SRS is the general name for a series of world-famous audio patented technologies. The full name of SRS is Sound Retrieval System, which means sound reduction system.
The essence of SRS audio technology is based on the principle and mechanism of the human ear perceiving spatial information in acoustic signals. It extracts and restores the spatial information lost during the transmission process of acoustic signals (including compression and decoding). Recreating a true surround sound experience for listeners, as if they were in a live three-dimensional sound field, is called "visible" sound.
Not only that, SRS technology also incorporates the human ear canal effect into the entire auditory system and dynamically processes audio signals based on the human perception model, achieving a real and natural effect that exceeds the physical limits of terminal speaker components. experience.
Its representative technologies SRS TruSurround XT, SRS TruSurround HD, SRS WOW, SRS WOW HD and SRS Circle Surround Headphone are widely used in flat-panel TVs, speakers, media players, personal computers and mobile phones Telephone and other areas.
Working principle of SRS 3D
1. SRS 3D stereo processing
SRS? 3D processing can restore the spatial information lost during sound recording and playback. Its processing uses the characteristics of the human auditory system, because as long as people use their ears to listen to sounds, the sound transmission characteristics of the human ear are a part of the entire sound transmission system that must pass through. At the same time, the SRS 3D system on the one hand avoids the additional accumulation of negatives in the phase-sensitive frequency band, and on the other hand it is effective over a wide range, so that the listener is no longer restricted to the central axis position of the two loudspeakers.
In a stereo signal, the leading edge sound provides equal signal energy in the left and right channels (L, R), thus forming a "sum" signal, the L R signal. In contrast, environmental noise including reflected sound and edge sound will generate a complex sound field, which is unevenly distributed in the left and right channels, forming a "bad" signal, that is, an L-R signal. Although these two signals are generally just "one" composite signal audibly, they can be separated and processed separately, and then mixed into a new composite signal that will include the stereo recording process. Unable to provide spatial information. Since most of the information that provides directionality is contained in the difference signal, they can be processed to restore the contained directional information to its original state. The processed difference signal can also be amplitude enhanced, thereby further increasing the width of the sound image.
The two stereo signals L and R can be decomposed into a combination of sum signal (L R) and difference signal (L-R):
L = 1/2(L R) 1/2( L-R)
R = 1/2(L R) 1/2(R-L)
Among them, R-L is the inverse signal of L-R.
After processing the original stereo signal, the final SRS output is like this:
SRSL = K0L K1 (L R) k2(L-R)p
SRSR = K0R K1(L R) K2(R-L)p
Among them:
K0 is the pass-through gain parameter of L and R;
K1 is the L R gain parameter, also known as the center gain parameter;
K2 is the L-R gain parameter, also known as the spatial gain parameter;
Figure 3 is a curve of the correction transfer function, which can It is used to correctly play back the sound produced by a sound generator (such as a speaker) directly in front of the listener but with the psychological sound source located to the side of the listener.
Figure 3: Correction transfer function that makes the psychological sound source position of the sound emitted by the sound source in front of the listener be located at 90 degrees to the side
The SRS correction curve is based on the correction transfer function in Figure 3 Based on this, other factors are compensated, such as ear canal vibration and stereo bass compensation.
Benefits of SRS 3D
Most differential signals contain rich intermediate frequency information, but they cannot be enhanced indiscriminately because the human ear has a higher sensitivity to intermediate frequencies. The sensitivity will make us always feel stronger mid-frequency sounds. In order to prevent the processed signal from becoming rough and harsh due to excessive mid-frequency, the playback intensity of this part of the sound must be limited, and the high-frequency and low-frequency sounds surrounding this part should be strengthened. The selective enhancement provided by the SRS 3D stereo system effectively controls the spectral content of the processed signal, creating a wider stereo image perception without the harshness and image drift caused by indiscriminate enhancement of poor signals. shortcoming.
In addition to reducing the roughness of the sound, SRS 3D also provides advantages that traditional 3D processing technology does not have. It allows the listener to easily break through the cover of direct sound and perceive a sound field with vivid performance, strong background feeling and real sense of presence. However, in a traditional recording environment, the ambient sound that can contribute to a sense of presence is often covered by direct sound, and people will not be able to perceive the same intensity of ambient sound during playback as compared to the scene. In general, ambient sound tends to be in the frequency band with lower amplitude of the difference signal. Appropriately strengthening this part of the frequency band in the differential signal can liberate the ambient sound from various coverings, thereby simulating the feeling of a real scene.
Selective enhancement of differential signals also provides a wider listening area. The higher intensity part of the difference signal is often concentrated in the intermediate frequency, and the wavelength of the intermediate frequency sound happens to include a length similar to the outline of the human head between the ears. One of the results of SRS selective enhancement is to fully reduce the problem of stereo image drift caused by the indiscriminate enhancement of poor signals, and the best listening position is no longer limited to the center position of the speaker.
Mono to Stereo Synthesis
In addition to creating a 3D sound image in a stereo source, people often need to expand a mono sound source into a wider pan format.
The first step in converting a mono audio signal to 3D sound is to synthesize a stereo signal. This is accomplished by the SRS 3D mono system using a technique that utilizes isophase filters. Two sets of filters are added to the original mono signal, resulting in two channel outputs that are 90 degrees out of phase. This phase shift remains constant in the 100Hz to 20kHz frequency band. According to the priority effect, the human ear will think that the leading signal is direct sound, while the lagging signal is environmental information. Therefore, the pilot signal is similar to the sum signal LR in a standard stereo signal, while the lag signal is similar to the difference signal L-R.
By analogy, the pilot signal and the lag signal are decomposed through traditional addition and difference techniques to synthesize the left and right stereo signals. This stereo signal is then fed into SRS 3D processing as described previously. Because the synthesized L, R, LR and processed L-R signals are generated from a mono signal, the correlation between them remains unchanged, so there is no need to modify the L R and L-R signals ("center" and "center"). Sense of Space") for user control. In addition to the left and right signals, the L R signals are also output for subsequent processing.
Because the 90-degree phase difference between the equal-phase filter output signals is only maintained in the frequency band above 100Hz, while the phases of the output signals begin to converge below 100Hz. In order to solve the problem of bass imbalance and diffusion in adjacent frequency bands, the stereo left and right outputs of SRS 3D are high-pass filtered by a second-order filter, so that the synthesized stereo sound is mainly in the frequency band above about 150Hz. The LR output signal synthesized by SRS 3D becomes a mono bass signal after passing through a low-pass filter, and then undergoes phase conversion to ensure its consistency with the high-pass signal.
Finally, the high-pass filtered left and right composite signals and the low-pass filtered L R bass signals are combined at the output end to generate the final left and right composite signal output. The result is a highly focused bass output.