2. Understanding the Internet Low Bit Rate Coder
Jan Linden Vice President of Engineering
Global IP Sound
Presented by
Jan Skoglund
Sr. Research Scientist

3. iLBC – Background info Development started in Summer 2000
Contributed to IETF as an internet draft in Feb 2002
Accepted as work item in IETF AVT group Mar 2002
Contributed to CableLabs RFP in June 2002
Improved version to IETF, Fall 2002
ECR submitted in May 2003
Support for 20 ms frames spring 2003
Successful interoperability events
Past Working Group Last call in IETF Jan 2004
April 2004 added as a mandatory codec in PacketCable 1.1
December 2004 IETF process finalized (became Experimental RFC 3951 and 3952)

4. Design Principles Free of 3rd party IPR Packet independency
extensive experience in speech coding patents by design team
patent and research situation monitored since 2000
has been public in IETF since March 2002 and reviewed by independent speech coding researchers
Packet independency
no coding interdependency between frames
increased packet loss robustness
suitable for IP networks
Linear Predictive Coding
well know highly successful coding model
novel coding techniques of residual signal

5. iLBC Features Sampling Rate: 8 kHz
Supports 30 ms and 20 ms speech frame modes
Bitrate
13.3 kbps (399 bits, packetized in 50 bytes) for 30 ms frames
15.2 kbps (303 bits, packetized in 38 bytes) for 20 ms frames
Computational complexity (TI C54x)
30 ms frames: appr. 18 MIPS/channel
20 ms frames: appr. 15 MIPS/channel
Memory
400 Words/channel state memory (RAM)
less than 4 kWords table memory (ROM)
Stack and program memory requirements similar to other low bit rate codecs (e.g. G.729A)

6. The Core iLBC method Start state encoding
Gain-shape waveform matching forward in time
Gain-shape waveform matching backward in time
Pitch enhancement
Packet loss concealment

7. iLBC Encoding
Incoming speech
Packets to network

8. iLBC Decoding
Decoded speech
Packets from network

9. 20 ms vs 30 ms sub-blocks
20 ms frame size mode - 4 sub-blocks with the total length of 160 samples
30 ms frame size mode - 6 sub-blocks with the total length of 240 samples

10. 20 ms vs 30 ms mode – bit allocation
240 samples encoded to 399 bits = 13.3 kbit/s (50 oct)
160 samples encoded to 303 bits = 15.2 kbit/s (38 oct)
Parameter
Bits
LPC
Start state position
Start state scale
Start state samples
Shapes
Gains 40 4 6
174
115 60
Total
399
Parameter
Bits
LPC
Start state position
Start state scale
Start state samples
Shapes
Gains 20 3 6
171 67 36
Total
303

11. Advantage over CELP
original
iLBC
g729
g723
State
recovery
PLC

12. iLBC Performance vs G.729A & G.723.1 old version from Winter 2002
Source: Dynastat

13. iLBC Performance
Equivalent or slightly lower performance than G.729E in clean.
Improved robustness to packet loss compared to G.729E.
iLBC showed better than G.728 in other testing.

14. Implementation Fixed Point Source Floating Point Source DSP Source
Significant signal processing skills necessary
Quality / efficiency trade-off
~ 6 Months
Optimization skills
~ 4 Months

15. iLBC Specifications
Available in floating point , fixed point ANSI C, TIc54x, TIc55x, TIc64x,…
Supports 20 and 30 ms speech frames
Algorithmic delay: Same as frame size
Sampling Rate: 8 kHz
Bit rate: kpbs for 30ms and 15.2 kpbs for 20ms
Product
Frame size
Complexity (max)
Program Memory
Data Memory Static
Data Memory Dynamic
Encoder
Decoder
Fix
Per channel
GIPS iLBC TIc54x
20 ms
11.5 MIPS
4.1 MIPS
17.6
2.4
1.4
2.3
30 ms
13.5 MIPS
4.4 MIPS
GIPS iLBC TIc55x
7.5 MIPS
3.0 MIPS
15.6
8.8 MIPS
3.1 MIPS
Memory in kWord16