Actual bitrate set by VBR exceeds our expectations

AI & Data Science Computer Vision & Image Processing
1

Dear NVIDIA Support Team,

I am writing to seek assistance regarding an issue we’ve encountered with our video encoding process using NVIDIA’s encoding technologies. Despite configuring our encoder to use Variable Bitrate (VBR) settings, the actual bitrate exceeds our expectations. Additionally, our configuration sets both vbvBufferSize and vbvInitialDelay parameters to zero.

Issue Details:

  • Expected Behavior: Achieve target bitrate as specified in VBR settings.
  • Observed Behavior: Actual bitrate is consistently higher than the target.
  • Configuration Parameters:
    • RateControlMode: VBR
    • vbvBufferSize: 0
    • vbvInitialDelay: 0
    • avgBitrate:25Mbps
    • maxBitrate:30Mbps
    • Actual Bitrate received: 49Mbps

System Information:

  • Operating System: Windows 11
  • NVIDIA GPU Model: NVIDIA RTX A2000 8GB Laptop GPU
  • Driver Version: 553.62
  • Encoding SDK Version: NVENCAPI_MAJOR_VERSION 11, NVENCAPI_MINOR_VERSION 1

Steps Taken:

  1. Configured the encoder with VBR settings and zeroed vbvBufferSize and vbvInitialDelay.
  2. Initiated the encoding process on various video samples.
  3. Monitored the output bitrates, noting consistent elevation above the target.

We would appreciate any guidance or recommendations you can provide to help us achieve the desired bitrate control in our encoding process.

Thank you for your attention to this matter.

Best regards,
Mehul

2

Hi @mehul.kumar I am not an expert in this space, but remember seeing somewhere in the past that setting vbvBufferSize and vbvInitialDelay to 0 means they use their default values, rather than using 0 as the rate and the delay.

I’ll reach out to the computer vision team and ask them to confirm or deny!

Best,

Sophie

1 Like
3

Thanks for the clarification @sophwats,
Yes please, if you can confirm the same behavior with the computer vision team it would be of great help.

Thanks,
Mehul

4

Hey @mehul.kumar,

from the vis team:

How long was the stream and how is the bitrate measured is an important factor here.
Setting the vbvBufferSize and vbvInitialFullness to zero means they will take default values (which can be rather large, easily corresponding to a few seconds at the maximum bitrate)
That buffer size of a few seconds can easily distort the bitrate measurements

Best,

Sophie

5

Thanks for the confirmation @sophwats!

Thanks,
Mehul

6

I know about vbvInitialDelay and vbvBufferSize, what is vbvInitialFullness?

Best,
Mehul

7

Depending on the API, vbvInitialDelay can be expressed in time (delay) or bits (decoder buffer fullness), with the vbv buffer size representing the maximum vbv delay
If measuring bitrate as a moving average within a N seconds window with a vbv buffer size of M seconds, the maximum bitrate is roughly (maxBitrate * (N + M) / N)
I believe setting the vbv buffer size to zero results in a default vbv buffer size that is typically a few seconds (typically, the larger the better from a quality point of view)