NAGRA means "will record" in polish (honest ! no pun !!!).

Stéfan Kudelski, born in Poland, moved to Switzerland where he began designing and constructing portable tape recorders with no compromise whatsoever, with the best components available, which is still the company's motto today.

NAGRA I 1951 runs with a mechanical motor
NAGRA II 1957 runs with an electric motor
NAGRA III 1958 will rapidly catch on within the film industry. Thanks to a wire linking it to the camera, it can record sound with the same speed fluctuations as the camera.
But it's in 1962, with the arrival of the
NEOPILOT system, which can be optionaly fitted on the NAGRA III, that production recording can free itself from the necessity of being connected to the camera by a wire.
  1959 First recording of dialogue for film : Marcel Camu's Orfeu Negro
NAGRA IV 1968  
(S & N)
1970 fans of MISSION : IMPOSSIBLE love it. Quartz driven motor, 1/8" tape (the same format as analogue audio cassettes). The N model is mono full track, the S model is stereo.
NAGRA IV.2 1971 three microphone inputs, quartz driven motor.
NAGRA IV.S 1971 the standard of motion picture sound recording for nearly two decades !!! Records in stereo, with two microphone inputs, quartz driven motor. And in the great gadget tradition of Kudelski : go-go gadgetto reel adapter !
  1981 you can now buy a TC card and a centre time-code head block option for your NAGRA IV.S.
NAGRA T 1981 1/4" studio tape recorder, can handle any tape including neo-pilot and central time-code tapes.
NAGRA D 1992 4 channels, 24 bit, digital recorder on 1/4" tape, with rotary heads. An RS422 link can let you optionaly handle transport commands and sound reports data with a PC (the software is named : NADCOM).
ARES-C 1995 16 bit tapeless recorder on Flash-RAM PCMCIA cards. MPEG-2 (Mono and J-Stereo), G722, u-law and A-law reduction algorithms. Option for a built-in ISDN CODEC.
ARES-P 1999 miniature 16 bit tapeless recorder on Flash-RAM PCMCIA cards. Compatible with the ARES-C.
NAGRA D-II 1999 upgrade of the NAGRA D. 24 bit converters, 96 kHz capability (on two channels only). Redesign of the tape path. Upgrade "windows style" of the NADCOM software.

"What is the Neo-Pilot system ?"

The point of the system is to control the playback speed of a tape recorder, according to the inevitable fluctuations that occurred during the recording, thanks to a "pilot-tone". Even though that signal could be recorded on the centre track of a NAGRA IV-S (format named "centre time-code : see further), it's on a full track mono format (such as the NAGRA III or IV and IV-2) that the system is most spectacular by its simplicity.

Mono full track means that the recorder has just one winding and gap on its head block, covering the entire width of the head, therefore creating just one track on the tape, hence the term : full track ("full tape" would have been more appropriate).

During the recording, a quartz oscillating at 50 Hz (or 60 Hz if you're in a country with a 60 Hz mains) inside the machine delivers a stable sine wave, at the same frequency, recorded on the tape by two gaps. One of the windings is in phase while the other is out of phase.

For the playback head, that frequency doesn't exist since the sum of the magnetic fields of the two sine waves cancel themselves in the single gap of the audio playback head.

During a recording, the speed variations of the capstan motor will result in a "stretching" or a "compression" of the pilot tone's wave length, once it is on the tape :

The wavelength on the tape is the reflection of the speed variations of the tape during the recording.

When time has come to play the tape, the capstan speed can be controlled by a comparison between the internal oscillator's frequency and the pilot tone read from the tape's frequency
(only the top gap is used when reading the pilot tone, the purpose of the lower one is to make the pilot-tone's frequency "inaudible" for the audio playback head).

The "comparison system", without going into the details of a PLL circuit, manages to keep these two signals in phase, by speeding-up or slowing down the capstan motor therefore reproducing the speed variations the tape recorder had done when recording. This guaranties that the durations are exactly identical to the moment of the shooting.

"What is a Centre Time Code format ?"

It's a special stereo recording format, on 1/4" tape, with an additional track in the middle of the Left and Right track, hence the name "centre time-code". This third track is used to record sync signals, such as a pilot tone described above, or more recently, an LTC time code.

The NAGRA IV-S can be fitted, optionaly, with a 4 head block, comprising the usual audio erasure/record/playback heads. The fourth head, between the audio record and playback heads is there to record/playback the centre track.

TC head between record & playback heads

Note the thin black line in the centre of the sync head, indicating the position of the centre track.