Compute metadata overhead

[tdaede]

We can compare the size of the standalone iamf streams to the component bitstreams (or their bitrates) to compute the overall metadata overhead.

[sunghee-hwang]

@jwcullen @yeroro @yilun-zhangs,
is it still an open issue or was it already addressed? or will it be postponed?

[jwcullen]

We did not make a formal report of bitrate overhead. I'm not sure how useful it is without actual real-life samples and even then the overhead depends on what features are used. I see 3 main components of overhead as 1) Descriptor OBUs 2) Parameter Blocks 3) Audio Frame header.

1. For descriptor OBUs the % overhead depends on how often they appeal. Many test vectors encode ~500 ms which means I expect they have a higher descriptor overhead than typical real-life samples would have.

2. Parameter block overhead depends on how granular control the content creator wants. If they use a lot of subblocks or cannot use param_definition_mode == 0 then the overhead is increased. I'm not sure what would be representative here without real-life samples.

3. This can be approximated by hand based on the frame size. There is additional overhead if large audio frame IDs are used - even though they usually would not be necessary.

I am deferring to the working group to decide if we need to do more on this.

[sunghee-hwang]

Let me calculate the overhead based on the following assumption.

• Simple Profile
• One set of Descriptors
• Codec: Opus
• Input 3D audio signal: 7.1.4ch
• bitrate: 32kbps or 64kbps per channel
• Frame duration: 20ms
• No parameter block
• No scalable channel audio

1. Overhead for Descriptors: Total 137B

• IA Sequence Header OBU: 8B
• Codec Config OBU: 22B
• Audio Element OBU: 16B
• Mix Presentation OBU: 91B (based on one language(English) & label length 20B)

2. The size of audio_frame: 48KB (for 32kbps) or 96KB (for 64kbps) per second.

3. OBU header overhead of Audio Frame OBU: Total 1050B per second

• Number of Audio Frame OBU: 350B per second (50 frames per second x number of substreams)
• OBU Header size: 3B

As a result, the overhead is

• Bitrate 32kbps per channel: (137 + 1050 x S + 6 (trimming info)) / 48K, about 2.5% for S = 1 and 2.2% for S = 240 sec
• Bitrate 64kbps per channel: (137 + 1050 x S + 6 (trimming info)) / 96K, about 1.25% for S = 1 and 1.1% for S = 240 sec

[sunghee-hwang]

I think that we can postpone this to compute the overhead for further cases.