Documentation
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Overview ¶
Package rangecoding provides a Range coder for the Opus bitstream
Index ¶
Constants ¶
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Variables ¶
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Functions ¶
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Types ¶
type Decoder ¶
type Decoder struct {
// contains filtered or unexported fields
}
Decoder implements rfc6716#section-4.1 Opus uses an entropy coder based on range coding [RANGE-CODING] [MARTIN79], which is itself a rediscovery of the FIFO arithmetic code introduced by [CODING-THESIS]. It is very similar to arithmetic encoding, except that encoding is done with digits in any base instead of with bits, so it is faster when using larger bases (i.e., a byte). All of the calculations in the range coder must use bit- exact integer arithmetic.
Symbols may also be coded as "raw bits" packed directly into the bitstream, bypassing the range coder. These are packed backwards starting at the end of the frame, as illustrated in Figure 12. This reduces complexity and makes the stream more resilient to bit errors, as corruption in the raw bits will not desynchronize the decoding process, unlike corruption in the input to the range decoder. Raw bits are only used in the CELT layer.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Range coder data (packed MSB to LSB) -> :
+ +
: :
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: | <- Boundary occurs at an arbitrary bit position :
+-+-+-+ +
: <- Raw bits data (packed LSB to MSB) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Legend:
LSB = Least Significant Bit
MSB = Most Significant Bit
Figure 12: Illustrative Example of Packing Range Coder
and Raw Bits Data
Each symbol coded by the range coder is drawn from a finite alphabet and coded in a separate "context", which describes the size of the alphabet and the relative frequency of each symbol in that alphabet.
Suppose there is a context with n symbols, identified with an index that ranges from 0 to n-1. The parameters needed to encode or decode symbol k in this context are represented by a three-tuple (fl[k], fh[k], ft), all 16-bit unsigned integers, with 0 <= fl[k] < fh[k] <= ft <= 65535. The values of this tuple are derived from the probability model for the symbol, represented by traditional "frequency counts". Because Opus uses static contexts, those are not updated as symbols are decoded. Let f[i] be the frequency of symbol i. Then, the three-tuple corresponding to symbol k is given by the following:
k-1 n-1
__ __
fl[k] = \ f[i], fh[k] = fl[k] + f[k], ft = \ f[i]
/_ /_
i=0 i=0
The range decoder extracts the symbols and integers encoded using the range encoder in Section 5.1. The range decoder maintains an internal state vector composed of the two-tuple (val, rng), where val represents the difference between the high end of the current range and the actual coded value, minus one, and rng represents the size of the current range. Both val and rng are 32-bit unsigned integer values.
func (*Decoder) DecodeSymbolLogP ¶
DecodeSymbolLogP decodes a single binary symbol. The context is described by a single parameter, logp, which is the absolute value of the base-2 logarithm of the probability of a "1".
https://datatracker.ietf.org/doc/html/rfc6716#section-4.1.3.2
func (*Decoder) DecodeSymbolWithICDF ¶
DecodeSymbolWithICDF decodes a single symbol with a table-based context of up to 8 bits.
https://datatracker.ietf.org/doc/html/rfc6716#section-4.1.3.3
func (*Decoder) Init ¶
Init sets the state of the Decoder Let b0 be an 8-bit unsigned integer containing first input byte (or containing zero if there are no bytes in this Opus frame). The decoder initializes rng to 128 and initializes val to (127 -
(b0>>1)), where (b0>>1) is the top 7 bits of the first input byte.
It saves the remaining bit, (b0&1), for use in the renormalization procedure described in Section 4.1.2.1, which the decoder invokes immediately after initialization to read additional bits and establish the invariant that rng > 2**23.
Source Files