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AMWA-TV / bcp-006-02

[Work In Progress] AMWA BCP-006-02: NMOS With H.264
https://specs.amwa.tv/bcp-006-02
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Questions about Sender (transport) `bit_rate` #13

Closed garethsb closed 1 year ago

garethsb commented 1 year ago

The Sender resource SHOULD indicate the target bit rate (kilobits/second) including the transport overhead of the H.264 stream. The value is for the IP packets, so for the RTP payload format per RFC 6184, that includes the RTP, UDP and IP packet headers and the payload. The bit_rate integer value is expressed in units of 1000 bits per second, rounding up. The Sender's transport bit_rate value MUST derive from the Flow's encoding bit_rate value and according to the Flow's constant_bit_rate attribute it corresponds to a constant bit rate or a maximum bit rate.

  1. What does "MUST derive from" mean?

  2. If we have Flow (codestream) bit_rate and separate Sender (transport) bit_rate, should we add constant_bit_rate flag also to Sender. Theoretically, perhaps a transport mapping of a (non-strict) CBR codestream produce a strict-CBR transport bit stream?

https://github.com/AMWA-TV/bcp-006-02/blob/400435c00a99f516d3f5147eb5248ed80b5ab42a/docs/NMOS%20With%20H.264.md?plain=1#L149

alabou commented 1 year ago
  1. The Sender bitrate cannot be lower or equal to the encoding bitrate as there is an overhead ... it cannot be a random or unrelated value .. maybe "derive from" is not specific enough ...
  2. The constant_bit_rate concept is well specified in the H.264 specification and applies to the Flow. The specification controlling the transport (ex ST2110-22) defines the transport behavior. I'm not sure we should add the constant_bit_rate flag for a Sender.
garethsb commented 1 year ago
  1. I think it's a simple fact that the transport bit rate cannot be less than the codestream bit rate so I don't think this needs mentioning again after the first sentence describes transport overhead nicely already, or its own conformance language.
  2. Maybe then, instead of the last sentence, something like "The transport bit_rate indicates a constant bit rate or a maximum bit rate depending on the constant_bit_rate of the associated Flow."
alabou commented 1 year ago

Will be resolved as:

"The Sender's transport bit_rate indicates a constant bit rate or a maximum bit rate depending on the constant_bit_rate attribute of the associated Flow."

alabou commented 1 year ago

resolved by PR #19