Audio Restoration
What is audio restoration about ?
Just as with antiques in general, what matters first is to keep within limits, not overdo it. Do just what it takes to restore the object to its original aspect but keep the patina of time. For a disc or a cylinder, restoring will mean cleaning the sound from all unwanted noise (clicks, crackle, hiss), perhaps readjusting the frequency curve to make the words of a song more intelligible. But never try to remove ALL the noise because the original recording would lose all of its "antique" character. Equally it would be nonsense to add reverb and stereo to a 1910 record, although it has been done on some commercial "oldies collections"...
Therefore, let us clean what we can without removing anything from the original sound and without disfiguring it with incongruous and anachronistic effects !
How to hook up you phonograph to your computer !
There is no major difficulty in the hook-up. Plug in the amplifier LINE OUT to the sound card LINE IN and the amplifier LINE IN to the sound card's LINE OUT. Now you will be able to record and playback on your stereo system as you would do with just any old cassette recorder. But your PC with adequate software will do lots more things than a tape recorder !
Audio restoration software
Plenty of audio processing software is currently available. Most are commercial solutions, relatively expensive, like DC ART or Sound Forge. But there are also cheaper programs and a free software called Audacity, which has the same features as the "serious" ones. I will therefore recommend Audacity to all beginners. After using a free software for a while, you can decide whether the pleasure of audio restoration is worth investing in more expensive and more performing software.
The small demo hereafter will give an idea of an old record restoration performed with Izotope RX 2. The example I have chosen is "La Violettera" by Emma Liebel from a hill-and-dale Twenties Pathé disc. A record in rather poor condition... Here is what we get directly from the magnetic cartridge without any processing. We get as much noise as music : low frequency rumble, medium-high frequency surface noise + clicks and crackle, in other words a very bad signal/noise ratio. The following steps will have to be performed in the right sequence as the final result depends on it.
Step 1 : Cancelling the RIAA curve
If your turntable is plugged into the PHONO input of your amplifier, the signal undergoes a frequency curve correction following the RIAA specification which is useful only for microgroove records. Our 1920 acoustic record has nothing to do with that curve and we must cancel it by applying an inverse RIAA curve. The DC ART paragraphic filter does that quite simply : just select the "reverse RIAA" pre-set and the trick is done ! Now we have recovered the original treble and better clarity . But the level of the clicks and crackle has increased too. Don't panic ! This will make detection easier for the filters we are going to use next.Step 2 : Impulsive noise suppression
Impulsive noise means all those clicks and pops which can really spoil the listening pleasure of old records. They can be visualised as short amplitude peaks that some complex algorithms manage to distinguish from the useful signal and to eliminate for the most part. The click is deleted and replaced by part of the adjacent signal. In some cases the elimination is radical. In other cases it may leave some glitches but so much quieter than before. The DC ART "de-clicker" is quite easy to use but it takes some practice before you can get what you want. Impulsive noise differs from a cylinder to an LP. You can even ruin the music if you set the controls too high. For the beginner, the cure can be worse than the illness. However, this is what you get with some practice...Step 3 : continuous noise reduction
Continuous noise is the surface noise of a record (or hiss for a tape or cassette). It is basically white or pink noise, made of random frequencies that run through the entire recording. Any sound processing software has a "noise reduction" function that can reduce continuous noise dramatically. The first thing to do is select a part of the recording with only noise and no music, in fact the first grooves of the record. The software then makes a spectral analysis of that sample. Afterwards, the software will search through the entire wav file and eliminate everything similar to the selected noise sample. Here again practice makes perfect ! If you want to reduce the noise by 60 dB, you can of course but the music will then sound strange and completely unreal. It is safer to reduce by 10 dB and not alter the music. And as I said a slight surface noise is not unpleasant : it is the patina of time...Step 4 : final filtering
Our old Pathé disc has no useful frequencies below 150 Hz neither above 4000 Hz. So why keep low frequencies which are just rumble and noise from the recording stylus that was used long ago. The same applies to the high frequencies that contain only a part of the surface noise. So let us use the band-pass filter and cut everything below 250 and everything above 4000. I took advantage of this final filtering to boost the frequencies around 3000 Hz in order to improve the voice presence, but this is a personal choice that can be criticised. And there you are !. We won't go any further because "better" is sometimes worse ! The sound is now much, much better than it was before processing...And finally : what you should not do !
All sound processing programs have reverb functions that are fun to use. Therefore, I just had fun and here is the iconoclastic stereo version of "La Violettera". Just to give an example of what you should NOT do with an old record ! On the other hand, you can add a bit of reverb and stereo to a monophonic sixties single that seems too flat.....
The ultimate demo : How to deal with hopeless cases !
This venerable brown wax Bettini cylinder of 1900 was in a rather poor condition, with lots of mouldy patches on the whole surface, but it had to be saved at all costs : Charme d'Amour, sung by Mademoiselle Vivier.. !
I have combined the unprocessed sample and the processed sample of that song in a "stereo" wav file, so you can play both channels simultaneously or use the balance of your stereo system to switch rapidly between each of them in order to compare. The upper graph represents the unprocessed signal, as it came out of the electric phonograph. The numerous clicks and glitches are clearly visible (and audible ! ). The lower graph shows the processed signal. Not really a miracle, but it sounds a lot better.
I have used Cool Edit for clearer waveform views, but the restoration has been performed with DC Art 32. The clicks and scratches have been manually processed one by one by zooming and selecting only the unwanted glitch and using the "interpolate" function to replace the noise by a sample of the adjacent signal, thus making no "hole" in the music. This is a long, painstaking and tedious method but far superior to the automatic declicker, unable to cope with such hopeless cases !
Now here is the whole restored song (MP3 942 Ko) Charme d'amour, one of the greatest hits of Paul Delmet (1862-1904) on a remarkable post-symbolic poem by Maurice Bouquay. See lyrics here.