PERFORMANCE EVALUATION OF EVS AUDIO CODER FOR ORIANTAL AND ORCHESTRAL MUSICAL INSTRUMENTS Yasser A. Zenhoma, Micheal N. Mikhaelb, Eman Mohammed c, Hala A. Mansourd a b
Modern Academy for Engineering and Technology, Cairo,
[email protected]
Benha University, Faculty of Engineering at Shoubra, Cairo,
[email protected] c Modern Academy for Engineering and Technology, Cairo,
[email protected] d Benha University, Faculty of Engineering at Shoubra, Cairo,
[email protected]
ABSTRACT A new 3GPP audio coder, this coder called Enhanced Voice Services (EVS), which is a high definition full band audio coder (HD), that provides new features for improving the real-time audio communication systems. The ordinary speech coders operate in the time domain. To improve the quality of the audio encoders, these encoders operate in the frequency domain. EVS introduces a new novel switching between speech and audio coder without adding any artificial errors and with negligible delay. EVS produces highly encoding quality for speech, music, and mixed content. The EVS audio quality has not been evaluated yet for oriental and orchestral musical instruments. This paper evaluates the EVS audio coder performance for oriental and orchestral musical instruments. Extensive testing done over 10 musical instruments and more than 80 audio recorded signals at different output bitrates. The evaluation was done by both subjective (i.e. P.800 MOS) with 22 naïve listeners and objective evaluation technique (Perceptual Evaluation of Audio Quality PEAQ algorithm). Both of MOS and PEAQ results shown that the average quality of the EVS audio coder for orchestral musical instruments is slightly better than the quality for oriental musical instruments at different output bitrates. Keywords: EVS, VOLTE, Perceptual Audio Coder, MOS, PEAQ, Oriental Music.
1. INTRODUCTION As known that the channel resources are limited in terms of its bandwidth and the corresponding quality of the reconstructed speech or audio signals for any modern telecommunication system [1], [2]. Third Generation Partnership Project 3GPP organization and other nonprofit organization have been encouraging the development of speech and audio coders, to provide higher speech and audio quality with low bitrate [3]. Unfortunately, the previous coders (i.e. before EVS) cannot produce a nearly transparent quality for both audio and speech signals with lower bitrates. 3GPP introduced TS 26.441 Enhanced Voice Services (EVS) speech and audio coder. This coder provides low complexity with low delay (for encoding and decoding audio and speech signals), robustness against different speaker Languages and robustness against channel frame losses, this coder used in a mobile telecommunication network (i.e. LTE) [4] [5].
Martin Dietz, Markus Multrus, and et al have evaluated the speech quality performance of the EVS coder for the English language at different output bitrates. They found out that EVS improves the quality of the encoded speech signals [5]. Anssi Rämö and Henri Toukomaa have evaluated the speech quality performance of EVS for the Finnish language at different output bitrates. They found out that EVS provides higher speech quality at the lower bitrates Compared to previous coder types[6]. Christina Gamal, Michael N. Mikhael, and Hala A. Mansour have evaluated the quality performance of EVS against different speech-language at different output bitrates. They found out that EVS provides more stable performance for American English over the Arabic Language [7]. M. Maruschke, O. Jokisch, M. Meszaros, F. Trojahn, and M. Hoffmann have evaluated the quality performance of EVS audio coder for violin musical instrument and pop music at different output bitrates. They found out that EVS introduces good quality for input musical signals [8]. Guillaume Fuchs and et al have evaluated the quality performance of the EVS audio coder, the coder operates at WB and SWB, this test was done for musical and mixed content input audio signals at different output bitrates. They found out that the EVS improves the quality performance of the decoded music signals with low-delay encoding time comparable to different encoder types at low output bitrates [9]. This paper aims to evaluate the quality performance of EVS audio coder, this evaluation for different oriental and orchestral musical instruments at different output bitrates. The EVS in this evaluation operates at full band (FB) encoding mode at different output bitrates. The input signals to EVS coder were clean musical instrument records, these records sampled with rate 48 K samples per sec, the number of musical records equals nine records for every musical instrument each with 9 sec length. This paper organized being with background in section 2, EVS musical coder evaluation results in section 3 and the paper conclusion in part 4. 2. BACKGROUND 2.1.EVS Overview The study and the design of EVS audio coder were started in 2007, these design by many international organizations, while the first emergence of this audio coder was in September 2013 [6]. EVS operates with different input signal bandwidth, also at different encoding modes as illustrated in the following table 1 [5]. Table 1: Different encoding modes of EVS audio coder.
Encoding mode
Input signal BW
sampling rate
Output bitrate
NB WB
20 Hz to 4 KHz 20 Hz to 8 KHz
8000 sample per sec 16000 sample per sec
7.2 to 24.4 Kbps 7.2 to 128 Kbps
SWB FB
20 Hz to 16 KHz 32000 sample per sec 20 Hz to 20 KHz 48000 sample per sec
9.6 to 128 Kbps 16.4 to128 Kbps
Figure 1 presents the significant main building blocks of the EVS coder. The preprocessing stage is using to help the EVS encoder to select the best encoding mode and encoder scheme for efficient encoding performance [5] [10]. This step includes high pass filtering, resampling operation, pre-emphasis filtering, spectral analysis, bandwidth detection, time domain transient detection and windowing reshaping.
Figure 1: Main blocks of EVS Encoder.
Resampling is used to convert the input audio sampling frequency to the operating sampling rate of the selected encoder scheme. Pre-emphasis filter is first order high pass filter as described in Equation (1), where pre(z) is the pre-emphasis filter output in z transform, μ is a pre-emphasis filter coefficient, where μ=0.68, 0.72 and 0.9 in case of EVS operates with an internal sampling rate of 12.8, 16 and sampling rate higher than 16 KHz [10]. This filter is used for emphasizing the higher frequencies of the input signal magnitude among the low magnitude frequencies. This filter improves the overall signal-to-noise ratio. p𝑟𝑒(z) = 1 − μz −1
(1)
The sudden changes in the input audio signals are called transient, the Timedomain transient detection on of the most significant operation used to detect these transient signals. The windowing shape of the EVS coder changes according to the selected encoder scheme (i.e. symmetric and asymmetric) [5]. Also, the windowing size changes from long to short windowing size in case of input transient audio
signals, the changing in the windowing size and shape for using the benefits of time-domain aliasing cancellation (TDAC) [10] as shown in Figure 2.
Figure 2. The changing of windowing size for efficient encoding of the transient frames.
The various encoding schemes of EVS shown in Figure 1, Linear Prediction (LP) encoder with Bandwidth Extension (BWE) scheme, this encoder scheme to encode the input speech signal. Frequency domain scheme to encode the music and mixed content input audio signals. The EVS using Modified discrete cosine transform (MDCT) as a time to frequency domain transformation to increase the efficiency of the frequency domain encoder scheme as well as maximize encoding gain. EVS includes interoperable encoder scheme with AMR-WB, the output bitrates of EVS in this encoding scheme are (6.6, 8.85, 12.65, 14.25, 15.48, 18.25, 19.85, 23.05 and 23.85) kbps [4]. 2.2. ENCODING TECHNIQUE FOR MUSICAL SIGNALS In the frequency domain encoder scheme of the EVS audio coder includes two encoding techniques (i.e. MDCT) based Transform Coded Excitation (TCX) and High-Quality MDCT (HQ-MDCT). The selecting between these two encoding techniques depends on the input signal type, input signal bandwidth and output bitrate as illustrated in table 2. The MDCT Transform represented by equations (2), where 𝑥𝑤 is the input samples after windowing operation that have 2n numbers, 𝑥𝑐 is the output coefficients that have h= 0, …,n-1numbers. The inverse MDCT represented by Equation (3), 𝑥̃ 𝑤 is the reconstructed sampled signal and k=0,…., 2n-1 as the reconstructed samples [11] [12]. 𝑘=2𝑛−1
𝑥𝑐 = ∑ 𝑥𝑤 cos[ 𝜋 𝑘=0
(𝑘 + (𝑛 + 1)/2)(ℎ + 1⁄2) ] 𝑛
(2)
ℎ=𝑛−1
𝑥̃ 𝑤 =
2 (𝑘 + (𝑛 + 1)/2)(ℎ + 1⁄2) ] ∑ 𝑥𝑐 cos[ 𝜋 𝑛 𝑛
(3)
ℎ=0
Table 2: Different encoding schemes for different input signal.
2.3. MUSICAL INSTRUMENTS CLASSIFICATIONS The human audible system receives speech, music, mixed content or any frequencies from 20 Hz to 20 KHz. Each musical instrument has fundamental frequency and harmonic frequencies. These fundamental frequencies range listed in table 3 [13]. Table 3: Fundamental frequency ranges classification
Frequency Region Bass Region Mid-Bass Region Midrange Region Upper Midrange Region First High End Region
Frequency Range 20 Hz to 140 Hz 140Hz to 400Hz 400 Hz to 2.6 kHz 2.6 kHz to 5.2 kHz 5.2 kHz to 12 kHz
second High End Region
12 kHz to 20 kHz
The musical instruments classification of according to the material of the musical instrument, like brass instruments (Wind or Woodwind Instruments), String Instruments, Percussion Instruments, Keyboard Instruments. One more classification method is based on the culture of the region in which musical instruments were born, such as the musical instruments used in the Middle East countries (i.e. Egypt, Jordan, Yemen, Saudi Arabia…etc.), they classified as oriental musical instruments. The musical instruments using in western countries (i.e. USA, Germany, Greece, China…. etc.), they classified as orchestral musical instruments. The oriental musical instruments like lute, simsimiya, rabab, kanun and Kemence, these instruments are string oriental musical instruments. There are other oriental woodwind musical instruments such as kaval, nay, and zurna. Also, there are another oriental Percussion musical instruments such as Tabla, Daf, and Tambourine. The orchestral musical instruments like violin, guitar, cello, harp, and viola, are string orchestral musical instruments. There are another orchestral woodwind musical instruments such as cornet, French horn, oboe, Shakuhachi flute, pan flute, and recorder. There are another orchestral Percussion musical instruments such as snare drum, hang, and bass drum. Moreover, the organ and piano classified as orchestral keyboard musical instruments [14]. 3. EVS MUSICAL CODER EVALUATION RESULTS 3.1. Evaluation Procedures The performance of the EVS audio coder evaluates for many different oriental and orchestral musical instruments shown in table 4. The clean record signals (i.e. noiseless) used for the evaluation process. These musical records are nine records for every musical instrument with 9 sec length for each, these records sampled at 48 Kbps. Table [4]: Musical instruments under test. Instrument name Instrument type Music type Fundamental frequencies Piano 28 Hz to 4.1 KHz Keyboard Orchestral Organ 16 Hz to 7.04 KHz Tabla Oriental 65 Hz to 1.2 KHz Percussion Snare Drum Orchestral 1 KHz to 2 KHz French Horn Woodwind Orchestral 110 Hz to 880 Hz
Kaval Saxophone Oboe Violin Lute
Oriental Orchestral Orchestral Orchestral Oriental
String
200 Hz to 4.5 KHz 55 Hz to 1KHz 250 Hz to 1.5KHz 200 Hz to 3.5KHz 100 Hz to 2.6 KHz
EVS encoder operates in this evaluation at full band encoding mode, the output bitstream fed to the EVS decoder to reconstruct the original signals. These reconstructed signals finally applied to evaluation techniques (i.e. MOS and PEAQ). The subjective quality evaluation method depends on human opinion for the reconstructed signal, while the objective quality evaluation method (i.e. PEAQ) depends on the software algorithm for easy evaluation of the quality and to avoid time wasting. In this paper as illustrated in Figure 3, the subjective quality evaluation achieved by 22 naïve listeners, included 3 expert listeners. This evaluation results using (ITU) recommendation, P.800 the mean opinion score (MOS), with an Absolute Category Rating (ACR) methodology. This evaluation rate from 1 to 5, where 1 is very annoying, 2 is annoying, 3 is slightly annoying, 4 is perceptible, but not annoying and 5 is imperceptible [15]. The objective quality evaluation (i.e. PEAQ) algorithm, this algorithm depends on the human auditory system. This evaluation result is the Objective Difference Grade (ODG), the evaluation rate from 0 to -4, where -4 is very annoying, -3 is annoying, -2 is slightly annoying, -1 is perceptible, but not annoying and 0 is imperceptible [16]. Reference Musical record
EVS Encoder operates at FB encode mode
Ideal
---------Channel
EVS decoder
Reconstructed musical record
Listening test depend on MOS (ACR) rate from 1 to 5 PEAQ algorithm rate from -4 to 0
Figure.3: Experiment procedure for Subjective and Objective Evaluation test.
The ODG results from PEAQ calculate by comparing the reconstructed signal of the audio coder to the original signal. 3.2. MOS (ACR) Evaluation Technique Results: EVS was evaluated only for orchestral violin musical instrument as described in [8]. The evaluation results were compared with this research results. The comparison clarifies the results similarity, therefore the similarity ensures the procedure that used in this research operate in the right direction. The Subjective
Evaluation results illustrated in Figure 4, show that there are slightly better quality results for the Violin orchestral musical instruments over the Lute oriental musical instruments for all output bitrates. As the more, the bit rate increased the more the quality of both instruments becomes better which is the logical result.
MOS (ACR)
5 4 3
2.33
2.12
2.73 2.38
3.73
3.3
3.23 3.08
4.33
4.00
3.80
3.99
3.6
3.28
2 1 0 16.4
24.4
32
48
64
96
128
Output Bitrate in Kbps Violin
Lute
48
3.9 3.9 3.8 4.1
32
3.8 3.7 3.7 3.8
3.3 3.5 3.5 3.4
24.4
3.6 3.6 3.8 3.6
3.2 3.1 3.3 3.1
16.4
2.8 2.9 3.1 3.0
2.6 2.9 2.7
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
2.6
MOS(ACR)
Figure.4: ACR evaluation results for string musical instruments at different output bitrates.
64
96
128
Output bitrates in Kbps French Horn
Kaval
Saxophone
Oboe
Figure.5: ACR evaluation results for woodwind musical instruments at different output bitrates.
The MOS results for woodwind musical instruments illustrated in Figure 5, show that, for different output bitrates except 48, 96, 128 Kbps output bitrates, the orchestral saxophone musical instrument has a higher evaluation quality than the other musical instruments. At 48 and 96 Kbps output bitrates, the saxophone musical instrument and oriental kaval musical instrument have encoding quality equal to each other. However at 128 Kbps output bitrates, the saxophone has the least encoding quality. At 96 and 128 Kbps output bitrates, the orchestral oboe
MOS(ACR)
musical instrument has a higher encoding quality than the other musical instruments. At 16.4, 24.4 and 48 Kbps output bitrates, the orchestral French horn musical instrument has the least encoding than the other musical instruments. 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
2.7 2.8
16.4
3.1 2.9
24.4
3.6 3.6
3.2 3.3
32
3.8 4.0
3.8 4.0
4.0 4.1
64
96
128
48
Output bitrates in Kbps Tabla
Snare Drum
Figure.6: ACR evaluation results for percussion musical instruments at different output bitrates.
The Subjective Evaluation results for Percussion musical instruments illustrated in Figure 6. Show that, for different output bitrates except at 24.4 and 48 Kbps output bitrates, the orchestral snare drum musical instrument has a higher evaluation quality than the oriental Tabla musical instrument. At 24.4 Kbps output bitrate, the Tabla musical instrument has higher encoding quality than the snare drum musical instrument. But at 48 Kbps output bitrate, the musical instruments equal to each other in the encoding quality. 5
MOS(ACR)
3
4.0
3.8
4 2.5 2.6
2.8 2.8
3.4
3.1 3.2
4.1 3.6
3.9
4.3 4.1
2 1 0 16.4
24.4
32
48
64
96
128
Output bitrates in Kbps Piano
Organ
Figure.7: ACR evaluation results for Keyboard musical instruments at different output bitrates.
In this paper, the orchestral organ musical instrument used to play oriental melodies, these melodies able to use them as oriental musical records. The MOS
Evaluation results for two Keyboard musical instruments illustrated in Figure 7. Show that at 16.4, 24.4 and 32 Kbps output bitrates, the encoding quality for organ musical instrument is higher than the encoding quality for the piano musical instrument. But in the other output bitrates, the piano musical instrument has the higher encoding quality. As the bit rate increased the quality of both instruments becomes better. 3.3. PEAQ Evaluation Results Analysis:
ODG
16.4
0 -0.5 -1 -1.5 -2 -2.5 -3 -3.5 -4
24.4
32
48
64
96
-0.7
-0.4
128 -0.4
-0.1
-1.6 -1.6 -2.5 -2.4
-3.6 -3.6
-3.5 -3.4
-3.3 -3.1
Output bitrates in Kbps Violin
Lute
Figure.8: ODG evaluation results for string musical instruments at different output bitrates.
The Objective Evaluation results (i.e. PEAQ algorithm) for string musical instruments illustrated in Figure 8. Shown that, at 16.4 and 64 Kbps output bitrates, the lute and violin musical instruments nearly have the same encoding quality. But for the other remaining output bitrates, the lute musical instrument has lower ODG evaluation values than the violin musical instrument, that means the lute musical instrument has higher encoding quality than the violin musical instrument.
-1.7 -2.8
-3.2 -3.3
-2.8 -3.2
French Horn
-0.5 -0.2 -0.4
128
-0.6
96
-0.8 -1.1 -0.5 -0.9
64
-2.1 -1.6 -1.4
48
-2.2 -2.4
32
-2.1
24.4
-3.1 -3.5 -3.3 -3.5
-3.6 -3.7
-3.3 -3.6
ODG
16.4
0 -0.5 -1 -1.5 -2 -2.5 -3 -3.5 -4
Output bitrates in Kbps Kaval
Saxophone
Oboe
Figure.9: ODG evaluation results for woodwind musical instruments at different output bitrates.
The Objective Evaluation results for woodwind musical instruments illustrated in Figure 9. Shown that, at 16.4, 24.4, 32 and 48 Kbps output bitrates, the French horn musical instrument has lower ODG than the other musical instruments. At 64 Kbps output bitrate, the oboe musical instrument has lower ODG than the other musical instruments. But at 96 and 128 Kbps output bitrates, the saxophone musical instrument has lower ODG than the other musical instruments. At 16.4 and 32 Kbps output bitrates, the kaval and saxophone musical instruments have the same ODG evaluation values. Also at 24.4 Kbps output bitrate, the kaval and oboe musical instruments have the same ODG evaluation values. But at 48, 64 and 96 Kbps output bitrates, the kaval musical instrument has higher ODG than the other musical instruments, that means the kaval musical instrument has lower encoding quality than the other musical instruments. 16.4
24.4
32
48
64
96
128
0 -0.5
-0.4 -0.3
-0.3 -0.2
ODG
-1 -1.5 -2
-2.1 -1.9
-2.5 -2.6
-3
-3.0
-3.5 -4
-1.8 -1.6
-3.6
-3.3
-3.0
-3.4
Output bitrates in Kbps Tabla
Snare Drum
Figure.10: ODG evaluation results for percussion musical instruments at different output bitrates.
The PEAQ Evaluation results for percussion musical instruments illustrated in Figure 10. Show that, at different output bitrates, the snare drum musical instrument has lower ODG than the Tabla musical instrument. But at 96 and 128 Kbps output bitrates, there was a slight difference between these two musical instruments.
ODG
16.4
0 -0.5 -1 -1.5 -2 -2.5 -3 -3.5 -4
24.4
32
48
64
96
-0.6 -0.9 -1.8
128
-0.7
-0.5
-0.6
-1.8
-2.3 -2.9
-2.8
-3.3 -3.7
-3.2
-3.5
Output bitrates in Kbps Piano
Organ
Figure 11: ODG evaluation results for keyboard musical instruments at different output bitrates.
The Objective Evaluation results for two keyboard musical instruments illustrated in Figure 11. Shown that at different output bitrates, the organ musical instrument has lower ODG than the piano musical instrument. But for the highest two output bitrates 96 and 128 Kbps, the difference between the qualities of these two musical instruments is very small. Also as the more, the output bitrates increases as the more the quality of these musical instruments increase.
ACR
3.4. Average Overall MOS Results for Oriental and Orchestral Musical Instruments 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0
16.4 Average of orchestral 2.630455 instruments Average of oriental 2.499817 instruments
24.4
32
48
64
96
128
2.902306
3.189305
3.572167
3.771133
3.905333
4.0886
2.781683
3.149027
3.44828
3.555757
3.726483
3.989817
Output bitrates in Kbps Average of orchestral instruments
Average of oriental instruments
Figure 12: The ACR average evaluation results for oriental and orchestral musical instruments.
Figure 12 represents the average subjective evaluation results for oriental musical instruments and orchestral musical instruments at different output bitrates. The results show that, at different output bitrates, the EVS ables to encode the orchestral and oriental musical instruments with highly encoding quality. The EVS quality performance for orchestral musical instruments is slightly better than the quality performance for oriental musical instruments. 3.5. Average PEAQ Results for Oriental and Orchestral Musical Instruments Figure 13 represents the average objective evaluation results for oriental musical instruments and orchestral musical instruments at different output bitrates. The results show that at different output bitrates except for 128 Kbps, the objective evaluation results introduce the same evaluation values of subjective evaluation results. At 128 Kbps output bitrate, the quality performance for orchestral musical instruments is lower than the quality performance for oriental musical instruments. 0 -0.5 -1
ODG
-1.5 -2 -2.5 -3
-3.5 -4 Average of orchestral instruments Average of oriental instruments
16.4
24.4
32
48
64
96
128
-3.46011
-3.22259
-2.89779
-2.13845
-1.48513
-0.63853
-0.38863
-3.59815
-3.4301
-3.10568
-2.46397
-1.835
-0.63221
-0.30713
Output bitrates in Kbps Average of orchestral instruments
Average of oriental instruments
Figure 13: The ODG average evaluation results for oriental and orchestral musical instruments.
At 128 Kbps output bitrate, the difference between the quality of orchestral and oriental musical instruments nearly equal 0.8. 3.6 Voiceprint Representation A voiceprint or spectrogram is a visual description for the frequency domain varies with time domain. The spectrogram by the matlab software for one melody of orchestral violin musical instrument signal and the EVS encoded signals for this melody, at different output bitrates are illustrated in Figure 14. The spectrogram by
the matlab software for one melody of oriental lute musical instrument signal and the EVS encoded signals, at different output bitrates are illustrated in Figure 15. These two spectrograms describe that the EVS audio coder able to encode different string musical instruments. Also, EVS provides a higher encoding quality for the violin musical instrument melody with medium output bitrate such as 48 Kbps. However, the highest quality for the lute melody introduced with higher output bitrates. EVS encoded signals at 96 and 128 Kbps output bitrates and the original violin and lute records nearly have the same frequency resolution which reflects the fact that, as the more the output bitrate increases, the frequency resolution increases but the noise decreases.
Figure 14: Spectrogram representation of violin signal and EVS encoded signals at different output bitrates.
Figure 15: Spectrogram representation of lute signal and EVS encoded signals at different output bitrates.
As shown from the spectrogram of lute musical instrument, the lute musical record contains sharp tones which make it difficult to be encoded, therefore better encoding quality for this instrument done with higher output bitrate 4. CONCLUSION The subjective (i.e. MOS) and objective (i.e. PEAQ) encoding quality evaluation techniques are used to study the performance of the EVS Audio Coder. The EVS used in this study operates at full band encoding mode. Different inputs signals as clean oriental and orchestral musical instruments record used in the evaluation process. The EVS output was tried out at various output bitrates to be able to study the impact of both the input musical instruments and different output bitrates for the EVS coder. The results prove that the EVS coder produces highly encoding quality for the oriental musical instruments signals as well as the orchestral musical instruments signals. The encoding quality of the orchestral musical instruments signals is slightly higher with range from 0.1 to 0.2 than the encoding quality of oriental musical insrtruments. The results of the spectrogram illustrated that the quality of the EVS encoded outputs affected by the nature of the music signal production from the musical instrument itself and its frequency band.
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