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—The CELT codec has been merged into the IETF Opus codec and is now obsolete—

Codec Feature Comparison

Codec Rate (kHz) bitrate (kbps) frame size (ms) total delay (ms) Robustness license
CELT 32-96 24-128 (mono)
40-160 (stereo)
<5.8 (44.1 kHz)
<5.3 (48 kHz)
<8.7 (44.1 kHz)
<8.0 (48 kHz)
packet loss, bit errors open-source/
free software
G.722.1C (Siren14) 32 24, 32, 48 20 40 packet loss, bit errors no charge, but not open-source
AAC-LD 16-48 16-128 from 10 to 11.6 from 20 to 50+ packet loss proprietary, MPEG
Fraunhofer ULD 32-48 48-192? <10 <10 limited proprietary, non-standard

Codec Quality Comparison

These are the results of MUSHRA tests comparing CELT with AAC-LD, G.722.1C and MP3, with 10 listeners. We used version 0.3.2 of the CELT codec with 5.8 ms frames (256 samples) and 2.9 ms (128 samples) look-ahead, for a total of 8.7 ms at 44.1 kHz. As for AAC-LD, we used Apple's implementation, which has 11.6 ms frames and 23.2 ms look-ahead, for a total of 34.8 ms delay at 44.1 kHz. The MP3 implementation is LAME in CBR more and is only included for reference purposes only.

The first graph is for the 48 kbit/s listening test.

The second graph is for the 64 kbit/s listening test. Note that G.722.1C is still at 48 kbit/s in that test because it is the highest bit-rate it supports.

—The CELT codec has been merged into the IETF Opus codec and is now obsolete—