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Basic Information About Audio Technologies

The following chapter explains different concepts of modern audio technology including some technical background. This information is given here because the AudioCutter Cinema supports audio technologies wich are compatible to the well known, today used audio formats:



Dolby-Surround«

The first system and the first system useable and payable for home usage was Dolby-Surround« with matrix encoding. It was followed by Dolby-Surround ProLogic« some years later. This second, now well known system offers 2-dimensional room sound (width-stagger like known from stereo and the additional dimension depth) for home. Nearly all sources delivering stereo sound can be used for it: video- and audio cassettes, stereo telecasts and also a normal computer together with a stereo soundcard and the AudioCutter Cinema.

Multichannel talkies are known since the seventies in cinemas. Together with the introduction of stereo VCR's Dolby« offered his first Dolby-Surround«-decoder in 1982. The first system based on a simple matrix decoding method which was replaced by an integrated solution later: the ProLogic« decoder chip. Together with this better solution the breakthrough of home cinema and surround sound started, because it offered a much better quality for a lower price than the old matrix-method.

The decoding algorithm uses attributes of analogue signals to filter the surround informations out of a stereo signal. All parts which have the same amplitude and the same phase alignment are used for the center channel, parts with the same amplitude but a opponent phase alignment belong to the surround channel and all remaining parts (with different amplitudes but the same phase alignment) belong to the right or left channel.

A problem of this method is: nearly every stereo stream contains information which can be filtered for the rear and center channel also if no surround encoding took place before. Thats why a stereo signal normally is a mixture of much different sound sources - and this "chaotic" mixture will always contain phase shifted parts which can be misunderstand as a surround signal by the decoder.

The AudioCutter Cinema offers a new encoding algorithmus wich is compatible to all existing decoders and which is optimized for digital audio processing. It can be used very easy: First a sound has to be positioned using the Positionseditor at a static position or as a moving sound which changes its position over time. Depending on the selected output and encoding method the program calculates the audio information which belongs to every single speaker in the room and encodes the resulting signal (if soundmode 2.0 Surround was selected). The resulting replayed stereo signal can be decoded using a normal surround decoder.

One important fact are the technical specifications of Dolby-Surround«. Caused by its working principle it is a limited audio system comparing to the other available ones (see following sections). The decoding-mechanism creates up to 4 different channels out of a stereo signal. Beside the known channels left and right a center channel will be added. This channel can be configured in differnt ways: it can monitor the full frequency spectrum (mode "WIDE") or it can work with a limited frequency spectrum (mode "NORMAL") which monitors all frequencies >100 Hz. The working mode should be selected depending on the quality of the used center speaker.
The surround channel is also one single channel only, that means there is absolutely no difference between the decoded channels surround left and surround right. So both rear speakers are connected with the same channel. Depending on the used amplifier some technical tricks may be used to create some acoustic differences between them. An other fact is, the specification for Dolby-Surround« prescribes a frequency range from 100 Hz up to 7 kHz only for this channel.

Dolby-Digital« (=Dolby-AC-3«)

Starting from the Dolby-Surround« technology a better sound system was developed for large cinemas. Until today this new audio system became a new standard: Dolby-AC-3«, also called as Dolby-Digital«. A major difference is the maximum number of available audio channels. AC-3 offers up to 6 discrete channels: the known ones for left, right and center, two surround channels, now splitted into real surround left and surround right and an additional low frequency channel for deep bass effects. These channels are often specified using numbers. 5.1 says, the maximum number of available channels (for the standard AC-3) is used. "5" designates the channels for L, C, R, SL and SR and ".1" is the abbreviation for the effect channel LFE (= low frequency effects).

The fundamental approvements in comparison to Dolby-Surround« are the much better frequency ranges (there are no more limits except in the LFE which is used for frequencies below 120 Hz only) and the real distinguishability between the channels surround left and surround right.

There is an other major difference for AC-3. The number of 6 channels now cannot be encoded into a stereo stream. So this system isn't backward compatible to stereo or mono. The up to 6 channels are compressed using a lossy audio compression algorithm and then packed into a bitstream which contains all neccessary information. These bitstreams can be found at different data carriers like DVD or the original cinema films. The used compression algorithm is similar to the MPEG-compression, it reduces the size while removing parts of the sound which can't be heared. The compression ratio of AC-3 is also selectable and results in bitstreams with a lower or higher required data rate.

dts« - Digital Theater Surround«

Beside AC-3 there exists an other audio system which was developed later and which is more modern therefore. It also doesn't owns the problems of the ProLogic«-system and it offers a higher quality than Dolby Digital. Thats why in cinemas the audio data of it are delivered by a CD and in home applications the audio data are compressed with a lower compression ratio. This audio system is called "dts«" which stands for Digital Theater System.

The other technical features of it are identical with Dolby-Digital«. It offers up to 6 channels (5.1) and knows a difference between surround left and right. One of the first spectacular actions of this format could be seen in movie "Jurassic Park«", which is well known for his fantastic visual and also audio effects

Dolby Surround EX«

This is a further development of the normal Dolby Digital«. "EX" stands for "EXtended Surround" and points to the fact that an additional surround channel is used here. This channel works like an additional center channel but at a rear position. The reason for it is the same like for the front center: positioned (surround) sounds can be heared and located much better also from persons who sit outside of the middle. But in fact this audio system is useable meaningful only in large rooms like cinemas because there is a minimum distance required between listener and the surround center box.

dts ES«

"Digital Theater Surround Extended Surround" is also a 6.1 audio format, which uses an additional surround channel. Beside the rear channel all technical specifications are nearly identical to the "normal" dts«.



THX«

THX« is a shortcut for "Tomlinson Holemans Experience" and is NOT an other audio system but a kind of quality signet from Lucasfilm« Inc. for audio systems. To make it short this quality sign implies for a cinema: you will get a good sound and also a good picture. Beside some other technical facts, the (primary, high-quality-guaranteeing) THX« standard requires following from an audio system:

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