Recording AC3 sound

[doctor15]

I have been reading alot about video capture lately, and was trying to figure out if this was possible.

I have a Sound Blaster Live 5.1 w/ spdif in and out. If i get a capture card, would it be possible to record off my digital cable in dolby digital 5.1? I have the proper outputs and inputs, but I dont know of any software that would allow me to do that. My guess is I'ld have to use a program to record the AC3 stream from sound card, at the same time as another program is recording video from my capture card. If this would be possible, I'ld definitly buy a capture card.

Anyone know how this can be done?

[Greg12]

You need a dobly digital encoder, which comes with Sonic Scenarist or DVDIt.


TO make true surrond sound, you need 5 or 6 mono wave files, however it won't sound different because in true surround sound, each of the wave files is recorded from a different part of the stage.

A simple 2 channel is all you will make, anyway, AC-3 makes a nice compression, better than using PCM.

[doctor15]

Thanks for the reply!

Im probably wrong, but my understanding is dolby digital comes from my digital cable encoded, and gets decoded when my home theater reciever gets the signal. I can get dolby digital sound simply through a coax digital cable from my cable box to my reciever, so if i want to record the stream coming from the coax digital out on my cable box, and later play it back through my reciever (I also have coax digital run from my computer to reciever), why would it need to be encoded. I would want to play it back in the exact same format as it came from my digital cable in.

Sorry if i didnt explain that very well, and dont know what im talking about. Basically I think the signal is encoded my digital cable box, and decoded by my reciever, so the computer just needs to save the signal to the hard drive.

If you could explain why it needs to be encoded, that would be great.

[Greg12]

yes you can but I think it's decoded and to re-assemble it, you need the actual 6 mono wave files that were used to produce it.

[Nelson37]

I am also interested in this question. If you have Digital Audio Out on the cable box, that should be an AC-3 signal ready to be DEcoded. It should be possible to capture this signal and have a true 5.1 Surround file.

SFAIK, there is no software available that will do this. I would think it would have to be a complete capture program to avoid synch issues.

As a test, try using the digital input on the SBLive and the audio apps that come with it to record some true 5.1 audio, and see what happens. If the card will record, then test to see if it is true 5.1 AC3.

[Greg12]

i know of no software able to break the incoming audio down into it's componante wav files for re-assembly.

[doctor15]

I dont really understand why I would need to break it down for reassembly, but ill try the encoder thing, and see if that does what I want.

An example of what I was thinking, is if I rip a dvd, I get one .ac3 file that contains the dolby digital audio for that movie. THis then plays back on my reciever, which decodes it. Cant I just record a similar file from the digital in?

[Nelson37]

IF you can capture an AC3 file, I believe Besweet or something will convert an AC3 into 6 wav files, However, to burn to a DVD I think the AC3 file is all you need.

The first question is Can it be Captured?, and then Is It an AC3 file?

[Nelson37]

Doctor15, or anyone who has tried this, I am finally getting a Cable box with Digital Audio Out (after they told me they didn't have any and I pointed out their lobby demo unit does). I have the SBLive with Digital In.

Has ANYONE had any success capturing a surround sound file?

[BJ_M]

you can simply record the ac3 file as a wav file set to 48khz sample 16bit mono ... it will sound like crap (loud) but record it at -6dBA level .. this file can be renamed xx.ac3 and is usable .. it will be minus some information in the headers of the origanal but you should be able to still remultiplex it ... you can do this with DTS signal also ...

[Nelson37]

Perusing the SBLive Docs, it DOES say you can record from the SPDIF-in, but also says it DOES NOT support "AC-3 compressed SPDIF". It also requires the Digital I/O option, which I do not have.

BJ_M, how are you getting the signal to your Sound Card? Do you know if what comes out of a Digital Cable SPDIF (RCA-Type) is "AC-3 compressed SPDIF" ?

If my AMP had digital in (which it doesn't), and I then used the RCA out jacks to my SBLive, would I at least get a downmixed Prologic signal? Some thought my cable box with NO digital sound out would do this, but it appears it does not. Would a Digital amp accept the input from new Digital Cable SPDIF and then output something other than an "AC-3 compressed SPDIF"?

I don't want to buy the IO connector if it does me no good, just want to record the best audio I can. All help greatly appreciated.

[Nelson37]

New Cable Box - Motorola DCT 2000 - w/RCA SPDIF, does seem to give a better Prologic signal to my receiver, Front and Rear channels are now definitely distinct and crisp, although lower in volume. This is on broadcasts with "Dolby 5.1 surround sound".

My amp has no Digital In. The Motorola docs say you must connect the "baseband" RCA stereo in addition to the RCA SPDIF as the SPDIF does not carry analog. This is explained in connecting to a VCR, with no SPDIF. Would this indicate that the SPDIF is only carrying a portion of the total signal, or is it the only connection I need for all sound channels?

Motorola docs say SPDIF can output "AC-3 SPDIF" or "uncompressed PCM", but I think the PCM is only for music channels as I see no way to select output type other than "Surround" or "Digital Surround".

Sound Blaster WILL record PCM from optional IO card, but docs say it will NOT accept "Compressed AC-3". Am uncertain about Hercules Game Theater XP.

My goal here is to CAPTURE a true "surround" sound file, and then downmix it to Prologic 2, as this seems the best option for SVCD.

ANY INFORMATION ON THIS TOPIC GREATLY APPRECIATED.

[pacoreguenga]

SPDIF (rca) connector carries ALL channels at the same time on a single wire.
This is what MULTIPLEX stands for.
You can MULTIPLEX any given amount of different signals into a digital stream, just as you can do with Analog RF and A/V signals. You can even MULTIPLEX all TV channels into a single digital stream which can then be sent via satellite using a single TRANSPONDER, (a TP can handle 6 or more streams at once without problem, BTW) which can send this stream down to your sattellite dish, and again wil be DEMULTIPLEXED at home, where your base unit will decode only the A/V signal you desire (the MULTIPLEXED channel).
In the same way, an ac3 multiplexed digital stream contains from 1 to 7 audio channels which your receiver must then decode to their corresponding speakers.

There are many ways to capture 5.1 audio to your computer. One is having a 5.1 capable sound card and a multitrack recording software, like Digidesign's Pro Tools Free (It supports up to 8 tracks) or N-Track Studio. If your soundcard can't decode 5.1 to elementary streams, try a TRUE multitrack soundcard (like the old VIGRA) and record with the above mentioned programs from your receivers' multiple channel outputs to your soundcard inputs.

You can export each channel as a wav file, and encode it back in any ac3 encoder software.

[Nelson37]

Thank you for the software tips.

I understand multiplexing. As you point out, the AC-3 stream can contain anywhere from 1 to 7 channels. At least 3 wiring diagrams for various components indicate, and in one case flatly stated, that the two baseband RCA cables are necessary IN ADDITION TO the SPDIF connection to transmit all 6 channels. As this will complicate my capture solution, I was attempting to determine if this wiring method was unique to this card or common.

[BJ_M]

strange -- we got tonnes of dts and dolby equipment around here and all of only require 1 cable to transmit .. mind you a lot of the stuff is for theatre dolby and dts (which are both different than whats on dvd) but we have comsummer stuff here as well ..
but maybe that equipment you have has some weird hookup ... but for sure you can have at least 5:1 on the digital signal ..
to record it in analog you need a DA convertor .... to record it digitally you would record the digital stream , i dont know how in a pc but i just use a da88 or dat machine ..

[Nelson37]

This isn't stuff I own, but on-line manuals I am browsing. Aparrently the analog ie either for analog VCR or at least the 3-cable setup is not common.

It looks like my choice is between the $21.00 Hoontech add-on to my SoundBlaster or the Hercules Game Theater XP ($75.00).

Are there any rules on when a SPDIF will output PCM or Compressed AC-3? Will standard Audio Coax cable function on the SPDIF?

Has ANYONE captured an AC-3 stream on their PC without using custom studio equipment.?

[doctor15]

I actually have that same cable box, and the hoontech addon card. I have a normal audio coax cable run from my cable box to my computer. On any channel that doesnt have dolby digital, the box outputs PCM, so there is no reason to hook up the analog outputs (at least w/ my cable provider). I havent tried to capture AC3 yet as I'm still waiting for my caputer card to come. I can however listen to audio on my computer from my cable box, so I know the connection will work.

If someone could suggest a good program to capture audio with, I'll go ahead and try it. All I have is creatives crap.

[Nelson37]

Thanks for your response, Doctor15. I am still concerned about Creative's manual stating that the SPDIF on the add-on IO card accepts uncompressed PCM and NOT "Comptessed AC-3".

Do you get sound on the computer with both PCM and AC-3 streams? Is there any indicator what the sound card is receiving, as in stream type?
Or an indication what the Cable box is outputting? If you can hear it both ways then it should capture.

SFAIK, it has to be captured as a WAV file, then I think BeSweet will convert to AC3, which I want to convert to Prologic II Mpeg audio for SVCD. Hopefully, the audio capture can be done with the video but making 2 passes would not be too bad, if it works.

[doctor15]

[quote="Nelson37"]
Do you get sound on the computer with both PCM and AC-3 streams? Is there any indicator what the sound card is receiving, as in stream type?
Or an indication what the Cable box is outputting? If you can hear it both ways then it should capture.
[quote]

Wow, im really glad you pointed that out before I recieved my capture card. I just tried tuning into channels that my reciever reports as PCM (normal analog channels), and I get sound. The I switch to channels reported as Dolby Digital 2 ch, (HBO and every channel I would want to record) and there is no sound from my computer.

I guess I wont be capturing video now..

[Nelson37]

Wait a minute, we ain't dead yet.

Exactly how are your cables connected? If I am right, the card will pass thru AC-3 on the outboard connectors for an encoder but using the spdif -in you may not hear it as it's not decoded yet? Or at least maybe not thru the standard outputs.

A Couple posts above somebody mentioned some apps which appear to be able to cap AC-3 as multitrack wav files. Try one of those or any WAV cap program and see if there is anything at all captured. Remember to set SPFIf-IN as default, or I guess you already have that as PCM works.

I really hope this works or I'm gonna go with the Hercules for $75.00 and see if that works. I'll help all I can and will post all results if I go with the GameTheater XP.

[doctor15]

Can someone please point me to a place where I can download some software to try capturing the audio?

I tried plugging my amp into spdif out, and passing the dolby digital sound through my sound card to the amp, and i still only got sound on analog channels.

[Nelson37]

Doc - I have seen some reliable info that this WILL work.

Goldwave, the Creative app, even windows sound recorder should do it. Anything which will record a WAV file.

There should be a SPDIF which is on the card, NOT on the breakout box. The one on the box is for pass-thru, the SPDIF Line-in is on the add-on card. Make sure you select the SPDIF Line-in in control panel.

I don't think there will be any output from the card in this setup. Just record a few seconds, do PCM channel to make sure it works, then try an AC-3 channel. I'm not sure what will play the AC-3 file, but if you can play the PCM and there is filesize to indicate something captured, then the AC-3 file can be processed for DVD or SVCD, or even VCD w/Prologic 2!!

[BJ_M]

i tried it with an old creative live card with the optional digital input (this was the first generation card and the digital in was a seperate card) ..

it recorded fine from the digital out on a dvd player to the digital in .. i captured ac3 stream and was able to play it in windvd ...

[doctor15]

sweet!

Thank you guys so much.

I'm going to order a capture card as soon as I confirm this.

[Nelson37]

I am getting Digital Audio, PCM captures OK. But when I cap what should be an AC-3 stream, I get rapidly pulsing static. Same when I try an AC-3 sample or a WAV converted to AC-3 with Besweet.

It sounds like somebody is running a dirt bike next to your AM radio, without the backround music, just the static pulses. All 3 AC-3 files are similar, tempo varies a little.

I've tried audio from Cable and DVD, nothing seems to play or convert these files.

Besweet processes the file, but no conversion. Headac3he says the file (renamed from .WAV to .AC3) has an invalid ac3-header. It accepts the downloaded sample file OK, but still no conversion.

I must be missing some small step. Am using XP Pro and a Phillips Accoustic Edge card, any ideas?

[ahfairley]

That's what an AC3 file sounds like when you play it back with a non dolby player. What happens if you mulitplex the file with a video file and write or burn it and play it with a player that will decode dolby sound?

[BJ_M]

did anyone read this?

http://alive.singnet.com.sg/tech/passthrough.htm


to fix your ac3 file if there is no header use ac3fix or add a wav header to it to use it in v-dub with ac3wav

[Nelson37]

PowerDVD apparently doesn't work on XP Pro, even with the update. Headac3ache could not find AC-3 header, or Sanity Check" after running AC3FIX. AC3DEC gives errors on 98, won't run on XP. Besweet will make 6 WAV files but all are pulsing static. AC3WAV no help.

PCM captures give error in Headac3che "Could not find data chunk". PCM caps have some noise.

Size is not as expected, 30 sec PCM cap was 600 K and 30 sec AC-3 was 5,000 K. Also I do seem to get 5 channels from PCM even with the QSurround effects turned off.

I'm less concerned with playing AC-3 than converting, even without Player it should convert to a wav or Dolby Prologic.

Am using standard Audio RCA Coax, will try a Digital Cable as it seems I am dropping parts of signal.

Is there any software which will analyse/repair AC3 or WAV files? Or a good, actual AC3 WAV to download. Tried several, most are bad file or same as my caps, One seems to be a plain WAV - it plays everywhere but seperates into 6 identical WAV files???

[doctor15]

Just so you guys know, im still following the forum, and am having pretty much the same problems with ac3 files. My PCM captures are fine.

Also, to anyone following this, if you have a SB Live, do not use the newest drivers. Everytime after capturing ac3 I got a BSOD and had to reinstall drivers. The older ones (w/o the new interface) worked fine.

[Nelson37]

Tried a VIDEO cable on RCA connection, this has ELIMINATED noise on PCM caps - still problems with AC-3. Get a lot of unexpected frame size errors, others which looks like I'm dropping info or have a non-standard AC-3 file. I have yet to be able to play one, much less convert it.

It looks like an AC-3 file has some specific header parameters and some zero padding, etc. I'm thinking of BBmpeg and how it corrects scan offsets and packet sizes, and De- and Re-Muxing my ATI video caps; is there any sort of "rebuild" utility or something similar for AC-3? AC3FIX doesn't seem to do what I need.

Maybe I need a BFH.