View source for CELT/IETF

{{Obsolete|The CELT codec has been merged into the IETF [[Opus]] codec and is now obsolete}}

== 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=

== Examples ==

== 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)

[[Category:Opus]]