jparnold: CBR vs. VBR (apologies for my mistake)

[GeorgeW]

Hi John,

I must apologize for accidentally removing your post on CBR vs. VBR.

And to everyone else who participated in that thread, I sincerely apologize to also (John, Trevor, Brian).

Somehow, I made a reply, and the site would not respond, So I submitted the reply again, only to find my post double listed. So I tried to remove one of the double posts, and I must have removed the entire post by mistake

Sorry Guys

Regards,
George

[jparnold]

Could the people who posted the last 2 replies (posted between 2300GMT 1st Mar and 0200GMT 2nd March please post again - or was that you George?

[sjj1805]

These things happen.
There were two posts in the thread that I found of interest - in fact in relation to 2 of these replies I created a pointer to the thread in our FAQ.
If it is possible for Devil and Graham skier-hughes to recreate their replies it would be very much appreciated.

Steve J.

[jparnold]

Thanks Steve
However I have the link that Devil left and can remember most of what skier-hughs wrote but if they want to reproduce (for other followers of this post) then go ahead.

[GeorgeW]

Could the people who posted the last 2 replies (posted between 2300GMT 1st Mar and 0200GMT 2nd March please post again - or was that you George?

Hi John,

Those two posts were probably me (they were probably the same/duplicate post I mentioned).

I was mentioning that your idea of going with 7,700kbps video CBR plus LPCM audio would give you a combined bitrate of 9,236kbps.

That could be working fine on your dvd player, but for other dvd players, they sometimes have problems during playback for burned dvd's at bitrates over 8,000kbps (combined) -- and even if it plays in normal speed ok, sometimes using FF/REW can cause stutter during playback.

I often recommend trying to keep total bitrate <= ~7,500kbps (just to be conservative). But that's for a wide distribution.

Regards,
George

[davidddr]

is there any advantage in setting the bit rate above 8000, I only add around 1 hour of video to each dvd and want to get the maximun quality possible

Dave

[etech6355]

davidddr,
It depends how good your source video is. Rule of thumb is the higher the bit-rate the better the quality, but there can be a point where going higher may not improve quality if your source isn't that good. Like converting old VHS tapes to dvd you may not see any improvement going above 6000kbs.
Sometimes the difference in quality between 8000kbs & 9500kbs can only be seen on a large screen TV.
My dvd players don't have any problems playing high bit rates like 9500kbs.
Another reason I use high bit rates is if I need to re-encode them at a later date.

You can do the math yourself.
A standard dvd can fit 4.370 gigs. I use 4000MBytes - 4200MBytes as the base number, this leaves a safety factor for motion menus, background audio etc. Plus I don't like to burn a dvd to it's limits. Usually I like to leave about a 1/4" blank space if possible on the outside rim of the dvd (if possible). Can't always do this though.

This Calculation
(Your_DVD_Expressed_In_MBytes / 1_Minute_in_MBtyes ) = Size on DVD.
(4200MBytes / 1_Minute_in_MBtyes ) = Size on DVD.
So if you encode a 1 minute video of your material at whatever settings and the file is 60MBytes then.
(4200 / 60 ) = 70 Minutes on the dvd.
(4300 / 60 ) = 71.6 Minutes (DVD Maxed out)
(4000 / 60 ) = 66.6 Minutes ( A good safety factor, I use this calculation a lot).
You can also use a bit-rate calculator.

So if you encode 1 minute test files will give you a good idea of the final size.
Different programs will also give you different results depending on their mpeg encoding methods.

[jparnold]

I have noticed that when I compress the audio using LCPM then the MAXIMUM bit rate I can use is 8264 where as when I compress the audio using MPEG I can select a much higher bit rate for the video so that goes with what I read in earlier posts.

I have rendered all 5 projects using 7800 CBR and LCPM and authored the DVD and used GSPOT to show me the attributes of the rendered (MPG) and the DVD (VOB)files and GSPOT indicates that the video part is around 7800 (actually 7801) yet in the window marked "Container" it displays SYS BITRATE 10080kb/s VARIABLE.
How can that be as I rendered using CONSTANT bit rate? Is GSPOT not accurate? Also would the 10080kb/s be correct?
How much 'space' does LCPM use (sorry I have forgotten - it was in one of the posts which accidentally got deleted)

[Devil]

OK, the relevant link is here.

Generally, remember that bigger is not always best. As far as bitrate is concerned higher is often worst. If you want to be reasonably sure of a DVD¡ÓR playing on most set-top players, then never exceed a combined video+audio bitrate of 7000 kbit/s, 7500 kbit/s at the outside. And remember LPCM hogs a full 1536 kbit/s, so that this leaves so much less for the video bitrate. Remember also that a DVD is viewed at a normal viewing distance and not with your nose glued to the screen, looking for pretended artefacts. Remember, as well, that CBR gives constant quality. VBR doesn't and is best used only for long projects (say > 2 h). Finally, remember that it is a mistake to try to be more royalist than the king.

[GeorgeW]

I have noticed that when I compress the audio using LCPM then the MAXIMUM bit rate I can use is 8264 where as when I compress the audio using MPEG I can select a much higher bit rate for the video so that goes with what I read in earlier posts.

That is because the max combined bitrate for video+audio on SD-DVD is 9,800kbps (9,800kbps minus 1,536kbps (LPCM bitrate) = 8,264kbps).

I have rendered all 5 projects using 7800 CBR and LCPM and authored the DVD and used GSPOT to show me the attributes of the rendered (MPG) and the DVD (VOB)files and GSPOT indicates that the video part is around 7800 (actually 7801) yet in the window marked "Container" it displays SYS BITRATE 10080kb/s VARIABLE.

Some software will put the MAX bitrate for the container for SD-DVD (which is 10,080kbps -- the extra above the 9,800kbps is to handle other things in the stream such as subtitles and BOV's, etc...)

Regards,
George

[skier-hughes]

I've been meaning to write out my piece ona web page for a while, so I'll do it later and post a link to it and if Steve would like to add it to the faq or whatever great.
That's of course assuming I'm thinking of the right piece, I can't remember what I had for dinner yesterday, let alone what I wrote in replies to people

[jparnold]

Thanks for all your input.

I have a rather large plasma TV so want the best quality video I can get.

That begs another question. I always thought that video quality was mainly sharpness.
But what does 'quality' include and what is the first to noticeably suffer when we render at a lower bit rate? I can't remember what the maximum bit rate for VHS is but surely we want to render above what VHS is or otherwise what is the point of DV (other than lossless copying)?

Someone (previous post) suggested that some 'pressed' (manufactured) DVDs (movies) are as low as 4000kbs yet most of these (manufactured) DVDs I have viewed have excellent sharpness yet they all share the same resolution as my home made movies (720 X 576 - PAL). So how can this be?
I guess they use a far higher quality rendering software - is that the reason?

[2Dogs]

Most pressed movie DVD discs actually use variable bitrate with around 9500kbps. I think references to lower bitrates are probably confused with episodic TV shows put out on DVD rather than Hollywood movies.

You also have to take into account that Hollywood has access to far superior cameras, giving better quality source video, as well as multi-pass encoding.

[Devil]

OK, I think there may be a misunderstanding.

Hollywood blockbusters are frequently encoded at an average of 4000-5000 kbit/s, but they can peak to 9000 kbit/s. If you have a still image, you can encode, under some circumstances, at 2000 kbit/s and it will look no different to 9000 kbit/s, because all the P and B frames are blank. You cannot compare the 20-odd pass dedicated hardware encoders, costing 6 figures, used to make them with what is essentially a bundled thing on a PC. The bitrate AND quantisation on commercial films are tweaked to optimum and vary even with a single shot. Unlike home-built DVDs, the transitions are quasi ignored, whereas they have the highest bit-rates on your DVD-Rs. Another difference is that the GoPs are variable on commercial products and are fixed on yours.

IOW, there is no comparison.

And VHS has no bitrate because it has no bits; it is not digital, it is analogue. It has a poor quality with a horizontal resolution equivalent to about 200 lines or 400 pixels, compared to 720 or 704 for digital TV. If converted to digital, there is little advantage in using a high bitrate. In reality, there is a serious disadvantage in that it can emphasise the noise level. The S/N ratio of many tapes is abominable.

[2Dogs]

I just don't understand where this "low bitrate" myth comes from.

Lets consider the notional Hollywood movie, encoded at an average bitrate of 4000kbps. Let's assume that the audio tracks take a further 384kbps.

The average movie is about 90 minutes long, so that would require a whopping 3GB of disc space.

You could fit 2 hours 18 minutes of video onto a single layer disc at that bitrate.

In fact the average Hollywood movie uses a dual layer disc, and the movie content generally takes up about 7.5GB. The rest is taken up by extras and "fluff".

Again, checking with the bitrate calculator, you will find that, once more assuming 384kbps taken up with audio tracks, the average video bitrate can use the maximum allowed by the DVD standard of 9406kbps. In fact you could fit 1 hour 53 minutes of video encoded at that rate onto a DVD9 disc.

So I maintain that most Hollywood movies use a video bitrate around 9000kbps, not 4000kbps.

(but of course if a movie with all its extras has been ripped to a single layer disc, you would expect to see such low bitrates)