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Old 21st June 2002, 06:29   #1  |  Link
LoKi128
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AC3 real-time encoding

As I understand it, if you use a multichannel OGG (or some other non-AC3 format) for your audio, you cannot use the digital output of your sound card straight into a DD receiver, because the multichannel audio does not get reencoded into a DD stream.

But now that there is an AC3 encoder out there, could it perform the duties of "recompressing" all the channels back into a DD stream so that any file format can be sent down digitally to the receiver.

I understand that recompressing of audio will degrade it, and if this is done that would have been the third time (AC3->OGG->AC3), but since we are not really storing the 3rd stream, we could use a pretty large bitrate to minimize the artifacts.

I guess this would mainly help in the convenience area. Instead of having 7 cables from the PC to the receiver (in case of an HTPC, 6 audio + 1 video) you would only need 1 video and 1 audio cable. But other advantages would be that any audio format can then be used for multichannel sound.

Something else that would be neat is that if a settings panel is created, we could create different virtual audio devices and then send each different stream to a different speaker in a system. This way, you could have a cheap way of multi-room sound (cheap DD receiver, front channels for one room, rears for the other, or 5 different mono channels).

Anyway, I think the main problem would be CPU usage. After all, you will be decoding the video stream, decoding the audio stream, plus reencoding the audio stream all at the same time. Maybe the current 2GHz processors can do it, but my lowly 400MHz celeron can barely keep up with a XviD stream

So... would this be at all possible? Thanks!
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Old 22nd June 2002, 19:14   #2  |  Link
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It is possible. I do it. I have no idea what the nForce southbridge reencodes at but I am satisfied with the quality. As far as playing back surround sources, I've only played back AC3 files which "pass-through" and vorbis or MP3 files which play as prologic sound. All in all things sound great using the DD encoder of the nForce and the optical digital hookup from my motherboard to my receiver. I'm satisfied with this setup.
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Old 12th July 2002, 09:12   #3  |  Link
tateu
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AC3 DirectShow Encoder?

So, are there any AC3 Encoder DirectShow filters available? Or is it possible to modify the source code to BeSweet's ac3enc.dll to work as a realtime Directshow filter? I know a small amount about DirectShow programming in C++ but have not yet written any of my own filters.

The reason I ask is that I have written a DirectShow based capture application for my Dazzle DVC-II mpeg card and it captures to multiplexed mpeg-2 with mp2 or LPCM audio and to separate video and audio files in mp2 or wav format. It would be nice to also put a DirectShow AC3 encoder in the capture graph to cut out the intermediate step of capturing to wav and then converting to AC3.

Thanks for your help.
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Old 12th July 2002, 09:22   #4  |  Link
DSPguru
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@tateu
if you'd like to write a direct-show ac3 encoder, i can guide you thru the sources, and we'll work on it together.
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Old 12th July 2002, 10:03   #5  |  Link
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Where is MaTTer ? His heart should be jumping up and down when reading this !!!!
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Old 12th July 2002, 18:12   #6  |  Link
tateu
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DSPguru,

Yes, I would like that. As I said, I don't know a lot about it, but any help you can provide would be very much appreciated. Thanks.
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Old 12th July 2002, 18:20   #7  |  Link
DSPguru
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i can help you with AC3 encoding and with digital signal processing, but not with windows programming. that's not my forte..
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Old 12th July 2002, 21:19   #8  |  Link
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Well then, before I waste any of your time helping me with the finer details of the audio encoding, I think my first order of business should be to figure out how to write a DirectShow filter and get it connected to my capture graph/wav files. I am still in the very infantile stages of my programming skills and I'm afraid any help you could provide at this stage would probably fly right over my head.

If I am able to get that working, I can then come back and ask for your assistance with the actual audio encoding functions.

Again, thanks for your offer of help, hopefully I will be able to take you up on it.
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Old 12th July 2002, 21:30   #9  |  Link
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okay, good luck! and let's hope for the best .

Dg.
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Old 13th July 2002, 07:35   #10  |  Link
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@tateu :

have a look at the example filter 'gargle' ( or similar ) from Windows SDK .... this seems to be a good start.
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Old 13th July 2002, 17:48   #11  |  Link
tateu
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Yes, thank you. That is exactly where I started. I'm afraid it's going to be awhile (if ever), though, so if for some reason anyone is holding their breath for this...please breathe in deeply now.
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Old 16th July 2002, 22:34   #12  |  Link
ChristianHJW
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^bump ... be assured i wont loose this thread tateu .. any progress ? everything else we could help with ?
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Old 17th July 2002, 06:31   #13  |  Link
tateu
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I won't lose it either but progress is going to be slow. I don't have a whole lot of time, at the moment. I just got hit with atleast a weeks worth of heavy overtime at work and I'm also trying to get a new version of my DVC-II capture app ready to release.

But I did spend a couple of hours this weekend going through the sample SDK filters; I didn't have much luck, though. I wasn't exactly sure what I was looking at. I know there are input and output pins and they must have a Mediatype associated with them but that's about all at the moment. I sort of know how to setup an array of Mediatypes and I thought I had multiple input pins setup, but only one showed up on the filter. Also, my main goal is to get this connected to the PCM output of my capture card and I was having some trouble with that.

I probably have several hundred hours put into my capture app and, in the beginning, I had a few 30+ hour weekends of nothing but DirectShow code. I'm still not very proficient with it and there are tons of c++ things I know absolutely nothing about, but I've been able to get the job done and have a working capture app that I have released for public use. I need to be able to spend some of that same quality time of trial and error testing with the sample filter code and hopefully I will start to gain some understanding of it all. I have the source code for a couple of other freely available filters also that will hopfully help me.

But I just don't see this happenning quickly. Even without a lot of overtime, I usually only have time enough for programming on the weekends.

As for help...I don't even know enough yet to be able to ask any questions. I suppose you could buy me a winning lottery ticket and then I could quit my job and have more time to devote to this...
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Old 17th July 2002, 16:54   #14  |  Link
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tateu,

you are in the same process as our new MCF dev team member myFUN is ... he is grabbing his way throught Dshow SDK to code the MCF parser filter. May i invite you to join on IRC, openprojects , #mcf . May be interesting to talk to him. We also sometimes have the pleasure to have Ingo Ralf Blum on our channel ( he is logging very often, but not posting too much ), and he can be looked at as being the absolute Dshow expert, maybe only rivalled by Nic . It should be well possible to get some great input from Ingo, if you are interested to do something here.

( IRC client : www.mirc.com )
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Old 19th July 2002, 09:34   #15  |  Link
BlackSun
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On Eugene's homepage you'll find the source code for PCM to VOB, he use this code to send MP3 to the Hollywood+ SPDIF Output. You now have everything you need

Good luck, I'm really interested in this !
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Old 19th July 2002, 12:44   #16  |  Link
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For your convenience, here the link to Eugene website, the PCM2VOB source code will help you a lot

http://eugene7777.tripod.com/

Unfortunately it's unavailable atm (the website) :/
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Old 20th July 2002, 00:13   #17  |  Link
tateu
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BlackSun,
Thanks for the link. I have the code now and will see if it can help me.


ChristianHJW,
Thanks for the invitation. Are there any specific days and times that are better to stop by? Since this is a global community, obviously times when I am available do not correspond to everyone else.
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Old 20th July 2002, 17:41   #18  |  Link
spyder
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Any time is good, but since most of the MCF team is located in Europe, the best times here(I am in Central US Time Zone) are between 10:00 AM and 5:00PM(8:00AM and 3:00PM for you??). This is during the late evening there and most are back from work already.
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Old 24th July 2002, 14:52   #19  |  Link
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Quote:
Originally posted by tateu
BlackSun,
Thanks for the link. I have the code now and will see if it can help me.
Keep us informed
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Old 27th July 2002, 05:22   #20  |  Link
scherian
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Hey,
Does anyone have any idea what kind of processing power is required to encode 5.1 or 2.0 AC-3 in real-time?
Also, is there anyway to output nForce encoded DD5.1 to a file instead of sending it straight to the SPDIF-out?

thx
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