EVRC in RTP in the static load format rfc3558,4788,5188

1 EVRC

Protocol Evolution: 3558->4788->5188

EVRC0 net charge format see 3558

EVRC-WB net charge format see 5188

EVRC codec algorithm based on loose code excitation linear prediction (RCELP) algorithm, with linear pre-and error control functions, EVRC family of various codes see the following diagram:

2 Multi-frame format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| RTP Header [4] |

+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

| r| r| LLL | NNN |  MMM |  Count |  TOC |  ...  | TOC |padding|

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| One or more codec data frames, one per TOC entry |

| ....                              |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Rr:reserved, two-bit reserved, temporarily unused;

LLL: Staggered length, 0 for a special staggered frame, bundle frame, that is, the frame spacing between the frames is 0;

NNN: Staggered frame index;

MMM: Description of the mode request number;

The number of COUNT:TOC, for example, C, the number of TOC is c+1

Padding: The fill bit that is introduced for byte alignment, it is obvious that if the TOC item is an odd number, do not fill the bit, and the even number is required;

TOC: Table entry, that is, the frame type, the following table is the value of the various rates, the current value is not within the frame of the table is illegal;

0 1 2 3

+-+-+-+-+

|FR type|

+-+-+-+-+

Value Rate Total Codec data frame size (in octets)

---------------------------------------------------------

0 Blank 0 (0 bit)

1 1/8 2 (BITS)

2 5 (valid; not EVRC)//3558 definition not, but the following protocol has been supported

3 (BITS)

4 1 (171 bits; 5 padded at end with zeros)

5 Erasure 0 (should not being transmitted by sender)

3 single Frame net charge does not take the lead field format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| RTP Header [4] |

+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

| |

+ Only one codec data frame +-+-+-+-+-+-+-+-+

| |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

By the length of the net charge in distinguishing which mode

4 Multi-frame multiplexing types

A. Staggered frame interleaving

Count>0,count+1 Frame multiplexing

Lll=l, every two frame interval l+1

First RTP Packet in Interleave Group:

Lll=l, nnn=0

Frame 0, Frame l+1, Frame 2 (l+1), Frame 3 (l+1), ... for a total of

B Frames

Second RTP Packet in Interleave Group:

Lll=l, Nnn=1

Frame 1, Frame 1+l+1, Frame 1+2 (l+1), Frame 1+3 (l+1), ... for a

Total of B Frames

As explained in the agreement, the use of staggered frames can reduce the rate of data loss, but if there are drops, sound quality is exponential decline, quality degradation can be understood, but why can reduce the data loss ratio, depending on where to come from. Faq

B. Bundle Frame

Count=0, with staggered frames, is only 0 frames in multi-frame intervals.

5 Letter of order negotiation content

M=audio 49120 RTP/AVP 97

a=rtpmap:97 evrc1/8000

a=fmtp:97 fixedrate=0.5//optional

a=maxptime:120//optional, multi-frame multiplexing frame number must be less than Maxptime value for package duration

FMTP properties that can be taken:

Maxinterleave, maximum staggered frame number;

Dtxmax,dtxmin,silencesupp,hangover for mute suppression, see 3551,3558;