1

CELP / FS-1016 – 4.8kbps Federal Standard in Voice CodingMehmet Umut Demircin

demircin@ece.gatech.edu

2

History

Code excited linear prediction (CELP) first introduced by B.S. Atal and M.A Schroeder at the 1984 ICC.

In 1988 DoD selected the CELP algorithm developed by AT&T Bell Laboratories as the basis for Federal Standard 4.8 kbps voice coder. (FS-1016)

Produced low-rate coded speech comparable to that of medium-rate waveform coders.

3

What is new in CELP?

Analysis-by-Synthesis Linear Prediction Excitation sequence is selected from a codebook by closed-loop

optimization. Adaptive and stochastic codebooks.

Long-term Linear Prediction Pitch (fine) structure of the speech is predicted.

Perceptual Weighting (Filtering) Shapes the error such that quantization noise is masked by high-

energy formants. A Hybrid Coder Other Standards: VSELP, LD-CELP

4

Short-term Linear Prediction:

Generic CELP Coder Block Diagram

Choosing the Excitation SequenceLong-term Prediction:

Open Loop: Closed-Loop:

20≤ ≤ 147

Perceptual Filtering

5

Linear Prediction (LP)

Computed for 30ms frames. Captures the formant structure. 10th order autocorrelation LPC is performed. LP parameters are represented with Line Spectrum Pairs

(LSP). Quantize using 4 bits for each of f2 – f5 and 3 bits for each of

the others (34 bits in total) from empirically determined probability density functions.

Smooth filter transitions by linearly interpolating a new set of LSP frequencies every ¼ frame.

6

Code Books

Codebook originally consisted of Gaussian sequences; 1024 vectors 40-samples (5ms)

Schroeder and Atal claim prediction errors after short-term and long-term prediction have Gaussian pdf.

Exhaustive search.

7

FS-1016 Standard Block Diagram

8

FS-1016 Codebook

Contains two parts: Adaptive Codebook Stochastic Codebook

9

Adaptive Codebook (ACB)

It is a delayed version of previous excitation samples multiplied by a gain, f. The value of m is in the range 20 ≤ m ≤ 147 7 bits (400Hz > ⇒ f0 > 54Hz).

10

Stochastic Codebook (SCB) Each code-vector contain 60 samples. Stochastic codebook contains 1082 independent

random values from the set {–1, 0, +1} with probabilities {0.1, 0. 8, 0.1}. The values of k is in the range 0 ≤ k ≤ 511 9 bits are needed.⇒

The code-vector (k-value) that gives the minimum weighted error is selected.

11

Weighting Filter

Listener will not notice noise at formant frequencies due to higher energy.

Errors at noticeable frequencies are emphasized.

12

Error Protection

Not all the bits of CELP parameters affect the speech intelligibility to the same degree.

Most significant bits of the ACB are protected with (15,11) Hamming code.

13

FS-1016 Bit Allocation

14

Quality Comparison

15

Implementation IssuesCODEBOOK SIZE CODE SEARCH TOTAL COMPL. QUALITY (DAM)

128 2.4 MIPS 6.8 MIPS 65

256 4.8 MIPS 9.2 MIPS 66

512 9.5 MIPS 13.9 MIPS 67

1024 18.9 MIPS 23.3 MIPS 68

120 ms delay, Average complexity ~16 MIPS LPC-10e: 157.5 ms delay and ~7 MIPS MELP: 122.5 ms delay and ~40 MIPS

16

References

A. Spanias, ``Speech coding: A tutorial review,'' Proceedings of the IEEE, vol. 82, pp. 1541-1582, October 1994.

Welch, Vanoy C., Thomas E. Tremain and Joseph P. Campbell, Jr., "A Comparison of U.S. Government Standard Voice Coders", IEEE Military Communications Conference (MILCOM) Conference Record, 1989, p. 269-273.

Manfred R. Schroeder and Bishnu S. Atal, "Code-Excited Linear Prediction (CELP): High-Quality Speech at Very Low Bit Rates," ICASSP 85

Campbell, J. P., Jr., T. E. Tremain, and V. C. Welch. "The Federal Standard 1016 4800 bps CELP Voice Coder." Digital Signal Processing 1, no. 3 (1991): 145 - 155.