I was wondering whilst looking at a piece of modern film what all the wiggly lines, strips and "holes" around the film actually did and it got me thinking about sound. A couple of months ago I did a piece about how film is shown and thought "why not do one about sound"?.

First a little history lesson.

Sound in film started almost 25 years before The Jazz Singer was released in 1904, when a Frenchman, Eugene Lauste, recorded sound onto a piece of film

In 1905 he built a complete experimental apparatus for recording and reproducing pictures and sound simultaneously on the same film. The actual recording device, referred to as a 'sound grate', consisted of two slotted iron grids through which a light beam passed on to the film. One of the grids was fixed while the other, under the influence of the field on an electromagnet modulated by the output of a microphone, slid up and down over the surface of the fixed grid. This resulted in a variable density-type of sound track.[ http://www.amps.net/newsletters/issue22/22_lauste.htm]

Then, in 1906, he received a UK patent (number 18057) for this method of recording and reproducing sound. There was a major problem with this method of reproduction, inertia, making it impossible to respond to high frequencies. By 1910 he was at an impasse and his funding had disappeared. A breakthrough came later on in the year. The answer was to use the principles of the string galvanometer. He connected a microphone to a section of silicon bronze wire and played a gramophone record and was amazed at the clarity that was reproduced.

This format for recording looks very strange today with ½ of the film taken up with the soundtrack. This produced an optically synchronised soundtrack 15 years before The Jazz Singer.

The technology used for the sound on The Jazz Singer was "Vitaphone sound-on-disk". Now with modern formats we have gone back to this "sound on disk" format, but using CDs to store the information instead.

Modern film is crowded with information for different formats. Today there are 4 different soundtracks imbedded into one 35mm frame. You can see, therefore, there is not that much space left to get many more formats onto the frame.

Optical soundtrack:

If you look at a modern piece of film you will see on the right hand side (as it goes through the projector) two "wiggly lines". This is the optical soundtrack that is read by a photoreceptor set 20 frames ahead of the image. The reason for this is at this point the film is no longer being jerked through the projector a frame at a time but is now running freely. The first modern optical soundtrack was standardised in 1930 in the area between the sprocket holes and the image itself. This is the oldest of the sound tracks on modern film and as such is compatible with all sound projectors. The modern stereo soundtrack is quite recent, first being used in 1974 with the film Stardust

Magnetic soundtracks (not used today)

In the meantime, between the introduction of an optical soundtrack and modern digital soundtracks, there was a lot of use of magnetic soundtracks. This was thought of as the way forward in the 50s and 60s but, due to the high reproduction costs, equipment costs, maintenance costs, short lifetime and the lack of fidelity, these systems had a short lifetime and the industry went back to the old crackly mono optical soundtrack. It seemed a great idea but when it came to be produced there were too many costly problems.

Dolby Digital

We now have to jump forward 62 years to 1992 when Dolby Digital - also known as Dolby Stereo Digital (SR-D) was introduced. This is an optical digital track that uses a six channel digital optical soundtrack, in addition to a four channel SR (Spectral Recording). This will now produce a true "moving" sound with independently controlled left, right, center, a left and right surround with a 6th channel for bass. This is stored as a matrix between each sprocket hole on the same side as the analogue stereo track

This may leave you thinking "Why do I see some cinemas advertising Dolby SR when you only mention it in passing?"

The answer is quite simple. Dolby SR is a simpler version of Dolby Digital. This system was first used in 1987, with Innerspace and Robocop being some of the first films to be released using this sound format.

DTS

First used in 1993 for Jurassic Park, DTS consists of an optical timecode track on the film linked to a CD player that uses this timecode signal to sync the audio track. This system has the advantage of not suffering from loss of fidelity over the lifetime of the print, as the audio is stored on a media that is not physically touched when read and as such has a far longer lifetime that the actual film itself.

SDDS (Sony Dynamic Digital Sound)

First used in 1993 with The Last Action Hero, SDDS originally stored 6 tracks of digital sound on 2 optical stripes on each edge of the film. This has now been increased to 8 tracks. It encodes the sound in the same format that Sony Minidiscs use, ATRAC,

You may be wondering why THX hasn't been mentioned. Thanks for asking. The simple answer is that it's not a sound system! Rather a way an auditorium is configured to respond in a particular way to the sound. So remember when you go to see your next film in THX that the only way it differs from an auditorium playing in DTS is that the auditorium is configured to the specifications laid down by THX

For the more technically inclined the specifications for the systems mentioned above as are such:

DTS:
DTS has a compression ratio of 4:1, a dynamic range of 96db and a frequency response of 20-20,000Hz; DTS Coherent Acoustic Coding is flexible and can combine lossy compression (data beyond normal hearing range is removed) with lossless compression (data is sampled and restored), capable of a 24-bit linear PCM data rate, although most common is a 20-bit data rate that is higher quality than the 16-bit rate used in compact discs, and at 240 kb/s per discrete channel or 1040 kb for all six channels, it is faster than the 384 kb used by Dolby AC-3.

Dolby Digital
SR-D digital sound film format added 6 digital optical tracks, recorded between sprocket holes, to the 4 existing Dolby SR analog optical tracks on the edges of the film strip - SR-D has a compression ratio of 10:1, a dynamic range 120db, a frequency response of 20-20,000Hz, and a 16-bit data rate of 384 kb.

SDDS
This puts 6 or 8 tracks of digital sound on 2 optical stripes on each edge of the film strip, recorded on the cyan layer beneath the other emulsion layers, using the lossy ATRAC algorithm of the Sony Minidisc technology with a compression ratio of 5:1, dynamic range of 105db and a frequency response from 5-20,000Hz. It is compatible with a backup standard optical soundtrack such as Dolby SR.

The technical data has come direct from the manufacturer's website or other sources on the internet