View Full Version : AC3 (DD 5.1) Realtime Encoding

2005-04-18, 06:35 PM
This was originally posted in the support thread, but it seems a better fit here.

I used to use AC3Filter for my AC3 needs, however, I could never get it's realtime encoding to work correctly. While doing some research I found out that the most recent versions of FFDshow enable realtime AC3 encoding. I don't have my A/V receiver hooked up with analog outputs, and only connect through the digital output, so it gets annoying not being able to hear my center channel when playing strereo movies and shows through the PC. So for those that want to use the realtime encoding of FFDShow Audio here is how I got it done. Reven can verify the steps for 5.1 Analog as I helped him get that working. You also will need a 2005 version of FFDShow. I have this working on an Athlon XP 2400+ with 256mb, sound blaster live, and connected to a Kenwood THX A/V receiver via COAX digital input.

For analog output of 5.1, you need the following filters enabled:


The following filters need to be in this order:


Mixer tell it to mix the channels as 3/2+LFE
LFE enable the cross over and set it to about 120
Volume enable this, and set to Normalize, and View current volume level. Good for debugging purposes.

Now if you want to go to a complete digital route, you'll need one of the most current versions of FFDShow as it includes the ability to do Real Time encoding to AC3 output.

Codecs in FFDshow need to be setup as the following:
MP3/MP2 disabled
DTS - SPID/F or enable this to the default handler if your digital receiver doesn't handle DTS.
Uncompressed - all supported types

Filters in the following order:
Resample - 48000 (it is a good idea to resample to the correct AC3 Supported audio level).
Mixer - 3/2+LFE
LFE - 120
Volume - Normalize, View current volume levels

Output - set this to AC3

Now whenever an a video is played, if it has an MPEG 2 or MP3 stream, the uncompressed audio will be converted to a 3/2+LFE stream, and when OUTPUTed will be encoded to an AC3 digital stream sent out through the SPID/F digital output.

AC3Filter has similar support for realtime encoding, and doesn't produce very good results (lots of pops and hisses). The AC3 encoding seems to be pretty good for FFDShow Audio.

2005-04-18, 11:26 PM
I'll have to give this a try on "production" machine. I probably wont get time to try it until the weekend.

2005-04-20, 04:12 PM
I've been using the digital setup for the last several days, and really like it. Much nicer to hear voices from the center channel instead of the left and right channels.

If you have Quicktime Alternative installed, it'll even convert the audio from the quicktime files to AC-3 streams as well.

2005-04-20, 04:40 PM
I've justed configure mine remotely using VNC, but its 4:40AM (thats dedication for you) so I resist the urge to fireup the TV and wake up the whole how. I'll try it a little later in the morning and report back.

2005-04-20, 05:11 PM
I just followed the setup...everything took but when I set output to AC3, it wouldn't do audio and bomb out GB. I turn off that one thing and go back to integers, it's fine.

Mine is an analog setup, going into a sony w/ prologic 2 (so I opted to use the PRII for mixer). I set everything as drescribed, and in places there were choices:

ffdshow ac3 to liba53
turned off ac3filter
Uncomp to all supported
dind't check any item in Mixer (norm, voice, etc)

I then went to I/O, and checked AC (which unchecked all integers). Boom. Changed bitrate to default for tv. Boom. I turn off and itegers back on, fine.

Btw, thanks for putting this together. Wiki should collect the "tips from users" like this as an expanded manual.

2006-03-21, 12:44 AM
capone, i had this problem. gbpvr kept crashing if i set Output to AC3 in ffdshow audio configuration. figured out that i had the wrong version of ffdshow. i downloaded ffdshow with SSE support. what my AMD 64 supported was SSE2. so i downloaded ffdshow with SSE2 support and now things work fine.

http://www.free-codecs.com/download/FFDShow.htm (read the notes part)

2006-03-21, 05:06 AM
spoke too soon. that wasn't the problem as i still have gbpvr crashing. windows media player is also crashing too so it's something to do with the codec i think.

2006-03-21, 05:32 AM
ok. i think i have everything working. my only problem is mp3's are also affected. that is, music is coming from center channel. i did disable mp3 codec in ffdshow. isn't that suppose to do the trick?

2006-03-21, 12:32 PM
My setup was slightly different.

- I have MP3 set, the I also set all AC3-related (DTS, LPCM, etc) to ffdshow codecs and not spdif
- checked only 16 & 32 interger in processing
- set resampling to 3/2 w/ LFE checked
- checked only AC3 under output, and set to 224 (same as mpeg out)

2006-05-15, 02:50 AM
finally got s/d-dif out to work with the ac3 filter control panel very nice stuff wish they would update it in the next few years. :PPP
now what about dts audio?

2006-05-15, 03:41 AM
hurray for ffdshow!!!!!!!!!!!!!!!!!!!!

2008-02-27, 12:38 AM
I don't know if I'm missing a setting or it simply can't do it. I'm using a digital receiver with DTS and PLII and connected with the optical cable.

I can get all AC3 and other kinds of stereo turned into PLII, but it's also turning the broadcast 5.1 AC3 into PLII. If I set it to keep broadcast AC3 as-is to preserve 5.1, TV broadcast stereo AC3 goes to the receiver digitally and stays in stereo. I would have thought the receiver could do it on its own, but maybe not. The best I can do on the receiver itself is 5-channel stereo.