace minerals. The other was a more expensive imported naturale Italian sea salt. So, of course, I performed a taste test. To my huge surprise, there was a very significant, undeniable difference in taste. The locations of sensation on the tongue were also different. The Italian brand easily won out. It was smooth, pleasant, and the flavor finished evenly. By comparison, the other brand was strident, piercing, and had a synthetic chemical-like flavor and feeling. Both immediately evoked the idea of salt -- but, side-by-side, the differences were almost like that between real sugar and an artificial sweetener. I presented this taste test to a handful of other people, and all the reactions have concurred with mine. I will certainly never think, or rather not think, of salt the same. The cookbook writer was correct. I've found it does make a subtle but worthwhile impact on the end product, making the eating experience just a little bit more enjoyable, especially after all that hard work in the kitchen.
So what does salt have to do with audio dithering? Dither and salt both speak to the issues of marketing, personal enjoyment, and conventional wisdom.
Conventional wisdom says that all salt tastes the same. It's salt! NaCl. Sodium chloride. How could one box of NaCl taste that much different from another box of NaCl? Conventional wisdom scoffs at the idea that one brand might taste much better than another. Conventional wisdom laughs at the idea of spending five dollars for a can of it, rather than a buck for that old Morton's.
Conventional audio engineering wisdom says dither's a bit like salt -- you add a pinch of noise to "enhance the flavor." Conventional wisdom says that you'd be crazy not to dither when mixing down to a 16-bit master. I say: Conventional wisdom isn't all that wise. Stop bothering with dither! Unlike salt -- it has no impact on the end product.
I know what a lot of people will say to this: But, I've heard the improvements of dither with my own ears! I've done the taste test. I've read the book. And, I've seen the movie!
For example, you can go to this website and evaluate all the major players in dithering technology. It's true -- you can absolutely hear the improvements in their examples. And, that particular algorithm with the most votes does sound the best. But, let's read the fine print about the chemical bleaching agents: "54 dB of gain has been applied after dithering." Since the audio isn't audibly clipping, this means there was at least 54 dB of headroom in the original digital signal. 54dB of gain equals a multiplication factor of 501. In binary, this equates to shifting the digital word almost 9 bits (501X = 2 to the power of 8.97 bits). In other words, the original audio had about 9 unused bits of headroom. 16 minus 9 equals 7. We're listening to the equivalent of 7-bit audio here. So, what does this listening test prove?
That dither is highly effective for 7 or 8 bit audio. How many recording engineers do you know sending out 8 bit masters?
Why did the creators of this test use such bit-reduced audio? They had to or the test never would have worked. Quantization noise, which dither is meant to mask, is not humanly audible for typical, normalized, 16-bit audio, especially not in the everyday listening environments where we experience music. 16-bit audio has over 90dB of dynamic range -- this is the difference between the maximum signal amplitude and the quantization noise. In other words, the quantization noise is 90dB below what you're actually listening to. That's huge! Think about that in real world terms: 85dB SPL is a standard peak calibration level for movies and is also the threshold of hearing damage, at long durations. Under this condition, 90dB below that is well under the generally accepted "threshold of human hearing". Expecting to hear the effect of dither is like expecting to hear the fluttering of a fly through the din of a jackhammer -- it's just not going to happen. Maybe... just maybe, with a highly un-normalized Classical CD from the 80's listened to in an anechoic chamber with the lowest-noise-possible audio reproduction gear, you just might barely detect hints of quantization noise on the quietest passages. But, I'm dubious -- who do you know that has an anechoic chamber for a living room anyway?
To further this assertion, why is it so difficult for audio engineers to figure out how to apply dither properly? There's been numerous magazine articles, white papers, tidbits in the many home recording books, and thread after thread on the internet forums asking: "How do I dither?", "When do I need to dither?" , "Should I re-dither?", "Does dither work with floating-point audio, or only fixed-point?", "On what insert do I put the dither plugin?" "Can I EQ the audio after dithering?" And, these questions never end.
If dither makes such a perceptible improvement to fidelity, then shouldn't it be completely obvious when that plug-in gets latched into the correct slot and the audio is flowing through the correct path? Shouldn't the standard, somewhat flippant, internet forum answer of "Just use your ears!" be applicable here? It's not -- no one ever says this with regards to dithering. That's because, it's hard to stand behind such a statement with any confidence about a technology that does not exist.
A while back I discovered that the TDM POW-r dithering plugin that's provided with Pro Tools had a bug, making it incompatible with my L2007 limiter plugin. The POW-r plug-in would stop producing dither when activated on the same DSP as the L2007. (My limiter, unlike most others, does not provide integrated dithering, so I've always suggested to people that they just use the POW-r plugin.) I discovered the bug on my own, more or less by accident, while using an FFT analysis plugin. But, guess how many users called to report it themselves? Zero. Just like myself, no one ever heard this on their own. Only with tweaky analysis tools is dither measurable.
Finally, let's consider the end product. When have you ever listened to a CD and thought, "Wow! This is a really incredible album. But, damn! The dithering algorithm that the mastering engineer used is total crap! I can't believe they used POW-r type 3 instead of type 2!" Never. No one has ever had this thought. Regardless of its questionable efficacy, no one can reasonably argue that dither has an impact on the experiential enjoyment of music. Unlike other aesthetic choices of audio production (compression, distortion, etc.), no one has ever argued that dither ruined or enhanced their ability to enjoy a musical work.
The casual listener is capable of perceiving all subtle aspects of music production, whether or not they speak the language of audio engineering. It may be difficult for them communicate what they are hearing. If the vocal is slightly distorted, or if the phase is mutated due to excessive equalization, they may wave their hands around or find a peculiar phrase that best fits the tenuous categorization scheme their mind has created for this language-less reality. Likewise, we can communicate and agree about the flavors of salt, even though no one has previously defined a rigorous categorization scheme of salt. But, dither -- the only people on the planet perceiving and discussing dither are those people who have been informed of its existence -- because dither does not exist in anyone's reality until someone gives it a name.
So, where does the mythology of dither originate? Well, the problem is, it is a mathematical truth. There's no denying that. You can write down and solve the equations. Dither does work in the abstract sense: it decorrelates the quantization noise from the carrier signal. But, again, this theory is only applicable to the human experience for very low-bit, highly-quantized audio signals -- simply put, when the quantization noise gets "really loud", you can hear it.
If dithering makes no difference, how has the concept lived on for so long? I think because it is mathematically validated, it has allowed the engineers at pro audio companies to say, with confidence, to their marketing departments that the inclusion of dithering has made their product better over the competition. That's great! I imagine marketing people love positive specifications and catch-words which they don't have to create themselves. Anything that is easily quantified and can be succinctly composed into the marketing text is great for the unimaginative salesperson. Examples of such marketing-driven myths are common in the technology market. 64-bit floating-point audio! Awesome! How in the world could that not be better than 32-bit floating-point audio? 64 is twice the size of 32. Six Megapixels is of course better 5 megapixels! (Like with most bullet-points and talking points, these metrics ignore the subtleties and/or disregard all the other equally important elements that were compromised in reaching that isolated specification.)
But, I think the primary reason dither lives on is because the public itself has embraced it wholeheartedly, beyond simple influences of marketing. Why? I believe it's a bit like humanity's attachment to the supernatural. People seem to posses, myself included, an overwhelming desire to imagine their perceptions of the world to be more subtle and more magical than they actually are. Even though no one has ever truly witnessed the effects of dither, the public has faith in its power. And, since it's impossible to prove or refute something that you cannot hear (or see, or touch), the ghost of dither lives on...
Interesting perspective.
So what does salt have to do with audio dithering? Dither and salt both speak to the issues of marketing, personal enjoyment, and conventional wisdom.
Conventional wisdom says that all salt tastes the same. It's salt! NaCl. Sodium chloride. How could one box of NaCl taste that much different from another box of NaCl? Conventional wisdom scoffs at the idea that one brand might taste much better than another. Conventional wisdom laughs at the idea of spending five dollars for a can of it, rather than a buck for that old Morton's.
Conventional audio engineering wisdom says dither's a bit like salt -- you add a pinch of noise to "enhance the flavor." Conventional wisdom says that you'd be crazy not to dither when mixing down to a 16-bit master. I say: Conventional wisdom isn't all that wise. Stop bothering with dither! Unlike salt -- it has no impact on the end product.
I know what a lot of people will say to this: But, I've heard the improvements of dither with my own ears! I've done the taste test. I've read the book. And, I've seen the movie!
For example, you can go to this website and evaluate all the major players in dithering technology. It's true -- you can absolutely hear the improvements in their examples. And, that particular algorithm with the most votes does sound the best. But, let's read the fine print about the chemical bleaching agents: "54 dB of gain has been applied after dithering." Since the audio isn't audibly clipping, this means there was at least 54 dB of headroom in the original digital signal. 54dB of gain equals a multiplication factor of 501. In binary, this equates to shifting the digital word almost 9 bits (501X = 2 to the power of 8.97 bits). In other words, the original audio had about 9 unused bits of headroom. 16 minus 9 equals 7. We're listening to the equivalent of 7-bit audio here. So, what does this listening test prove?
That dither is highly effective for 7 or 8 bit audio. How many recording engineers do you know sending out 8 bit masters?
Why did the creators of this test use such bit-reduced audio? They had to or the test never would have worked. Quantization noise, which dither is meant to mask, is not humanly audible for typical, normalized, 16-bit audio, especially not in the everyday listening environments where we experience music. 16-bit audio has over 90dB of dynamic range -- this is the difference between the maximum signal amplitude and the quantization noise. In other words, the quantization noise is 90dB below what you're actually listening to. That's huge! Think about that in real world terms: 85dB SPL is a standard peak calibration level for movies and is also the threshold of hearing damage, at long durations. Under this condition, 90dB below that is well under the generally accepted "threshold of human hearing". Expecting to hear the effect of dither is like expecting to hear the fluttering of a fly through the din of a jackhammer -- it's just not going to happen. Maybe... just maybe, with a highly un-normalized Classical CD from the 80's listened to in an anechoic chamber with the lowest-noise-possible audio reproduction gear, you just might barely detect hints of quantization noise on the quietest passages. But, I'm dubious -- who do you know that has an anechoic chamber for a living room anyway?
To further this assertion, why is it so difficult for audio engineers to figure out how to apply dither properly? There's been numerous magazine articles, white papers, tidbits in the many home recording books, and thread after thread on the internet forums asking: "How do I dither?", "When do I need to dither?" , "Should I re-dither?", "Does dither work with floating-point audio, or only fixed-point?", "On what insert do I put the dither plugin?" "Can I EQ the audio after dithering?" And, these questions never end.
If dither makes such a perceptible improvement to fidelity, then shouldn't it be completely obvious when that plug-in gets latched into the correct slot and the audio is flowing through the correct path? Shouldn't the standard, somewhat flippant, internet forum answer of "Just use your ears!" be applicable here? It's not -- no one ever says this with regards to dithering. That's because, it's hard to stand behind such a statement with any confidence about a technology that does not exist.
A while back I discovered that the TDM POW-r dithering plugin that's provided with Pro Tools had a bug, making it incompatible with my L2007 limiter plugin. The POW-r plug-in would stop producing dither when activated on the same DSP as the L2007. (My limiter, unlike most others, does not provide integrated dithering, so I've always suggested to people that they just use the POW-r plugin.) I discovered the bug on my own, more or less by accident, while using an FFT analysis plugin. But, guess how many users called to report it themselves? Zero. Just like myself, no one ever heard this on their own. Only with tweaky analysis tools is dither measurable.
Finally, let's consider the end product. When have you ever listened to a CD and thought, "Wow! This is a really incredible album. But, damn! The dithering algorithm that the mastering engineer used is total crap! I can't believe they used POW-r type 3 instead of type 2!" Never. No one has ever had this thought. Regardless of its questionable efficacy, no one can reasonably argue that dither has an impact on the experiential enjoyment of music. Unlike other aesthetic choices of audio production (compression, distortion, etc.), no one has ever argued that dither ruined or enhanced their ability to enjoy a musical work.
The casual listener is capable of perceiving all subtle aspects of music production, whether or not they speak the language of audio engineering. It may be difficult for them communicate what they are hearing. If the vocal is slightly distorted, or if the phase is mutated due to excessive equalization, they may wave their hands around or find a peculiar phrase that best fits the tenuous categorization scheme their mind has created for this language-less reality. Likewise, we can communicate and agree about the flavors of salt, even though no one has previously defined a rigorous categorization scheme of salt. But, dither -- the only people on the planet perceiving and discussing dither are those people who have been informed of its existence -- because dither does not exist in anyone's reality until someone gives it a name.
So, where does the mythology of dither originate? Well, the problem is, it is a mathematical truth. There's no denying that. You can write down and solve the equations. Dither does work in the abstract sense: it decorrelates the quantization noise from the carrier signal. But, again, this theory is only applicable to the human experience for very low-bit, highly-quantized audio signals -- simply put, when the quantization noise gets "really loud", you can hear it.
If dithering makes no difference, how has the concept lived on for so long? I think because it is mathematically validated, it has allowed the engineers at pro audio companies to say, with confidence, to their marketing departments that the inclusion of dithering has made their product better over the competition. That's great! I imagine marketing people love positive specifications and catch-words which they don't have to create themselves. Anything that is easily quantified and can be succinctly composed into the marketing text is great for the unimaginative salesperson. Examples of such marketing-driven myths are common in the technology market. 64-bit floating-point audio! Awesome! How in the world could that not be better than 32-bit floating-point audio? 64 is twice the size of 32. Six Megapixels is of course better 5 megapixels! (Like with most bullet-points and talking points, these metrics ignore the subtleties and/or disregard all the other equally important elements that were compromised in reaching that isolated specification.)
But, I think the primary reason dither lives on is because the public itself has embraced it wholeheartedly, beyond simple influences of marketing. Why? I believe it's a bit like humanity's attachment to the supernatural. People seem to posses, myself included, an overwhelming desire to imagine their perceptions of the world to be more subtle and more magical than they actually are. Even though no one has ever truly witnessed the effects of dither, the public has faith in its power. And, since it's impossible to prove or refute something that you cannot hear (or see, or touch), the ghost of dither lives on...
Interesting perspective.