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AC3 bitrate - quality.

 
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espentan



Joined: 25 Feb 2004
Posts: 6
Location: Oslo, Norway

PostPosted: Wed 25 Feb 2004, 8:28    Post subject: AC3 bitrate - quality. Reply with quote

I'm about to burn a 70 minute home movie onto a DVD-R, and I've decided to use AC3 audio, but what is a good bitrate for 2 channel AC3? The default setting in most encoders I've seen are 192kbp, but this seems a bit low to me.

Any recommendations?
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BretL



Joined: 16 Feb 2004
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PostPosted: Wed 25 Feb 2004, 17:31    Post subject: Reply with quote

I've used 224k with good results and use this setting on most everything. If it's a project where audio is a major concern I've used as high as 448k. Just make sure you adjust the video bit rate accordingly.

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RMN
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Joined: 04 Feb 2003
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Location: Lisboa, Portugal

PostPosted: Thu 26 Feb 2004, 2:14    Post subject: Reply with quote

For stereo, 192 Kb/s should be fine, but if you can go higher, no reason not to (at 384 Kb/s it will be virtually indistinguishable from uncompressed audio).

Most commercial movies use 384 Kb/s for 6-channel surround, which adds up to 64 Kb/s per channel (although you can't really make this kind of comparison, because some of the data is shared by two or more channels).

RMN
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espentan



Joined: 25 Feb 2004
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Location: Oslo, Norway

PostPosted: Thu 26 Feb 2004, 13:34    Post subject: Reply with quote

Thanks for the advice, both of you.
Since I only have about 68 minutes of video and an entire DVD-5 disc to fit it on, I think I'll go with 384 Kb/s. The video will look excellent either way...

Have a good one!
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neil wilkes



Joined: 26 Dec 2003
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Location: London, England

PostPosted: Thu 4 Mar 2004, 21:57    Post subject: Reply with quote

If you only have 68 mins of video, I'd stick with 16 bit 48KHz PCM.
Dolby Digital should be 192KB/sec, or 0.192MB/sec for stereo, and 448 for 5.1 surround - according to Dolby Labs, and I guess they should know.
Don't use the higher rates, as you may well run into compatibility issues with some players. Not all players are created equal.

RM - I've never seen a commercial movie with DD 5.1 at lower than 448, Dolby will not let you use the logo if you do this.
The lower rate you quoted would be acceptable for ProLogic encoding, which is matrix driven and an entirely different animal.
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RMN
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PostPosted: Sat 6 Mar 2004, 23:00    Post subject: Reply with quote

neil wilkes wrote:

Dolby Digital should be 192KB/sec, or 0.192MB/sec for stereo, and 448 for 5.1 surround - according to Dolby Labs, and I guess they should know.


If there was a single "right" value, the encoders wouldn't let you use a different one, they would simply pick the "right" one based on the number of channels. As with video, the amount of compression you can apply to audio without noticeable loss depends on the source. Some types of audio compress better than others.

I don't think Dolby would have anything to object to someone using 256 or 384 Kb/s for stereo, for example. Those values (192 Kb/s for stereo and 448 Kb/s for 5.1) are probably just the ones that they feel will produce good quality with any type of source.

neil wilkes wrote:

I've never seen a commercial movie with DD 5.1 at lower than 448, Dolby will not let you use the logo if you do this


I'm pretty sure I have a few DVDs with lower AC3 bitrates (though they may be just 3- or 4-channel, I'll have to check).

Meanwhile, from Dolby's encoding guide:

Quote:
Although AC-3 algorithms are independent of the number of channels coded, current implementations have standardized on the SMPTE-recommended 5.1 channel arrangement: five full bandwidth channels representing Left, Center, Right, Left-Surround, and Right-Surround; plus a limited bandwidth low-frequency Subwoofer channel. AC-3 conveys this channel arrangement with a high degree of transparency at data rates as low as 320 Kbps.


So even if they want you to use 448 Kb/s to be able to use the logo, it seems they admit that, at 320 Kb/s, 5.1 sounds good enough.

Anyway, to use the logo you need to make an agreement with Dolby, it's not enough to use the recommended bitrates.

neil wilkes wrote:

The lower rate you quoted would be acceptable for ProLogic encoding, which is matrix driven and an entirely different animal.


Considering that many people here are making DVDs from consumer-level audio (net movies, MiniDV cameras, Hi8, etc.), meaning below-average source quality, with little or no audio post-production, I'd say that even 192 Kb/s is overkill; it's unlikely to make any noticeable difference from 128 Kb/s.

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neil wilkes



Joined: 26 Dec 2003
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PostPosted: Sun 7 Mar 2004, 13:46    Post subject: Reply with quote

Good points well made.
I guess that I am quoting direct from Dolby 'cos we have signed the TDA agreement, and I was told by them to use 192/448 or they would not pass the encoding & allow logo use.

I can't help myself always wanting to use the so-called "max quality" settings.
It gets me into trouble though - just found (the hard way) a set of 2 discs MPEgged up using Encore's "automatic" preset.
Disc 1 does not work correctly in the customers drive/settop, whereas disc 2 is perfect.
Disc 2 is slightly longer, so a bit of analysis showed a bitrate of 8.5 for disc 1 using Automatic. Seems the Auto preset uses 8Mb CBR, as that preset gives the same results. Reencoding now using CBR/7 instead.
Hope this will work. 8 discs down so far, as they all play on my gear but not his, it's a worry!
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RMN
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PostPosted: Sun 7 Mar 2004, 17:58    Post subject: Reply with quote

Some old players don't like video bitrates above 8 Mb/s. And some players have problems with recordable DVDs in general (which a lower bitrate can help mask, because it gives the player time to retry).

Of course, could also be some problem with the encoder itself. Have you tried TMPGEnc or CCE?

RMN
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neil wilkes



Joined: 26 Dec 2003
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PostPosted: Sun 7 Mar 2004, 18:25    Post subject: Reply with quote

TMPGEnc is superb - I use it all the time.
Used the MainConcept Adobe one for rapidity, and am regretting it.
BTW - if I view the encoded file with Teco ltd's Bitrate viewer, it shows a huge peak at the end of the file, an enormous 13Mb/sec.
What could be causing this?
I am about to email their support to find out if they know the reason too, but any advice would be welcome.
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RMN
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PostPosted: Sun 7 Mar 2004, 23:08    Post subject: Reply with quote

Probably some non-video data. Some encoders use the first or last frames to store project settings, etc., in "user data" blocks. The authoring program should ignore it.

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neil wilkes



Joined: 26 Dec 2003
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PostPosted: Mon 8 Mar 2004, 9:01    Post subject: Reply with quote

Thanks again.
The disc seeme to work, let's hope it works in the customers' system!
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