Learn how to restore the sound of your vinyl when you archive it.

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Overview of Audio Restoration
Vinyl is a relatively delicate medium which is easily damaged by the presence of dust and grit which becomes embedded in the record groove during normal play. A disc can also become damaged by the careless lowering of the stylus or by other hard objects. All these types of damage cause so-called 'impulsive noise' disturbances which produce sounds ranging from tiny high frequency ticks to loud unpleasant clicks and plops. Particularly annoying are the repeating clicks which occur when the damage has spanned several adjacent record grooves and which consequently repeat once per revolution of the record.

Audio restoration is the name given to the process of removing these disturbances and reconstituting the original waveform. The following illustration shows a typical click (in blue) overlaid by the restored waveform.

Audio restoration is a two stage process. Initially the wave file is scanned using an algorithm which discriminates between wanted musical content and unwanted noise. This is a far from trivial task because some musical instruments, especially low frequency brass and reed instruments, have waveforms remarkably similar to typical clicks. The waveform can be analysed in either the time domain, the frequency domain, or a combination of the two. Generally speaking, the goal is to identify instantaneous deviations in the data which are statistically significant. These 'outlier' events are usually indicative of an impulsive noise spike.

The second stage involves excising the unwanted portion of the wave and replacing it with an approximation of the pre-damaged waveform. Generating the replacement waveform is another difficult procedure. If the noise spike is very short and only spans a few samples, then linear or bezier interpolation may provide a sufficiently accurate replacement. However, in the case of longer duration impulses, a more sophisticated approach is necessary if the noise is to be effectively masked. The techniques used to mask these longer impulses include:

• muting.
• inserting a combination of frequencies which the ear finds difficult to recognise.
• generating a waveform based on the frequencies present in the surrounding wave.

Wave Corrector uses the third of these options. The example illustrated above shows how the corrected wave matches up with the surrounding wave and blends naturally with it.

Mis-Operation of Audio Restoration Software
Because click detection is an imprecise process, it is quite common for audio restoration software to produce imperfect results. This is because the software must balance the conflicting requirements of detecting as many clicks as possible whilst at the same time not being triggered by musical forms resembling clicks. When this balance is incorrect, the result is:

• false negatives - the failure to detect and correct a click even though it is audible to the human ear.
• false positives - detecting and adversely modifying parts of the music where the ear does not perceive any clicks.
• a combination of both the above.

Ganymede's Wave Corrector software provides a number of features which allow the user minimise these adverse effects. These features include the ability to re-scan selected blocks of the wave file as well as the facility to insert, delete and adjust individual click corrections. By these means, most of the problems usually associated with audio restoration can be overcome.

Wave Corrector User Options
Wave Corrector provides a minimalist set of user options for click detection The Wave Corrector Options dialogue shown below allows the user to set just sensitivity and detection mode. (Note, the other two settings affect track detection and are not covered in this tutorial.)

Click Threshold
The click threshold can take values from 1 to 5 with a default value of '3'.

Note, you can also set the click threshold to '0'. This turns off click detection completely and allows you to use Wave Corrector simply for its track splitting functions.

The default setting (3) should be suitable in the majority of cases and novice users should use this setting whilst they gain experience of using Wave Corrector.

Settings 1 and 2 are the least sensitive settings and may occasionally be required if the source material is such that you need to globally reduce the number of 'false positives' being detected.

Settings 4 and 5 should only be used with care. If your source recording is exceptionally noisy with a very large number of clicks, then these settings will sometimes yield an improved result. However, this will be very dependent on the musical content. These settings reduce the Wave Corrector's ability to discriminate between music and noise and therefore will only be useful if the musical content itself is relatively un-noise-like in its characteristics. Piano, strings and human voice can all be safely scanned at settings 4 and 5. However, if the music contains low frequency reed or brass instruments, cymbals or similar percussion instruments then these settings should be avoided.

In many cases it will be better to scan at setting 3 and then 'super-scan' those parts of the wave file which are still noisy. 'Super-scan' is one of Wave Corrector's advanced features designed to allow the audiophile to enjoy better results than a purely automatic process is able to achieve.

Subsequent sections describe these advanced features in more detail.

Click Detect Mode
The default Click detect mode is 'Stereo'. In this mode each channel of the stereo pair is scanned independently and clicks on either channel will trigger a correction to be generated.

Wave Corrector also provides its so-called 'Mono' detect mode. This is only usable if the vinyl source is a mono record and if a stereo cartridge was used to create the digital recording. In such circumstances, it is possible to use the stereo difference channel (left minus right) to enhance the discrimination between music and noise. This is because a mono record should produce no difference channel signal and therefore all the difference signal is theoretically due to noise. It is recommended that you only experiment using this mode once you are confident using Wave Corrector's default 'Stereo' mode.

Wave Corrector Advanced Features
On occasions, it is possible that Wave Corrector will over-correct or under-correct particular sections of the Wave File.

Over-corrected sections will exhibit distortion introduced by the correction process itself. Usually this will be manifested as the leading edges being taken off percussion instruments making their attack sound slightly duller than it should. Under-corrected sections will still have unwanted noise present after the file has been scanned.

With Wave Corrector, you can manually intervene to rectify these sub-optimal sections. The simplest method is to mark a block and then to either re-scan or delete the corrections in that block. At a more detailed level, you can insert, delete or adjust individual correction. In order to carry out these actions, you need to navigate to the particular part of the wave file which requires manual intervention.

Navigation and Auditioning in Wave Corrector
Wave Corrector uses a slightly unconventional means of navigation which is described here.

To move directly to an arbitrary new location, single click with the left mouse button over a point in the Overview window. This will re-centre the main display on that point. Note, if there is an existing correction in the vicinity of the point clicked, the program will centre the display on that correction; otherwise it will centre on the point itself.

If the location you want to move to is already visible in the main window, you can double click over the point in the Main window to re-centre the display. As before, if a correction already exists in the vicinity of the point double clicked, that correction will be selected.

You can also click on any correction in the Correction List to immediately centre the display on that correction.

Use the horizontal scale control (or the numeric keypad cursor 'left' and 'right' keys) and the vertical scale control (or the numeric keypad cursor 'up' and 'down' keys) to zoom in and out to a suitable viewing scale.

By means of these navigation controls, you can quickly and easily move around the wave file to audition or edit the Wave Corrector output.

Use the audition controls to audition whatever is visible in the Main window. Note, if the Main window is zoomed in such that less than 2 seconds of waveform is visible, then the audition commands default to a playing a 2 second clip centred on the main window.

You can also audition using the 'Play' controls. In this case, the entire current track is played, ie whatever is visible in the Overview window. During play, you can move to an arbitrary new location in the track by clicking over it in the Overview Window. When the end of the track is reached, play loops back to the beginning and repeats endlessly until the 'Cancel' button is pressed.

By means of these navigation and audition controls, you can quickly and easily move around the wave file to locate sections which might require manual intervention.

Block Operations
Block operations allow you to mark a block of the wave file and then to re-scan or delete corrections in the block.

To select a block, drag with the mouse in the Main window whilst holding down the left mouse button. The selected block is highlighted in blue.

NB, do NOT attempt to select a block whilst the mouse cursor is displayed as an E-W arrow. This form of the mouse cursor indicates that the mouse is hovering over the edge of a correction and if you press the mouse button at this point then 'Correction Adjust' mode is entered instead of 'Block Select' mode.

Having selected the block, you can now perform one of three actions:

• delete corrections - this deletes all corrections in the block and restores the original waveform.
• re-scan - this deletes all the existing corrections in the block and re-scans using a new click threshold. Thus you can selectively re-scan the block at a higher or lower sensitivity than the global setting for the entire file.
• super-scan- this leaves all the existing correction in place and re-scans the corrected block to mop up any remaining clicks. Again, the re-scan can be done a t a higher or lower sensitivity than the global setting for the file.

Using these block operations, it is possible to quickly fine-tune the operation of Wave Corrector to accommodate sections of music which are particularly difficult to correct.

Finding False Positives
One of the main uses of the block operations is to remove the 'false positive' corrections which the program has applied to particular sections of a wave file music where the music is difficult to distinguish form noise. Sometimes these mis-corrections are difficult to locate since the audible effect is very often minimal. You may say that in such cases the mis-corrections might as well be ignored but if you are a perfectionist Wave Corrector provides a useful option called the 'correction signature' to help you find them. To activate the correction signature, select 'Signature' from the View menu.

The signature is a thin red line which rises and falls to indicate 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 positives. Such areas should be auditioned and false corrections removed if necessary.

Adjusting Individual Corrections
At a finer level, it is also possible to manually adjust individual corrections. Adjusting a correction enables you to alter the shape of the corrected wave and hence to affect its audibility.

Wave Corrector automatically generates corrections based on its estimates of the precise position and width of each click. The Wave Corrector display indicates these click parameters by means of a light yellow band. By adjusting the position and width the yellow band, you can alter the corrected wave. The display below shows an example of this. The left-hand image shows the corrected wave (in red) as automatically determined by Wave Corrector. The right-hand image shows the same wave after it has been manually adjusted. You can see that by manually adjusting the correction, you can obtain a better match of the underlying wave.

Images above are Before Adjustment (left) and After Adjustment (right).

To adjust the correction which is currently active, select the 'correction adjust' command either from the Corrections menu or by clicking its toolbar button. As a short cut, you can also enter correction adjust mode by clicking the left mouse button over the edge of the click.

To adjust the width of the correction drag the correction edge whilst holding down the left mouse button or use the cursor up/down keys. To adjust its position, use the cursor left/right keys.

During Correction adjust mode, the screen background colour changes to blue-green and a confirmation box id displayed to enable you to accept or reject your adjustments.

Finding Critical Corrections
Wave Corrector categorises corrections according to the estimated severity of the underlying click. Generally speaking, only the most severe corrections are likely to require user adjustment. These are highlighted in red in the Correction List. Medium severity corrections are highlighted in yellow, and the least severe ones are highlighted in green.

You can jump immediately to the next or previous 'red' correction using toolbar buttons provided for the purpose. This enables you to quickly audition the critical corrections to decide whether any manual intervention is necessary.

Deleting and Inserting Individual Corrections
It is also possible to manually delete and insert individual corrections.

To delete the currently active correction, select the Delete Correction command or use the keyboard shortcut (Delete Key). A confirmation dialogue asks you to confirm the deletion.

To insert a correction, select the Insert Correction command or use the keyboard shortcut (Insert Key). Then click with the left mouse button in the Main window over the point where you want the correction inserted. Once inserted, you can adjust the correction using the adjustment procedure described above.

Such manual corrections are highlighted in blue in the Correction list.

It is also possible to insert corrections 'on the fly' during track playback. To do this, simply hit the Space Bar at each instant you want a correction inserted. When you have finished playing the track, you will need to go back and adjust these manually inserted corrections to position them exactly over the clicks you want to correct. Before adjustment, these 'cued' corrections are highlighted in white in the Correction List. They are initially set to zero width and hence do not have any effect until they are manually adjusted.

Derek Higgins represents Ganymede Test & Measurement, the home site for Wave Corrector, the vinyl to CDR audio restoration software http://www.ganymede.hemscott.net/

 

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