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MPEG4 rate control fro Dm648

Purvin Pandit
Prodigy 110 points

I have been experiementing with the DM648 SP MPEG-4 encoder rate control setting (CBR v/s VBR).

My observation is this:

* I set the default RC = IVIDEO_LOW_DELAY and target = max = 2Mbps

Observation: The rate was varying close to around 2Mbps ( 1940 - 2050). 

* Then I set default RC = IVIDEO_STORAGE and target = max = 2Mbps
Oberservation: The rate was varying much more but always stayed well below 2Mbps (1.230 - 1.500).

* I set the default RC = IVIDEO_LOW_DELAY and target = max = 4Mbps
Observation: The rate was varying much more than at 2Mbps more like VBR below.

* Then I set default RC = IVIDEO_STORAGE and target = max = 4Mbps
Oberservation: The rate was varying much more (3.700 - 4.300).
The VBV setting was 112 (default). Why is there so much variation between these rates. Can you please point me to what I might be setting wrong? Also I wanted to confirm that:
IVIDEO_LOW_DELAY = CBR
IVIDEO_STORAGE = VBR
Thanks,
-Purvin

  • 0
    TI__Prodigy 220 points

    Purvin,

    IVIDEO_LOW_DELAY : This is more suited for CBR and this can skips frames.
    IVIDEO_STORAGE : This is more suited for VBR kind of applications, and does not skip frames.

    The rate control on DM648 achive the bit rate on an average basis, but there could be instatntaneous
    bit rate fluctualtions as you pointed out based on the content.
    The difference between these 2 options in this software is that in IVIDEO_LOW_DELAY, we skip frames if the content
    is tough or hard to encode based on the rate control statistics.

    Can you check if you achieve the average bit rate (over intervals of few seconds) ?

    Regards,

    Keshav

  • 0 in reply to Keshava Prasad
    Prodigy 110 points

    Thanks Keshava.

    What we have noticed is that in the VBR mode we are almost never able to see the rate reach near the max value specified when going from a low motion (static) to high motion (dynamic) content. It is will more or less be around the target BR.

    Target = 1Mbps & Max = 4Mbps.

    Is this expected?

    -Purvin

  • 0 in reply to Purvin Pandit
    TI__Prodigy 220 points

    Purvin,

    The rate control only uses target (average) bit rate parameter. The max bit rate parameter just sepcifies the maximum bit rate the codec can handle and it is not used for any rate control purpose (this parameter, "max bit rate" is not applicable for rate control, hence you can ignore this parameter).

    The bit rate will fluctuate aroud the target bit rate specified. The encoder will try to achive this average bit rate over a period of time. The behavior you saw is expected.

    regards,

    Keshav