Octasic Inc.

4101, Molson Street Montreal Quebec Canada jean-marc.valin@octasic.com

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General This document provides a quick overview of a prototype codec combining a linear prediction layer (SILK) with an MDCT-based layer (CELT). These codecs are used because of the authors' familiarity with the source code, but it does not prevent inclusion of code from other codecs as well. This is a work in progress.

We propose a hybrid codec based on a linear prediction layer (LP) and an MDCT-based enhancement layer. The main idea behind the proposal is that the speech low frequencies are usually more efficiently coded using linear prediction codecs (such as CELP variants), while the higher frequencies are more efficiently coded in the transform domain (e.g. MDCT). For low sampling rates, the MDCT layer is not useful and only the LP-based layer is used. On the other hand, non-speech signals are sometimes adequately coded using linear prediction, so for music only the MDCT-based layer is used. In this proposed prototype, the LP layer is based on the SILK codec and the MDCT layer is based on CELT codec. These codecs are used because of the authors' familiarity with the source code, but it does not prevent inclusion of code from other codecs as well. This is a work in progress.

There are three possible operating modes for the proposed prototype: A linear prediction (LP) mode for use in low bit-rate connections with up to 8 kHz audio bandwidth (16 kHz sampling rate) A hybrid (LP+MDCT) mode for full-bandwidth speech at medium bitrates An MDCT-only mode for very low delay speech transmission as well as music transmission. Each of these modes supports a number of difference frame sizes and sampling rates. In order to distinguish between the various modes and configurations, we need to define a simple header that can used in the transport layer (e.g RTP) to signal this information. The following describes the proposed header. The LP mode supports the following configurations (numbered from 00000...01011 in binary): 16 kHz: 10, 20, 40, 60 ms (00000...00011) 12 kHz: 10, 20, 40, 60 ms (00100...00111) 8 kHz: 10, 20, 40, 60 ms (01000...01011) for a total of 12 configurations. The hybrid mode supports the following configurations (numbered from 01100...01111): 48 kHz: 10, 20 ms (01100...01101) 32 kHz: 10, 20 ms (01110...01111) for a total of 4 configurations. The MDCT-only mode supports the following configurations (numbered from 10000...11101): 48 kHz: 2.5, 5, 10, 20 ms (10000...10011) 32 kHz: 2.5, 5, 10, 20 ms (10100...10111) 16 kHz: 5, 10, 20 ms (11000...11010) 8 kHz: 5, 10, 20 ms (11011...11101) for a total of 14 configurations. There is thus a total of 30 configurations, so 5 bits are necessary (with 2 codes unused) to indicate the mode, frame size and sampling rate (MFS). This leaves 3 bits for the number of frames per packets (codes 0 to 7): 0-3: 1-4 frames in the packet, each with equal compressed size 4-6: 1-3 frames in the packet, with different compressed sizes, which need to be encoded 7: The first frame has this MFS, but others have different MFS. Compressed sizes need to be encoded. The compressed size of the frames (if needed) is indicated -- usually -- with one byte, with the following meaning: 0: No frame (DTX) 1-251: Size of the frame in bytes 252-255: A second byte is needed. The total size is (size[1]*4)+(size[0]%4)+252 The maximum size representable is 255*4+3+252=1275 bytes. For 20 ms frames, that represents a bit-rate of 510 kb/s, which is really the highest rate anyone would want to use in stereo mode (beyond that point, lossless codecs would be more appropriate).

Simplest case: one packet

Four frames of the same compressed size:

Two frames of different compressed size:

Three frames of different *durations*:

In hybrid mode, each frame is coded first by the LP layer and then by the MDCT layer -- which discards the frequency components already encoded. Both use the same instance of the range coder to encode the signal.

The codec needs to take appropriate security considerations into account, for example as outlined in and .

This document has no actions for IANA.

IAB This document provides an overview of possible avenues for denial-of-service (DoS) attack on Internet systems. The aim is to encourage protocol designers and network engineers towards designs that are more robust. We discuss partial solutions that reduce the effectiveness of attacks, and how some solutions might inadvertently open up alternative vulnerabilities. This memo provides information for the Internet community. All RFCs are required to have a Security Considerations section. Historically, such sections have been relatively weak. This document provides guidelines to RFC authors on how to write a good Security Considerations section. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.