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  Broadcom Listening Test

• In December 2010, Broadcom conducted an ITU- BS.1116-style subjective listening test comparing different configurations of the CELT-only mode of the IETF along with MP3 and AAC_ LC. • 17 listeners participated. • 10 diverse full-band audio tracks with 44.1 kHz sampling used: – 2 pure speech – 2 vocal – 2 solo instruments – 1 rock-and-roll – 1 pop – 1 classical orchestra – 1 jazz • The goal was to compare different configurations of the latest CELT at that time (version 0.9.1, as of November 2010), and compare them with the two reference MP3 and AAC_LC at 96 kbps.

Copyright © 2011, Broadcom Corporation. All rights reserved. 1 Codec Conditions Tested

• 6 codec conditions were tested, all with constant bit-rate (CBR): – Reference 1: MPEG-1, layer 3 (MP3) codec at 96 kbps – Reference 2: AAC Low Complexity profile (AAC_LC) codec at 96 kbps – 96 kbps CELT 0.9.1 with pitch prefilter/postfilter on, 23 ms frame size – 96 kbps CELT 0.9.1 with pitch prefilter/postfilter on, 5.8 ms frame size – 96 kbppppps CELT 0.9.1 with pitch prefilter/postfilter off, 5.8 ms frame size – 64 kbps CELT 0.9.1 with pitch prefilter/postfilter on, 5.8 ms frame size • It was later realized that CELT 0.9.1 was only optimized for 48 kHz and not for 44.1 kHz sampling. • After the test, CELT was optimized for 44.1 kHz in addition to 48 kHz. • After re-processing the audio test files with 44.1 kHz-optimized CELT, there was noticeable audio quality improvement. • Later, CELT went through further audio quality enhancements. • As a result, the CELT test scores (on a 5-point scale) presented on the next slide can be considered lower bounds of the scores of the current version (0.11) of CELT.

Copyright © 2011, Broadcom Corporation. All rights reserved. 2 CELT Listening Test Result (Audio Quality Degradation Relative to Uncoded Original)

0.00 -0.11 -0.20 -0.17

-0.40

-0.60 -0.60

-0.80 -0.79 -0.86 -1.00

-1.20

-1.40

-1.60 -1.58

-1.80

-2.00 CELT_96kbps_on CELT_96kbps_on CELT_96kbps_off CELT_64kbps_on MP3_96kbps AAC_LC_96kbps _23ms _5.8ms _5.8ms _5.8ms 95% CI-L -0.61 -0.06 -0.09 -0.47 -0.68 -1.37 95% CI-U -0.97 -0.16 -0.25 -0.72 -1.04 -1.80 Mean -0790.79 -0110.11 -0170.17 -0600.60 -0860.86 -1581.58 • “on” or “off” in the codec conditions above indicates whether the pitch prefilter and postfilter were on or off. • 95% CI-L and 95% CI-U indicate the lower and upper bounds of the 95% confidence interval.

Copyright © 2011, Broadcom Corporation. All rights reserved. 3 Conclusion

• 96 kbps CELT 0.9.1 with a frame size of 23 ms was rated significantly better than 96 kbps MP3. • 96 kbps CELT 0.9.1 with a frame size of 23 ms was rated roughly equivalent to 96 kbps AAC_LC; with 44.1 kHz optimization and further enhancements, CELT 0.11 is expected to be no worse than AAC_LC. • 96 kbps CELT 0.9.1 with a frame size of 5.8 ms was rated slightly better than 96 kbps MP3, even though its codec delay is much lower than that of MP3. • The pitch prefilter/postfilter method provided statistically significant audio quality improvement for the 96 kbps CELT 0.9.1 with a frame size of 5.8 ms.

Copyright © 2011, Broadcom Corporation. All rights reserved. 4   

                 

         

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