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Old 10th January 2003, 00:57   #1  |  Link
Quilty
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Recording AC3 *a challenge for the best and brightest!*

I am trying to make some DVDs of some laserdiscs I have. Now - my sound card has an SPDIF in, and I wanted to record the AC3 track. I could not do it directly from the RF of the laserdisc player as it is RF. Then I hit upon an idea to hook my Amp up, which has an ac3 rf in and also a standard SPDIF out. Sure enough it worked a treat.

The stuff it recorded into a 48khz stereo file WAS Ac3 and could be run back out to thr amp. I recorded it in Sound Forge 4.5 and then saved it as RAW. One thing that was odd was that because it thought it was PCM it left gaps in the audio track - so basically it has the AC3 frames and then space and then the next frame etc. And it is about 3 or 4 times the size it should be.

Now THE CHALLENGE! Who knows of a piece of software that will take my RAW file and rebuild the AC3 into a proper AC3 file suitable for burning to dvd WITHOUT recompressing it (obviously after all this I don't want to loose any quality by having to recompress)

Can anyone else offer any helpful advice? Surely I cannot be such a "pioneer" ...

Cheers,

Quilty
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Old 10th January 2003, 07:15   #2  |  Link
DSPguru
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1. prboably BeSplit could eat it as 'ddwav'. read q24 in the faq.
2. please post a small slice of this raw, so i can analyze it.
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Old 10th January 2003, 08:30   #3  |  Link
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What sound card are you using? I want to do the same thing with DTS laserdiscs which I would imagine would be much less trouble due to it being the same bitrate as PCM... I thinking of getting an Audigy 2 for the task.

Also... what software did you use to record the stream?
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Old 10th January 2003, 14:01   #4  |  Link
Quilty
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I asked for the best and brightest .... and you did not disappoint

It worked! Many, many thanks! Ah the joys of a certain sci-fi trilogy in proper AC3 .... ahhhhhh.

I wouldn't try it with the Audigy ... I think all the creative cards resample the signal and kill the AC3. You need a card that does pure digital copies of the SPDIF bitstream. Mine is a C-Media soundcard built onto the motherboard.

No clue how one would go about fixing this with DTS. What seems to happens with the AC3 is that it gets a packet each fraction of a second and then nothing for a fraction of a second before the next AC3 packet. Hence you end up with a file that is rather like an Ac3 WAV with the gaps in there. It's not possible to rename it to .ac3 then author with this as the program cannot seem to read the length of the ac3 file. Also the file is about 4 times the size it should be due to all the redundant gaps in the stream.

But besliced cut it to perfection!

Well done!!!

Best,

Quilty
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Old 10th January 2003, 15:54   #5  |  Link
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Cheers
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Old 13th January 2003, 01:35   #6  |  Link
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Thanks for the heads up on the Audigy... I was just about to get one of these, glad I saw your post first! Might look into something from M-Audio instead.
What software did you use to record the stream?
Also are you using the ac3 track for a dvd project? I'm just curious how it goes because coming from LD the stream will be 44.1khz rather than 48... and I'm going to face the same problem when I start doing the DTS LDs.
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Old 15th January 2003, 10:28   #7  |  Link
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Quote:
Originally posted by Quilty
It worked! Many, many thanks! Ah the joys of a certain sci-fi trilogy in proper AC3 .... ahhhhhh.

I wouldn't try it with the Audigy ... I think all the creative cards resample the signal and kill the AC3. You need a card that does pure digital copies of the SPDIF bitstream. Mine is a C-Media soundcard built onto the motherboard.
I wanted to do the exact same thing with my SW LDs also.

There was another thread in this audio forum where a guy explained how he had managed to record the AC3 spd/if signal out of his Home Theater amp via CoolEdit.
His sound card was a rather high-end one, which allowed to record digital sound without assuming that the stream was a 2-channels stream. At that time, he had insisted on the fact that, if your sound-card let you adjust the volume of the recorded digital stream, it meant that it was assuming the stream was a 2-channels PCM stream and it was no good.

You said you recorded the digital stream under Sound Forge as a raw PCM stream, right? Does this mean that you were able to adjust the volume of the stream during recording? If yes, how do you adjust it? Is it possible to use the volume meters often included in sound editing software?

My sound card is a Hercules GameSurround Fortissimo II, which has an optical digital in connector, but I know its digital-in is meant to record 2-channels PCM sound and the volume can be adjusted.
Do you think it will work like your sound card or does it mean that it resamples the stream?

If anybody can help...
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Old 12th February 2003, 06:42   #8  |  Link
Jeff D
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I'm looking to do the same thing with the AC3 tracks from a laserdisc trilogy.

What software did you use? I'm borrowing a soundcard with optical and coax digital in from Turtle Beach. Apparently the sound card was OEMd by some other sound card manufacturer. I don't have the model nubmer, but TB only released 2 cards like this and I believe they are almost the same card.

Right now I'm just putting the pieces together.My plan was to do an AC3 capture. Then convert the AC3 to the 6 audio waves to edit it so I can match the waves to the captured waves. I'll then take the edited waves and encode them back to AC3. The reason for these extra steps is becase I don't currently have a way to capture AC3 directly with a video capture.

Any advice on tools would be great. Thanks!
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Old 18th February 2003, 21:59   #9  |  Link
rishardc
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Has anyoe worked on this since the initial postings? I am quite interested in this myself.
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Old 18th February 2003, 23:52   #10  |  Link
DIggedy
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I'm keen to work on this myself but still can't figue out what soundcard to buy... anyone have any recommendations?
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Old 14th April 2003, 17:41   #11  |  Link
HeinePaul
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Any luck capturing AC3? What software?

How goes the battle to capture AC3 files through coax SPDIF? In my case, I'm trying to timeshift DD5.1 audio and Svideo from my digital cable set top box. The video, thanks to software found here, is a snap. But the folks at Dolby are making the audio a bit more difficult.

While I have purchased an M-Audio Revolution for playback, it doesn't have an SPDIF input. And it was bought with the intention of replacing the Live! Platinum with LiveDrive that I also have.

I'm willing to buy a card that will do NOTHING but bit-perfect SPDIF input if there are software applications that will eventually result in an AC3 file. I've seen lists of full-featured sound cards that will apparently work, but it's the software combination that I'm really curious about.

What do you folks recommend? I don't want to spend $500 on software unless I have to.

Is it a matter of money, or am I still on a quest for the Holy Grail?

Last edited by HeinePaul; 14th April 2003 at 17:45.
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Old 8th May 2003, 23:05   #12  |  Link
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The cooledit technique works great. As far as a good cheap card with bit-for-bit accuarate capture, the prodigy 7.1 from audiotrak works well for me.

I was wondering if any of you guys have been able to play the ac3 stream from S/PDIF (on my computer -- NOT on an external decoder) in real time, and if so, how?
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Old 19th August 2003, 22:36   #13  |  Link
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LD audio, record at 44.1 and resample vs. record at 48

I know digital audio on laserdiscs is 44.1khz, like CDs, while DVDs have 48khz
so my question is, when i record digital audio from an LD for use on DVDR what would be better: record at 44.1 and resampel later or record at 48?
a favour 44.1 because the first thing i wanna do is a dpl2 upmix and so i want the audio to be as exact as possible
but maybe i overlooekd something, so please post if you thin kyou can contribute to the subject (also generally if no upmix or otehr editing is planed)
thx
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Old 19th August 2003, 23:00   #14  |  Link
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Keep 44.1, it will be better.
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Old 20th August 2003, 00:49   #15  |  Link
E-Male
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first of all WTF is my post doing in this thread??

second: AFAIK I CAN'T keep 44.1 on DVD
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Old 20th August 2003, 09:17   #16  |  Link
Fr4nz
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Quote:
Originally posted by E-Male
first of all WTF is my post doing in this thread??

second: AFAIK I CAN'T keep 44.1 on DVD
Because your argument was already discussed.
Anyway go for 48khz if you don't have another choice. That's simple
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Old 20th August 2003, 19:07   #17  |  Link
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Re: LD audio, record at 44.1 and resample vs. record at 48

Quote:
Originally posted by E-Male
when i record digital audio from an LD for use on DVDR what would be better: record at 44.1 and resampel later or record at 48?
I have never seen a spec that says that LD digital audio is always 44.1 kHz. It could conceivably be 48 kHz, as SPDIF can carry either as well as other sample rates. But it doesn't really matter:

How are you recording the audio from the Laserdisc? Via analog 2-channel, SPDIF digital 2-channel, or RF AC3 5.1 channel?

2-channel Analog L/R Recording
------------------------------
1. Record at 48kHz
2. DPL2 Upmix to 5.1
3. Encode & author

2-channel Digital SPDIF Recording
---------------------------------
1. Record digitally via SPDIF. The sound card should force you to record this at the same sample rate as the input.
2. DPL2 Upmix to 5.1
3. If the samply rate wasn't 48 kHz, use Sound Forge or CoolEdit Pro to resample all 6 .wav files to 48kHz
4. Encode & author

RF AC3 5.1-channel Recording
----------------------------
Look Here
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Old 20th August 2003, 19:54   #18  |  Link
E-Male
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it's 2 channel PCM audio
I want to record via optical spdif

"The sound card should force you to record this at the same sample rate as the input." <-- really? I'll try that, but i think mine let's me choice

and in every LD FAQ I read it says that LD has 44.1
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Old 21st August 2003, 03:35   #19  |  Link
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Yes all audio, be it PCM, DD or DTS on Laserdisc is definitely 44.1khz.
Record natively in 44.1khz to avoid problems down the track, and resampleto 48 afterwards.
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Old 21st August 2003, 06:18   #20  |  Link
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Quote:
Originally posted by DIggedy
Yes all audio, be it PCM, DD or DTS on Laserdisc is definitely 44.1khz.
Oh, but you're wrong! I don't know about PCM or dts, but DD on LD is NOT 44.1kHz -- it's 48kHz. 100% guaranteed.
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