Convert Vinyl Records and Tapes to Digital Audio

User Manual Part 4.  Managing Click Corrections

• 4.1 Understanding Click Concealment
• 4.2 What are Click Corrections
• 4.3 Auditioning a Click Correction
• 4.4 Adjusting a Click Correction
• 4.5 Inserting New Click Corrections
• 4.6 Removing Click Corrections
• 4.7 Correcting Very Severe Clicks and Plops
• 4.8 Inserting Cue Markers
•  4.9 Spectral Methods for Finding Clicks
• 4.10 Block Operations

4.1 Understanding Click Concealment

Click concealment is the name given to the process whereby individual clicks are removed from a recording and replaced by a wave shape that is less offensive to the human ear. Because the original signal is obliterated by the click, concealment algorithms construct a replacement waveform by analysing the known waveform on either side the click.

Wave Corrector's automatic mode of click concealment is more than adequate for general use. However, audiophile users archiving treasured record collections may wish to achieve even cleaner results by utilising Wave Corrector's waveform comparison and editing functions.

These notes are provided to enable you to use these features to the full.

The Limitations of Click Concealment

All click concealment programs must balance the conflicting requirements of detecting as many clicks as possible whilst at the same time not being triggered by musical forms resembling a click. When this balance is incorrect, the result will be:

• false negatives - the failure to detect a click even though it is audible to the human ear.

• false positives - detecting and modifying parts of the music which the ear does not perceive as a click.

• a combination of both the above.

Wave Corrector's advanced features help to overcome these limitations.

False Negatives

False negatives are relatively rare with Wave Corrector. However, if a click has slow rising and falling edges, then Wave Corrector may fail to identify it, particularly if the click detection threshold is set at a relatively insensitive value.

These missed clicks can usually be tracked down by using Wave Corrector's 'Block Re-scan' operation. However, if this does not work, it is possible to manually insert one or more individual corrections. This is quite difficult because you need to learn how to visually identify a click from the shape of its waveform. Having identified it, you then insert a correction precisely over the centre of the click and adjust the width to obtain the optimum audio quality. See §4.5 Inserting New Corrections for a description of this process.

False Positives

False positives are rather more numerous than false negatives, but they are much easier to deal with. The vast majority can in fact be ignored since they impart only minor changes to the waveform that are usually inaudible.

However, if you wish to obtain maximum fidelity to the original recording. any corrections which do not appear to serve a purpose can be easily deleted using either the Block - Remove Corrections command or else the Delete Correction command if you just want to remove a single correction.

To help you identify the location of false positives, Wave Corrector provides the option of displaying a 'correction signature' in the Overview window. The signature indicates the rate at which Wave Corrector is making corrections during the course of the wave file. A high correction rate during medium to high amplitude music is probably indicative of the false operation of the click detector. Such areas should be auditioned and false corrections removed if necessary. See §4.6 Removing Corrections for a description of this process.

Adjusting Corrections

Although the majority of Wave Corrector's auto-corrections are inaudible at normal listening levels, there are occasions when a click is not completely removed. To minimise correction artefacts, Wave Corrector retains as much of the original waveform as possible and this can sometimes allow small portions of a click to slip past the corrector. Usually, this will only affect the highest magnitude clicks highlighted in red in the correction list. (These can be located using the Find Correction command.)

For these situations, Wave Corrector allows you to make fine adjustments to corrections. A correction can be made wider or narrower; and it can be shifted to the left or the right. By these means, the audible artefacts of the correction can be minimised. During adjustment, use the Audition commands to optimise your result. See §4.4 Adjusting Corrections for a description of this process.

4.2 What are Click Corrections

When Wave Corrector detects a click, it removes it from the waveform and substitutes a replacement waveform. In Wave Corrector, this replacement waveform is called a 'Click Correction'.

Each Click Correction has a unique timestamp and is recorded in the Correction List at the right-hand side of the program window. A light yellow band at the centre of the main display indicates the width of the selected correction. Click Corrections can be auditioned and adjusted via the user interface.

To select a Click Correction for audition or adjustment use one of the following methods:

1. Click over the required correction in the Correction List.

2. Click over the required correction in the Overview Window.

3. If a non-selected correction is visible in the main window, double clicking over it will cause it to be selected.

You can also step through the correction list with the Cursor Up/Down arrow keys.

Once a correction has been selected, it appears at the centre of the main window.

Depending on the width of the correction and the amplitude of the waveform, you may need to alter the horizontal or vertical scale using the Control Bar controls to optimise your view. You can also use the cursor keys on the numeric keypad to set the horizontal and vertical scale.

As well as zooming in to reveal the detail of a particular correction, you can also zoom out to view as much of the surrounding wave as you require. This enables you to view and audition other clicks in the vicinity of the selected correction.

4.3 Auditioning a Click Correction

You can audition the current correction at any time by selecting the 'Audition' command on the Corrections menu and selecting either the 'Audition - Original' or the 'Audition - Corrected' option. There are also toolbar buttons for these functions, or the numeric keypad keys '1' and '3' can be used as shortcuts.

During auditioning, the second status bar pane from the left indicates whether the original or corrected version is being played. You can instantly switch between original and corrected versions by clicking over this pane with the mouse.

4.4 Adjusting a Click Correction

If required, the placement and width of the currently selected correction can be adjusted in order to optimise the shape of the corrected wave. This is done using the Corrections - Adjust/Insert command.

Having selected the correction, simply drag the correction edge with the mouse to widen or narrow it. To move it to the left or right use the left/right cursor keys. (Note you can also adjust the width using the cursor up/down keys if you prefer.)

During adjustment, you can audition the changes you make. To save your changes select the 'Apply' button; otherwise, select 'Abort'.

Note: Corrections cannot be adjusted after digital filtering has been applied (eg the hiss filter). Therefore, make sure you have finished adjusting corrections before applying any digital filters. Alternatively, you can use the Remove Filtering command to restore the Adjust/Insert capability.

4.5 Inserting New Click Corrections

Although Wave Corrector automatically generates corrections, it will occasionally be necessary to manually add corrections because of imperfections in the click detect algorithms.

If a click is still audible after your wave file has been scanned and corrected, it can often be found by highlighting a block around the area where you can hear the click; and then selecting the Block - Rescan or Block - Superscan option using a more sensitive click detect threshold.

If this method fails however, you can manually add a correction using the Correction - Adjust/Insert command.

The following step by step procedure explains how this is done.

1. Centre the main window display as nearly as possible over the click you want to correct. (Note that you re-centre the display by double clicking over the new centre position you require.)

2. Expand the horizontal scale, re-centring if necessary until the rogue click is identified by its waveform shape. Use the 'Audition' command and its synchronised moving cursor to help you locate the click precisely.

3. When you think you have identified it, select the 'Adjust/Insert' command. The program enters adjust mode.

4. Now fine-tune the width of the correction to match the characteristics of the click. If necessary, you can also nudge the position of the correction to the left or right with the cursor left/right keys. Use the 'Audition' command to evaluate the effectiveness of your adjustments.

5. When you are satisfied with the result, click 'Apply' and your new correction will be inserted into the correction list.

6. If your identification of the click was mistaken, simply click 'Abort' to return the normal waveform screen.

See also §4.8 Inserting Cue Markers for another tool to help you manually insert corrections.

4.6 Removing Click Corrections

Occasionally it will be necessary to remove a correction (or a group of corrections) because the click detect system has misfired on actual music.

Individual Corrections can be removed using the Corrections - Remove Current command.

If you need to remove a contiguous group of corrections, highlight a block around the corrections you want to remove and select the Block - Remove Corrections command.

4.7 Correcting Very Severe Clicks and Plops

If a click is very severe, it may not be possible to remove it completely using the standard Wave Corrector procedure. This is because too much of the original waveform may be obliterated for the program to be able to reconstruct the waveform. In this case, the click can be removed effectively using the Cut & Splice command.

Cut & Splice removes the entire click from the waveform and cross-fades between the sections immediately before and after the cut. 

To use the command, centre the screen on the click to be removed and select 'Cut & Splice' from the Corrections menu or use its toolbar button. Use the Cut & Splice audition buttons to preview the effect of the operation. By adjusting the size and position of the block you should be able to make the click inaudible.

It may also be necessary to alter the length of the cross-fade (Splice Length). A short cross-fade is more likely to be audible but will have minimal effect on the tempo of the music. A long cross-fade will produce a very smooth transition but a larger section of the music is removed (100 msec). This can have a noticable effect on the tempo and should therefore be used sparingly.

4.8 Inserting Cue Markers

Cue Markers are placeholders that you can add to the Correction List. They are useful for marking a particular point in the waveform to return to later.

Cue Markers can be used to mark a point where you hear a residual click when playing back the corrected wave. They can also be used, for example, to mark a point where you wish to insert a track marker.

To insert a Cue Marker, use the Insert Cue command on the Corrections menu. Alternatively, use the TAB key shortcut. The marker will appear in the Correction List highlighted in white.

You can also insert cue markers during playback mode. During playback, press the TAB key at any time to generate a new Cue Marker. You can freely insert markers in this way until you cancel playback.

Note that during playback, you can jump back or forward (for example to repeat a section) by clicking over the point you want to jump to in the overview or the main window. If you just want to jump back a few seconds, you can use the Backspace or Enter key shortcuts.

Cue Markers are converted to ordinary user corrections when you use the Adjust/Insert command. They are converted to Track Boundary markers when you use the Split Track command.

 4.9 Spectral Methods for Finding Clicks

Wave Corrector provides two methods to manually find clicks using spectral analysis of the wave. Musical forms tend to consist of discrete frequency peaks usually with a harmonic relationship; ie the peaks tend to be evenly spaced across the frequency range. Noise, on the other hand, has no coherent structure and all frequencies tend to be present in approximately equal amounts. These characteristics can help you recognise a noise event when it is not obvious from the waveform display in the main window.

Wave Corrector provides a simple spectrum view and the more informative spectrogram view.

The Spectrum view shows the frequency spectrum at a single point in the file. In Spectrum view, the x-axis represents frequency and the y-axis represents amplitude. Separate traces are shown for the left and right channels. In the following illustration, a spectrum is shown where a click is present on the right channel (red) and not on left channel (green).

Figure 9. Frequency Spectrum Display

In Spectrogram view an entire sequence from the wave is shown in a single image. In this view, the x-axis represents time, the y-axis represents frequency, and the colour of a point represents its amplitude. In Spectrogram view, a click is usually visible as a vertical bar of a single colour.

Figure 10. Spectrogram Display

4.10 Block Operations

Block operations are available from the Block menu on the main menu bar.

Block operations enable you to perform corrections and other tasks on a portion of the wave file rather than on the entire file.

To create a block, drag with the mouse in the main window over the area you want to re-process. The block will be highlighted in blue, and corrections in the block are indicated by a blue vertical line the correction list.

Note, having selected a block, you can fine-adjust the edges of the block or move the entire block to the left or right by dragging with the mouse.

To cancel a block, click with the mouse anywhere outside the block. Note, to avoid accidentally cancelling a block, hold down the CONTROL key during block adjustment.

To modify the corrections within the block use one of the following three functions:

Remove Corrections: removes all corrections from the block.

Re-Scan: removes all the existing corrections from the block and re-scans it using the parameters you supply when you select the option.

Super-Scan: is the same as re-scan except that it superimposes the new corrections on top of the existing corrections rather than removing them first.

Note: These operations are not possible after digital filtering has been applied (eg the hiss filter). Therefore, make sure you have finished editing corrections before applying any digital filters. Alternatively, you can use the Remove Filtering command to re-enable the block commands.

There is also a Block command called Cut and Splice. This command removes the entire block from the waveform and cross-fades between the sections immediately before and after the block. The command is named after the customary practice of audio engineers to physically cut a section from an audio tape and then splice the tape back together.

Shortcut:

Whenever a block is selected, you can right click the mouse to activate a menu of block options.

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