I haven't worked with this for so long that I forgot how I did it before. Do I risk clipping if I normalize 5.1 audio if it's listened to on headphones or thru a 2.0 mixer like in ffdshow?
If there's a spot where there is high-volume content on all 6 channels, will this clip when played on stereo?
+ Reply to Thread
Results 1 to 6 of 6
-
-
I'm no expert on electronics but..... probably not. I'd assume while it's in the digital domain clipping isn't an issue, it'd be how much headroom there is when amplifying the downmixed audio, and I suspect there'd be enough for the audio not to be clipped. And some 5.1ch could already have peaks at 0dB in places anyway.
I've downmixed 5.1ch audio to stereo MP3 after it's been normalized on ocassion, then checked the volume with MP3Gain, and the highest peaks I've seen are around +7.5dB (remember MP3s can store values above 0dB) but I've not heard them cause clipping on playback. Not heard it, as opposed to not causing it. I'm pretty sure the Dolby spec for downmixing AC3 says after downmixing (where the rear channel volume is supposed to be reduced by 3dB or 6dB), a further 7.5dB of gain reduction should be applied to prevent any chance of clipping, which is in line with what I've experienced when downmixing 5.1 audio with peaks already at 0db, but that's really worst case scenario. I guess if you're paranoid you could enable ffdshow's volume filter and run it at -7.5dB, or if ffdshow is doing the downmixing, it's Mixer filter has a "Normalize Matrix" checkbox which I think should produce a similar result.
Actually.... I just compared enabling the normalize matrix in the mixer with not enabling it and using the volume filter instead, and the normalize matrix sounds like it reduces the volume by somewhere around 12dB to me. I'm not sure why so much, I've not really thought about it until now as I don't use it, however I'm curious now so I'll have to try to investigate the downmixing formula is uses at some stage. Unless someone else knows....Last edited by hello_hello; 16th Nov 2013 at 06:55.
-
Out of curiosity I think I've worked out what the numbers in ffdshow's mixer filter matrix represent. I assume they're a percentage. So without the "normalize matrix" option checked when downmixing 5.1ch to stereo, the front and rear left/right channels are mixed without reducing their volume. The centre and LFE channels are mixed using 0.707, or 70.7%, which would mean they're reduced by 3dB.
With "normalize matrix" checked, the front and rear left/right channels are reduced to 29.3%, or a reduction of 10.6dB. For centre and LFE channels it's 20.7% or -13.6dB, so once again they're reduced by 3dB more than the other channels. I looked at the script MeGUI uses for downmixing and as it turns out, it uses exactly the same formula.
I'm still not quite sure why ffdshow's normailze matrix reduces the volume as much as it does, but I'd assume it's a kind of "worse case", "all channels at 0dB when downmixed", no clipping scenario. -
Thanks for the detailed input. I've decided to leave the audio track stock and let others normalize it as they see fit which I suspect is what I did last time which is probably why I have no memory of how I did it.
But I suspect that downmixing non-clipped 5.1 audio to stereo does indeed cause clipping if the decibels of all 6 channels are high enough and they don't need to be that high to clip when combined during downmixing.
I don't hear clipping when playing 5.1 audio on stereo headphones (no pre-processing obviously) though. -
None - check how 5.1 is rematrixed (for example Dolby standard) - each channel provide only fraction of level to overall downmixed signal. However as a principle You should always normalize to at least -3.01dB or better to -6.02 dBFS to avoid system clipping.
-
Yeah but the Dolby standard for downmixing probably only applies to hardware players and not to using ffdshow or a soundcard etc for downmixing. Unless the decoder applies a volume reduction I'd assume the Dolby standard doesn't generally apply to PC playback.
Out of curiosity I tried normalising some DTS audio (no downmixing). The normalised version was obviously a little quieter than the original, which somewhat surprised me. Thinking maybe MeGUI's normalising was doing something odd I converted it to AAC with eac3to. Checking the eac3to log file I found these entries:
[a02] Clipping detected, a 2nd pass will be necessary. <WARNING>
[a02] Applying -2.78dB gain...
I've seen that warning many times before, so I tried another dts movie audio track and an ac3 audio track from a TV capture. Same warning each time, although in those cases the reduction was only around 0.5dB.
As I rarely normalise audio myself I'd not thought about it all that much until now, but maybe normalising is likely to reduce the volume as well as increase it.
Similar Threads
-
Normalizing Multiple Audio
By enim in forum AudioReplies: 72Last Post: 9th Apr 2016, 10:15 -
Audio Normalizing
By dvddemon in forum AudioReplies: 7Last Post: 26th Sep 2011, 21:02 -
questions about audio normalizing and audio encoding
By codemaster in forum DVD RippingReplies: 2Last Post: 25th Sep 2011, 21:22 -
Normalizing audio help
By 1031982 in forum AudioReplies: 10Last Post: 23rd Jun 2009, 22:42 -
Normalizing Audio
By evilbubbles in forum AudioReplies: 2Last Post: 7th Dec 2008, 15:04