How does the realtime mode in libaom perform for live streaming?

The realtime mode in libaom—the reference AV1 encoder implementation—is designed to deliver the high compression efficiency of the AV1 codec while maintaining the low-latency and high-throughput requirements essential for live streaming and Real-Time Communications (RTC). By heavily optimizing the encoder pipeline, bypassing computationally expensive partition types, and aggressively prioritizing speed over exhaustive search tools, libaom in realtime mode successfully achieves sub-frame encoding times. While it historically carried a reputation for being too slow for live deployment, continuous algorithmic updates have turned it into a viable, bandwidth-saving alternative to traditional codecs like H.264 and VP9 for live broadcasts.

Architectural Trade-offs and Speed Presets

In standard high-efficiency modes (such as --good or VoD encoding), libaom evaluates a vast matrix of block partitions, complex compound prediction modes, and multi-frame temporal filters to maximize data compression. For live streaming, this exhaustive approach is impossible. Activating the --usage=realtime flag completely shifts the encoder’s behavior.

The performance of libaom in this mode is dictated by the --cpu-used parameter, which generally ranges from 5 to 11 for live environments.

• Speed 5 to 7: Focuses on maintaining a balance between high visual quality and computational cost. It leverages basic AV1 tools but disables highly complex asymmetric partitions.
• Speed 8 to 11: Drastically strips down the encoding process. The encoder disables rectangular partition searches, limits motion vector search ranges, and heavily utilizes early-termination strategies to process frames quickly enough for 30 or 60 frames-per-second (fps) execution.

Bandwidth Efficiency vs. Compute Cost

When properly tuned, libaom’s realtime mode delivers roughly 30% to 40% better compression than H.264 at equivalent visual quality tiers. This compression advantage directly translates to lower ingest and egress bandwidth costs for live streaming platforms.

However, this efficiency comes at a higher CPU cost. Even in its fastest realtime presets, libaom requires significantly more compute power than legacy encoders like libx264. To mitigate this, developers frequently deploy libaom alongside hardware extensions—such as Arm Neon/SVE2 or Intel AVX2 vector instructions—which allow multi-threaded software architectures to handle 1080p live streams on modern multi-core server processors.

Handling Real-Time Challenges

Live streaming applications encounter unpredictable network fluctuations and unique visual content, both of which libaom’s realtime mode addresses via specialized tools:

• Low-Latency Rate Control: Realtime mode incorporates single-pass rate control (CBR or constrained VBR) that strictly prevents bitrate spikes. This keeps the stream within the network’s throughput limits and avoids frame drops.
• Screen Content Coding (SCC): For game streaming or live desktop sharing, libaom dynamically activates SCC tools. These tools optimize the encoding of sharp edges, text, and flat graphics, providing massive quality boosts and bitrate savings over standard camera-captured video.
• Zero-Latency Reference Frames: Unlike traditional multi-pass configurations that rely on future frames for forward prediction, the realtime mode restricts motion estimation to past reference frames. This eliminates encoder-induced frame delay, facilitating instantaneous video transmission.