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The finite-register rational coder is within two bits per block

Statement

For the rational source and length- $n$ frames of Exact finite-register arithmetic coding for rational block sources, $E [ℓ (X^{n})] \leq n H_{2} (p) + 2.$

Facts & Assumptions

Given: A product source block $X^{n}$ , its rational frequencies, and the pointwise length bound of the finite-register coding theorem.

[F1]

The rational finite-register arithmetic code is prefix-free and lossless: $ℓ (x^{n}) \leq - \log_{2} p^{n} (x^{n}) + 2$ for every block.

[F2]

Finite discrete sources and product blocks: $p^{n} (x^{n}) = \prod_{t = 1}^{n} p_{x_{t}}$ .

[F3]

Base-two entropy of a finite source: $H_{2} (p) = - \sum_{i} p_{i} \log_{2} p_{i}$ .

[F4]

Information units and logarithm-base convention: $\log_{2} t = \ln t / \ln 2$ .

[F5]

Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm: $\ln (x y) = \ln x + \ln y$ for positive $x, y$ .

Proof

1.1F1F2givenalgebra

Take the expectation of [F1] under the finite product law [F2], obtaining $E [ℓ (X^{n})] \leq E [- \log_{2} p^{n} (X^{n})] + 2$ .

2.1F2F3F4F5step 1.1algebra∎

By [F2], [F4], and [F5], $- \log_{2} p^{n} (X^{n}) = \sum_{t = 1}^{n} - \log_{2} p_{X_{t}}$ . For each coordinate $t$ , summing $p^{n}$ over blocks with $x_{t} = i$ gives $p_{i} (\sum_{j} p_{j})^{n - 1} = p_{i}$ , so $X_{t}$ has law $p$ . Linearity of finite expectation and [F3] then give $E [- \log_{2} p^{n} (X^{n})] = n H_{2} (p)$ . Substitute in step 1.1.

Depends on

Used by

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Dependency tree · two levels

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Sources

Scott D. Roweis, CSC310 Lecture 8x: Arithmetic Coding Without Blocks, version 1.1 (standard reference, not scraped)
Stanford EE376A, Information Theory Course Notes, Chapter 4: Lossless Compression (standard reference, not scraped)