[maemo-developers] Playbin volume bug for OGG and WMA playback

[From: Martin Grimme]

Hi,

this is very interesting. Thanks for your investigation and detailed
description. I have always wondered why I get distorted sound with Ogg
and gstreamer on Maemo. Now it makes sense.


Thanks a lot,
Martin


2009/6/10, Nick Nobody <me at nikosapi.org>:
> This has been bugging me for quite some time and I think I've finally
> gotten to the bottom of it. In this message I present my (limited)
> understanding of how gstreamer works, what this annoying bug is,
> potentially why this bug occurs and perhaps how to fix it.
>
> Lets get on with it.
>
> There are several ways that audio gets played through gstreamer, the
> audio can either be decoded on the device's processor or it can be
> offloaded to one of a few different hardware decoders (DSPs).
> Gstreamer's playbin provides a very easy way for the developer to not
> have to worry about writing pipelines for every possible type of audio
> that can be played back by their application. For MP3 and AAC audio,
> gstreamer uses hardware decoding provided by dspmp3sink and dspaacsink
> respectively. OGG and WMA audio must first be decoded to PCM (decoderbin
> takes care of this nicely) and then get passed to dsppcmsink.
>
> On maemo, the audio sinks (dspmp3sink, dspaacsink, and dsppcmsink) all
> have their own "volume" property (an integer between 0 and 2^16) and
> another property named "fvolume" which seems to simply be volume/2^16 (a
> float between 0 and 1). Either of these can be set, they accomplish the
> same goal; controlling the volume level.
>
> Gstreamer also provides a nice volume control element that you can add
> in a pipeline to presumably control the volume before it gets to the
> audio sink in the event that the audio sink doesn't have a volume
> property. The documentation says that the volume control element can
> take a value from 0 to 10 but during my testing it seems that anything
> over 1 distorts. So in reality you'll only be using it from 0 to 1.
>
> The Problem.
>
> If you create a pipeline using playbin to playback an ogg or wma file,
> you end up with a volume control element stuck right before dsppcmsink
> (see image #1).  In this case playbin's volume property is connected to
> both the volume control element and the dsppcmsink "fvolume" property.
> This is were it gets weird, when you set the volume property of the
> playbin, the volume control element gets set to that exact number BUT
> the dsppcmsink's "fvolume" property gets set to one tenth of that
> number.
>
> Big deal, right?
>
> Wrong. This means that you can only set the output volume to about 10%
> before it begins to distort. For example, if I set the playbin's volume
> property to 3 (referring again to image #1), the volume control element
> gets set to 3 (we have distortion at this level) and the dsppcmsink's
> "fvolume" property is set to 0.3. The maximum value we can set the
> playbin's volume property is 1 (instead of 10, like when playing mp3s),
> anything else sounds bad.
>
> The Solution.
>
> There are two ways I can think of to solve this problem. Make the volume
> control element and the dsppcmsink share the same volume property or
> remove the volume control element entirely for playbins. I just don't
> really know how to go about fixing it...
>
> I suspect this issue has something to do with the way dsp sinks work on
> maemo's gstreamer. When using a dspmp3sink or a dspaacsink via a
> playbin, the volume property is a number between 0 and 10 which gets
> divided by 10 and used as the sink's "fvolume" value (see image #2).
> Could it be that someone simply forgot to divide by 10 in the volume
> control element code?
>
>
> Here are some gstreamer pipelines to help you experiment with this
> problem yourself (use these on your NIT).
>
> The following two pipelines sound exactly alike (very distorted), the
> first using playbin and the second using decodebin, a volume control
> element and dsppcmsink. The second is essentially the same as what
> playbin creates, which is wrong.
>
> gst-launch-0.10 playbin uri=http://tinyurl.com/524rwv volume=6
>
> gst-launch-0.10 gnomevfssrc location=http://tinyurl.com/524rwv \
>   ! decodebin ! volume volume=6 ! dsppcmsink fvolume=0.6
>
>
> Now if we change the volume element's volume setting from 6 to 0.6,
> there's no distortion:
>
> gst-launch-0.10 gnomevfssrc location=http://tinyurl.com/524rwv \
>   ! decodebin ! volume volume=0.6 ! dsppcmsink fvolume=0.6
>
>
> These images were generated on an n810 running Maemo 4.1 with the
> ogg-support 0.9 and gstreamer version 0.10.22 (the version that comes
> with Maemo isn't able to generate images of pipelines).
>
> Image #1 (OGG file, playbin, volume=3): http://imgur.com/H1Uiy.png
> Image #2 (MP3 file, playbin, volume=3): http://imgur.com/oEB4Y.png
>
>
> I'm pretty sure this is a bug, but if not please let me know what I'm
> doing wrong here. Or perhaps someone could explain how Nokia's media
> player handles volume or pipeline creation since it doesn't seem to
> experience this bug.
>
> It would be a shame to have to force the application developer to work
> around this problem by either worrying about creating different
> pipelines for different media types or just limiting playbin's volume to
> 1. I guess my point is, we should be able to take advantage of
> gstreamer's awesome playbin simplicity on Maemo.
>
> Thank you for your time and patience,
>
> nick
>
> _______________________________________________
> maemo-developers mailing list
> maemo-developers at maemo.org
> https://lists.maemo.org/mailman/listinfo/maemo-developers
>