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Review: AJA HELO Streaming & Recording Device

The AJA HELO H.264 Encoder is an excellent entry into the existing AJA line of products. Don't let the small form factor fool you--this device packs advanced compression control, streaming, recording, and scheduling features for a reasonable price.

Stored Presets, Profiles, and Templates

Unlike most H.264 encoders, the AJA HELO allows two tiers of saved settings. At the lowest level, any compression and destination settings can be saved as one of 20 profiles---10 profiles for recording, and ten for streaming. At the highest level, presets allow you to take a snapshot of the current configuration of all settings, including current Recording Profile and Streaming Profile.

By default, the profiles are empty and numerically named as Streaming Profile 1, Recording Profile 1, and so on. Any changes you make to Streaming Profile Settings or Recording Profile Settings are automatically saved to the active preset. You can easily rename a profile by clicking the profile’s name to the right of the Streaming Profile Name or Recording Profile Name field.

Likewise, presets are initially named Preset #1, Preset #2, and so on. To store all of the current settings on the HELO to a preset, click the Presets item in the left navigation menu. Then, click the Save button next to one of the Preset entries. You can rename the preset to something more meaningful, such as a location name or client name. If you need to load the stored settings for a preset, click the Recall button. You can also export and import presets for backup/restore purposes, or migrating settings to another HELO, perfect for multi-camera setups of the same event.

Templates are fixed encoding settings that AJA has created to provide starting points for your Streaming or Recording Profile. When you are adjusting a profile, you can import one of the stored templates by selecting the template name in the Streaming (or Recording) Template combo box and clicking the Apply button. The HELO includes templates based on resolution, bitrate, or standard, such as 1080p High 10Mb, 720p Mid, NTSC, PAL, VGA, among others.

Advanced Preset Control

No encoder review is complete without discussion of the compression settings available on the device. In addition to the typical encoder settings such as frame rate (fractional to source), dimensions, average video/audio bitrate, and GoP size, you have access to several settings as shown in Figure 6 (below).

Figure 6. Compression settings

  • P Interval: You can control the frequency of P frames in relation to B frames with this setting. The default value of 1 specifies that all frames in a GOP after the I frame are P frames. If you choose 2 or 3, you will be adding B frames into the GOP. In my testing with RTMP streams and H.264 playback on web browsers, any value other than 1 caused a disruption to the image.
  • H.264 Encoding Profile: You can enable a specific profile for your HELO output, including Constrained, Baseline, Main, and High profiles. The default is Main, but you may opt to use High profile for better compression at a given bitrate, especially if you’re transrating lower-quality adaptive streams on an ingest server.
  • Slices Per Frame: You can control how each frame is segmented, or sliced, for encoding and decoding. The general rule of thumb is that more slices enables faster parallel processing of encoding frames, but for a dedicated hardware encoder, the default value of 1 slice per frame is likely your best option. You can choose from 1, 2, 4, or 8 slices.
  • Encoding Rate Mode: You can control how strictly the HELO adheres to the video bitrate specified in the profile with this option. For most live streaming applications, you will opt to use the Constant mode, to keep the average bitrate in check. If you’re employing further transrating on an ingest server, you may want to use Variable mode to allow the encoder to more flexibility with the bitrate.
  • Entropy Coding: You have access to both CAVLC and CABAC entropy modes with the HELO. For higher quality video at equivalent bitrates, CABAC is preferred when using Main or High profiles. CAVLC should be used only with Baseline or Constrained.

Note: All audio encoding by the HELO is performed at 48kHz.