Tuning Transcode Latency

Latency tuning allows for trade off among end to end delay, video bit rate, GOP composition, etc. Interactive applications that require low latency, can do so by setting relevant parameters in both decoder and encoder. In the latter case, it is understood that lowering the latency comes at the cost of increased bit rate, for a given visual quality.

Decoder Latency

For streams without B frames, decoding latency can be reduced by enabling both the -low_latency and -splitbuff_mode decoder options in FFmpeg or the low-latency and splitbuff-mode options in GStreamer.

The low latency decoding mode is not compatible with B frames. If the input streams contains B-frames, the low_latency option should not be used.

When using the XMA APIs, the splitbuff_mode should only be used if the application can always send a complete Access Unit in one shot to the decoder.

Encoder Latency

Xilinx encoder performs multi-objective optimization with set constraints on bit rate, GOP topology, visual quality measures, etc. As such, it may be tuned to achieve a compromise between latency and quality or be specialized to ultra low latency.

Guidelines on Encoder Latency Configuration

Encoding latency can also be reduced by trading off compression rate or visual quality. The following table lists the encoder options which can used to reduce to that effect.

Encode Options

Notes

Look Ahead Depth
For best visual quality, it is recommend to set the lookahead depth to 1/3 of the frame rate; however, this also increases latency proportionally to the requested depth. It is possible to reduce latency at the expense of visual quality by reducing the lookahead depth.

QP Mode
Any option other than uniform requires extra processing and hence will increase the pipeline delay.

Scaling List
Disabling this option skips over a visual quality refinement process and as such will reduce the pipeline delay.

Adaptive Quantizations (AQ)
Temporal and Spatial AQ require extra processing stage and will increase the pipeline delay. Disabling AQ will improve encoding latency.

Rate Control
Setting this to any option other than low latency will result in extra delay.

Number of B frames
It is understood that for every inserted B frame there will be a frame period delay.

Ultra Low Latency (ULL) Mode

Ultra Low Latency (ULL) encoding is enabled by setting -disable-pipeline flag in FFmpeg or ultra-low-latency in GStreamer.

Notes

  • The ultra low latency encoding mode is not compatible with B frames. Requesting the insertion of B frames by the encoder will result in an immediate termination of the pipeline when ULL is enabled. The user must choose between encoding in ULL mode or adding B frames to the encoded stream.

  • When ULL is enabled, the maximum achievable real-time rate is 4kp30.

Considerations for AVC encoding

Encoding to AVC using the ULL mode affects the maximum density and resolution per device. The FFmpeg -avc-lowlat and GStreamer avc-lowlat options must be set as a based on the desired density or resolution, as explained below.

  • If avc-lowlat is set to 0 (default), only a single accelerator core can be used per stream. This implies that only resolutions up to 1080p60 are supported. However, multiple streams may be active on a given core, which means that density is maximized.

  • If avc-lowlat is set to 1, multiple accelerator cores can be used per stream. This implies that all resolutions are supported. However, only a single stream may be active on a given core which limits overall density on the device.

Given that each accelerator core is capable of encoding at an aggregate rate of 1080p60, it is usually best to use the following avc-lowlat settings when encoding to AVC in ULL mode:

  • If the resolution is 4kp30 or higher, set avc-lowlat to 1 (as more than 1 core is needed for these resolutions)

  • If the resolution is 1080p60 or lower, set avc-lowlat to 0 for of 1080p60 or lower (to maximize density)

Furthermore, when encoding to AVC in the ULL mode, realtime performance is only guaranteed for 1080p60 and not beyond.

These considerations do not apply when encoding to HEVC in ULL mode.

Example Use Cases

The following scripts exemplify typical use cases for each possible low latency pipeline configuration:

  1. To achieve low latency with control over visual quality refer to:

  2. examples/u30/ffmpeg/tutorials/07_ffmpeg_transcode_lowlatency.sh to obtain ultra low latency transcode

  3. examples/u30/ffmpeg/quality_analysis/latency_test.sh to measure decode and encode latencies.