Change Speed

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Peter 09Jun14 later: ToDo-2 Steve suggested in an email discussion with me that he felt this page should remain in the Wiki - and "as with the "Remastering" pages I think it would be good if the page made much use of links to the manual to supply the "how" details (what buttons and keys to press)."
This page is mainly about the Audacity Change Speed effect which changes the speed of a selection, also affecting length and pitch. The page also touches on other methods for changing speed or tempo.
 
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Contents

Change Speed basics

  • The Change Speed effect operates on a "percent change" basis. Use the slider or enter the desired speed change.
  • Length, pitch and frequency content are all affected when you use Change Speed. When you slow a track down, all its frequencies become lower, and when you speed it up, they all become higher. It's just like when you change the speed of a cassette or gramophone record.
    • A 100% speed change is a ratio of 2. It halves the length of the track (makes it 50% as long), and doubles the frequency (which corresponds to raising the pitch by an octave).
    • A -50% speed change is a ratio of 0.5. It doubles the length of the track (makes it 200% of the original length), and halves the frequency (which corresponds to lowering the pitch by an octave).
    • A 50% speed change is a ratio of 1.5. It reduces the track length to 66.67% (to two decimal places of accuracy) as long, raising the pitch by a fifth (7 semitones).
To slow down or speed up the tempo of a track without changing the length and pitch, use Effect > Change Tempo or Effect > Sliding Time Scale / Pitch Shift. Note: There can be minor length changes with Change Tempo. See SoundTouch for an explanation of this.
  • Audacity has presets for changes to and from common speeds for gramophone records.
  • Technically, Change Speed resamples the audio so that the sample rate of the length- and pitch-changed audio remains the same as before. Resampling is required because when speeding up, the samples are "squashed" into a shorter time. This would otherwise increase the sample rate because there are more samples per second of audio. So Audacity downsamples (discarding some samples) so that the sample rate of the track is brought back to its original rate. When slowing down, the samples are "stretched" to a longer duration. This would otherwise decrease the sample rate, so Audacity upsamples (adding/interpolating extra samples) to retain the same rate as before.

    Resampling is very slightly lossy. Generally you would never notice this, except if you speed up then change the speed back to the original speed by slowing it down again. This would cause a loss of high frequencies as explained in Undoing a speed change below.

Calculating speed changes to produce length or pitch changes

You can calculate the speed change needed to produce a required length or a required change in pitch as follows:

Length change

( ((original length) - (target length)) / (target length) ) * 100 = Percent Change

For example, if your original length was 3 minutes 21.730 seconds and you want to speed it up so that the length reduces to 3 minutes 15 seconds, the calculation is:

((201.73 - 195) / 195 ) * 100
Step 1: 201.73 - 195 = 6.73. Step 2: 6.73 / 195 = 0.034513. Step 3: 0.034513 * 100 = 3.4513

So enter 3.4513 in Percent Change.

Let's assume now you have a 3 minute 15 seconds track and you want to slow it down so that it's 3 minutes 25 seconds long:

((195 - 205) / 205 ) * 100
Step 1: 195 - 205 =-10. Step 2: -10/205 = -0.04878. Step 3: -0.04878 * 100 = -4.878

So enter -4.878 in Percent Change.

To avoid making calculations, you can instead:
  1. Open Effect > Change Tempo (which changes tempo without affecting pitch)
  2. Type your required length in the "Length (seconds): to" box
  3. Write down (or select and copy) the value that appears in "Percent Change:"
  4. Cancel Change Tempo, open Change Speed, enter or paste the value obtained in Change Tempo and click OK.

Pitch change

Sometimes you may want to slow down or speed up to produce a particular change in pitch.

(2 ^ (semitones change/12) - 1) *100 = Percent Change

^ means "to the power of" (most calculators will have this function). Just use a negative semitones change if you are moving down in pitch. So for example, if you have a selection you want to speed up so the pitch goes up 5 semitones:

(2 ^ (5/12) - 1) *100
Step 1: 5/12 = 0.416667. Step 2: 2^0.416667 =1.33484016. Step 3: 1.33484016 - 1 = 0.33484016.
Step 4: 0.33484016 * 100 = 33.484016

So enter 33.484016 in Percent Change.

Now let's say we want to slow down a selection to bring the pitch down a fifth (or 7 semitones) :

(2 ^ (-7/12) - 1) *100
Step 1: -7/12 = -0.583333. Step 2: 2 ^-0.583333 = 0.667420081. Step 3: 0.667420081 - 1 = -0.33258.
Step 4: -0.33258 * 100 = -33.258

So enter -33.258 in Percent Change.

To avoid making calculations, you can instead:
  1. Open Effect > Change Pitch (which changes pitch without affecting tempo)
  2. Type your required pitch change in the "Semitones (half-steps):" box
  3. Write down (or select and copy) the value that appears in "Percent Change:"
  4. Cancel Change Pitch, open Change Speed, enter or paste the value obtained in Change Pitch and click OK.

Change Speed Nyquist Plugin

You may find it easier to use an experimental Nyquist plugin called change-speed.ny. Using this you can:

  • Stretch audio to a specified length (also affecting pitch) instead of working out the required percent change in speed
  • Specify the new desired speed as a percentage of the current speed, instead of as a percent change in speed.

Undoing a speed change

As Change Speed is slightly lossy, it's best if possible to use Edit > Undo if you want to undo the speed change, rather than apply a further Change Speed effect. However, Audacity's Undo is stepwise, so any edits made since the speed change will be lost if you undo the change.

If you need to undo the speed change by applying an opposite speed change, note that this needs more calculation than just applying the same percentage change in the opposite direction. For example if you applied a 50% speed slow down to make the track twice as long, you need the reciprocal of the change (1/0.5 = 2) or in other words a 100% increase to bring the track back to original speed. It's often simplest to just to recalculate using the formulas above (or use Change Tempo or Change Pitch to make the calculation for you).

Note that because slowing down a track with Change Speed makes all its frequencies lower, it's not a good idea to use Change Speed to increase the speed then Change Speed again to slow it back down. If the track has a sample rate of 44100 Hz, the useful bandwidth before the slowing down is from about 20 Hz to 20000 Hz (theoretically up to 22050 Hz). This comes about because a given sample rate can only contain frequencies up to half that rate. If you slowed down by 50% to undo a speed up of 100%, the useful upper frequency limit comes down to about 10000 Hz, with any sound above 11025 Hz totally lost. The only way you would not lose high frequencies in this situation would be if the audio did not contain frequencies above 11025 Hz in the first place (this might be the case with speech).


Alternative ways to change speed, length and pitch together

Set Rate

Set Rate is in the Track Dropdown Menu (click in the track name to left of the waveform). Unlike Change Speed, this changes the sample rate of the track - choosing the new sample rate determines the new length and pitch. Increasing the sample rate squeezes the existing samples together into a shorter time, so playback will be faster and higher pitch. Decreasing the sample rate spreads the existing samples out over a longer time, so playback will be slower and lower pitch.

For example, to double the track length and lower pitch by an octave you would choose a rate half that of the current track rate shown under the track name; to halve the length and raise pitch by an octave, you would choose a rate double that of the current track rate.

Unlike Change Speed, changing the rate does not resample the track, so is completely lossless - the samples are just moved to different positions along the Timeline. Set Rate can thus be completely reversed at any time, even if you speed up the track by choosing a higher rate then slow it down to original speed by choosing the original rate. This is because when you increase the sample rate you always proportionally increase the higher frequencies that can be carried, so lose none by reducing the rate back to the original. When you resample upwards, you cannot increase the higher frequencies in the audio beyond half the sample rate of the track, but still lose the higher frequencies when you resample downwards.

Play-at-Speed

Audacity has a "Play-at-Speed" feature located at Transcription Toolbar. Move the slider to choose a slower or faster speed than normal, then click the green triangle button to left of the slider to play at that speed.

The benefits of changing speed with Transcription Toobar are that you don't have to change the audio data in the track by running an effect, or worry about reverting the change if you want to export the project at its original speed.

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