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@maemo.org https://lists.maemo.org/mailman/listinfo/maemo-developers
