Bitrates for encoding and AC3 Audio??

[franco]

I have found these bitrates suggested for a 2 hour DVD from VHS footage "average 4800 - maximum 6800" would that be about right?
I have been watching my encoded footage through Windows Media Player and it all looks the same.

Lastly, since my audio is captured from VHS should I set the audio bitrate to 224 or less?? I plan to convert to AC3 for authoring and I want good compatibility. Would 192 or 160 for AC3 audio cause compatibility problems when I author??

[teegee420]

Where did you get those numbers from? The average bitrate looks about right but I'm not sure why that max bitrate was suggested. In the future, do the math yourself with this Bitrate Calculator. You might want to encode with a 352x480 resolution because you can't use a higher bitrate. If you use 720x480 you may end up with some blockiness. Personally, I wouldn't go any lower than 192kbps for AC3 audio just for quality's sake.

[W_Eagle]

I use 192kbps for AC3 audio from analog Satellite TV input and get excellent results, maintaining Dolby Prologic. So, I don't see any use going any higher, definitely not form VHS source.

[franco]

The suggestion for the bitrate and audio at 224 all seemed to be suggested for maximum compatibility. I am hoping to sell some DVD-R discs so it is a bit more of an issue.

Maybe the person was wrong, but either way I can't see any difference between different bitrates in windows media player, watching MPEG2 from VHS footage.

[adam]

Compatibilty has nothing to do with it. The DVD standard supports up to 448kbits for Dolby Digital AC3 audio. This rate or anything lower is equally compatible on all hardware setups.

For 2 channel audio the typical rates are either 192 or 224kbits. If you only have one audio track then just use 224kbits. The amount of space saved by using anything less is going to be negligible, and your video bitrate is high enough as it is. As for the video max, there is no reason to go that low, it lowers quality and gains you nothing. With 224kbit audio I'd say use a max bitrate of around 9400kbits, and even that is being conservative.

Since compatibility seems to be a primary concern for you try reading the DVD Demystifed FAQ (http://www.dvddemystified.com/dvdfaq.html) and maybe some of the technical notes on mpeg.org. As long as you fall within the requirements listed for the DVD standard, your disk will be fully compatible on all DVD compliant hardware. There's no reason to have to trade off one resolution/bitrate whatever for another. The DVD standard supports well more than you need for a VHS capture.

[franco]

Thanks for that, Adam. Very comprehensive.

[offline]

I use 192kbps for AC3 audio from analog Satellite TV input and get excellent results, maintaining Dolby Prologic

Hey W_Eagle,

Did you mean to say ".. maintaining dolby surround 2.0."

AFAIK there is no support for Dolby Pro Logic in AC3. DPL needs LPCM or mpeg-2 stereo audio. If you are getting DPL 1 from a stereo AC3 source then I like to know how

[adam]

DPL is purely a decoding method. It requires no special encoding matrix like Dolby Surround does. All a DPL decoder requires to output DPL audio is a stereo signal. That can come from LCPM, mpeg audio (if the decoder supports mpeg audio of course), or AC3.

[offline]

Thx for your reply adam.

DPL is purely a decoding method. It requires no special encoding matrix like Dolby Surround does.

DPL II may be a pure decoding method but you can argue that DPL 1
is not as it greatly benifits from special analog mixing of the stereo track.

All a DPL decoder requires to output DPL audio is a stereo signal.

Yes, but again without special mixing the "surround" effect is pretty well absent.

That can come from LCPM, mpeg audio (if the decoder supports mpeg audio of course), or AC3.

Here is my point of interest. An A/V receiver will detect and decode LCPM, and mpeg if supported, and allow the output to either be left alone or mixed with a number of signal processor settings - one of which is the DPL chip. With AC3 I did not think you could have that option -> i.e. there is no option to feed AC3 through DPL in a single receiver. Am I wrong?

[adam]

DPL II may be a pure decoding method but you can argue that DPL 1 is not as it greatly benifits from special analog mixing of the stereo track.

Yes, but again without special mixing the "surround" effect is pretty well absent.

EDIT: Woops had some major mistakes in my post.

Both DPL and DPL II are matrix decoding methods. Your right, DPL requires that the audio be Dolby surround sound mixed. But this can definitely be done quite simply with AC3, and most audio sources you will run into will already be mixed for Dolby Surround. In the example above, if he took his audio off his broadcast and encoded it to AC3 he should have no problem getting Prologic output.

DPL II is also a matrix decoder but it doesn't require the audio to be anything more than 2 channels.