1) Field
The invention essentially relates to a method of producing more than two different electric time signals from one first and one second electric time signal. The invention has a particularly advantageous application in the field of sound processing, to transform a stereophonic sound signal and a multi-band sound signal such as, for example, the system referred to as 5.1 which is broadcast using at least five speakers. In an audio phonic system which is broadcasting a 5.1 signal, each speaker is designed to broadcast a sound signal which is different from the other signals being broadcast.
2) Description of Related Developments
In practice, 5.1 signals are generally broadcast by audio phonic systems that are inside a cinema, an apartment or a car. Such systems provide for a listener, situated in the centre of the space which is delimited by the 5 loud-speakers, the sensation of being surrounded by a rich sound which is coming from five different sources. The simultaneous broadcasting of five or six different sound signals, by the same number of independent speakers, conveys a certain surrounding quality to the sound signal.
Alternatively, with a classic stereophonic system, the listener does not have this sensation of being surrounded and of depth of sound. In reality, the listener only has the impression that the sound coming from the loud speakers is being disseminated, since the number of signals and sound sources is generally limited to two in a stereophonic system.
One of the goals for some of the existing methods is therefore to transform stereophonic sound signals into 5.1 sound signals in order to achieve the best possible listening comfort. A 5.1 signal is broadcast by a system comprising at least five speakers: a central speaker, two (left and right) speakers and two rear (left and right) speakers. A sixth speaker can be added to this system in order to handle low frequencies.
In a first approach, to obtain a 5.1 signal from a stereophonic signal, it would be possible to duplicate the two stereophonic signals on the five speakers. However, duplication such as this would not provide the sensation of being surrounded which is sought by a listener. In reality, even if the number of sound sources are multiplied, the number of different signals being broadcast are not, therefore, this richness of sound is not achieved.
In other known methods, the monophonic components of the stereophonic components contained inside the sound signals of a stereophonic system are separated and broadcast using five speakers.
More exactly, in these methods, the monophonic components of the original stereophonic sound signals are detected and the corresponding signal is broadcast using the central speaker. In addition, to produce front sound signals, the monophonic component of the original sound signals is subtracted and the obtained sound signals are broadcast using front speakers. To produce rear sound signals, the components in phase opposition of the original sound signals are detected and the obtained sound signals are broadcast using rear speakers. The phase opposition sound signals give the impression that the sound being broadcast is coming from behind, or that it is further away from the listening point than the other sounds. One of the aims of these methods is therefore to establish good sound discrimination between different sound signals in order that each speaker broadcasts its own particular sound.
To produce these five sound signals, a method is known in which a filter is applied on the stereophonic and electric time sound signals. However, this time processing involves the use of compressors which possess relatively long response times. These long response times cause pumping, that is to say a sharp variation in intensity, in particular on the left and right channels when the central monophonic signal goes from a high level of sound to a low one. The left and right front sound signals include the monophonic component which is greatly reduced when it is loud in the centre and which becomes foremost when reduced in the centre. However, there is a certain inertia between reduction and increase of the monophonic component. This inertia gives the impression of soundlessness at certain times.
Moreover, this process does not make it possible to obtain good rear stereophony. To obtain rear signals, a same electrical sound signal is broadcast on both the rear speakers. The rear signals thus include stereophonic signal components in phase opposition, but which are mutually monophonic.
A method is also known in which one sound signal is more clearly disassociated from another. To this effect, one of the steps of this method is to remove some of the obtained signal components which are below a threshold. This step permits the reduction of a measured discordance between two adjacent speakers. This discordance characterises the separation between two adjacent speakers. However, the pumping effect is still present.
Another method is known for processing sound in which a filter envelope is capable of changing over a period of time. However, this method has a degree of instability. Over a period of time, the sound sources situated around the listener appear to move. With such a method, it is not possible to obtain the same sound effect throughout the duration of the broadcast. This method does not therefore provide a very enjoyable sensation of sound variation for the listener and does not respect the sound effect desired by the creator of the original sound track.
A method is also known in which a strong reverberation is applied to the stereophonic sound signals. This reverberation corresponds with echoes which increase in density. The method therefore provides a sort of virtual surround effect but cannot give the same richness supplied when broadcasting five different sound signals around the listener. All the sound signals have a commonly held modification information. In this method, there is no real discrimination between the information of five sound signals, rather the tonality of musical pieces is adjusted. As a result, the nature of the originally broadcast work is again being altered.
The invention is proposing, in particular, to provide an improvement in the discrimination between different sound signals, at the same time as resolving these problems relating to pumping and to respect for the original work.