Sound

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[1]Download a pdf copy of the Sound Format Guide.

Sound Formats Guide

Often learning about all of the different sound formats can prove quite challenging and this is not surprising due to the large amount of information related to each individual one. I have written a guide that covers everything you need to know (with a little bit more information for those more technically inclined). This is by no means a comprehensive guide but should prove adequate for most projectionists at filmsoc. I have italicised information that is not essential for you to memorise but you should be aware of.

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Starting from the left (edge of the film) and working towards the right.

SDDS

Sony Dynamic Digital Sound is the grainy one on the EDGES of the print. It is printed in the deepest layer (the cyan layer which explains why it is a bluey/greeny colour). The reason for it being printed in the cyan layer is to reduce the possibility of scratching i.e. the other layers of colour offer some protection. SDDS runs on both edges of the print therefore providing two steady streams of data and so is less likely to stop being read due to minor damage or scratches unlike Dolby Digital.

SDDS offers up to 8 channels of sound (left, right, centre, bass/sub-woofer, right surround, left surround, left-centre and right-centre) however we only run 6 channels since we do not have left-centre and right-centre speakers.

Pros The sound data runs on both edges so if the SDDS reader has trouble reading one edge it can try reading the other one. A fairly low compression ratio (approx 5:1) which means that less of the original sound recording is lost when it is reconstructed.

Cons SDDS is on the edge of the print and often the edge of the print is more likely to suffer damage and be covered in tippex, dirt etc... The sound data is printed directly onto the film and hence can quite easily be damaged by bad film handling. Cannot mute the sound by pressing mute on the CP500 remote (the grey box that has all of the different sound format buttons on and is next to the changeover controls). Hence if a changeover is missed then you must drop down to SRD and then mute or just press the CD button.

More information I stated above that the sound data runs on both sides of the print, this is technically true but each edge does NOT consist of identical sound data. The edge to the left of the Dolby SRD sound data consists of the following channels: sub-woofer, right-surround, right-centre, right and a mixture of the left channels centre along with a compressed copy of the centre channel. Whereas the other edge consists of centre, left-surround, left-centre, left and a mixture of the right channels along with a compressed copy of the sub-woofer channel. Hence if there is a problem reading one edge of the print the SDDS unit will do its best to reconstruct the channels from the data it has available on the one edge it can read. Therefore there will be a drop in sound quality but this would not be noticeable to the average person’s ears.

Also worth noting is that for additional data security the two sound tracks on the edges of the film are separated by 17 frames. This is so a single splice or series of missing frames will not result in a total loss of data.

Dolby Spectral Recording Digital

Often referred to as SRD (Spectral Recording Digital) or just Dolby Digital. It was first introduced in 1992. This sound format is shown by the small Dolby logos printed between the sprockets/grainy blocks. I shall refer to these as blocks.

Pros The sound data is located in between the sprocket holes and this is generally considered to be a safer location than the edge of the film.

Cons Sound data is printed directly onto the film and hence can quite easily be damaged by bad film handling. The highest compression ratio (approx 12:1) which means that compared to the other digital sound formats it will reconstruct less of the original sound recording when decoded. The dynamic range is the lowest of the digital sound formats meaning that there is less range between the loudest undistorted sound and quietest. Think of this in terms of classical music where you may have a triangle and cymbals in the same piece if the dynamic range is low then you probably won’t hear the triangle being played. Also the cymbals will sound quieter.

More Information Dolby Spectral Recording Digital will fail (drop down to Dolby SR) when the Dolby SRD reader is unable to read 5 consecutive blocks.

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The next three formats are the analogue sound formats and are the trickiest to distinguish between. Could be either Mono, Dolby A or Dolby SR.

Mono

Consists of just one channel (centre) and is often found on films dating before 1975. Mono comes in three forms:

The image above shows the so called variable density form. You are probably used to variable area sound tracks where the sound is represented by squiggly lines getting fatter and thinner. However, it is also possible to represent analogue sound by altering the density of ‘blackness’ and hence controlling how much light gets detected by the sound reader. The variable density format is very uncommon (due to inherent problems with noise that it created). Hence you are far more likely to come across the variable area form.

The two forms above show the variable area approach. Since mono is only one channel it is possible to just have the one variable area ‘strip’ as indicated on the left. However it is also possible to have two IDENTICAL variable area strips, as indicated on the right.

Mono can be distinguished between the other two stereo sound formats (Dolby A and Dolby SR) since the two lines that run down the film are identical. Often they look very similar but any slight discrepancy indicates that it is not Mono.

Finally if a print is re-struck it is possible to have a Dolby Spectral Recording Digital mono track i.e. you still have the mono analogue form but also have a digital track in between the sprockets that accompany it.

Dolby A

Introduced around 1975. Dolby A has two distinct ‘squiggly’ lines but does not consist of just two channels (left and right). Using some clever techniques, the decoding system is able to output a centre and surround channel.

Dolby Spectral Recording (Dolby SR)

Not to be confused with Dolby SRD (Dolby Digital) since it is NOT digital but analogue. Dolby SR improves on Dolby A by having a higher dynamic range. This sound format was introduced around 1986 but some films released after this date were still recorded in Dolby A e.g. Reservoir Dogs was released in 1991 but was only recorded in Dolby A since it was a low budget film. You have to be very careful since you can not distinguish between Dolby A and Dolby SR by looking at it. The golden rule here is that if you are unsure you should play a reel of the film through and select Dolby A and Dolby SR and listen to which sounds better in the auditorium i.e. NOT in the proj box.

Dolby SR was introduced to replace Dolby A since it had better noise reduction (reduced the amount of unwanted signal).

Finally Dolby A tracks will only play in Dolby A, however Dolby SR will play in either (it is backwards compatible) so if in doubt play in Dolby A.

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DTS

Unlike all of the other sound formats which have the sound information printed directly onto the film, DTS (Digital Theatre Systems) has a time code printed on instead. The actual sound is played from a CD and is kept in sync with the picture on screen by use of the time code.

Pros A low compression ratio (approx 4:1). The sound data is not stored on the film but is on a separate CD.

Cons If the CD is badly scratched then it cannot be used. Often the soundtrack CD doesn’t arrive with the film reels but if you make up the film in time then it may be possible for the films officer to obtain one.

More Information Should the DTS time code be unreadable for more than 40 frames then the sound track will drop down to Dolby SR.

Finally

There is no one digital sound format that is vastly superior to another one and most audience members probably would be unable to notice a difference in sound quality between the various ones. Although most projectionists do have a preferable sound format. The only things worth bearing in mind is the condition of each individual sound track for e.g. is one badly scratched or are there copious amounts of ‘gunk’ on the edge of the reel. As well as constantly checking during the film to see if the sound track is failing and changing to another sound format if necessary as not all damage is visible. Apart from that it is entirely up to you.