CELP / FS-1016 – 4.8kbps Federal Standard in Voice Coding

1. CELP / FS-1016 – 4.8kbps Federal Standard in Voice Coding Mehmet 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. Long-term Prediction: Short-term Linear Prediction: Open Loop: Closed-Loop: 20≤  ≤ 147 Generic CELP Coder Block Diagram Perceptual Filtering Choosing the Excitation Sequence

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 Issues • 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.