[Kdenlive-devel] Questions about audio effects

Dan Dennedy dan at dennedy.org
Wed Sep 1 07:31:40 UTC 2010

Hi Hugh,
Rate scaler - may be useful in very small amounts; sometimes repeats,
sometimes not.
Sox band works but does not sound the same as sox play utility
Sox bass = bad (works fine with sox play utility, must be a MLT bug)

On Tue, Aug 31, 2010 at 9:37 AM, Dan Dennedy <dan at dennedy.org> wrote:
> 2010/8/31 Hugh Tebby <hugh.tebby at gmail.com>:
>> Hi,
>> With the 0.7.8 release approaching, I think the non functionning audio
>> effects should be removed. In fact, according to the list I made it seems
>> that most problems come from the Sox audio effects. It could be an issue
>> with the sox plugins I have on my computer, but if it isn't I would suggest
>> deactivating all sox plugins for the time being (along with Reverb and Rate
>> Scaler which don't seem to work).
>> Does anyone disagree with this ?
> I disagree because the sox effects work for me, and you presented this
> as all-or-nothing proposition. I was testing all audio effects
> recently when I added pan and balance, and I remembered this email.
> However, I did not undertake a rigorous study, but I was left with the
> impression that nearly all of them worked as expected although some do
> have dubious value. And there was only a couple that just did not seem
> to work correct at all. For example, ones that have anything to do
> with time such as stretch do not work correctly. I will do a more
> thorough review of them as well as the history of this thread and get
> back to you.

I found that I could only get some of the Sox effects to work
decently. Then, I started looking into Bass in particular. Besides
perhaps being redundant with Equalizer, I noticed that I could
reproduce the problem with melt, but not the sox play command. Then, I
compared the Band(-pass) filter between melt and sox and noticed a big
difference as well. Something indeed broke in the way MLT is passing
the parameters to sox. I will investigate further, and I do hope to
get it resolved by release. Read on for more comments...

>> 2010/6/25 Hugh Tebby <hugh.tebby at gmail.com>
>>> Hi everyone,
>>> I've listed all the audio effects that I have, and here's what I find :
>>> Working perfectly:
>>> - Declipper: I suppose it works... not sure how to test
>>> - Vinyl
>>> - Room reverb
>>> - Pitch Scaler
>>> - Pitch Shift
>>> - Volume
>>> Tweek needed:
>>> - Equalizer: Not enough controls (8 ?), and vertical sliders would be
>>> better

There is another ladspa EQ with 15 bands: mbeq_11197.so.
I encourage you to make an effects xml file for it while I work on the
sox bridge.

>>> - Gain: expecting dB instead of %, should be under "audio correction"
>>> folder

The underlying mlt filter does accept a dB value, but I just had to
commit a change to let it accept negative values for db. However, if
we change it, then existing projects will have insane dB values
without doing some document conversion stuff, for which I think we are
out-of-time (I do not know anything about that).

>>> - Limiter: seems to work... but no idea what it really does (and what is
>>> the release time ?)
>>> - Mono to stereo: left to right only, options for right to left would be
>>> good

already done a few weeks ago.

>>> - Mute: just gain effect with fixed gain to 0 and not modifiable (weird
>>> from a user pov)

I agree to drop it.

>>> - Normalise: gain effect with fixed gain to 0, not modifiable and
>>> normalise option activated (weird from a user pov)

Um, yeah, this is a hack where the non-modifiable parameter is a
side-effect. I will fix it and add some more parameters. Also, maybe
we should call this "Dynamic normalize" because it is not exactly
likely normalizing an audio file. Rather, it was designed for live
broadcast, so it normalizes over a sliding window with a smoothing
buffer for the gain changes. What do you think?

>>> - Phaser: Ok, but Rate doesn't seem to affect the sound
>>> - Sox band: width OK, but Center Frequency doesn't work
>>> Not usable:
>>> - Sox strech: Window ok but too powerful, Factor does nothing. Sox
>>> recommends using the Tempo effect instead of stretch
>>> - Sox Pitch Shift: awful sound
>>> - Sox Bass: awful sound
>>> - Rate Scaler: Works but ugly (repeats every second or so ), user should
>>> be able to change the 'segment' size (if possible)
>>> No effect:
>>> - Sox Echo
>>> - Sox Flanger
>>> - Sox Gain
>>> - Sox Phaser
>>> - Sox Reverb
>>> - Sox Vibro
>>> - Reverb
>>> Propose that all non fonctionning or unusable effects be removed before
>>> the next release (either blacklist or moving the xmls away into another
>>> folder for the time being). I'll look into the "tweak needed" effects later
>>> on to see if I can do something.
>>> Unfortunately the LADSPA plugins website is down, so no documentation is
>>> available for those effects...
>>> Cheers,
>>> Hugh
>>> 2010/6/4 Hugh Tebby <hugh.tebby at gmail.com>
>>>> Thanks for your answers Dan. I suppose I'l have to dive down the rabbit
>>>> hole into MLT wonderland !
>>>> I'll be looking into this in the next few weeks. Actually I think that
>>>> even the simple first step of disabling effects that don't work at the
>>>> moment would be quite useful, so I'll start off with that.
>>>> 2010/6/3 Dan Dennedy <dan at dennedy.org>
>>>>> 2010/5/31 wille <wille.yyz at gmail.com>:
>>>>> > Yes, left/right stereo balance.
>>>>> >
>>>>> > It would be very useful a feature that allow split a stereo audio clip
>>>>> > in
>>>>> > two mono clips.
>>>>> Well, now you have requested two different new features when Hugh was
>>>>> volunteering to do some cleanup. The lack of balance/pan is already
>>>>> well known and on my todo list. I praise Hugh's interest in
>>>>> volunteering for some cleanup and would be saddened to see him
>>>>> distracted.
>>>>> One thing you can do today is to split the audio and video and add the
>>>>> clip again on an audio track. Then, use the channelcopy filter on both
>>>>> audio clips to isolate left vs. right. Finally, you can use the
>>>>> keyframable volume effect on each to do panning. Unfortunately, today
>>>>> channelcopy is exposed in Kdenlive as simply "Mono to stereo," but you
>>>>> can alter the effect XML file for that to add "to" and "from" channel
>>>>> parameters (0 is left, 1 is right). ("Mono to stereo" may seem like a
>>>>> strange name, but it was added specifically for the case where a clip
>>>>> only has a mono track and you do not want it both channels and not
>>>>> just left.)
>>>>> MLT and Kdenlive does not support the notion of a different number of
>>>>> channels per track. So your second request is much longer term, if
>>>>> ever, especially considering that I am still working on proper
>>>>> multichannel (>2) support. I have been doing a lot with multichannel
>>>>> in the past year for broadcast applications of MLT, but there are
>>>>> still some gaps to add it in Kdenlive. One of these is surround
>>>>> panning, which would also be capable of stereo.
>>>>> > 2010/5/31 Hugh Tebby <hugh.tebby at gmail.com>
>>>>> >>
>>>>> >> Do you mean left/right stereo balance ? I don't think there's such an
>>>>> >> effect available in Sox or Ladspa... Would probably have to be done
>>>>> >> directly
>>>>> >> with MLT...
>>>>> That is correct.
>>>>> >> 2010/5/31 wille <wille.yyz at gmail.com>
>>>>> >>>
>>>>> >>> By the way, I feel the need of a Balance audio effect or a similar
>>>>> >>> tool.
>>>>> >>>
>>>>> --
>>>>> +-DRD-+


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