What Can Restoration Achieve?
Enhancing or repairing problem audio in the analogue domain has always been difficult, and every method employed had unwanted side-effects. Modern specialist digital processing can now achieve remarkable, sometimes seemingly magical improvements with many issues that used to be regarded as ‘something you just have to live with’. However, heavy-handed or inappropriate use can still make matters worse, so an experienced engineer and accurate monitoring are essential to judge the balance between perceived improvement and unwanted side-effects.
For restoration software to work as intended, it is essential that the audio is captured from the original source at 24-bit resolution and at least 44.1kHz, ideally 88.2kHz or 96kHz sample-rate. Recordings at lower resolution (e.g.mp3) will rarely give good results. Similarly, files that have already had some destructive processing applied (such as hiss reduction) are best avoided in favour of the original recording if further enhancement is deemed necessary, particularly EQ or dropout repairs.
We have the Cedar range of hardware processors, plus the best of the more surgical plug-ins and stand alone software applications (iZotope, Waves and Wave Arts), and the experience to know which will work best for the multitude of issues that can now be successfully addressed.
PLEASE NOTE: we specialise in restoring music and speech recordings, mainly of studio origin, with a view to release or further production work; also domestic recordings of sentimental or historic importance. We do not work on covert recordings for forensic purposes.
A particular problem with analogue tape, but hiss can also be an issue on digital recordings: for example a dynamic mic used to record a very quiet sound source, incorrect use of a ‘pad’ switch or with vintage equipment. Significant improvements can usually be achieved, but over-processing produces very undesirable side-effects; better to leave just a little hiss as it sounds more ‘natural’.
The Cedar ‘DHX’ is extremely effective at reducing background hiss, but we also use a selection of software solutions where hiss is more intrusive or sporadic. We can reduce hiss in stereo mixes considerably by careful processing, but if a multi-track tape is available much better results are achieved by de-hissing each individual track separately before remixing. Settings can then be tailored to each instrument for maximum hiss reduction and minimum side effects.
We can remove short duration clicks perfectly by careful ‘surgical’ processing. The most serious (like a badly scratched record) need more careful treatment but can usually be successfully disguised using a variety of techniques. Editing around such loud clicks may be a better option.
Crackles are more difficult, but significant improvement can usually be achieved with careful adjustments. ‘Spectral’ repairs can also yield excellent results.
We occasionally get asked to ‘clean-up’ vinyl records for re-release, where the original master tape has been lost or damaged. It is essential that the cleanest possible copy is obtained, ideally unplayed, and with a spare for safety, as surface noise (usually due to re-cycled vinyl being used for the pressing, or ground-in dust from playing) can vary hugely. We have a record washing machine to improve the odds of a success, and a fresh high-resolution transfer is always going to yield better results than one made on domestic equipment.
This is the classic ‘overload’ distortion, for example where recording levels have been set too high on a digital recorder and the waveform peaks are literally cut off.
We use modern software that is incredibly good at reconstructing the lost peaks and results are usually excellent – in most cases we can remove clipping distortion completely, but results are clearly dependant on the severity of the overload. The best results are obtained by working on the original recording since the shape of the waveform is changed if processed with any EQ or compression, reducing the effectiveness of the process. Similarly, affected tracks on multi-track recordings need to be ‘de-clipped’ individually before remixing. Sometimes de-cracking is also required to disguise any residual noises.
A dropout is an interruption to the correct audio playback, where either dust or contaminant momentarily gets in between the tape and the head. With analogue tape this causes a drop in volume and loss of high frequencies; with digital, clicks and glitches or even complete loss of audio. If this loss of head contact happened during the recording then the dropout is permanent, otherwise careful cleaning of the tape before transfer is usually effective.
Dropouts can also be caused by damage to the tape, such as a crease or flaking of the magnetic layer from the backing. Creased tape can usually be played successfully with careful use of a pressure pad to maintain head to tape contact. However, a pressure pad can introduce other problems so the affected sections must be replaced by surgical editing only where required. Editing around the damaged section may also work very well, if, for example, the dropout is on a riff that is repeated elsewhere in the song.
If the stereo width of the recording is narrow, the dropout is only on one channel and also very brief, then substituting the good channel for the duration can work well. If not too severe, it may also be disguised by precise automation of the channel volume. As a last resort, reconstructive software can fill the gap by analysis of the audio either side of the ‘hole’ and generating a suitable infill.
We use all of these techniques to remove or significantly reduce the audibility of dropouts in any tape recording.
Mains hum or buzz is a common problem when working with analogue equipment and frequently goes unnoticed until the recording is played on headphones or full-range speakers. We can usually remove hum very successfully with modern specialist software, and with minimal side effects. Great care is taken to ensure no wanted bass is lost in the process.
The Cedar ‘De-crackler’ unit is also exceptionally good at removing low-level high-frequency buzzes like those resulting from thyristor lighting dimmers, frequently encountered on live recordings.
Extraneous noises, such as a ‘P’ blast on a vocal mic, feedback on live recordings, a knocked mic stand and similar unwanted sounds, can be very effectively removed by ‘spectral repair’ software. We use iZotope RX4 Advanced for such ‘surgical’ noise removal.
As with any form of audio restoration, all of these repairs are best effected on the specific track of the multitrack recording where the noise occurs rather than the stereo mix, but improvements to both are usually possible.