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Recording: digital audio Digital Audio Tutorial: Misinformation

Digital audio tutorial
Introduction to digital audio
What sounds best?
Music compression
Common computer digital audio formats
Perceptual encoding schemes
Chart 1: bit depth and sampling rate chart
Chart 2: audio file formats
Chart 3: compression schemes
Glossary of common digital audio terms
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musiq.com digital audio
There are quite a few misnomers about digital audio, what sounds better, and why. The main source of misunderstandings has been the blame of manufacturing companies, who create new boxes with a host of numerical features (144 bit DAC's! 1,096 x oversampling!) without any real understanding from the consumer what the difference might be (between the 144 bit DAC's and lowly 16 bit jobs). So here are a few misinformations about digital audio:
  1. higher bit depth means better sound.
    This can be true, but recent comparisons of 24 bit DAC's have found that many of the supposed 24 bit converters really only captured 17-19 bits of sound, since they were poorly made and the analog circuitry feeding them couldn't handle that resolution of signal. In a test of DAT machines, one independent research firm found that the Tascam DA-P1, with 16 bit converters, was universally considered better than the Panasonic SV-3800, with 18 bit converters. In this case, the Tascam unit had very high quality 16 bit converters.
    Some companies are advertising "true 24 bit sound," and I don't know what this means. It doesn't appear, to my listening, to have more or less truth than the fact that good converters cost more, regardless of the bit depth.
  2. higher sampling rates mean better sound.
    Even less true than #1, since the human ear can really only hear up to 24 khz, and most loudspeakers can only accurately reproduce frequencies up to 15 khz! The reality is that in the "real world," we are exposed to much less high frequency content than low frequency since low frequencies carrier over a longer distance. As such, having 96 khz recordings may appear to have a level of "clarity" not found in earlier recordings, but this could as easily be simulated with a positive high shelving filter.
So with that in mind, let's break down what affects sound quality in digital audio.
  1. quality of components
    The most crucial points for digital audio recording are having high quality converters which are external to the computer. A 24 bit 96 khz converter mounted on a PCI card inside the computer will pick up a phenomenal amount of radiation (read: noise), as much (if not more) than 16 bit 44.1 khz converters. Having an external converter with a digital input into the computer sound card is the most important issue to address. If you can afford it, extremely high quality converters like the Apogee and TC Electronics models make a world of difference, and work at 16, 20, and 24 bit.
  2. sample rate conversion
    You lose a lot of fidelity and accuracy if you need to convert sample rates. Take this scenario: someone records a concert on DAT at 48 khz, you bring it into your computer at 96 khz, work with it and output to 44.1 khz. You have 2 sample rate conversions in the process, each which cause more distortion in your signal than if you did everything in 44.1 khz. If your primary output is 44.1 khz, save yourself the hassle and record, mix, and output to 44.1 khz.
  3. bit depth is more important than sample rate
    If you wish for a high-fidelity alternative to 16 bit 44.1 khz, it is better to increase the bit depth (to 24) than to increase the sampling rate. Converting bit depth does not introduce any distortion (unlike converting sampling rates), though you can lose some very soft detail when stepping down to 16 at the end.
Why does bit depth matter more than sampling rate? It's a matter of the human ear, which is only accustomed to hearing and resolving frequencies from 20-20,000 hz., but can resolve about 130 dB of dynamic range. The dynamic range of CDs and 16 bit digital audio is, at best, 90 dB; the dynamic range of 24 bit digital audio is 109-120 dB, depending on the quality of the converters. You see, the DVD audio standard allows a wider variety of amplitudes, or volumes, than CDs (which is its main improvement). Read about compression (the loss of sound quality) here...

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