CELT/IETF

Intro

• CELT description and design goals
• CELT RTP profile
• Open issues

About CELT

• Two types of codecs
  • Speech codecs with low-medium quality and low delay
  • Music codecs with high quality and high delay
• CELT aims for both high quality and very low delay
• Perceptual transform (MDCT) codec
• Developped within the Xiph.Org Foundation
• Reference implementation is open source (BSD-licensed)
• No royalties, avoids known patents in the field

Why low delay?

• Low delay is critical to live interactions
  • Prevents collisions during conversations (higher sense of presence)
  • Reduces or remove the need for acoustic echo cancellation
  • Allows synchronization for live music performance
• Increased range
  • 1ms reduction = 200 km more in fiber

CELT features

• Sampling rates from 32 kHz to 48 kHz
• Total algorithmic delay from 2ms to 20 ms (8 ms typical)
• Configurable frame size from 64 samples to 512 samples
• Bit-stream not frozen yet

Comparison with existing codecs

• Summary of quality figures, along with delay and bit-rate

draft -- Important Facts

• The bit-rate is implicitly encoded in the size of the compressed data
• The decoder must know:
  • The frame size (decompressed samples) being transmitted
  • The size of each compressed packet

RTP

• Lacing values and all?

SDP

• SDP parameters
  • ptime
  • b=AS: ?
• CELT-specific parameters
  • frame-size=
  • mapping=
  • low-overhead=

Open issues

• Need to negotiate the frame size
• CELT would be more flexible with configuration data (~100 bytes)
  • Configuration packet transmitted at regular interval
  • Incrmentally tramsmitted (one byte at a time)
  • base64-encoded in SDP parameters

Future work

• Freeze the bit-stream
• Complete RTP profile
• Possible standardization (IETF or other)