Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Matt]

When I play HDTV with AC3 audio, and decode the AC3, the peak audio level is low.

From watching the "Peak Level" in DSP Studio, it stays around a few percent.  I can safely add 11-15 dB without any clipping.  This is during the loudest part of commercials (which are louder than the program).

I wouldn't expect this much headroom in the signal, so I'm wondering if we should bake in a +10dB (or whatever) gain?  Or if it's attenuating channels to provide LFE headroom, or something else?

Does anyone have the tools to make a sample 5.1 file that plays at 100% on all the channels?  It would be interesting to see what the decoded levels looked like.

I haven't compared ffdshow to AC3Filter to LAV, but I should do that too.

Thanks for any insight or sample files.

[Matt]

This link is helpful:
http://ac3filter.net/wiki/AC3_overflows_in_movies

The samples do go right up to 100% (and a bit more with AC3Filter, whereas ffdshow clips to 100%).

It would be interesting to run some movies and television through the Audio Analyzer to see what their peak level is.  If we could do this, we could use that information at playback to set levels better.

[raym]

Matt,
I don't know if it helps but perhaps MC you could leverage from this. I use AC3Filter for TV in particular. It's one decoder that seems to do a very good job of normalising audio in real-time. I use a mixture of it's DRC setting (usually set to +15db or thereabouts) and I also enable the auto gain control and voice matrix options.  The end result is even levels across a variety of audio formats (particularly problematic with DVB-T streams).

On a related topic (sorry to ask a question) but say I have a DTS source and I have "Output surround sound as DD" enabled, is it possible for MC to repackage the data into a DTS stream in these cases?

[glynor]

I can make you the file with test tones on all channels if you need it.  Just post back tomorrow.

[jmone]

I have to say I've never noticed a difference in pressure levels from DTV streams (either AC3 or MP2 here in oz) and Video Files

[jmone]

Matt - Just did a test with DTV on a MP2 and and AC3 track and found:
LAVAudio: Same apparent sound pressure
FFDSHOW: AC3 was noticeably quieter than the MP2 Channel & that outputted by LAVAudio (though it may be due to the componding DSP effects of FFDSHOW then JRiver).

[jmone]

Nope - the test Ch is a 2CH AC3 and the sound pressure is defiantly lower with FFDSHOW than with LAVFilter and non AC3 Channels.

...another reason to recomend LAVAudio over FFDSHOW! 

[Hendrik]

I don't know what ffdshow does to the data, but i can tell you that LAV Audio does not touch the decoded data at all.
Any processing option in ffdshow active?
ffdshow set to "avcodec" for ac3, and not a52? (different decoders could produce different values)

[jmone]

That was it.  FFDSHOW was set to a52 not avcodec.  Once it was changed to avcodec the vol level jumped up to "normal"

Thanks

[Matt]

That was it.  FFDSHOW was set to a52 not avcodec.  Once it was changed to avcodec the vol level jumped up to "normal"

Thanks

We'll make the auto configuration use avcodec instead of a52.

I'm still wondering if an option to boost AC3 and DTS by +10 dB.  I have yet to find a source where clip protection would actually engage, but even if it did, it works well and I don't think it'd be a problem.

Because it's also a problem that I watch television with the level set high enough to damage my speakers and ears if a full level signal did come through the pipe.

To be clear, I don't want dynamic range compression.  If the source actually goes up to 100%, I'm happy to let it use the full spectrum.  But when the source only goes up to 10%, it's silly to push the sound card or amplifier harder than necessary.

[glynor]

Because it's also a problem that I watch television with the level set high enough to damage my speakers and ears if a full level signal did come through the pipe.

I agree.   That's a problem I've run into often.  My TV recordings (AC3 audio) always force me to set my stereo to ~0db.  But if I play music out of MC at that volume, it blows your socks off.

I'll try switching to avcodec.  That's a good tip.

PS.  Perhaps this is what SageTV does dynamically on-the-fly?  I've never had the AC3 volume issue when playing files directly out of SageTV.

[Matt]

http://yabb.jriver.com/interact/index.php?topic=64174.0

[Matt]

avcodec honks like a goose on seek, so we're going to switch back to a52.

I reported this a long time ago:
http://yabb.jriver.com/interact/index.php?topic=56116.0

The volume difference should be a non-issue now that normalization is available.

[jmone]

If you get time can you try LAVAudio as this is built on the some decoder.