Recording in stereo before digital re-enhancement

Noise may be largely caused by tiny specks of dirt and steel or by little cracks and holes on either one side of the groove or the other. When a recording of a mono 78 is made in stereo and replayed, many of the clicks and scratches will be heard coming from either one speaker or the other. The actual music will be heard from the centre. One simple way to test the severity of surface noise is to invert one channel and then superimpose it on the other, an easy operation using a digital editing program. The music, being added in antiphase, will largely vanish but the clicks will still be there. How convenient it would be if the noise from the left and right channels could be eliminated leaving just the music in the centre! Unfortunately, there is no possible way of using superposition of channels to achieve this.

The decision whether to record in stereo and later convert to mono, or to record and work throughout in mono, will depend partly on the following considerations.

Disadvantages of stereo include:

• longer processing time
• larger files
• some de-noising algorithms may introduce phase shifts which will appear as additional distortion after the two channels are combined into mono

Advantages of stereo include:

• some impulse noises can appear as signals in anti-phase on the two tracks, and so cancel out when converted into mono - although of course they would not appear if recorded in mono by connecting L and R in parallel!
• some records have heavier groove wear on one side of the track than the other so the conversion to mono can be adjusted to give greater weighting to the cleaner channel
• noise appearing on only one channel only is reduced to 50% of its initial amplitude after conversion to mono
• some click-reduction algorithms give rise to artifacts that were not present in the original wave form. If these defects appear on one channel but not the other, their amplitude will be halved on the final mono track.
• in theory an overall noise reduction of about 3dB is possible by de-noising each stereo track and then converting into mono
• You may wish to use your preamplifier for stereo vinyl discs!

Versions of Adobe Audition from 1.5 onwards have an effect called "centre channel extractor" which carries out the operation of eliminating most sound except the music in the centre channel as described at the top of this page. Preliminary experiments with this effect are producing very promising results for certain types of records, most notably with those in which the signal on each side of the groove is similar even though there may be a significant difference in the noise in each channel.

Many types of software may be found which allow Karaoke users to remove the vocals from a track leaving just the backing. This works by eliminating the sound common to both tracks, as described at the top of this page. As the vocalist is usually located in the centre, it is the voice which is removed. There is hardly any software which can extract or isolate the vocal, removing the left and right spatial information.One exception is Elevayta Extra Boy Pro, a VST plug in which works in some audio editors and can produce some good results in cleaning up spatially separated noise. Another software package which does the same sort of operation in a somewhat different way is Voice Trap from Clone Ensemble.

In general, the worse the original record quality, the greater the gain for recording and cleaning in stereo before final conversion into mono.