Its also worth noting that the louder recorded music is, the wider the resulting grooves will be on an LP: the groove shows the actual sound wave. So in order to fit certain material onto the 25-minute side of a disc, producers faced tough decisions. They might reduce certain bass frequencies to make the grooves smaller, for example. These alterations (now superfluous) are all noted in the job files, and give Ramsay and his colleagues vital information for a comparison of the original LP release with the master tape.

Having read the matrix card and job file, the time then comes to find the original tapes, stored in tin boxes at the EMI archive in Hayes, Middlesex. These are brought to Abbey Road to be repaired, if necessary, and then digitised.

Various problems can occur to magnetic tape. Sometimes the glue from the editing process will come loose and accidentally stick parts of the tape to each other, or the joins might break apart during rewinding. Another difficulty is sticky-shed syndrome caused by the deterioration of the binder (a glue along the entire length of the tape which holds the magnetic, recordable part of the material onto the plastic strip). Only a baking of the tape can help in this case. Fortunately the latter has not affected any of Callass masters, although many edits have had to be repaired.

Sometimes the edits are extremely plentiful. When Ramsay plays the final scene of Manon Lescaut from the master tape, there are little white strips of splicing tape every few seconds. Each time the tape is rewound they can come a little looser. Sometimes they break altogether, and then it is Ramsays job to repair them.

Callas didnt only record on tape for LP; she recorded on wax for her earliest 78s on Cetra, and these wax discs were used to create shellac 78s. These recordings were originally leased to EMI for release on the Parlophone label in the UK, and are also being remastered as part of this project. Engineer Andy Walter is working on this in another part of the Abbey Road complex.

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First he tries to find the right kind of stylus for the disc. There are rules about which stylus is appropriate for which year 2.7 thousandths of an inch, or 4.0 thousandths of an inch, say but in practice you just try them all until you find the one which works best, the one which will get as much information off the disc as possible, he explains. The sound is then digitised for RetouchTM editing, but because shellac is such an inherently noisy medium, it is run through a two-stage automatic declicking programmes one for larger clicks, one for smaller which can remove about 25,000 clicks per second. (The declicking programme is made by a company called CEDARTM, which also developed RetouchTM.) Theres also a programme to reduce surface noise and hiss. Its always a juggling act about what you take out and leave in. Some people think that the noise is authentically inherent to the 78 medium thats simply what shellac should sound like. I feel that as long as we dont alter the voice, were doing a good service.

Allan Ramsay agrees, and stresses that the remastering process should not interfere artistically with the original recordings. We just want to present the recording as it was meant to be heard as they heard it in the control room, and just as it was approved. Were simply correcting faults just as the original producers would have done, had they had the technology.

Walter plays a 78 of Casta Diva, and then switches between the original copy and the declicked version. Its like a veil being drawn aside. When the clicks and noise disappear, its as if the singer is suddenly standing much more clearly in front of you, he says. And this is actually a beautifully recorded 78. Shes right there in front of the microphone, blasting away, but it sounds great.

While Andrew Walter is dealing with the 78s (and chasing collectors, trying to track down a version of the Liebestod which was never released in the UK), Ramsay is working on the tapes. Firstly he prepares the tape machines for correct playback. The remastering engineers select an EMI Calibration tape dating from the period of the particular recording and containing a series of tones in the form of long beeps. This tape is played and the frequency response of the tape machine is adjusted accordingly.

Pitch is another matter of concern. Tape speeds werent computer-controlled then, and were slightly at the mercy of the frequency of the mains electricity and the tape transport, says Ramsay. In fact, many LPs from the era were sharp, simply because the tape machines were running a bit fast when the

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lacquer was cut. And sometimes two tape machines would run simultaneously at different speeds during a session. If both tapes were used for editing, the pitch would jump between edits. Again, it was just the best they could do at the time. Now we can try to fix those problems.

The tape recording is then played through an analogue-to-digital converter, which makes the sound available for digital editing. When the tapes were first digitised in the 1980s for CD release, the information was stored on video tape, and these tapes were used as the basis for the 1997 remastering. Now Ramsay and his team are returning to the original analogue masters for all their work.

The next stage is to open the digital files on the computer workstation and begin the process of editing and retouching. RetouchTM is a programme that creates a visual realisation of the recording by splitting it into multicoloured sound waves which travel horizontally across the computer screen. (Imagine an ultra-sophisticated electrocardiogram, and youll get a rough idea of what it looks like.) Any unwanted sound, such as the distant beeping of a car horn or traffic noise, can be visually isolated and removed without affecting the music.

People have sometimes criticised Callass voice for its wobble: her producer Walter Legge humorously suggested that it could even induce seasickness. Its fascinating to note that part (but not all) of the problem was simply due the inadequacy of the recording equipment of the time. Her voice was so powerful that microphones and tape were occasionally unable to cope with it, and the result was an electrical bubbling or popping between the harmonics of her voice. The bubbling occurs, amongst other occasions, during the fortissimo climax of In questa reggia in her 1957 recording of Turandot, conducted by Serafin. The bubbles, which appear as little black lines between the separated harmonics on screen, are not an accurate representation of her sound, so Ramsay carefully removes them with RetouchTM and restores Callass voice to how it originally sounded at the recording venue (in this case, La Scala.)

Once digitally uploaded and converted into RetouchTM visualisations, the masters are ready for cleaning up. In Tosca, a high descending squeal from a malfunctioning amplifier at the recording session is present. (It looks like a thin sloping black line on the computer screen, and is easily erased.) Prior to RetouchTM the only way to try to fix this kind of fault would be to cut all the high frequencies changing the whole sound, says Ramsay.

On some LPs, made in Kingsway Hall (where Callas recorded Il barbiere di Siviglia, the second Lucia

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di Lammermoor, and the disc of Mad Scenes) London Underground Central Line tube trains can be heard rumbling away below the venue. The rising buzz of an accelerating Vespa is heard from the streets outside La Scala during the stereo Norma too: it looks like a rising black line on screen. All these sounds can now be removed, with a few simple clicks of the pointer and without affecting the music.

Sometimes an original edit isnt quite correct. If a little too much tape was left in (creating a double attack on the note), it can be corrected by removal. If a little too much was cut out, the problem is magnified. If a tiny snippet is missing and theres a rush in tempo, all the tools in the world may not be able to fix it, says Ramsay.

On recital discs, a problem can occur between tracks. The original editors inserted strips of plastic leader tape (blank white tape) in the spool between arias, so that there would be rills (wide separating bands) on the LP when it was cut. On an LP such as Maria Callas Sings Rossini and Donizetti Arias, the surface noise of the vinyl masks the sudden silence, says Ramsay. But when you play the original tape, or hear it digitally, it really jars. So we have to find some atmosphere the sound of the empty room from the Salle Wagram in Paris where she recorded it to insert at those points. (When the exact atmosphere from the original session is unavailable, Ramsay will search for a close match from a library collection of ambience sounds.)

Magnetic print-through is another hazard. Sometimes when the music is very loud, it prints through to the adjoining piece of tape during storage on the spool. The audio is recorded as a magnetic field. The louder the music, the stronger the field, says Ramsay. It can affect the next layer of tape, and sound like a pre-echo, or post-echo, depending how the tape was stored. There is an echo of this sort during an orchestral climax in Il trovatore: it becomes visible on screen and can be straightforwardly rectified with RetouchTM.

Once the audio is fully cleaned up, it is passed where necessary through an equalising (EQ) desk to adjust the bass or brightness or add other effects required by the producer just as it would have been in the 1950s. There is a software programme which digitally recreates the effects of a manual mixing desk with all its faders and dials, but Ramsay prefers the old, non-virtual analogue version. Theres a warmth to the sound with analogue EQ which we like, but well use digital if its more appropriate. Were lucky to have the option.

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The recording is then tracked, or PQ-ed. Ramsay explains: A CD can contain various sub-channels of information as well the audio. These sub-channels are named P, Q, R, S, T and so on, up to W. The P and Q channels are used to store, amongst other things, the track start & timing information. Even this process has been overhauled. Some of the CDs from the 1980s, the mono Tosca among them, were assigned only two tracks, and this would now be considered too few. More PQ tracks have been added, and the PQ plot of each opera has been rationalised across the entire project. For example the plot is now the same in both the mono and stereo versions of Tosca, Lucia, Gioconda and Norma.

For CD, the digital files are then processed by a sample-rate converter and dither, which reduces the output to 44.1 kHz and 16 bits. These files are sent to the CD factory, so that a glass master (the CD equivalent of an LPs metal matrix) can be made. For the downloadable high-definition release, the files are left alone and remain at 96kHz/24 bit.

Is Ramsay ever worried about overstepping the mark? Is he concerned that some of the technical clean-up alterations might be considered as a form of the artistic interference he is so keen to avoid? If we ever have a doubt about something, we leave it alone. And if weve remastered something well, nobody should be able to notice it. Were simply removing the inefficiencies of the recording process of the time. All we want to do is remove the specks of dust, as it were, and wipe the glass clean. Were presenting the recording in the best possible light, and as the artists would have wanted it to be heard.

Warwick Thompson, 2014