[Mp4-tech] Bitrate Vs Delay

[Gary Sullivan]

Not quite correct.
Actually there are several different definitions of delay, and it is best to ask for more detail about what someone means when they refer to delay.
In videoconferencing, delay ordinarily means the amount of time between when you move and when the person you're talking to sees you move.
In broadcast video, delay may mean the amount of time between when you punch the button on your remote control to switch to a new channel and when the new channel shows up on your TV (or when the picture that shows up reaches adequate quality, or when the picture both shows up and starts moving properly).
Those are different things.
Higher encoding bit rate does not mean higher delay.  In fact one of the best ways to reduce delay is to increase the channel bit rate that you're operating over, which allows frame rate to increase and other sources of delay to be reduced.
For example, uncompressed PCM video has very high bit rate and very low delay.
There are many sources of delay in a video codec system.  Here is a non-exhaustive list for videoconferencing applications:
1) the time between when you move and when the camera shutter integration time stops
2) the time it takes to dump a picture from the camera into the capture device
3) time for pre-processing in an encoder for noise reduction, etc.
4) re-ordering of pictures prior to encoding, e.g., for conventional B picture use
5) the time it takes an encoder to encode a picture and turn it into compressed bits
6) extra buffering delay used to allow the encoder to vary the number of bits it spends on different pictures
7) extra buffering delays introduced in the encoder to make its implementation easier
8) the time it takes to multiplex or packetize the video and perform channel coding
9) the time it takes the bits to get from the encoder to the decoder
10) the time it takes to perform channel decoding and demultiplex or depacketize the video
11) the time it takes a decoder to process the compressed bits into a decompressed picture
12) re-ordering of pictures prior to display, e.g., for conventional B picture use
13) the time it takes to post-process decoded pictures, e.g., for deblocking
14) time delays added so that the relative timing of the output pictures matches the input timing
15) time delays added for synchronization with other events such as audio sync
16) extra buffering delays introduced in the decoder to make its implementation easier
17) the time it takes the decoder to get the output picture onto the display
It's hard to get all that to add up to less than a few hundred milliseconds on average, particularly if your video frame rate is relatively low.
In videoconferencing, sources 4 and 12 are ordinarily eliminated, and sources 6 and 14 are also greatly compromised and all the others are minimized to the extent feasible for the implementation.
In broadcast, you also need to think about channel acquisition refresh time.
In DVD-style playback, you also need to think about random-access seek time.
High frame rates (which tend to require high bit rates) can reduce delay dramatically.  Delay gets pretty awful at a few frames per second.
A videoconferencer's dream is to operate beyond 30 fps.  I think no videoconferencing systems today have that capability.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> Hans-Juergen Bardenhagen
+> Sent: Friday, February 20, 2024 3:56 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: Re: [Mp4-tech] Bitrate Vs Delay
+> 
+> 
+> Hi Ravi,
+> 
+> on 20.02.04, 14:23 local time (received 20.02.04, 12:18 
+> GMT+1) you wrote:
+> 
+> > Can any one explain me what is the relationship between 
+> "bitrate" and
+> > "delay".
+> 
+> The higher the encoding bitrate is, the higher the encoding 
+> delay will  
+> be. Delay means the difference in time between starting the 
+> encoding and  
+> obtaining the output bitstream which can be several hundred  
+> milliseconds. To minimize this delay is important for all real-time  
+> applications like bi-directional communication in video 
+> conferencing  
+> systems, so you need specialized codecs for this purpose, 
+> e.g. the Low  
+> Delay object type for AAC general audio or others for speech content.
+> 
+> > Also, some one can explain me what this means or any good 
+> website for
+> > these understanding.
+> 
+> Maybe the Wiki of Audiocoding.com can give some help:
+> 
http://www.audiocoding.com/wiki/
ZZee ya, Hans-Jürgen
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