Bit Rate confusion!

My default import option is AAC 128 KBPS. I don't have custom settings or anything like that. For some reason, the last 3 or 4 albums I have imported have had varied bit rates. One song will be 126, and the next will be 130, and it's driving me crazy! Please help?
I'm using 7.5, in case that helps.

Yes this appears to be either a change in QuickTime or a bug in iTunes 7.5. Variable Bit Rate appears to be permanently turned on at the moment. For now you will just have to accept it or use a different encoder (like MP3).
See here for other discussion on this...
http://discussions.apple.com/message.jspa?messageID=6122846
Patrick

Similar Messages

Target & Max Bit Rate - A Little confusing Need file to be 15 GB Blu-Ray Help????

I have a 2 and a half hour project on PPro CS 5.5 that I want to encode in H.264 Blu-Ray. I want to get it to 15 GB.
What woud be the best settings to do so?
The Target Bit Rate and Max Bit Rate have a lot to do with it, but I am a little confused on what each of them really do.
I just want to get it to 15 GB and make sure the Estimated file size somewhat accurate.
Is 15 GB too big for a Blu-Ray?
Thanks in advance
PPro CS 5.5
Media Encoder 5.5.
Thanks in advance

Hi Jeff,
I just started to build just my menus with my QT files inside of them as a "Build to Folder."
I think I know why it's saying 7.1 GB used.
Since the menus in Encore stop and loop after 30 seconds, I did not like that.
I like to play a full song on each menu and then when the song ends, it starts again. If I did that on a standard 30 second menu in Encore, the song would stop every 30 seconds and start again. This was not acceptable to me.
This does not happen in DVD Studio Pro. In DVD Studio Pro, I can put in a full song with Videos in Video Zones, change the loop time to the length of that song, and the menu would play that song, and loop again when it was over.
I wanted to do this in Encore, so I had to make the menus a little longer than the song I wanted to put in them. I did that.
1) I went into After Effects with the background of the menu I wanted to use.
2) It comes in as a .psd file.
3) I then stretched it out to a length a little longer than the song. (For exaample, I'm using a song that is 4 minutes and 50 seconds, so I stretched out the background of that menu to 5 Minutes.)
4) I then rendered out that newly made 5 minute menu out of After Effects, using H.264 1440 x 1080i, 29.97 High Quality.
5) I then brought that newly made 5 minute background back into Encore and placed it over the original 30 second background in Encore.
6) Now the song and the videos inside the "Video Drop Zones" will loop until the song is over, stop, then the song will start again and the videos keep on looping. Just like in DVD Studio Pro
For this particular job I did this 6 times (1 for each sub-menu) depending on the length of the song I am using for each menu.
1) The first sub-menu is a 5 minute menu which, when rendered in After Effects, came out to be 1.5 GB Big
2) The second menu is a 4 minute menu which came out to be 1.2 GB
3) The third is a 4 and a half minute menu which is 1.35 GB
4) The fourth is a 3 and a half minute menu which is 1.05 GB
5) The fifth menu is another 4 minute menu which is 1.2.GB
6) The sixth menu is a 7 minute menu which is 2.1 GB.
When you add them up it actually comes out to be 8.4 GB just the menus without any videos in them.
So Encore has already compressed them down a little.
It's not the videos that are taking up much space, but these menus.
Since I am building my job in Encore to a folder, I want to see what the actual size comes out to be after it's compressd. This will give me an idea of how big I can make my 2 and a half hour wedding. The Target and Max Bit rate.
I hope that sheds some light on why Encore is saying that the menus and QT files are taking up 7.1 GB already.
I wish there was another way that I can have my menus play for the length of any song without having to go into After Effects and stretch each menu to accomodate the length of a particular song. This takes up a lot of space, but it's the way I like to make my menus.
Thanks again

Problem with bit rate ?

Hi there,
Using FCP7.
After compressing a FCP project with Compressor and burning DVDSP with DVDSP, I realized that scenes containing objects or people in rapid movement where showing jerked images.
The slow motion scenes are fine.
I checked the following
settings for DVDSP burning: VBR two pass;
averageBit rate 4,0,
max rate 7,0
1) the self-contained QT that I started with is not showing this problem.
2) the m2v file obtained from compressor (I use preset setting for high quality compression) does not show this problem when I view it with MPEG streamclip
3) I tried to burn directly the QT file with Toast 10 …get same problem if not worse !.
settings for Toast
automatic re-encoding
average rate 4,0 Mbps
max bit rate 8,0 Mbps
MPEG-2
I guess my problem comes from the bit rate I used ?
I am now confused and I am not sure I understood the impact of the bit rate setting on the quality of the resulting DVD, and I believe that using a high bit rate in DVDSP might result in problems for some clients to play the DVD on their player ?
Could it be something else ?
could someone please give me advice ?
Many thanks in advance
Ivan

I can only think of two possibilities based on the info available:
1. Old DVD player (choking on the bitrate spike)
2. You need to set some compression markers around the scenes while in FCP. This would flag Compressor to pay extra attention to that area. After you output the new clip, I suggest you use the standard High Quality preset (in Compressor) for your program duration e.g., "Highest Quality 90 minute". Don't get creative with the encoding set-up. And ALWAYS used AC-3 audio.

PROBLEM with 24/96 bit-rate USB DAC that worked fine until I plugged in two

I'm using a late 2008 15" Macbook Pro that I bought Jan 2009 right before the SD card models came out. Mac OSX 10.6.4 and iTunes 9.2 are installed. I have a Nuforce Icon HDP and a CEntrance DACport 24/96 bit-rate USB DAC (external sound cards) that still work great right now. Note - 16 bit/44 Khz is CD quality, and these 24/96 files are hi-resolution downloads that are closer to SACD or DVD audio.
I have been evaluating a new third 24/96 USB DAC plugged directly into the Macbook USB port (no hub) which has been working fine for over 2 weeks with 24/96 bit-rate lossless music files in iTunes. Audio Midi Setup is set for 24/96 to match the music file's resolution. After making suggestions to the manufacturer they made some improvements and sent me a new version to listen for the differences.
I listened to the new one for a day and it worked fine, but when I plugged in the original test sample into the second USB port at the same time as the new one in order to compare them, the sound became garbled and noisy. This never happened when either one was plugged in by itself. Note, if I make changes to the DAC's bit-rate in Audio Midi Setup I usually close iTunes first and then re-open it when I am done.
Upon hearing this new problem I unplugged both of the new USB DACs and plugged just one of them back in, but now it causes iTunes music playback to pause when one is plugged in and set for 24/96 in audio midi setup. Prior to plugging in both at the same time, either one worked just fine by itself. There is no reason when unplugging one or the other that they shouldn't start working again. But now I can't play any 24/96 music through either one anymore. In quicktime player the songs don't pause like with iTunes, but there is no sound coming through even with just one DAC plugged in at a time. Then I discovered that if I go into Audio Midi Setup application and lower the bit rate to 24/48 then iTunes will resume playing again and I can hear the music fine with iTunes or Quicktime player.
Basically, my Macbook Pro has now lost the ability to play 24/96 bit rate with either of these new USB DACs, when they both worked fine before I plugged them in at the same time. More confusing is that they use the same USB receiver chip as in one of the DACs that I mentioned above in the first paragraph, and both the HDP and DACport still work fine at 24/96 bit-rate. It's only these two new USB DAC that require me to lower the bit rate in Audio Midi Setup to 24/48 in order to listen through them, and this wasn't a problem as recently as yesterday!
The computer is acting like some setting needs to be wiped out - it's DEFINITELY NOT the USB DAC's that are faulty because they still both work fine at 24/96 on my wife's iMac (I'm afraid to try plugging in both at the same time to the iMac in case I screw that Mac up too).
I have reset the PRAM a few times, and this doesn't fix the problem. I thought maybe the com.apple.audiomidisetup.plist file could be corrupted and deleted it, but that didn't help. I don't know what to try next to get the Macbook to start fresh when trying to use these devices, short of having to re-install OSX.
Please, does anyone have a suggestion of how I can make the Macbook forget it ever saw these two USB DACs, and let me start fresh so they start working normally again? I do dozens of reviews for Head-fi.org and I can't afford to have my Macbook Pro compromised like this, which could prevent me from doing some reviews or evaluating new products.

UPDATE:
As noted in the first post, both of these newer USB DAC work fine at 24/96 on my wife's iMac, so it's clear that they didn't both become defective when I connected them to the Macbook Pro at the same time.
Resetting the Macbook Pro PRAM didn't help, so I removed the hard shell laptop case to get to the battery and remove it, then reset the SMC. After that I could use the original DAC at 24/96 again, but only IF it was plugged into the rear USB port. If I plugged it into the front USB port then the sound was garbled and filled with noise. This was a vast improvement over no sound at all.
Then I switched to the replacement/updated DAC, but I got no sound at all from either USB port at any bit-rate. And returning to the original DAC again now results in no sound again.
Sigh... I will reset the PRAM and SMC again, and see how far I can get with this. I'd still love some help. Thanks!

File sizes and bit rate slider

Hi,
I'm having two issues I'm hoping someone can help me with..
1) My file sizes are much bigger than they're apparently supposed to be. A 10 second clip at 720x576 takes up 32 MB. I've tried both quicktime h.264 and various avi codecs with roughly the same results. A larger 3 minute clip takes up over 500 MB. This is at 100% quality setting. I was told that they shouldn't take up more than 10% of that size so I'm not sure what's going on..
2) In order to solve it I've been told to use quicktime h.264 and adjust the bit rate slider to specific values, but PE7 won't give me that option, I can only move the quality slider. Setting that to a low value gives me extremely poor quality video. Using Sorenson codec allows me to set a bit rate, but at the recommended 1800 k/s I still end up with 22 MB for 10 seconds.
These videos were made through stills and transitions.. When I make similar stuff with windows movie maker the files are much smaller so something weird is going on for sure.
Hopefully I'm making a really obvious error that is easy to point out :D

This is for web use, but mind you there are plenty of sites that offer hd video so it's not a particularly high resolution..
I just don't understand why these files are so big considering the maths.
I've had 1800 kbit/s recommended as a decent compression level for web use. That's 225 kbyte/s or roughly 2.5 MB for 10 seconds. Yet as described above the files are 10 times that size. I don't understand why this is when I actually used this exact compression level.
Further, I was only able to specify bitrate compression with the Sorenson codec, not h.264, which is the one I actually want to use (and which I've been told should have adjustable bitrate in PE7 making me doubly confused..)

Export Settings - Minimum Bit Rate Question (MPEG2-DVD)

Hi,
Curious...just came over recently from FCP and I don't remember Compressor giving me an option to adjust the MINIMUM bit rate when exporting HD projects for DVD. I figured I'd just leave it where the preset has it (at 2.8mbps minimum)...BUT...then I hovered over it and saw an interesting popup/explanation of what it does which kind of confuses me. It says:
Higher values set a higher minimum quality, but reduce quality of more difficult scenes.
The first part makes sense so I figured I'd raise it...but then the second part makes me think I should keep it low. Sort of confuses me. Any thoughts??? FYI: My projects are Weddings with a fair amount of action, etc.
Normally, these are my settings depending unless I can't fit the project onto the disc in which case I adjust:
DVD
CBR at 7.5mbps
...or...
VBR 2pass
Target Bit Rate: About 6.8 or 7mbps
Max Bit Rate: Usually 8mbps
BLURAY
VBR 2pass
Target Bit Rate: 25mbps
Max Bit Rate: 30mbps
Unless someone tells me that I should raise these settings higher for better quality output (if the project size allows of course) these are what I've been using to get maximum quality out of my videos without jepordizing playback due to bit rate max for each media (which I think I read was 10mb and 40 or 50mbps respectively. I was just thrown by the minimum bit rate description above.
Thank you in advance for your help!

The whole notion of a minimum bitrate is crazy unless you have specific broadcast requirements that require you to pad out video to keep the connection alive.
If the encoder can express the image with zero loss in less than the minimum bitrate why would you pad it with zeros to get the bitrate high enough to meet the min?
The idea of a nominal/average bitrate is easy to understand.
The Maximum bitrate is often misunderstood though.
On some encoders the max rate sets the wiggle room (max - average) that can be used if there is a burst of required information.
On other encoders it sets the maximum rate at which the video buffer is allowed to fill as per the specifications. eg Blu-ray is capped at 40Mbps. The Video encode itself MAY EXCEED THAT 40Mbps for a split second but will only LOAD into the video buffer at up to 40Mbps.
For example, if you set constant bitrate at 20Mbps and no Maximum rate when the video first starts loading from the blu-ray disc to the video buffer it loads faster than the blu-ray maximum of 40Mbps. Thus you MUST have a maximum rate defined (for blu-ray compliance) even though it's a constant bitrate. Some encoders do this for you but some leave it up to the user to get right (and thus should provide a max slider and constant slider to set both).
Jeff- absolutely agree. It's very rare that you need to use all 40Mbps available to you. 20 and 30 can look great (depending on the detail in the sceen and how much motion there is).
VBR 2-pass does have value if you're trying to get down to lower rates like 10Mbps to fit a long title on a disc. If you don't action shots with sceen cuts will leave artifacts as they are bit starved.
I'm actually a big fan of Constant _Quality_ (CQ) vs CBR or VBR. You then know what quality you're going to get on every frame and from experience will know how big it will come out to be. CQ is also considerably faster to render because you have no rate control computations to do. x264pro has a CQ option for this very reason.
hope that helps.

Sample & bit rate

I have a two part question:
1) I have a Presonus Firebox that allows me to record up to 24 bit/96k. I am wonder if I should take advantage of this and use those settings in audio midi setup for when I record into GrarageBand? Is it worth the extra file size? Is there a noticeable quality gain? Will 96k work with GB?
2) I know GB is only 16 bit and that CD's that it will be mixed to are at 16 bit/44.1 kHz. But what about if I record a song in GB at 24 bit/96k, mix down to iTunes, and put it on my ipod as an AIFF will the increased quality show up there? What about on an Audio DVD made from itunes with iDVD.
Thanks for any info and/or opinions,
Paul

Not that it's relevant in the context of GarageBand, but it might be useful to clarify the effect of sample rate and bit depth.
The sample rate affects the frequency response. it is mathematically impossible for a sampling system to correctly render any frequency at or above half the sampling rate (trying to do so produces false frequencies - 'birdies ' - known as aliasing). 44.1kHz effectively limits your frequency response to about 21 kHz, 48kHz to about 23 kHz, and so on.
The bit depth affects the noise floor: 16-bit gives a theoretical noise floor of about -90dB below peak: 24-bit gives a theoretical noise floor of about -138dB below peak. -90 is pretty well inaudible: but remember that when adding tracks together the noise, being random, is added (3dB per track if they are the same level when mixed - of course the peak level increases too). The digital noise, if it becomes audible, is far more annoying in its sound than good old-fashioned tape hiss.
In practice, GB's restriction to 44.1/16-bit isn't really that much of a problem: of course 96/24 would be better, but you gets what you paid for!
Incidentally, don't confuse bit depth with bit-rate, which is a combination of the sampling rate and the bit depth, and is fixed for uncompressed files: but of course as you compress files with MP3 or similar you reduce the bit-rate by leaving out information, even though the sampling rate is the same.

Bit Rate AAC Issues

I have been having some issues / confusion : First off I'm not sure if my settings are set right or not. I am importing with the AAC Encoder set to custom.
Stereo Bit Rate : 192 kbps
Sample Rate : Auto
Channels : Auto
Check Mark in : Use VBR
Now is this a good setting? I want @ least cd quality in an AAC format.
Issue: Not just my Imported cd tracks but the tracks purchaced from the itunes store all seem to play @ a Quiet Volume having to crank up both the Machines Volume and iTunes to almost max, plus with the EQ Preamp put to +10dB. It could just be the headphones, they are old.
Any tips or help on settings would be great. Thanks!

You're welcome. I use the default import settings of AAC 128 Kbps. This works well for me, but each person is a little bit different. For example you like 192 better.
I also use Apple Lossless for the live music I download, but when I want to take that music on my iPod I transcode it to 128 AAc.

Streaming multi bit rate and single bit rate

I'm trying to simplify my setup. Sometimes I need to stream multi bit rate and sometimes I need to stream single bit rate. This is due to the internet connections I'm sending from sometimes have poor upstream and multi-bit rate is too much bandwidth to send... So.... We resort to single bitrate. As I understand my encoder and server settings are different for each of these... I created a single and multi-bitrate profile for Adobe Media Encoder. That was simple enough. My question/concern is in regards the server settings. I would like to have a server setting for multi-bitrate and a setting for single bit rate. My goal is to void making changes on the server. I simply want to load the desired Adobe Media Encoder Profile. So, do I need to create a dedicated event for single bitrate? Below is the syntax I send to my server from Adobe Live Encoder. Instead of using 'liveevent' for my event would I just give it a different name? Does this stand true for my .m3u8 files? I'm confused to how to name differnet event/streams....
livestream%i?adbe-live-event=liveevent&adbe-record-mode=record

Hi,
When you use a setting like: livestream%i?adbe-live-event=liveevent then the encoder expects multibitrate streams (the %i is replaced by a number so the streams published will be livestream1, livestream2 and livestream3). You cannot use the same setting in the encoder for single stream. Instead you'll have to use livestream?adbe-live-event=<event_name>. You could create a new event under the same application.
Hope this helps. Let me know if you have any other queries.
Thanks,
Apurva

Music Skips Some Songs After Choosing "Convert higher bit rate songs to 128 kbps" Option

I have so much music and choosing "convert to 128 Kbps" option makes it from 18 Gb to around 6 gbs but sometimes a few songs are ther

Meg St._Clair wrote:
"Convert" tends to be a confusing term. What iTunes does is make another copy of the track at the lower bit rate for transfer to the Shuffle. Your original, higher bitrate file remains untouched.
Actually, iTunes converts the track on-the-fly while loading it on the shuffle. NO copy of the downconverted track is kept. The same process will occur IF you decide to delete the track from the shuffle and then happen to want to put it on the shuffle again.

Video bit rate too high when try to build and burn. Audio is AC3

I am trying to burn a dual layer DVD and when I go to build/format the DVD it says that the video bit rate is too high. I am confused by this because this DVD was burned previously without a problem. Changes had to be made to the project and the files were compressed in compressor with the same settings and now they do not work. The audio is AC3 dolby. Is there some setting that needs to be changed in DVDSP? What am I missing here...
I am using FC Studio 2.
Thanks!
Natalie

I don't know exactly what the total length of the video is in minutes, but I do know that it is 6.8GB for a dual layer. Probably at least 3 hours. I don't know what the target bit rate is? Is what you are referring is to what the compression I used in compressor?
I am using the setting DVD Best Quality 150 minutes 3.7mbps for video and the AC3 for the audio.
I am recompressing all the videos again just in case there was a problem with one of the compressions. I don't understand why it isn't working this time when none of the settings or compression had changed.
Thanks!!

Blu-ray Error: "file already exists", Code: "6", "Audio buffer underflows. Total bit rate too high

Hopefully someon can help me out here - I'm stuck in my tracks. I'm reading through lots of threads on this topic, but don't understand what is going wrong here. CS6. Note: I have used Encore prior to put this material on a disc. I presently have a separate sequence paired down with only about 1/5 of the whole production. 1920x1080i, 29.97, H.264 Blu-ray, VBR 2-pass, PCM inside PP to produce .m4v/.wav. Here is what is confusing me - I tried a target of 10 and a max of 20 and I still get this error. I have tried different max bit rate "Default Transcode" settings in Encore as well. Note the time code listed "0.000000" - maybe that tells someone something. I don't really understand Encore yet, so maybe it is something I set wrong in Encore vs. my exports?

There are many similar, but not identical errors. And I assume you had already followed John's advice to review older threads. It is true they never quite pin a single set of issues down.
What specific version of Encore? Updated to 6.0.2.004? Encore staff once said updates solved some of these errors.
You say .wav for source files in Encore. But what format in the project? ac3?
Your versiono f the error is different from other one I see because it has a "audio buffer" rather than "video buffer" underflow.

Video Bit Rate

I have a question related to progressive Flash video or any
video for that matter. If my encoded bit rate for my video is at
80Kilobits per second and the total availalbe bandwidth available
is lets say 160Kbps, when my video is requested will the download
speed be based on full available network bandwidth or will my video
download be based on the encoded bit rate regardless of total
bandwidth available. I am bit confused about actually download
speed versus how much data is needed to play a second of video. The
reason I ask is I am trying to keep my bit rates at a percentage of
overall network bandwidth. Obviously I don't want to encode the
video with higher bit rates than available bandwidth per second but
I am really trying to understand what is happening with the
opposite scenario. When my video bit rate is cleary under the total
bandwidth available per second will the file download (not worried
about play) be based on total network bandwidth? Hope this makes
sense.

These are the crucial factors:
1. Length of the video in total minutes
2. Maximum and target bit rate for video in mbps
3. Encode audio aiff to ac3/Dolby2 at -31 normalization at 192 kpbs
4. Brand of blank media
5. No paper labels. We need this information.

Bit rate spikes above 7.3-maximum setting?

Hi
I have used a setting of 6.5 ave - 7.3 maximum-2 pass VBR with compressor 1.1 & AC3-192-audio. Burned a DVD-R of the DVDSP2 project and it plays fine in my set top player. However, I noticed when I watched the bit rate display (ave. & max. bit rates- which is an option on the set top player) while playing the DVD-R, the maximum bit rate spiked to 9.4 for a split second on one scene and would jump to 8 to 9 at other times for a split second on a few other scenes. Most of the time peaks were around 7. My question is will this present a problem for playback compatibility on some set top players? Or is this normal & not a problem for most set top DVD players as long as the bit rate spikes are very short in duration? I find it odd that the maximum bit rate setting of 7.3 that I used was over ridden that much to allow 9.4 as a peak-maybe the set top bit rate display is not totally accurate?
I have to burn 300 DVD-R's of this project for mail order distribution (will be burning at 1X using Taiyo Yuden media) so I need to make sure of playback compatibility with most DVD players. Can anyone shed some light on any issues that these sporadic high bit rates might present or is this not an issue for playback compatibility on most set top players?
Thanks in advance...
Della

Della,
Did you try the BitVice demo yet?
I'd be happy to help, so that your experiments with it will be fruitful. If you contact Innobits with any questions, I promise I will answer you personally. You are right in the assumption that bitrate graphs will not tell you any real truths. At least not those you were looking for:-(
Dear you all, Della included of course,
As one of the scientists behind the Bitvice MPEG-2 encoder, I am sad about the time we spend helping people clarify the crazy things they hear. Fortunately there are forums, such as this one, where hopefully clarifying the same things multiple times is not necessary. (Thus the invention of the FAQ)
At BitVice we treat our customers very well, and we are happy to help on this topic also.
There are so many misconceptions about maximum bitrate floating around on the Internet. Therefore, in a perhaps futile attempt, I decided that this might the time for putting my foot down. Hopefully, it may shed some light on the subject, at least to some of you.
No names, but I know that some of you are far better than me to explain technical things in a more "popular" form to members of this forum. If any of you wish to "translate" the essence of it into a more human readable format, please contact me off-line to get a "complete story".
I am not aware of any publically available diagnostic tool, for momentary (peak) bitrate graph calculation, that could be seriously used for MPEG-2 video stream evaluation against what is meant by "maximum bitrate", as defined in the DVD, or MPEG-2, specifications. Please, correct me if you think I'm wrong.
Bitrate graphs are generally created upon inspection of a .m2v file and based on some (non-standardized) time interval.
Depending on how (by which intervals) you are calculating a momentary bitrate of a .m2v file, it may very well vary between 1 to 40 Mbps, even for DVD compliant constant bitrate streams. The MPEG-2 group was obviously aware of this fact and therefore didn't even try to define what "momentary bitrate" is. And, INTERESTINGLY, there is absolutely no need for such a measure anyway, at least not where MPEG-2 is concerned. I hope the following will help to explain why that would be like weighing of a truck by measuring its length instead, regardless whether it was empty or fully loaded.
The DVD spec. has a 9.8 Mbps limit to the video bitrate, and 10.08 Mbps for the total bitrate. However, this has very little to do with what many bitrate graphing applications is trying to tell you. I'd say, unless they offer you to decide which maximum bitrate it is supposed to measure against, it is just pulling your leg. I think this is really sad. You didn't ask, or pay, to be falsely comforted, did you?
It really doesn't work the way you are led to believe. Actually, the MPEG-2 specification is completely silent about how to even calculate any "peak" bitrate (just because it is really beside the point). Instead, this rate problem is handled in a far more sofisticated and clever way, namely by looking at the decoder buffer fullness at all times. This seems to be completely ignored by most bitrate graphing utilities.
Don't get me wrong here, such utilities can still be rather useful for getting a general idea of which parts of a movie requires more bits than others, but that's about all they can do. For your own mental health, don't fall into the trap of thinking that the highest peak (momentary bitrate) is telling you what the max bitrate is. Sorry, if this sounds confusing at first.
The correct way of interpreting the term "maximum" bitrate is
NOT by comparing to any kind of bitrate graph. That is simply because MPEG-2 does not even define any way to calculate such a thing.
Instead, one needs to think of the "maximum" bitrate as the maximum CONSTANT sustained rate, at which a decoder is capable of reading (or pulling) the data from e.g., a DVD disk. Just think about it for a little while and you will understand that it is more a property of the decoder/player than of the MPEG stream (file) itself. Of course, the responsibility of meeting these requirements rests solely on the MPEG-2 encoder. However, and this is very IMPORTANT, the encoder is COMPLETELY FREE to chose ANY strategy or policy to ascertain that the buffer requirements are complied with. That alone, is enough to leave bitrate graphing utilities in the dark of what is really going on.
If the time it takes to display some already buffered frames (which could amount to say 50 to 300 ms, as counted in display time, depending on the situation) is too short for the next (big) frame to be pulled in its entirety from the disk, then the decoder buffer will underflow. Meaning; that no complete frame was received in the buffer by the time next frame was supposed to be decoded. This situation is where you can honestly say that the bitrate of the stream was higher than the decoder was capable of reading.
However, this is a very tricky thing to capture in a bitrate graph just by inspecting the stream, unless you know everything about the current state of the buffers in general (meaning that its recent history is important) AND more specifically knowing at which rate the decoder is supposed to be pulling picture data from the disk, including, of course, any imperfections in the DVD media.
Given this maximum sustained "pulling" rate, which actually corresponds to the "ceiling" setting in BitVice terminology, it is up to the MPEG-2 encoder to create a stream (.m2v file) that will never, neither overlow nor underflow the decoders' buffers. For those who think a step further on this matter, it means that there exists no common time (duration, measured along the display-time axis, or a number of frames) over which one can calculate a peak bitrate. So, however you choose to do; averaging number of bits over one second, over half a second, over a certain amount of frames or whatever, you will NOT find the correct answer to how the bitrate graph should look like, if you want to be able to interpret the peaks as reflecting what the max bitrate is.
Let's agree that an MPEG-2 stream NEVER has a constant frame rate, as seen by a decoder reading at a constant "maximum" bitrate. From this follows;
Assuming a duration, as counted in display time, by which you can divide the number of bits to get a bitrate, is rather pointless, and also wrong. However, that is generally what is assumed by a bitrate graphing utility.
An MPEG-2 video stream NEVER has a constant frame rate, as seen by the decoder. One frame may be swallowed 5 ms and the next may take several hundred milliseconds to completely enter the decoder buffer.
This is because every frame has an individual size, which may vary most considerably (even for CBR), but the decoder reads the stream of them at the same (normally maximum, see the PS below) bitrate. So, while the decoder is displaying one single frame on your TV monitor, it may be receiving and decoding several successive smaller frames. On the other hand, while receiving/decoding larger frames (e.g., I-frames) it will have time to display many previous frames on your monitor, before a single later, and bigger, frame has been completely read into the decoder buffer. I hope this explains why you should be careful about how you interpret any bitrate graphs, especially when maximum bitrate is concerned. (That's the primary reason why we have not designed such a tool for our customers yet).
Instead, and contrary to many other encoders, we have designed the bitrate control in BitVice in such a way that;
1) The target bitrate that you choose (the average over the whole movie) is enforced, extremely accurately, usually less than 0.1%. I have seen files generated by other encoders which have been 200% off the desired bitrate, though. (To get the true average bitrate of an m2v file; divide its number of bits by its duration expressed in seconds. Then you'll see what I mean. Extremely simple math for a 12 year old kid, although the numbers may be big and therefore tend to have many digits in them;-))
2) The "Ceiling" bitrate is NEVER exceeded, not even by one single bit/s. Now, frankly, if your bitrate graphing tool says something else, then it is telling you more about itself than about a file generated by BitVice.
There is sort of a contract between encoder and decoder, like this, where X is the maximum bitrate:
Decoder:
"I promise you that I am able to read your video stream at X Mbps, or less, for as long as you wish."
Encoder:
"OK, then I promise you that my video stream would never need to be read any faster than X Mbps, at any time, and your buffer will never overflow or run dry. However, if your buffer is less than THIS big (the needed buffer size is written at the beginning of the stream, normally 1835008 bits or 224 kB), then you can as well stop already.
I don't know how every other MPEG-2 encoder works in this respect, or if bitrate graphs could be of any help in evaluating them. The only thing I can promise is that with BitVice you can rely on its bitrate control to deliver what you asked for. You could use BitVice in your evaluation of different bitrate graphing utilities though, if you like.
I'll stop here, because I sense that even those of you who have read this far may want me to;-), but, this topic is enough to fill one whole chapter of a book that I may never find the time to write.
I realize that this post may seem controversial at first, so I'm prepared to follow up on any questions, suggestions or comments that you might have.
Kindest
Roger Andersson / Innobits AB, makers of BitVice MPEG-2 encoder for Mac
PS.
Of course, a decoder is assumed to completely stop reading (pulling) from disk whenever its buffer gets full, but then continues at full speed (maximum bitrate) again, whenever the buffer is NOT full.
So the true bitrate graph, according to the decoders' view, is be very easy to recongize, becaue it will just alternate between two values, 0 Mbps and the max bitrate.
Ds.

Controlling output video bit rate in FCPX

I ran into a problem: I created a mp4 file with Share/Export File… (settings: Computer, H.264 Faster Encode, 1920x1080 - edited from an HD camera): its video always freezes at a definite point when played on my BD Player connected to my HDTV through HDMI (audio instead continues on the frozen image).
It seem that the issue is a sudden video bit rate pick of almost 50 Mbps while its average is around 20-30 Mbps. It is not clear to me if this freeze is caused by the BD Player or by the HDTV not being able to handle such pick.
As a test I used Compressor to control the output bit rate (settings: average 20 Mbps and pick 25 Mbps) and this seems to fix the problem. To be more certain I should do more tests, but I'm confident I found a possible solution.
I'd like to know if there is a way in FCPX to control such picks (without using Compressor), either in the Share commands (but it seems to me that they don't provide enough setting controls) or in the timeline. By watching the Video Scopes I can see a quite strong signal in the area where the output file has the bit rate pick. But I don't know any way to have a measure useful to identify them before sharing and avoid a trial and error approach...
Piero
Below the critical point...

Tried to upload a PDF instead of PNG, but no luck. The picture shows a quite standard (I'd say...) image but with highly dense graphic of Y' component in the Y'CbCr Parade (in the limits of 0-100 IRE): much more dense than most images in the rest of the movie. So I assume this might mean a higher bit rate... but it's just an assumption.
Let me summarize my tests:
1st test: FCPX: Share/Export File.. (settings: Computer, H.264 Faster Encode, 1920x1080)
- copied the .mp4 file to USB drive
- USB drive into a BD player connected to HDTV through HDMI >> freezing effect
2nd test: FCPX: Send to Compressor - Compressor: Create Blu-Ray Disk (on Hard Drive) and Video Job with standard Video settings (Automatically select bit rates = ON)
- Played the .img disk with "Mac Blu-Ray Player" on my iMac >> same freezing effect in same position as before on HDTV
3rd test: FCPX: Send to Compressor - Compressor: Create Blu-Ray Disk (on Hard Drive) and Video Job with customized Video settings: Automatically select bit rates = OFF - Average = 20 Mbps - Maximum = 25 Mbps
- Played the .img disk with "Mac Blu-Ray Player" on my iMac >> NO MORE freezing effect (and smaller file by 30%)
I might do more tests (mainly on my HDTV) but I believe the point is the bit rate... and the only way to control it is by using Compressor.
So now my question: is there a color setting that helps in controlling such situation ? e.g. lower saturation, or lower exposure, maybe in mid tones or highlights, or what ?
Thanks so much for your help
Piero

Bit Rate confusion!

Similar Messages

Maybe you are looking for