The restoration procedure - Part 2

The exact sequence of restoration steps depends on the faults in the record. There will certainly be a lot of clicks and there may be some crackle. It will be more difficult to clean up if there is also distorition.
Clicks are impulsive noises of short duration, typically less than a few milliseconds in length.
Crackle consists of very large numbers of very small clicks close together
Distortion implies a badly reproduced waveform as a result of serious groove damage, either lateral or vertical. The latter type may induce rumble or thumps of low frequency
A procedure to clean up the digital recording is set out here. It is usually preferable to declick before removing any crackle or other noise because most click removal algorithms are more sensitive on an unaltered wave file. It is also preferable to apply noise reduction in two or more very small steps rather than one large one. This procedure is accompanied by examples of the sound file, in mp3 format, following each stage of processing. To listen to them, click on the link and if offered the option either choose Open to play the clip in your usual audio player or Save to put them on to your hard drive for future playing.

A brief section from a much played record with a lot of dust and dirt in the grooves (Plaisir D'Amour sung by Paul Robeson, HMV B9059, OEA8013□ recorded in 1939) can be heard here. It was transferred "flat", in stereo and without any equalisation introduced by the preamplifier using a sample rate of 44100 samples/second at 16 bits per sample.
The record was cleaned carefully as explained on the previous page and then left to dry until the shellac had hardened again. Cleaning produced a noticeable decrease in the both the number of clicks and their intensity, as can be heard here.

Next is the click removal computer alogorithm, in this case using Click Repair set to 50% click detection but without crackle removal. This is the result of the Click Repair declick session. If using Adobe Audition to remove clicks, It will be necessary to experiment with various setting in order to reduce them to a low level without introducing unwanted distortion. For material with a lot of small clicks, suitable settings might be: Auto Find All Levels with FFT close to the minimum size of 8, Pop Oversamples of 0, Run Size of 4, and Second Level and Pulse Train Verifications turned on. Some stubborn clicks may later have to be removed manually using Fill Single Click Now on the selected region. The Audition click removal algorithm is very slow at these settings.. If you wish to process a lot of records, this task may be automated by a scripting language such as the excellent open source software program AutoHotKey, which can simulate mouse movements and other commands. You could then record a dozen tracks and process them automatically overnight. Most audio editing software has a declick routine but not all are optimised for 78s. Experimentation with all the settings is advised. It is preferable to replace small clicks rather than large ones because the interpolation routines tend to work more accurately when used this way. This may mean treating large clicks as a series of smaller ones, increasing the time taken but getting a better final result.

Now some noise reduction may be applied. If the recording has a section containing noise but no music, such as the run-in or run-out, it may be used to obtain a noise print before applying the noise reduction algorithm. At this stage it is suggested than only partial reduction of noise should be attempted because too high a level may remove music too and can even introduce noise that was not present in the original. Here is the clip with the noise reduced using, in this case, the Sony noise reduction transform with the following settings: reduction mode 2; 0 bias; 90/50 speeds; 2048 windowing; sampled from 500Hz to top of range. If the noise reduction process removes too much bass, which may happen if the sample has a lot of rumble as is often the case at the lead-in, the noise sample may be taken before applying bass boost, then running the bass turnover compensation filter, and then applying the noise redution.

Crackle is high frequency noise composed of a lot of extremely short low level clicks. Commercially available decrackle routines may be used, and for severe crackle a useful piece of software is that produced by Acon. However, this particular record does not have enough crackle to justify a special procedure.

As already mentioned, how convenient it would be if the noise from the left and right channels could be eliminated leaving just the music in the centre! There are some ways of attempting this and the result of applying light settings of the centre channel extraction filter of Adobe Audition may be heard here. A similar result would be obtained by the use of a VST plug in transform called Extra Boy Pro or another recent introduction named Voice Trap. This step is not required if the original was recorded in mono. It is also not recommended if one side of the groove has suffered a greater loss of genuine music than the other because the outcome would remove music that was not common to both channels. However, it is very effective for records with high noise but little loss of signal on both channels.

Since this example was recorded at a sample rate of 44100 in case there was significant crackle (which is best removed by most software at this sample rate) it can be converted to 22050 before applying equalisation. This will halve the file size and reduce the highest frequency to around 10 kHz, well above what was recorded on most pre-war 78s. Most listeners would not notice much difference after downsampling and Indeed, a very slight amount of top-cut above about 7 kHz may reduced noise without affecting clarity, as may be heard here.

Now is a good time to apply appropriate equalisation. If it is left to later, the reduction of bass notes by the noise reduction filter may act too severely in reducing the unequalised low frequencies. The amount of equalisation required depends on the pre-amplifier and its ability to apply appropriate corrections to various types of 78s. If the recording was done "flat", as the sample used on this page was, a graphic equaliser set to the appropriate equalisation curve could be used. Getting the equalisation just right is more of an art than a science and to some extent it depends both on the original recording engineer's settings and the modern listener's preference. The final sound will also depend on the quality of the sound system used to replay it and, at least for those of us on the wrong side of 50, on the extent of our residual high frequency hearing response. Since the matrix code ends with a small square, the Blumlein equalisation curve witha bass turnover at 250 Hz and enhancement below this of 6dB per octave but without any change to levels above about 500 Hz was applied to produce this result. It may also be necessary to apply "de-rumble" with an algorithm such as the Audition Butterworth high pass 18th order filter set to a sharp cut-off at around 28 Hz to remove low frequency noise and low thumps.

On the basis that several small amounts of denoising may be better than a single large transform, a denoise algorithm may be applied again to treat the two tracks according to their respective noise levels. In this case the Audition noise filter, weighted to reduce more low frequency noise than high, may be used at a very low level. The next step is conversion to mono using both channels. On some badly worn records groove wear is worse on one side of the groove than the other, in which case a weighted mono conversion, or even only one channel, should be used to create the mono result. This example was weighted 50% right and 50% left because the left channel had the same number of clicks as the right channel. If heavy centre channel extraction had been used in the previous step but one, both tracks would be the same and equal weightings would be used. Next the wave may be normalised to a suitable level. This example shows normalisation of 95%.

The final stages may require some manual removal of stubborn clicks such as one at about 4 seconds into the sample, processing of other unwanted artifacts, and cutting and trimming the ends. However, the biggest bugbear at this stage is the removal of low frequency impulsive thumps. One way to detect and remove them using Adobe Audition is to view the wave in spectral view, zoom in to the lower frequency, look for bright coloured thumps, select and apply automatic single click removal. Then save the processed file either in Windows PCM (*.wav) format or as an mpeg3 file. It may also be advisable to retain the original file "raw" or "unedited" for archive purposes. For comparison here is the "before and after" treatment of this restoration project.

The whole restored record of "Plaisir D'Amout " may be heard here