Electrical audio signal processing systems and devices – Binaural and stereophonic – Pseudo stereophonic
Reexamination Certificate
2001-09-28
2004-08-03
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Pseudo stereophonic
C381S001000, C381S018000, C381S309000
Reexamination Certificate
active
06771778
ABSTRACT:
The present invention relates to a method according to the preamble of the appended claim
1
for converting signals in two-channel stereo format to become suitable to be played back using headphones. The invention also relates to a signal processing device according to the preamble of the appended claim
7
for carrying out said method.
Already for several decades the prevailing format for making music and other audio recordings and public broadcasts has been the well-known two-channel stereo format. The two-channel stereo format consists of two independent tracks or channels; the left (L) and the right channel, which are intended for playback using two separate loudspeaker units. Said channels are mixed and/or recorded and/or otherwise prepared to provide a desired spatial impression to a listener, who is positioned centrally in front of the two loudspeaker units spanning ideally 60 degrees with respect to the listener. When a two-channel stereo recording is listened through the left and right loudspeakers arranged in the above described manner, the listener experiences a spatial impression resembling the original sound scenery. In this spatial impression the listener is able to observe the direction of the different sound sources, and the listener also acquires a sensation of the distance of the different sound sources. In other words, when a two-channel stereo recording is listened, the sound sources seem to be located somewhere in front of the listener and inside the area substantially located between the left and the right loudspeaker unit.
Other audio recording formats are also known, which, instead of only two loudspeaker units, rely on the use of more than two loudspeaker units for the playback. For example, in a four channel stereo system two loudspeaker units are positioned in front of the listener: one to the left and one to the right, and two other loudspeaker units are positioned behind the listener: to the rear left and to the rear right, respectively. This allows to create a more detailed spatial impression of the sound scenery, where the sounds can be heard coming not only somewhere from the area located in front of the listener, but also from behind, or directly from the side of the listener. Such multichannel playback systems are nowadays commonly used for example in movie theatres. Recordings for these multichannel systems can be prepared to have independent tracks for each separate channel, or the information of the channels in addition to a normal two-channel stereo format can also be coded into the left and right channel signals in a two-channel stereo format recording. In the latter case a special decoder is required during the playback to extract the signals for example for the rear left and rear right channels.
Further, some special methods are known in order to prepare recordings, which are specially intended to be listened through headphones. These include, for example, binaural recordings that are made of recording signals corresponding to the pressure signals that would be captured by the eardrums of a human listener in a real listening situation. Such recordings can be made for example by using a dummy-head, which is an artificial head equipped with two microphones replacing the two human ears. When a high-quality binaural recording is listened through headphones, the listener experiences the original, detailed three-dimensional sound image of the recording situation.
The present invention is however mainly related to such two-channel stereo recordings, broadcasts or similar audio material, which have been mixed and/or otherwise prepared to be listened through two loudspeaker units, which said units are intended to be positioned in the previously described manner with respect to the listener. Hereinbelow, the use of the short term “stereo” refers to aforementioned kind of two-channel stereo format, if anything else is not separately mentioned. The listening of audio material in such stereo format through two loudspeakers is hereinbelow shortly referred to as “natural listening”.
During the last decade portable personal stereo devices, such as portable tape- and CD-players, for example, have become increasingly popular. This development has, among other things, strongly increased the use of headphones in the listening of music recordings, radio broadcasts etc. However, the commercially available music recordings and other audio material are almost exclusively in the two-channel stereo format, and thus intended for playback over loudspeakers and not over headphones. Despite of this fact, it is common to the portable stereo devices, and also to other playback systems, that they do not make any attempt to compensate for the fact that stereo recordings are intended for playback over loudspeakers and not over headphones.
When a stereo recording is played back over loudspeakers in a natural listening situation, the sound emitted from the left loudspeaker is heard not only by the listener's left ear but also by the right ear, and correspondingly the sound emitted from the right loudspeaker is heard both by the right and left ear. This condition is of primary importance for the generation of a hearing impression with a correct spatial feeling. In other words, this is important in order to generate a hearing impression in which the sounds seem to originate from a space or stage outside. When listening a stereo recording over headphones, the left channel is heard in the left ear only, and the right channel is heard in the right ear only. This causes the hearing impression to be both unnatural and tiresome to listen to, and the sound scenery or stage is contained entirely inside the listener's head: the sound is not externalised as intended.
Prior art methods, that are intended for improving the sound quality of two-channel stereo recordings when presented over headphones, come mainly in the following two types.
The first type of methods is based on the emulation of a natural listening situation, in which situation the sound would normally be reproduced through loudspeakers. In other words, the stereo signals played back through the headphones are processed in order to create in the listener's ears an impression of the sound coming from a pair of “virtual loudspeakers”, and thus further resembling the listening to the real original sound sources. Methods belonging to this category are referred later in this text as “virtual loudspeaker methods”.
The second type of methods is not based on attempting to create an accurate natural listening or natural sound scenery at all, but they rely on methods such as adding reverberation, boosting certain frequencies, or boosting simply the channel difference signal (L minus R). These methods have been empirically found to somewhat improve the hearing impression. Later in this text methods belonging to this category are referred as “equalizers” or “advanced equalizers”.
In the following, the virtual loudspeaker method and the methods based on different types of equalizers are discussed in somewhat more detail.
If sound is emitted from a loudspeaker positioned for example to the left side of the listener, it is possible to determine the sound pressures created at the listener's left and right ear. Comparing the loudspeaker input signal to the sound pressure signals observed at the listener's left and right ear, it is possible to model the behaviour of the acoustic path that transfers the sound to the listener's ears. When this is performed separately for both the left and right channels, it is further possible to realize signal filters, which can be used to process the loudspeaker input signals according to the behaviour of said acoustic paths. By processing the original signals using such filters, and playing back the filtered signals through headphones, ideally same sound pressures are reproduced at the listener's ears as in the case of listening the original signals through loudspeakers. The above described virtual loudspeaker method is thus, at least in theory, a scientifically justif
Faulk Devona E
Harvey Minsun Oh
Nokia Mobile Phones Ltd.
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