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Topic: H.264 Minimum Bit Rate? (Read 15281 times) previous topic - next topic
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H.264 Minimum Bit Rate?

Does anybody know what the absolute minimum CBR bit rate is that H.264 supports?

H.264 Minimum Bit Rate?

Reply #1 –
No.  CBR bitrate is rate control specific and is not defined by the H.264 standard, you can set it to whatever you want.  Heck you could set it to ridiculously low bitrates if you wanted.  But it is unlikely it will achieve your target bitrate.  I recommend play around  with different bitrates to find out what suits you needs best.  That is what is the best balance between quality and bitrate.  Any reason for using CBR and not 2 pass?  A 2 pass encode will always look much better.

H.264 Minimum Bit Rate?

Reply #2 –
The lowest possible bitrate is reachable in constant quantizer mode with quant 51. It won't be cbr, but you can get ridiculously low bitrates this way (30-40 kbps on full dvd resolution material). Obviously it will be unwatchable.

H.264 Minimum Bit Rate?

Reply #3 –
Quote
No.  CBR bitrate is rate control specific and is not defined by the H.264 standard, you can set it to whatever you want.  Heck you could set it to ridiculously low bitrates if you wanted.  But it is unlikely it will achieve your target bitrate.  I recommend play around  with different bitrates to find out what suits you needs best.  That is what is the best balance between quality and bitrate.  Any reason for using CBR and not 2 pass?  A 2 pass encode will always look much better.
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The reason I asked was because one of the new mobile TV systems (DMB) includes H.264 with HE AAC for audio, but apparently you can only access the HE AAC decoder if you transmit some video, and someone had contacted me that wanted to transmit radio stations using HE AAC on this system, so a bit rate of 0 kbps would be preferable. But failing that then using the absolute minimum bit rate for video would still be feasible if the bit rate was extremely low.

For example, if you just encoded a totally black still image that never changed, what do you think the bit rate might be?

H.264 Minimum Bit Rate?

Reply #4 –
Test it! Get x264 and feed it with black stream and use the largest quantizer available. It shouldn't take much of bandwith but I'd have a video stream with channel logo or something and a little bit more of bandwith.

H.264 Minimum Bit Rate?

Reply #5 –
I did a test by myself. A black video clip (25 Hz, 352x255, quantizer 26) resulted in a bitrate of 3.28 kbps. Hydrogenaudio's logo came up as 14.27  kbps (original resolution) a higher quantizer would bring that bitrate down without compromising the quality much.

H.264 Minimum Bit Rate?

Reply #6 –
Well, H.264 can go down to about 160bits per frame. If you use 1fps, it is not that much.

H.264 Minimum Bit Rate?

Reply #7 –
Yes, by all means you should just use constant quant if your just encoding something like a black screen.  Certainly would be worth the effort to do 2 pass or even cbr.  Play around with the quantizer till you find one that looks acceptable (won't take much for black as stated before) and low bitrate.

H.264 Minimum Bit Rate?

Reply #8 –
Quote
Yes, by all means you should just use constant quant if your just encoding something like a black screen.  Certainly would be worth the effort to do 2 pass or even cbr.
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Can you definitely use CBR?

Thanks everybody for their replies.

H.264 Minimum Bit Rate?

Reply #9 –
Quote
Quote
Yes, by all means you should just use constant quant if your just encoding something like a black screen.  Certainly would be worth the effort to do 2 pass or even cbr.
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Can you definitely use CBR?

Thanks everybody for their replies.
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Yes, although x264 uses abr by default you can use the video buffer verifier to ensure that the output is cbr.  I can't remember what the setting is exactly as I never use it but I'm sure you can figure it out or someone else can post it.  I assume you just set --ratetol to zero.