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Revision as of 22:09, 13 February 2011 by Stevethefiddle (talk | contribs) (Dither applied on Export - detailed explanation)
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"Dither" is intentional noise which is added so as to randomise the quantisation errors  (rounding errors) that occur when downsampling the Bit Depth of an audio stream to a lower resolution than the current format. The dither randomises the "arithmetical carry" on rounding the last bit to be used, rather than creating a repeatable, quantifiable rounding error. In this way the potentially audible distortion problems inherent in downsampling are minimised. The noise itself isn't audible unless heavily or repeatedly amplified.

Downsampling can occur when converting a file to a lesser bit depth, or in real time when Audacity plays audio at a lesser bit depth than the bit depth of the track. The bit depth of Audacity projects is set at "Default Sample Format" on the Quality tab of Preferences.

An example of downsampling when converting a file is when you export from a 32-bit float or 24-bit project to a 16-bit export format. Downsampling also occurs in a 16-bit or 24-bit project every time you edit a section of audio (thus writing to the project's audio data), because Audacity temporarily converts the samples to 32-bit float format before downsampling back to the current format. Conversely, dithering will never be applied within a 32-bit float project because no downsampling occurs. Also, the precision of float samples is good enough to justify not applying dither.

In a similar way, downsampling will occur with most sound cards every time you play audio in a 32-bit or 24-bit Audacity project, because most sounds cards are still 16-bit.

You can choose the type of dither to be used in Audacity when performing operations requiring downsampling, or you can turn dither off altogether by setting it to none. The dither settings are again in the Quality tab of Preferences. The setting for "real-time dither" applies to dithering done when playing back, and that for "high-quality dither" to file writing (editing or exporting). Note that applying dither can slow down processing (or potentially cause audio breakups when playing on slow machines) due to the extra calculations involved.

The rectangular and triangular types of dither are simply different methods for randomising the rounding process. The rectangular method can be thought of as like the roll of a die - any number has the same random probability of surfacing. The triangular method can be thought of as equivalent to a roll of two dice. The shaped dither means that the created noise is shaped so that the quantisation errors occur less in the frequencies we can hear easily, and more in very high frequencies that we cannot hear so easily. The general consensus is that shaped or triangle dither tend to sound the best.

Dither applied on Export - detailed explanation

Dither is only applied when converting from a higher bit depth to a lower bit depth. Exporting to WAV or other uncompressed audio format is virtually lossless in all situations. However there may be a very small amount of change to the audio data with some types of uncompressed audio export, depending on the settings in Audacity Preferences (see below).

  • Exporting to 32-bit PCM WAV format is 100% lossless - no dithering is applied.
  • Exporting to 24-bit PCM WAV format may be dithered with a peak dither level of around -130 dB
  • Exporting to 16-bit PCM WAV format may be dithered with a peak dither level of around -80 dB

When exporting to 16-bit, the maximum "error" is +/- 3 on a scale of −32768 to +32767. 16-bit audio can at best be only accurate to the nearest digit on this scale.

When Exporting to 24-bit, the maximum "error" is +/- 3 on a scale of -8388608 to +8388607. At the present time, state of the art audio electronics exhibit greater inaccuracies than are produce by dithering to 24-bit format.

Where the "errors" come from.

Audacity performs calculations using 32-bit float numbers, which provides incredible accuracy (considerably better than even the best top of the range sound cards). If Audacity needs to reduce the resolution to 16 bit, then there will be "rounding off" errors. If simple "rounding" is done by simply taking each sample value and converting it to the nearest 16 bit value, then a "steppiness" in the output will result which may be audible, particularly with extremely quiet sounds. To avoid this, Audacity applies "dither" in the conversion - this is often likened to "anti-aliasing" in graphics which is used to smooth the steppiness of curves. Rather than always rounding the values to the nearest 16-bit value, dithering will apply a degree of randomness to the rounding off so as to prevent causing steps.

Audacity has 3 alternative algorithms for how it applies dither and they are found in "Edit menu > Preferences > Quality". The dither settings apply to "High Quality Conversion". The available options are: None (no dither), rectangle, triangle and shaped. The differences between these settings are very subtle and you need to be listening carefully with good equipment to notice any difference. Generally, "triangle" or "shaped" will give the best (subjective) results.

  • If dither is set to "none", then conversions from one bit depth to a lower bit depth will be done by simple rounding.
  • If dither is enabled, pseudo random variations are applied to inexact conversions so that the rounding does not always occur in the same direction. The rounded number will be statistically more likely to be converted to the closest value at the lower bit-depth. The closer the sample value is to an exact number, the greater the probability that it will be converted to that number. This has a smoothing effect on converted values, producing a result that sounds closer to the intended audio signal than would occur by simple rounding. At very low amplitude levels, this randomization can be heard as very low level noise, but this can be minimized by shaping the probability curves, and hence the frequency content of the noise.
    • Rectangle dither produces very small amounts of randomization (in the order of +/- 1 bit). The distribution is such that the "dither noise" is approximately white noise.
    • Triangle dither produces a similar peak amplitude of noise as Rectangle, but the noise is shaped so that more of the noise occurs at very high frequencies where it is less noticeable.
    • Shaped dither has a higher peak amplitude than either Rectangle or Triangle, but the frequency content of the noise is shifted more into the very high frequency range where it is less intrusive.

When working in Audacity with 32-bit audio, dither should normally be enabled in the high quality conversion settings (it is by default). Dither will only be applied if the output is at a lower bit depth than the original data. Converting from a higher bit depth to a lower bit depth will inevitably have inaccuracies (because the lower bit depth is less accurate). Using dither just makes these inaccuracies less noticeable in the music. In most cases, triangle or shaped dither produce the subjectively best results.

Here's the exception to the rule: If you have recorded in 16-bit and are only doing simple editing (cut, delete, paste, trim...) and not doing any processing (amplify, equalize, frequency filter....) then for highest accuracy dither can be set to "none". In this case, because there are no 32-bit operations prior to export there is no benefit to using dither. Exporting a 16-bit track to 16-bit with dither set to "none" will be lossless. The same applies if exporting from a 24-bit track to an uncompressed 24-bit file format with dither disabled.

Whenever processing audio, the highest quality will be achieved by using 32-bit (float) throughout. Note that 32-bit audio files are twice the size of 16-bit audio files.