Truncate Silence

Peter 25Mar14: ToDo-2 the GUI here appears not to be consistent with other effects in Audacity. In other effects the units if a control are written encased in round brackets (though occasionally in square brackets) immediately after the control and before the data entry box; examples are Amplify and Bass and Treble. I'm not sure if this is something we have to worry about - in this case it may look better the way Steve has it now with the units after the data entry boxes. For me it does make the documentation here look a little clumsier than the other effects.

Truncate Silence automatically reduces the length of passages where the volume is at or below a specified level.

• Silences are detected that remain at or below the specified level for at least the specified length of time.
• The detected silences are then made shorter by automatically deleting a section from the middle of the silent region.

Two alternative methods for determining how much silence to remove are available: truncation and compression, as detailed below.

Accessed by: Effect > Truncate Silence...

• Throughout this description the words "silence" and "silent" mean sounds that are below the detection level.
• "Detected silences" mean sounds that are below the detection level for at least the specified length of time.

Operation Mode

There are two methods available for shortening the silence. The simplest method is by truncating the silence (default mode) and a more advanced method is by compressing the length of the silence.

Truncate Detected Silence

When this method is selected, silences are simply shortened to the specified duration (see example below).

When this option is selected, the "Truncate to:" (seconds) control is available lower down the interface and the "Compress to:" (percent) control is grayed out. All detected silences are shortened to the same duration.

Compress Excess Silence

This is a more advanced mode that allows silences to be shortened proportionally according to their original duration.

When this method is selected, silence in excess of the specified duration is reduced to a percentage of its original duration (see example below).

When this option is selected, the "Compress to:" (percent) control is available lower down the interface and the "Truncate to:" (seconds) control is grayed out. Because silences are reduced by a specified percentage the final duration of each detected silence varies according to its original length.

Detecting Silences

There are two controls that determine which audio will be treated as "silence":

1. Level (dB): to be treated as silence, the audio must be at, or below this level.
2. Duration (seconds): to be treated as silence, the audio must remain below the specified level for at least this amount of time; this value must be at least 0.001 seconds.

Reducing the length of detected silences

There are two controls that determine how much silence is removed, and hence the final length of each silence. Only one of these controls is available at a time, depending on whether the simple Truncate Detected Silence or the advanced Compress Excess Silence mode has been selected.

• Truncate to (seconds): when the audio remains below the specified level for at least the specified duration, it will be reduced to this duration.
• Compress to (percent): when the audio remains below the specified level for at least the specified duration, the excess silence is reduced. That is, if the original silence is 10 seconds longer than the specified minimum, then there is 10 seconds of excess silence. The percentage refers to this 'excess' silence, and not the entire detected silence.

Examples

Truncate detected silence example

To be provided.

Compress excess silence example

To be provided.

Threshold for silence

Audio at or below this amplitude will be regarded as "silence", so will be truncated. White space between audio clips is in effect absolute silence, so will always be truncated.

Ignore silence less than

Specifies the shortest length of silence that will be truncated by the effect. Silent passages of this length or greater will be truncated. Silent passages of less than this length will be left unchanged.

Compress silence by

A compression factor which proportionally reduces silences in the waveform that are longer than the "Ignore silence less than" length. Compression is only applied to that part of the silence that is in excess of the ignored duration, so for the default compression factor of 4:1 that "excess" silence would be compressed to a quarter of its original length. A ratio of 1:1 disables compression.

This setting has no effect if the "truncate to" length (below) is the same as or less than the "ignore" length.

and then truncate to

If the duration of a detected silence after compression is still greater than the "truncate to" length, it will be truncated to the "truncate to" length. The final duration of the detected silence will be this length unless it is already shorter. It may be convenient to think of "then truncate to" as the maximum length after compression/truncation.

• Setting the "truncate to" length to the same as the "Ignore" length will always reduce the truncated silences to this length.
• Silences longer than the "truncate to" length will remain if they were ignored by Truncate Silence because they were shorter than the "Ignore" length.

Compression or truncation is achieved by deletion from the middle of the detected silence.

Examples

• Simple usage: Setting both the "Ignore" and "truncate to" lengths to 5 milliseconds (ms) will truncate the silence to 5 ms. This is less than the length of a detectable silence, so will effectively eliminate it.

• Truncate length only: Set the "Compress silence by" factor to 1. Now any silence longer than both the "Ignore" and "truncate to" length will be reduced to the "truncate to" length and never any less than that.

• Proportional length only: Set the "Truncate to" length to some large value, like 1000000. Now that part of any silence greater than the "Ignore" length will always be compressed by the "Compress silence by" factor.

• Proportional truncation with compression factor: The resulting silence is calculated according to the following formula:

(output length) = ((ignore silence length) + (waveform silence length - ignore silence length)/compression)

with the constraint that output length can't be more than the "truncate to" length. So, setting the minimum to 33 ms and compression to 5:1, a silent passage 1033 ms long would be truncated to 233ms (33 + (1033-33/5)), unless "truncate to" was set to less than 233 ms (in which case truncation would be to that "truncate to" length).

As a real world example, setting the minimum to 100 ms, the maximum to 5000 ms and the compression factor to 4:1 will have the effect of doubling the speed of a speech track with no pitch change, while keeping about the same cadence as the original.

Limitations

Truncate Silence only removes audio, it does not reduce or eliminate noise in the silent sections that it keeps.

Avoid using Truncate Silence on selections which have fade outs or fade ins, since it may remove the quietest part of fades. If you need to add fades, apply Truncate Silence before adding fades.

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