When faced with any new audio restoration task our golden rule in Decca was, if possible, always use the original recording, second best would be a choice of “takes” which could be pieced together to give the best quality. We were fortunate in that much of the material we worked with was either from original matrix discs (stampers) kept at our record factory in New Malden or were from the dark end of our tape library known as the “graveyard”. From the 78 rpm matrix discs new pressings were specially processed for us onto vinyl however despite this many of these matrix discs had deteriorated over the decades and presented their own problems that needed solving as mentioned earlier regarding the Vera Lynn pressing. Some material was no longer available to us because of the passage of time (I understand part of the New Malden complex was destroyed by a bomb during the second world war and much material lost) in these instances we approached private collectors, often the BBC record library and borrowed commercial shellac pressings. Of course when faced with shellac the material is prone to severe deterioration over time and this causes multiple clicks and crackle as was the case with the Boswell Sisters mentioned earlier. Shellac is a very brittle material and often rare records came to us damaged with pieces missing from the outer edges. These were great challenges because it meant creating a new intro. By editing in passages from elsewhere in the record we could re-create the introduction often unbeknown to the listeners.
The restoration of this material was very much a manual process in those days and therefore very time consuming. The thought of efficient and effective devices for removing these clicks, crackle and background noise were just dreams that would not materialise fully until the digital age, fast processors and effective algorithms.
Initial attempts at an electronic declicker gave birth to a device known as a switcher which relied on two identical mono signals (such as a mono record signal being fed to a stereo cartridge), the individual signals from the two walls of the groove were simultaneously fed to the switcher which was constantly hunting for the cleanest of the two signals to switch to. In reality only loud clicks were processed with any real success, the system was incapable of differentiating between wanted material and unwanted low level ticks. Later developments then saw a digital device available called Sample and Hold. This was a step on from the early attenuator device mentioned last week. It still introduced a drop out but at constant amplitude to the last wanted signal. The drawback here was unpleasant noises that resulted from this process such as low frequency thumps, ticks and even distortion in places. Following this a more sophisticated device was introduced utilising a system called Linear Interpolation, which followed the same concept as Sample and Hold however this development allowed the process to match signal amplitudes before and after the click or tick. Linear Interpolation has developed now to a point where the algorithm incorporates what is known as signal modelling. This process analyses the signal over a period of time learns the “model” of the wanted signal, removes the offending click or tick and replaces the unwanted noise with a shaped waveform that fits the model of the wanted signal. This latter tool is highly sophisticated and one that Cedar equipment and software executes perfectly. Recently I was working on a rare Jazz recording for a customer, when played straight from disc the recording was tarnished with multiple clicks and, more annoyingly plenty of ticks. I demonstrated the power of the Cedar algorithms to a visitor who happened to be in the studio at the time, by switching between the A and then the B signal (A pre processing and B post processing) my visitor was astounded by the resulting quality – more about Cedar next time.
Tags: Audio Restoration, Damaged Materials, Tapes, Vinyl, Website