Background

Teradek decoders have always been able to convert incoming streams to any selected output format, and the 700 series decoders are no different.

For the 700 series platform, however, we discovered a drawback. Although the decoder would start in sync and be fine for hours, over longer periods of time, the format conversion element introduced drift between audio and video. In some scenarios, video could fall behind the audio by more than a second.

After thoroughly examining all the components in the decoder pipeline, we were unable to eliminate variances in the conversion element such that for all video format combinations, audio and video would stay synced indefinitely. We recognized that although format conversion adds flexibility, it is not necessary for many point-to-point broadcast applications. With this in mind, we decided to make it configurable.

Solution

In Cube, Slice, and T-RAX 8.2.5 firmware (700 series models only) we have introduced a setting called Auto Format Conversion on the decoder's audio/video output page (underneath the stream options on T-RAX). This setting allows users to disable format conversion if they experience issues with audio/video sync:

Auto Format Conversion is enabled by default, and allows the decoder to process and output incoming streams regardless of the resolution and frame rate. In previous versions this was not configurable, and was always on.

If you encounter drift between audio/video on the decoded output, this setting can be disabled. When disabled, sync should be stable as long as the formats match (e.g., if your Encoder is receiving a 1080i59.94 signal, and your Decoder has the Auto Format Conversion option set to Disabled, the video output signal must be set to 1080i59.94 on the Decoder). Audio and video playback will be negatively affected otherwise. The following should be considered:

• Interlaced video - If transmitting 1080i50/59.94, ensure the deinterlacer is disabled at the encoder. If it is enabled, p50/60 output should be disabled. Note that interlaced HEVC is not supported by software decoders such as FFMPEG or VLC.
   
• Playback speed will be incorrect if video frame rates do not match, and audio will drift quickly. ensure that frame rates match.