Film the episode! The film cameras use traditional "negative" film stock. To maximise efficiency and minimise cost, scenes are generally shot so that every one involving a particular location are all filmed together. This means that scenes as held on the negative are usually not in the correct order as required for the actual story-telling. (In fact for the series' first season multiple scenes shot consecutively in, for example, Cowley's office were often destined for use in different episodes!)

Spoken dialogue and background sound (footsteps, gunshots, explosions, vehicles driving by, wind, trees rustling, birdsong, etc) are recorded at this stage, though each of the two aspects on a separate magnetic tape as it may be desirable to subsequently replace the dialogue track for foreign-language sales.

Once all scenes for the episode have been shot, the recorded negative film stock from the cameras is cut into individual scenes as per the script and then reassembled into the correct story-telling order. Naturally a lot of the footage will be waste due to scenes having to be re-shot but also "good" scenes may need trimming a little to ensure the episode runs to around 49 minutes. The separate strips of negative that are to be included in the finished product are physically joined together using an adhesive.

(Experienced television scriptwriters develop a knack for how long his/her script needs to be in order to fulfill the timeslot. Occasionally an episode was found to overrun by a few minutes, so some scenes were considerably shortened or dropped altogether. Conversely some episodes from the first season severely underran, so required additional scenes to be written and shot.)

A (16mm negative) copy of the opening title sequence is also included at the appropriate point, as are a "textless" version (for reasons discussed later) of the end credits. The addition of these elements now bringing the total runtime to around the usual 50 minutes.

The end result of this stage is known as the "Master Negative" for the episode.

Because the Master Negative is physically delicate, given that it relies on adhesive to hold it together, its use should be kept to an absolute minimum (though, as we shall see, this didn't happen with The Professionals!). It is carefully handled in order to produce what's known as an "Intermediate Positive" (or "inter-pos"). Because the Master Negative is now assembled in the correct scene order, the inter-pos is a continual, single stream of film stock that doesn't use joins.

The sample image here shows the picture on the inter-pos to possess a strange orange tinge. We'll return to that shortly...

The Inter-positive can now be used as a guide for composing and performing of Laurie Johnson's incidental music themes. These are recorded on yet another (separate) magnetic audio reel.

Any sound effects that couldn't be created during the filming (eg sound effects of fist-fight punches which, of course, are not enacted for real during filming) are created at this point and added to the background sounds recording made during filming.

Due to a mix of primitive technology and the rush to get the episodes completed for transmission, the chaotic first season regularly suffered from poor conditions for recording the actors' dialogue. As such there was much reliance on it being re-recorded at the Harefield Grove studio. This was a tough process for the actors as they had to pronounce words in a way that matched their on-screen lip movements.

(There was much less need for dialogue re-recording from the second season onwards as the crew member Dave Crozier developed a superior means for recording audio on location, using a combination of wireless microphones hidden within the actors' clothing and a more "mobile" set-up for the recording machines.)

Back to the Inter-Positive and there is more work required. The orange hue is a special chemical base that allows film technicians to apply "colour grading". This is used to ensure colour tones are consistent across not just the entire episode but also individual scenes. Notable examples would be skintones, clothes, the deep blue of Cowley's later Ford Granada (yes, it's blue, not the black it appears to be in the old film prints!). During filming it's virtually impossible to keep these consistent from one scene to another due to differences in lighting levels, weather conditions and even different rolls of negative film stock. This is particularly true of long and/or complex scenes such as car chases that had to be filmed across different days.

At this stage captions are added to present the episode title, the copyright annotation and the names of the director and writer. The credits for the guest actors and film crew used in that particular episode are created on a separate, transparent piece of film stock and subsequently overlaid onto the (textless, as mentioned earlier) copy of the end titles.

While being very different beasts, the Inter-pos and Master Neg are hugely valuable for the long-term preservation of the episode - particularly if one of them is subsequently lost or damaged.

Of key importance here is that we now have a complete episode consisting of all its visual ingredients.

Meanwhile the audio elements are brought together on what's known as a "Triple Track reel" (or "Three-Stripe" as it is known in the USA). As its name implies, it is a single reel of magnetic tape that comprises three separate, parallel magenetic tracks. Each one is used to hold the dialogue, incidental music and sound effects/background sounds respectively. This approach not only presents the audio elements in sync with each other and the screen action but keeps them separable, too. It also makes the overall handling of the episode's "package" of visual and audio easier.

In terms of standard television transmission, there is another important benefit of the "triple-track" technique: it allows foreign-language markets to easily replace solely the dialogue track with their own spoken language (dubbed by other actors) if they prefer that to using on-screen dialogue sub-titling.

Importantly, a duplicate Triple-Track is also produced at this stage, for reasons which will become clear below.

The Inter-Pos is NOT used to create film prints for transmission. The reason for this is that such a valuable resource would become quickly worn out, particularly bearing in mind that for a show such as The Professionals, many transmission film prints were required when the show started selling overseas. Instead it is used to produce an "Intermediate Negative" – "Inter-Neg". (Remembering the "Negative -> Positive -> Negative -> Positive" chain of production.) The downside to this approach is that we are now two "generations" down from the original Master Negative, thus picture quality is starting to reduce (though, of course that's difficult to tell from a negative!).

Also note the white-coloured border around the image. When the Inter-Negs were made, they didn't wholly capture the entire picture area of the original frame. This is because the 16mm film format - which had been an industry standard since the early 1930s - that was used to shoot the episodes doesn't quite match the traditional 4:3 width:height ratio of television screens.

The Inter-Neg is used to create as many (positive) film prints for television transmission (at last!) and video release (and, between 2004 and 2012, DVD) as desired. The Inter-neg can be used in this way dozens of times but, of course, gradually wears out to a point where the resultant quality of the positive prints becomes unacceptable...

Indeed the example here reflects the muted colours and lack of detail and sharpness that the old prints suffer, having been produced from Inter-Negs that were already heavily worn. At this point the Inter-Neg would normally have been junked and a new one created from the Inter-Positive, as per Stage 6. Unfortunately London Weekend Television chose to keep using the existing Inter-Negs.

Another aspect to be considered here is that of breaks for adverts, which were usually bookended by the short drum roll and brass fanfare that is heard right at the end of the main theme, accompanied by a shot of the 'CI5' logo. These little clips are often referred to as "Interstitials". Although original transmission for the UK had two breaks per episode, overseas broadcasters - and, much later, UK satellite screenings - often required more. So the Transmission Print would have these inserted as per the broadcaster's requirements.

(Alternatively a broadcaster would not request any interstitials but carry out the insertions themselves - possibly using different imagery and/or music - after they had taken delivery of the Transmission Prints.)

The original interstitial - with the "typewriter" font - appears as an extra on Network's first season DVD and Blu-Ray box-sets.

When it came to the audio aspects for Transmission Prints, the broadcaster could simply request that the duplicate Triple-Track (created in Stage 5) is used to create a "mixed-down" monophonic (stereo television for the UK didn't start until the 1990s) magnetic reel where all three audio elements are combined (and thus subsequently inseparable) onto a single track...

Alternatively the Triple-Track duplicate could actually be dubbed straight onto the Transmission Print itself, if it is of a type known as a "Combined Print" or "Optical Sound Print". These possess a narrow, monophonic, magnetic audio track running parallel to the film frames. The advantage of this approach is that there is less physical material for the broadcaster to handle and store. The disadvantage of this media from the audience's perspective is that it offers poorer sound quality than the separate-reel option.

As with the Inter-Neg, the Triple Track duplicate is likely to be used many times and therefore wear out. At that point the original Triple Track can be used to make a new duplicate. Alternatively, to maximise sound quality, a new Triple-Track duplicate can be created from the original, physically separate recording reels. (However in reality this would be unlikely to happen as it would be very costly.)

With the problems indicated earlier, it was vital to minimise any possibility of further damage occurring when the Master Negative was processed by the BBC. Each scene-to-scene join was checked and, where necessary, repaired. Normally the aforementioned sprocket hole problems would need addressing, too, but fortunately the latest technology - covered below - removes the need for this extremely laborious task!

A chemical process was used to carefully clean off as much dust, dirt and grease as possible from the negative's top clear plastic layer.

The process was then followed up with ultrasonic cleaning. This is ingenious in that it sets up sound wave vibrations that cause any remaining surface contaminants to be literally shaken loose without risk of further damage to the film stock.

As discussed earlier, the Master Negatives for all the first season episodes had had the later "Car-Through-Glass" opening and "silhouettes" closing credits sequences grafted onto them. Network was keen to present the episodes with visuals as close as possible to their original 1977/78 transmissions and therefore looked to revive the "Assault Course" and "London cityscape" versions. Luckily the negatives for both of these had survived - the latter being usefully "textless"...

Captions that were unique to each episode - ie Episode Name, Writer, Director and the closing credits for guest actors and crew - all existed as text-only "transparent" film reels and were included in the repair and cleansing processes detailed above, to be handled later.

Although traditional production methods would demand creation of an Intermediate Postive and, subsequently, an Intermediate Negative in order to produce saleable prints, modern technology allows us to short-cut the process. The Master Negative, now repaired as much as safely possible, is carefully spooled up in a "Scanity" unit (manufactured by the Digital Film Technology company) to produce a digital positive "print" onto a computer Hard Disc. The Scanity incorporates a scanner which is capable of taking a High Definition image of each frame on the negative in turn (at around 25 frames per second, which means each fifty-minute episode can be scanned in "real time"). Another advantage of the Scanity is that it doesn't rely on the negative's sprocket holes, which was fortunate for this exercise as so many of them were damaged.

The title sequence and (textless) closing sequence were also inserted at the appropriate points, along with overlaying the aforementioned caption elements appropriate to the particular episode.

At this point, then, we have all the required visual elements assembled to reconstruct the episode.

Photographic and other flaws in the Master Negative that could not be eradicated as part of the cleansing process would, of course, have been carried over to the digital print. The most common problems were:

1. The last few frames of a scene and first few of the following one were discoloured, dirt-ridden and scratched where the aforementioned careless/incompetent use of splicing tape had been used to strengthen the physical join.
2. Debris in the original film cameras ("hair in the gate!").
3. Major defects caused by chemical deterioration of the colour emulsion layers.

The majority of these were handled by software tools, mostly via automated scan-and-repair processes. However the more major ones usually required manual intervention with the software. The example here is taken from 'Long Shot':

"Library" footage such as the Middle Eastern city which opens 'Long Shot' or the crowd riot scenes in 'Look After Annie' presented additional challenges. Although they were, of course, included as part of the High Definition scanning process, they were usually of inferior visual quality in the first place – and the Hi-Def treatment only highlighted the differences between them and the normal episode footage even further. Thus the BBC team applied extra effort to digitally enhance such scenes so that they more closely conformed with surrounding footage. Great care was needed here because too much enhancement would have rendered the picture rather "artificial"-looking. It's always a question of balance between quality and looking natural.

As discussed earlier, in traditional film preparation, the colour grading stage is carried out on the Inter-Positives. However because the new prints had been created from the Master Negatives, this meant that the colour grading had to be done again. A combination of human judgement and automated software process was used on the digital print to apply a consistent set of colours/shades across the episode.

(Actually, even if the Restoration Team had been able to use the (still-missing) Inter-Positives, they would likely have elected to carry out fresh colour grading anyway as modern digital techniques allow it to be achieved to a far higher standard than in the 1970s.)

It wasn't just the picture that Network and the BBC felt needed to be rejuvenated! Thankfully amongst the haul found at the British Film Institute, the aforementioned "Triple-Track" tapes had survived for all first season episodes. As discussed earlier, this medium provides the best of both worlds: all three elements synchronised together to provide the complete audio for the episode that can be played in tandem with the film print – but also the easy possibility to extract them individually. This is why it has been possible to accompany each episode with an "Isolated Music" option on the new DVDs and BluRays.

Digital copies were made of the individual audio elements and then software tools were used to clean them up and carefully enhance them, for example removing the "hiss" that traditionally accompanies old analogue tape recordings. The most obvious improvement over the old prints is the bass part of the sound spectrum, particularly in background sounds: for example when Charley Turkel climbs into the Mercedes in 'Old Dog with New Tricks', the door slams with mighty "thunk"!