What is the bit rate of video on the internet

[Steve Williams]

Hi Amir

First off, thanks for taking the time to answer questions on all things video in our new Ask Our Experts Forum.

I have often wondered if there is an accepted standard for the bit rate of video on the internet. Could you enlighten me (us) if there is indeed a standard and if so what is the bit rate

 

[amirm]

My pleasure Steve.

If one wants the specs for DVD and Blu-ray, it is one google link away. But the answer to the question you ask is not very easily found. The reason we can get the data on DVD/BD is that they are open standards and tools exist that readily display the data rates of content in them. Not so with online video, much of which is provided in encrypted mode with custom and proprietary players.

Let's put some things in context first:

1. Bit rate is a tricky thing. Video can be encoded using different modes. The way it is done for Blu-ray and DVD is called VBR or variable bit rate meaning that the bit rate of the stream is allowed to vary. This is a very good thing since sudden scene changes may require massively more data than slow changing ones. VBR mode allows the encoder to well exceed its average data rate to achieve better quality. Because of this, video for these formats is encoded by hand where an operator tweaks the scenes that don't look good by boosting its data rate.

In VBR encoding, you have two specs then: a) peak data rate and b) average data rate. Peak data rate for DVD is 10 mbit/sec (total for audio and video) and 48 mbit/sec for BD (40 for video and 8 for audio -- 3-D version allows more). Right away we see one way BD is superior to DVD in how it allows much higher peak rate although some of that is offset by the requirement of high definition video (which at 1080p, has 6X the resolution of DVD's 480p).

Average data rate of the disc will be much lower than peak. It is a parameter is determined by how long the movie and its extras are. The higher the average rate, the more space it takes.

As an example, a DVD may have average data rate of 4 to 5 mbit/sec relative to its peak of 10. A BD may have 15 to 20 relative to 48. So there is plenty of headroom there to keep the difficult scenes from falling apart.

The other encoding mode is called CBR or constant bit rate. This is a misnomer as video encoding is never constant. What CBR means in reality is that over a short period (usually a few seconds), the average rate must be maintained. In other words, if I said a CBR video has a data rate of 4 mbit/sec, it means that over that few seconds, it will always average to 4 mbit/sec. Within those few seconds, the peak rate may be lower or higher (e.g. a high of 8 and a low of 2). Contrast this with VBR mode above where the peaks and valleys and go on for for tends of seconds with no worry since we are not trying to cap their average.

What this means is that all else being equal, VBR quality wins over CBR for the same average bit rate setting. So one cannot compare a 4 mbit/sec VBR video to a 4 mbit/sec CBR. The latter will almost always have worse picture quality during difficult scenes since it cannot allow them to have high data rate for much time.

2. The current video delivery mode on the Internet for TV and movie programming is streaming. Streaming content mandates use of CBR encoding (beyond the scope of this short post to explain why). So right away we have a restriction here relative to physical discs.

3. Good streaming solutions (e.g. "Smooth Streaming" used by Netflix) don't use a single data rate or stream. Instead, audio and video is encoded into multiple bit rates from low to high. When you play the content, the player measures how fast it is able to fetch the data. If that speed is high, it will gradually ratchet up the stream that it is trying to play. So what may start at say, 1 mbit/sec, may end up at 3 mbit/sec. Then again, it may also stay at 1 mbit/sec for you, while running at 3 mbit/sec for me. As a result, the two of us could get radically different qualities and bit rates for the same content being watched.

4. Because of above, if you care about quality, try to curtail the use of your broadband connection for other purposed in the home. Use a wired Ethernet connection if you can especially if your wireless link is not high quality.

With those concepts out of the way, the rough data rates for video online are these (which is the combination of audio+video data rates):

1. Below DVD resolution, e.g. 320x240 (quarter resolution) youtube like video is about 300 to 500 kbps. With respect to youtube itself, it is not true streaming in that the youtube player will attempt to read ahead much of the video as it can (i.e. it is not doing "constant" streaming).

2. DVD resolution (i.e. 480p which is 640x480) is about 1 mbit/sec. Yes, the average is 4X lower than what is used on DVD. This is not as bad as it sounds as DVD uses a much older and inefficient video codec called MPEG-2. Online video is usually encoded using MPEG-4 AVC which has 2-3 X better efficiency at these data rates.

3. HD video: 720p to 1080p is from 2 to 5 mbit/sec depending on provider. Yes, as we DVD, not only do we use CBR here instead of VBR but cap the rate at well below BD. Since BD can and does use the same efficient MPEG-4 AVC we don't have the codec advantage either. Nor is online video hand tuned. Given all of this, it is remarkable that online video looks as good as it does in HD, or else, we tend to not be picky about video quality .

Here is an article I wrote that relates to this topic, covering the business and technical sides of online video: http://www.madronadigital.com/Library/FutureHomeVideoDistribution.html

And another one focused simply on data rates of various links and media: http://www.madronadigital.com/Library/Speed of things.html

Here is more on how we store the video samples themselves prior to compression: http://www.madronadigital.com/Library/Video Basics.html

 

[Mosin]

I get great streaming everywhere, except Netflix. The problem there is that the user is required to use the latest version of Silverlight, which has an absolutely insane memory leak on my machine. In fact, it qualifies as a "Sin Against God", if you ask my opinion. Older Silverlight worked for me. Oh, well. At least everywhere else is good. Besides, Netflix is going to HTML5 before too very long.

 

[Bill Hart]

Ironically, perhaps, the smoothest streaming I get is from Netflix. Both Vudu and Amazon HD suffer from buffering, break-up in audio, etc. I am using a hardwired connection directly to the router, less than 40 ft. I have experienced this with different service providers, different players (using the apps for the various media streaming sites) and even different locations, e.g. I suffered the same problems in Austin as I have in NY. In NY, I use FIOS and pay for a good amount of bandwidth. My suspicion is that it has less to do with ultimate bandwidth than with 'stability' of the signal, e.g. the Vudu system in particular seems to be particularly sensitive to fluctuations in available bandwidth. The problem was even more pronounced when I used Cablevision as an Internet provider- though they claimed big numbers on bandwidth, those seemed to be based on peak performance and not steady state. Is this a common experience? I have found this using Samsung, LG and Roku type players. Of course, if I 'buy' or 'rent' the film or program from the cable provider, there is usually no problem, but this is not Internet based delivery, e.g. in the case of FIOS, it is an optical cable that gets converted to 75 ohm coax, runs into the 'set top' box, then outputs in HDMI.

 

[audioarcher]

Ironically, perhaps, the smoothest streaming I get is from Netflix. Both Vudu and Amazon HD suffer from buffering, break-up in audio, etc. I am using a hardwired connection directly to the router, less than 40 ft. I have experienced this with different service providers, different players (using the apps for the various media streaming sites) and even different locations, e.g. I suffered the same problems in Austin as I have in NY. In NY, I use FIOS and pay for a good amount of bandwidth. My suspicion is that it has less to do with ultimate bandwidth than with 'stability' of the signal, e.g. the Vudu system in particular seems to be particularly sensitive to fluctuations in available bandwidth. The problem was even more pronounced when I used Cablevision as an Internet provider- though they claimed big numbers on bandwidth, those seemed to be based on peak performance and not steady state. Is this a common experience? I have found this using Samsung, LG and Roku type players. Of course, if I 'buy' or 'rent' the film or program from the cable provider, there is usually no problem, but this is not Internet based delivery, e.g. in the case of FIOS, it is an optical cable that gets converted to 75 ohm coax, runs into the 'set top' box, then outputs in HDMI.

Hi Bill, I think in most cases you are right about the problem being on the streaming services end. They can only handle so many users at a time. When they have too many users for their capability the service slows down. I think it's just a matter of the streaming companies needing to spend the money to upgrade their system. I only use Netflix but rarely have buffering issues. Sometimes the resolution is low but usually only at the beginning of the program.

Cable On Demand and like services are internet based but it's a private internet. They do not go over the world wide web. I'm guessing they just have a more robust system that they have more control over than the www. Cable phone service works the same way. Over their own private internet.