A stereo spreader is a very useful tool when trying to blend sounds of the same or similar frequency content and make them appear as if they where spread through the left and right speakers. The spreader function that is going to be described in this paper will vary a signals time and pitch characteristics resulting in a stereo spread signal, it is going to point the problems the code may have and how it can be improved.Architecture & Allied Art

Fernandez, Arturo

English

Sydney eScholarship

STEREO SPREADER REVIEWArturo Fernandez 420164911Digital Audio Systems, DESC9115, Semester 1 2013Post Graduate Program in Audio and AcousticsFaculty of Architecture, Design and Planning, The University of SydneyABSTRACTA stereo spreader is  a very  useful  tool  when trying to blend sounds  of the same or similar frequency content and make them appear as if they where spread through the left and right speakers. The spreader function that  is going to be described in this  paper will vary a signals  time and pitch  characteristics resulting in a stereo spread signal, it  is going to  point the problems the code may have and how it can be improved.1. INTRODUCTIONA stereo spreader effect also known as stereo widener make the stereo image of a sound appear wider within two speakers, therefore giving it more space within a mix of different sounds. This paper is going to  explain the approach given to  this effect and how it can be implemented, analyse the given results and suggest starting values for the possible user. Another algorithm will  be analysed  taking consideration of possible further improvements. 2. STEREO SPREADERThe input values  of this  function are delay L (ms), delay R (ms), pitch L, pitch R, and mix. A block diagram is provided so that the basic signal flow of the effect can be explained: Fig. 1 Block Diagram of spread functionAs shown in the diagram, the signal to be processed needs to be mono. This signal  will be sent to the left and right channels assigning  the specified delay  to  it. Then, it will  go  to  the pitch shift  and processed with the input values, then the dry mono signal will be processed  through the mixer and this will determine the amount of dry signal to be summed with the left and right  channels, the output signal  will  be a stereo file. This effect was tested  with different audio signals, a 1kHz sine wave, an acoustic guitar, vocal track, saxophone and a vocal track recorded through a telephone. The conclusions drawn to that was that in  the case of harmonic instruments, the input values for the function would need to remain relatively  low, as the pitch and time differences from the original signal and the processed one would start creating phasing problems, not necessarily canceling the signal, but would create comb filtering as well as  perceptual differences from where the sound was coming from, in the specific case of the acoustic guitar, the sound would feel  like it  was jumping from one speaker to the other, and never really settle in a specific space over time. In the other hand, melodic instruments  have higher tolerance for high values and when the widening effect  threshold is breached, the effect acquires harmonising qualities, which can  be used to create vocal harmonies, of even making an instrument deeper, for example a snare that cannot cut through a mix can be processed in a way that there is a slap-back delay with a lower tuned snare. Some of the starting values  to get good results out of the effect might  be setting  the left and right  delays somewhere around 15 and 30 milliseconds, then for the pitch values a good start would be -2 cents all the way up to -8 cents or +2 to +8, if the result of this is to fatten the sound then those values would be fine, otherwise calculating the actual pitch of the song in relation to the pitch shift could be used to create some sort of harmonic coherence.3. CONSIDERATIONS FOR FURTHER IMPROVEMENT OF THE CODEAccording to  the analysis done in the audio files, some important considerations need to be done with this effect. As mentioned before, depending on the desired product the effect’s threshold varies, and basically works better in  small values for some sounds and works well in higher values for different sounds. Considering the effects this effect has on the phase alignment of the audio file, and how this changes the perception dramatically then I decided to analyse a similar effect and point out some of the improvements based on this other algorithm.3.1 Virtual Sound Algorithm for Wide Stereo Sound StageThe output of this effect is  the widening of a stereo signal for use in two dimensional layouts. The main design implements three different FIR Filters; binaural synthesis through FIR filter convolution, crosstalk canceler in a form of high-order IIR Filter to maximise cancelation performance and a direct filter. The output of this algorithm is a 2x2 matrix(stereo signal).To preserve timbre and centre image, the original  signal is played back through the direct filter, this  filter matches  the time delay and output  level. All three filters  are later simplified into a low order FIR filter (panorama filter) in a 2 by 2 FIR filter matrix giving easy computation and a simple structure. (Kim, S. 2004.)Following  is an explanation of how each filter is structured, the math behind it and how they all sum together. Binaural Synthesis:HRTF's are convolved with the input audio data recreating a virtual sound image. This  filter would result  in two widely equally spread sources in front of the listener, left channel  of binaural signal is synthesised and  sent to the left  speaker, and the process is repeated for the right speaker. This is represented by the equation in fig. 1LL(z) RL(z)LR(z) RR(z)⎡⎣⎢⎤⎦⎥=LLii=1N∑ (z) RLi(z)i=1N∑LRi(z)i=1N∑ RRi(z)i=1N∑⎡⎣⎢⎢⎢⎢⎤⎦⎥⎥⎥⎥Fig. 1 Binaural Synthesis Mathematical RepresentationLLi(z)  and LRi(z)  represent  the HRTF's from the designated virtual source on the left side to left and right ear. To simplify the computation, instead of doing different  processes (four filters and four convolutions), the layout is built within  a 2 by 2 matrix which will  distribute the values in a more efficient way. (Kim, S. 2004.)Crosstalk Canceler:The widening effect  is played at its full  capabilities when using headphones, in the downside if the signal is  played back through loudspeakers, the crosstalk phenomena changes  the results. The crosstalk phenomena refers to the audio signals coming from the left  speaker delaying in the listener’s  head and arriving late to the right ear, this happens equally  to the left ear. This late signals give spatial cues to the main signal, therefore giving  a false representation  of what the resulting sound should be. In order to solve the crosstalk phenomena a crosstalk cancellation filter is applied to the algorithm; C(z) ≅ − i=1R∑ cibik z−(δ − k)k=1δ∑Dmin(z)H22(z) −H12(z)−H21(z)   H11(z)⎡⎣⎢⎤⎦⎥Fig.2 Crosstalk Canceller equationThe design of this function will be explained next. The direct inversion method is used. This method is represented  by the following matrix:C(z) = 1D(z)H22(z)   -H12(z)−H21(z)   H11(z)⎡⎣⎢⎤⎦⎥Fig. 3 Cross-cancellation matrixIn this matrix, the direct sound coming from the left and right speaker is represented by H11(z)  and H22(z)  accordingly. D(z)  would be the determinant (fig.3), it defines what signal is being cancelled in the left and right sources. D(z) = H11(z)H22(z)−H12(z)H21(z)Fig. 4 DeterminantIn order to simplify the inverse process, the determinant is separated into delay by using the root-finding algorithm and extracting the maximum phase subsystem(Dmax(z) ), doing this by  comparing the absolute values of the roots to 1. (Kim, S. 2004.)Direct Filter DesignThis filter helps the original signal preserve the timbre and centre channel image. This filter is represented by the following equation:A(z) = az −αFig. 5 Direct FilterThe complex variable is  represented by z , α  is the time delay and a  the output level.After all the filters are in place there is a minimisation technique used to  convert the three different filters  into low order FIR filters. What  this minimisation technique does is adding the binaural  synthesis and the crosstalk canceller creating a widening filter (Fig.6) and then adding this filter with the direct filter creating a "Panorama Filter" (Fig. 7) (Kim, S. 2004.)W(z) = C11(z)  C12(z)C21(z)  C22(z)⎡⎣⎢⎤⎦⎥LL(z)  RL(z)LR(z)  RR(z)⎡⎣⎢⎤⎦⎥Fig.6 Widening FilterP(z) = W11(z)+ A(z)        W12(z)W21(z)                W22(z)+ A(z)⎡⎣⎢⎤⎦⎥Fig. 7 Panorama FilterThe output stereo signal is represented by yL  and yR :yL = P11(z)L + P12(z)RyR = P21(z)L + P22(z)RFig. 8 Output stereo signal4. DISCUSSIONThe only main issue in  the original spreader function is the phase coherence. The other differences such as the pitch shift for harmonies is  a good parameter to have so that the effect  can have drastic changes but at  the same time keep phase coherence. In order to achieve this, a good implementation of the “Virtual Sound Algorithm” is  to use the direct filter in the original design of the stereo spread function, this way allowing the audio  to be have the phase correction needed and avoiding the unusual discrepancies in  the spatial  placement of sounds caused by this.5. CONCLUSIONGiven the results  of the first spreader function, a straightforward way of achieving the effect desired is clearly pointed out in the discussion, by  adding the direct filter to the original design of this  function. This direct  filter gives stability  to the phase coherence of the code, but at the same time keeping the ability to  acquire the harmonic qualities to create different layers of pitched up/down sounds.6. REFERENCES & BIBLIOGRAPHYKim, S. 2004. Virtual Sound Algorithm for Wide Stereo Sound Stage. In Audio Engineering Society. San Francisco, CA, October 28-31, 2004. New York: JAES. 1-6.Smith, S. 2013. Stuart Smith . [ONLINE] Available at:http://www.cs.uml.edu/~stu/. [Accessed 27 May 2013].Zölzer, U, 2011. DAFX: Digital Audio Effects. 2nd ed. West Sussex, United Kingdom: Wiley & Sons.

Stereo spreader REVIEW

https://ses.library.usyd.edu.au/bitstream/2123/9184/2/Review%202%20Arturo%20Fernandez.pdf

Stereo spreader REVIEW

Abstract

Similar works

Full text

Available Versions

Sydney eScholarship