CEDAR Single-Ended Noise Reduction System (part 4)
The CEDAR process works from a digital copy of the recording to be restored or enhanced. This means that there is some work to be done before sending the material to Cambridge Sound Restoration, CEDAR Audio Ltds subsidiary company, or running it through your own CEDAR processor.
Considering the case of 78 rpm records, the first thing to do is to get hold of the best possible copy of the recording. Obviously it makes sense to start with as few problems you can, but even simply playing a 78 is not as straightforward as it might seem, not if you want to get the best results out of it. Once you have a clean copy, the next problem is almost bound to be that the hole isnt in the centre, causing wow. CEDAR can do nothing about this, so the disc has to be correctly centred on the turntable. Next question: At what speed should the turntable run? Unfortunately, 78 rpm is not always the correct answer, and the label is not likely to be very informative on the subject. The speed could easily be 10% out and youll have to judge it for yourself. If these problems are not enough, theres plenty of work left in finding the correct stylus to get the most music out of the disc and the least noise.
Once the optimum playing conditions have been found, the disc can be transcribed to a digital medium such as DAT. After this stage, things become more straightforward.
The hardware part of the CEDAR system doesnt take up much desk space. In fact just enough for a Compaq Deskpro personal computer, monitor and keyboard, fitted with a large hard disk drive and the all-important CEDAR digital signal processing boards. The rest, as they say, is in the software. The software currently has four modules, a 1-pass scratch remover, a 2-pass scratch remover, a 512 band equaliser (yes, five hundred and twelve bands) and a noise reducer with equalisation of the noise free signal.
I was shown a system which had all of these modules.
The first step in restoring a recording is to load it in from a DAT master over the S/PDIF digital interface (there is also AES/EBU). CEDAR includes a hard disk recorder which stores audio segments as files on the disk. The presentation is very straightforward and I was able to operate it immediately. Once the file is on the disk, it can be Descratched to remove the major clicks. This is not a real time process, the software looks at the material and builds up a model of what the music should be like, and then takes away anything which it regards as not being music. The gaps where the clicks had been are filled by interpolation, resynthesis or adaptive splicing. The two adjustable parameters are Order, which roughly speaking is the power of the system and Threshold, which represents the degree to which a noise must deviate from the model of the music to be considered a candidate for removal. There are two ways to set these parameters, trial-and-error and experience. Setting the Order is fairly simple because if you make it too high, all you lose is time - the process takes longer. If the Threshold is set too high, however, then the process may develop an appetite for music as well as scratches. After processing with the Descratcher, which might typically take twice as long as real time, there should be no major clicks, but ticks, surface noise and hiss will remain.