Summary of S4-260142: On Complexity and Memory Constraints for ULBC

Introduction

This contribution addresses complexity and memory constraints for Ultra Low Bitrate Codec (ULBC) as part of the study in TR 26.940. The document aims to clarify previous discussions on measurement metrics and specific constraints, proposing concrete values for complexity, RAM, and ROM requirements.

Main Technical Contributions

Complexity Measurement Metrics

The contribution proposes using both MACS (Million Multiply-Accumulate Operations per Second) and Codec/Model Size together to characterize ULBC complexity, rather than relying on a single metric:

• Codec/Model Size: Directly impacts memory requirements and power consumption (more memory footprint requires more frequent DRAM access, leading to higher power consumption)
• MACS: More suitable for guiding computing hardware unit selection
• These metrics do not necessarily correlate, as different model architectures can result in very different MACS for the same model size

Memory Constraints Clarification

The document clarifies confusion from previous contributions (S4aA250253 and S4-251807) regarding the 5-10M parameters proposal:

ROM Constraints

• ROM characterized by overall Model Sizes across all operation modes
• Major impact is FLASH consumption in product design
• Minimal power consumption impact (only one model's parameters accessed at a time)
• Proposed constraint: < 15M parameters (relaxed from previously discussed 10M to support more operation modes)
• Enables support for ~5 operation modes (e.g., 2-3 bitrates for 2 different sampling rates)

RAM Constraints

• RAM characterized by maximum single Model Size (assuming no switching between operation modes)
• Proposed constraint: < 3M parameters
• With 15M ROM, this allows 5 operation modes
• Whether switching between operation modes will be supported is FFS

Complexity Constraints

MACS Reference Point

The contribution references the 2025 Low-Resource Audio Codec (LRAC) Challenge sponsored by Cisco Systems as a relevant benchmark:

LRAC Challenge Requirements:
- Sampling rate: 24 kHz
- Mono audio input
- Bitrate: up to 1 kbps (ultralow) and up to 6 kbps (low)
- Latency: 30 ms (Track 1) or 50 ms (Track 2)
- Compute complexity: ≤ 350 MMACS total; ≤ 150 MMACS receive-side
- Winner (ByteDance) used ~4M parameters

Proposed MACS Value

• While LRAC suggested 350 MMACS, the contribution proposes < 600 MMACS for ULBC
• Rationale: Slightly increased complexity enables better speech quality while remaining within target hardware (e.g., DSP) computational capacity
• Validation: Handcrafted 3M parameter codec (reduced from SoundStream architecture) achieved 600 MMACS

Proposed Design Constraints Summary

The contribution proposes the following specific constraints for ULBC:

1. Complexity:
2. Single Model Size < 3M parameters

3. < 600 MMACS

4. RAM:

5. < 3M parameters (assuming no switching between operation modes)

6. Whether switching will be supported is FFS

7. ROM:

8. < 15M parameters

Text Proposal

The contribution includes a change request to TR 26.940, Section 6.2 (Design Constraint Parameter), Table 6.2-1, adding the specific complexity and memory constraints detailed above to the "Complexity and memory demands" parameter row.