On 03.05.19 06:56, Alexander E. Patrakov wrote:
пт, 3 мая 2019 г. в 09:31, Georg Chini <ge...@chini.tk>:
On 03.05.19 02:22, Alexander E. Patrakov wrote:
чт, 2 мая 2019 г. в 23:45, Georg Chini <ge...@chini.tk>:
Hi,
I created a new module-plugin-sink and a small amplifier demo plugin
based on the attached header file. I did not (yet) drop max_latency,
disable_rewind and rewind_filter() because I am still waiting for more
feedback on the specification. I made it however more clear (using
Alexander's wording) that this should only be used if "real" rewinding
is possible.
Thanks for that.
However, the interface still thinks in terms of "number of channels",
which is, in the general case, wrong. E.g., a "proper"
module-virtual-surround-sink (not what we currently have) would sound
differently if given the same samples in "5.1" and "5.1 (Side)"
channel layouts.
I do not understand what you mean. How would you do it without
number of channels? You have to know how many input and
output channels are there, otherwise you can't process audio.
I am saying that, if we are going to support filters with non-trivial
interaction between channels, we would need to have a pa_channel_map
somewhere. In your header, this structure is not mentioned at all.
"5.1" and "5.1 (Side)" both have 6 channels. The hypothetical proper
virtual surround plugin would need to know if its input is "5.1" or
"5.1 (Side)", but, based on your interface, it can't.
I still don't see your point. The channel layout of input/output
and the playback device is known to module-plugin-sink, so if
a filter needs it, it can be passed as parameter. No need to
have it in the interface.
I wouldn't mind the number of channels, if we limit the interface to
"true" filters that have the same number of input and output channels
and don't care about the channel map. I.e., explicitly declare the
virtual surround sink as out-of-scope. But then, what useful things
are left in scope except the equalizer and maybe a dynamic range
compressor?
Why would there be any reason to limit it to filters with
equal number of channels?
Because that follows from the premise that the filter does not mix
channels. And without channel layout information (i.e. given only the
total number of inputs, but not even their order), it is impossible to
mix channels meaningfully.
See above.
Also, please make sure that, in the callbacks, the filter handle
always comes as the first parameter.
No problem, but usually in pulseaudio the userdata pointer
comes last in callbacks.
Ok, ok, the real idea was "be consistent" :)
Well, things are still at a very early stage and surely I will do
some cleanup once the general approach is accepted.
Regarding the virtual surround plugin, I actually have one more
business argument why, if implemented properly (and not how it is done
currently) it should be out of scope. The reason is that this plugin
with publicly available HRIRs only applies to headphones. From a
usability perspective, it should be switched off (or to a different
filter, specifically crafted for each set of speakers and the
listening room) when the audio is playing to something else than
headphones. The proposed interface does not have (and likely should
not have, because we want to keep it simple and stable) any place to
express such policy.
Why would that be a problem? You can have a boolean bypass
parameter that switches off the filter when needed. Or even a
choice that switches between different HRIR files.
Who would switch it (the virtual surround effect, I am not talking
about other effects) off? The idea is that it is not the user (because
with this particular effect it should really be automatic), and is not
the filter itself (because it cannot know, based on your interface,
that it is playing to speakers). So it has to be some third entity,
with enough knowledge both about the filter and the PulseAudio notion
of ports (to distinguish between speakers, headphones, and something
unknown). And, given that entity, it does not really make sense to
abstract the virtual surround implementation behind some common plugin
interface, it has already leaked.
As an example of prior art, please also consider the LFE channel
remixer - it is not implemented as a virtual sink precisely for this
reason.
As said above, there are parameters that can be passed to
the filter if needed and module-plugin-sink could propagate
changes in the playback device to the filter that way.
We could reserve special parameter names for this information,
because these controls would not be exposed to the user.
If you think channel layout and playback device should be
in the interface, I can integrate it, but using parameters looks
easier to me because this way we need no additional notification
functions.
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