Bandwidth attribute in MPEG-DASH manifest doesn't update

Wowza Streaming Engine
1

We’re publishing a live stream to Wowza using RTSP. When the transcoder add-on is disabled, we observed that the bandwidth attribute (shown below) in the MPEG-DASH manifest remains constant for the duration of the session even though the actual bitrate (of the encoded video) fluctuates over time. The bandwidth appears to only be estimated at the start of the session. But this initial estimate may not representative of the entire session, especially if the first few seconds of the live stream are static content.

For example (with non-relevant details omitted):

<Period>
    <AdaptationSet>
        <Representation id="p0a0r0" codecs="avc1.42c01f" sar="1:1" bandwidth="37000" />
    </AdaptationSet>
</Period>

The value 37000 doesn’t change for the duration of the session even though the encoded video bitrate fluctuates over time. Is this expect behavior or a bug in Wowza Streaming Engine? Should Wowza Streaming Engine update this value over time (like it does in the case of HLS playlists)?

2

Hello,

The value is an estimate, and this number staying constant is expected behavior since this number is used with the Adaptive Bit-rate delivery option that is available with MPEG-DASH.

This quote from the following article: http://www.streamingmedia.com/Articles/Editorial/Featured-Articles/How-to-Produce-for-Adaptive-Streaming-81020.aspx

"most authorities recommend encoding using CBR, or VBR constrained to a maximum of 110% of the average data rate. The problem with CBR, however, is that it can produce degraded quality in hard-to-compress sequences. For this reason, many producers use constrained VBR when encoding for adaptive streaming.

The potential negative impact of encoding with VBR will depend on many factors, including how close your streams are packed together and the constraint level used during VBR encoding. The most conservative approach is CBR, and for a first-time implementation, that’s where I would start. However, if quality is a problem, I would experiment with VBR constrained to 150%–200% of the target and see if that produces too many stream switches during your testing."

Are you using ABR in your implementation of MPEG-DASH or does this element not apply to your work-flow?

Regards,

Jason Hatchett