knotify and libcanberra Re: KDE Sound and Multimedia Plan

Ian Monroe ian at
Wed Dec 1 18:22:54 GMT 2010

On Wed, Dec 1, 2010 at 11:40 AM, Colin Guthrie <gmane at> wrote:
> 'Twas brillig, and Arnold Krille at 01/12/10 09:01 did gyre and gimble:
>> On Wednesday 01 December 2010 01:11:18 Alex Fiestas wrote:
>>> This is my list of needs:
>>> 2-Low latency easy concurrency
>>>      The time between the "play" function in our application and when
>>> the sound actually play is important for a lot of applications (KNotify
>>> for instance), we need to ensure the lower latency possible. Also we
>>> need to provide an easy way to play more than one sound in parallel
>>> (KNotify as app example again).
>> No, neither knotify nor multimedia players nor games or webcam-apps need low-
>> latency.
>> Of course they need a rather short time between api-call and actual playing.
>> But low-latency is the stuff where the time from soundcard_in-
>>> processing_in_cpu->soundcard_out takes less then 10ms. If you really need
>> this, use jack ( natively and drop any layers between.
>> What you need is a framework that plays files/media, regardless whether its
>> wav, mp3, ogg, flac, mp4, avi, wmv or flv. You also want easy access to webcam
>> and soundcard io to allow for phonon to be used in more cases then media
>> players.
>> But all that doesn't need latencies of 10ms or less. It will all be perfectly
>> well with latencies of 100ms. And you gain a lot in simplicity when you don't
>> have to deal with low-latency...
> [Apologies, I've ended up writing way beyond what I want and what Alex
> requested.... but I don't want to throw it away, so I'm going to send it
> anyway.... I've written two requirements right at the end, so skip to
> that if you don't want to read the rest, but I think it's useful :D]
> Yeah I have to second the "low latency" comments in this reply.
> To be honest, I really get P155ed off with a lot of application
> developers who make comments about *needing* low latency (and this is
> very much not directed at your good self Alex, just a general grumble!).
> The main issue is that latency is often something that you literally
> cannot control to any sensible degree and you have to cope as best you
> can with what you are given, but the one thing you cannot do is to
> mandate a system that has low latency - that is dangerous and just plain
> incorrect!
> To give some contextual examples:
>  Bluetooth headsets will almost always have a much higher latency than
> built in h/w sound card. Likewise for something like UPnP or Apple
> AirPlay etc. These outputs simple have a latency built into them and we
> have to live with it.
> Using these types of devices for games is likely going to require the
> "live with it" solution as you cannot delay the visuals to compensate as
> the user will likely be fragged due to "unresponsive input", but for
> media players (and this is where I've had most people (incorrectly) say
> they /need/ low latency), then the visuals *can* be delayed to
> compensate for the audio delays introduced by strange outputs (and or
> system load, other concurrent playing streams and any other thing likely
> to change latency)
> So with that grumble out of the way, I'd agree with Arnold here that low
> latency is not a requirement per-se (despite what people think) and in
> fact I'd go as far as to say we should in many cases advocate high
> latencies whenever possible: e.g. allow the application to pump 2s or
> more of data into the system and then just sleep, not bothering the CPU
> again until 1.9s later. This is a particularly attractive scenario on
> mobile devices such as mobile phones, netbooks and tablets etc. (on
> mobile phones particularly as often there is no UI to care about either,
> so you really can sleep properly without waking up to repaint the song
> time UI component!). Note that pumping 2s of pre-decoded audio into the
> audio system and having a latency of 2s does not preclude a quick (i.e.
> "instantaneous") response in any given use case. That 2s can be thrown
> away at any point and new data loaded, allowing users to skip forward
> and back at leisure without waiting for a 2s buffer to run itself out.
> I mention this because I've had people (e.g. from Intel) ask me in the
> past how to enable support for this kind of operation via Phonon API.
> The Phonon API does not provide any method to control latency like this
> even if the underlying backend does (gstreamer on top of PulseAudio is
> the only backend configuration that would work here AFAIK).
> So to be taken seriously and used in a mobile environment, Phonon would
> need to support a method to *request* specific latencies. Whether this
> is exposed to $APP or just dealt with internally in a semi-intelligent
> way, I don't know. And I say "request" as the application (or perhaps
> phonon itself in most cases) should always deal as gracefully as it can
> with the latency that it is actually given or occurs at any point in the
> future (e.g. a hotswap from a low latency local device to a high latency
> BT headset etc.). There is no escaping that requirement!

Maybe a generic way to set properties on a MediaObject.

> As for Knotify, I find it very hard to feel that current system support
> the current method of operation well. Knotify does indeed want a quick
> turn around on the event happening and the sound being output. This is
> already handled very well in Gnome systems via libcanberra on top of
> PulseAudio. libcanberra implements the FDO Sound Theme specification
> (similar to the icon theme) and while some people have previously
> reported a desire to reimplement it if this spec was to be used in KDE,
> I find that quite hard justify overall (the main arguments so far have
> very much been non-technical and personal which I think is a terrible
> method to judge something technical, but meh).
> It would be easy enough to write a phonon backend for libcanberra and
> adapt knotify to use libcanberra for event sounds rather than phonon
> directly. If a system uses PulseAudio, we could avoid the phonon layer
> completely and talk directly to PulseAudio which would gain the
> following advantages:
>  1. Sample Cache. Short sounds would be cached by the PA server for very
> quick and responsive playback of event sounds when they happen.
>  2. Ability to get positional sounds... if an event happens near the
> left side of the screen, it will come mostly out of the left speaker.
> Partly a gimmick but it's surprising nice in action :)
> Now, I've just done what I specifically tried not to do (i.e. talk about
> implementation rather than requirements!), but the requirements can be
> summed up thus:
>  1. Request latency requirements (or handle automatically based on
> category - e.g. games, communication = low latency, all others = high)
>  2. Respond quickly to play certain sounds for notifications etc.
> Hope I've not ranted too much :D

No, this sounds like a more useful thread. :)

Reusing existing technology rather then build our own -> sounds good
to me. Having a phonon backend for libcanberra would solve the "what
about Windows" problem of having kdebase-runtime depend on


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