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Subject: Re: [boost] Fwd: [asio]Extension for audio device service
From: adrien courdavault (adrien.courdavault_at_[hidden])
Date: 2013-04-24 08:35:52


>From all that I read that is BTW really interesting, I would like to
be clearer, and I have to put that in the document:
I want to do somethig simple.
1. audio streaming output only with native format supported by the
device and no mixer or DSP processing.
2. add input (only asynchrounous)
3. push/shared modes based on the existing mixer backends.

i don't want to do something that can does a lot of things
conversions of formats and mixing are well done in existing mixers.
for this reason too resampling is not part of the problem

Then it will be possible to write layers over that to make it higher level.

But really I just want something that can
1 list and adress audio devices and endpoints (given a certain audio
direction and channel nb - this is related to the problem of the bus
kind 1.0, 2.1, 2.0, 5.1 -)
2 connect to an audio device a streaming callback using the native
format supported by the driver
  a output
  b input
[3 later] connect to an existing mixer backend that allows more
formats but that is not streaming

thank you a lot for your feedbacks.

I put my comments inside your post.

On 24 April 2013 12:37, Brendon Costa <brendon.j.costa_at_[hidden]> wrote:
> Hi Adrian,
> I am interested in helping out with this. I have also been thinking about
> developing a boost library for use of audio hardware for about a year now.
> But have not had the time and motivation to get around to it.
I think it will take time indeed.
> The general idea of using boost::asio to provide an interface that is
> consistent with other IO in boost is a very good one if we can achieve the
> realtime requirements through it (which it sounds like we can from what you
> have said).
As I said, it looks possible from what I know (to fit the Boost.ASIO
design), I may be wrong but worse case, we have something that works,
and that does not follow Boost.ASIO design.
But I would prefer, and I really think we can do this like other boost services.

> I would suggest that you need to support multiple audio "backends" per OS
> when it comes to device selection. For example, on linux there is a
> plethora of choices and no one choice is "correct". The user should be
> given the option to choose which backend(s) to use/support. You will notice
> that some apps like to provide all options, and others work with only one
> backend.
Clearly I want to support all streaming solutions : Steinberg ASIO and
WASAPI on windows (minimum) and ALSA on Linux, and CoreAudio
Then in a second time work on the non-streaming modes (shared modes).

> I was thinking of proposing three related audio API's for boost a while
> ago. I do believe that to do this properly is a *LOT* of work (which is why
> I never really started).
I think this might be big, but that is also why we should stay focus
and clear on what we would like to do.
This is why I would like in the first steps to reduce the scope to
streaming modes,
and also the problem of cross compiling, is complew.
An in the first time I would prefer to only do MSVC on windows, and
gcc and clang on linux and mac. (not mingw on windows, because of the
complexity of building win32 apis)

> 1) Audio DSP Module
> This is for DSP related tasks and takes care of common things like audio
> format conversion, resampling etc.
> It is possible to write all of this I think in a platform agnostic way,
> however some platforms provide helpers for some of these things like MacOSX
> and maybe we should consider how we could use them or at least integrate
> with them.
> Possibly define both a compile time and runtime interface as both have
> advantages
> This would be used in the main interface for the play/record API for the
> pull/push callback
> Additionally this may expand in the future with all sorts of algorithms
> defined for audio processing. Though at first I would suggest it be a
> minimal of what is required to support the other two modules described
Yes all you say is right, but in the first place, I would prefer to
support the native formats offered by the deviec only.
I don't wan't to do a processing layer, like jack for instance, I
would really like to stay focus on the problem of opening the devices.
This means that the device may answer pretty often that is format is
16bit PCM int, and that it does not support floating point.
This what the different drivers familys do.
Then in a further step add the push/shared mode (which will obviously
offer way more formats).

> 2) Audio HW Play/Record Module
> This is for accessing the play/record of audio on audio hardware.
> I would suggest as you mentioned both push and pull modes. You can
> "emulate" one with the other though be it with a lot of work (and some
> latency) and some backends support both. So we should provide the interface
> for all backends but export a "capabilities" of the backend indicating what
> it supports natively (same as we should do for audio format).
Yes again I don't want to do emulations, or processing, I first want
to provide easy access to the supported natives formats by the devices
or the OS mixer.

> 3) Audio Mixer Module
> This is to control mixer settings including things like default device
> selections, volume controls, mutes, supported formats and device
> enumeration.
> Also, it should recieve events indicating external changes to these things.
> I have designed and implemented an API at work that did a lot of this
> (though the separation was not so well defined) and supported a number of
> backends including DirectSound, WASAPI, CoreAudio, portaudio, PulseAudio,
> OpenSLES.
> I might spend some time next week to writeup a proposal for a basic outline
> of what I think an interface could look like. Maybe we can compare ideas
> and notes?
Yes that would be a good idea.
Again, I will first focus on native streaming modes portability which
is my main issue to address
> --- snip ----
> errcode audio_port::open(audio_format &,const audio_direction,
> audio_device_mode ).
> --- snip ----
> Could you define what you mean by an audio_port?
Yes sorry, the audio port would the the i/o object created by the
service, it would target an audio endpoint. (not a device)
the name is probably not good.
The service when creating an audio io bject would have informations to
know which endpoint to open.
Probably the direction should not be in the open API, but when
requestiong the iobject to the service.

> Do you consider an audio_port to be an audio hardware device or a port on
> that device?
the endpoint, not the device.
I have to make that clearer in the document.
> For example: I consider a hadware device as representing a single physical
> device having one or more ports. Where a port is one of:
> * Recording port : Normal wave device input records from mic/line in etc
> * Playback port : Normal wave device output plays out to line out/speakers
> etc
> * Monitor port : Not always available but is similar to a Recording port,
> except it returns exactly what is going out of the device speakers (for
> example may include CD noise or MIDI or WAVE output from other processes).
> This is sometimes used for doing echo cancellation on input audio that
> cancels system sounds generated from other applications and not just your
> application.
> I also think some professional sound cards may have additional "ports" of
> type recording/playback. I think the MOTU is like that for example.
yes I have the same.
> If you take that definition of a port, then the direction may become
> un-necessary (at least I haven't seen a case where it would allow
> bi-directional).
i think we have the same point of view, we have to find a way to
address endpoint on a device.
Then the audio_port, the ioobject is created by the audio_service from
the addressing information you give.
The problem of addressing endpoints is a big issue, someone suggested
something like an ip addressing.
i mean this is not the same, but clearly you would like to say something like
auto my_audio_port =sercice.create("motu.output.default_stereo");
or something else
then he open function would only have the mode and format as argument.
> As for enumeration of "ports" it makes sense to have a simple function as
> you described that actually wraps something that is possibly more complex
> from the "Audio Mixer Module". I guess it *may* look something like:
> std::list<boost::audiohw::port>
> boost::audiohw::get_ports(boost::audiohw::recording_port|boost::audiohw::playback_port|boost::audiohw::monitor_port);
This is related to the problem of adressing, we should be able to get
a list of devices, and a list of audio endpoints on each device.

> What do you propose to be part of the audio_format struct?
> I can think of:
> * Channel Count : Integer >= 1
> * Data format : float32, int16, ...
> * Interleaving : yes/no
> * Sample rate: 48000
> * Channel mapping : (have a default defined mapping, but allow user
> overrides)
yes a bit like that.
but I don't nkow if channel mapping should be in that that depends
more on the addressiong of the endpoints I think
> There should also be the ability to define some form of latency parameters
> I think. Possibly even a place holder for extensible backend specific
> configuration (latency details could be part of that).
Yes, even if that something that I have to study more
> --- snip ----
> errcode audio_port::connect(audio_callback &);
> --- snip ----
> This makes sense, would you allow multiple callbacks to be connected to the
> single open port?
No, not in streaming mode.
because connecting several callbacks suggest that you have a mixer.

> For recording ports, I guess this is simple in that it just calls all
> connected callbacks with the same data
same question, it should probably be exclusive

> For playback ports, would you call multiple and then mix the audio into a
> block given to the device?
for the push/shared mode, which suggest a mixer, I would like to use
the backend provided by the OS in this kind of Mode.

> If so what are the restrictions on the audio produced by the callbacks?
> Should they provide exactly as much data as requested?
yes, when you do a realtime plugin, or application, using streaming,
you receive a buffer to fill and its size.
if you want to produce more audio data, then you have your own local
buffer to bufferize

this could be added in a further evolution, but I really want to keep it simple.

> If so, how much data does the callback request in one call? Is it fixed,
> variable, configurable (latency parameters I mentioned in the config)
> If they could return less than requested, then the mixing becomes more
> complex
no the device asks for the data it needs, and this depends on how the
device and the driver are made
latency is fixed, samplerate too

> Can you connect callbacks after you have started the device?
no you connect, start stop .

> I would assume that because the audio format has been defined already, then
> the callback can be verified based on the callback type.
> I.e. If defining a float32 format, then this could verify the callback data
> type is a float. Or you could go the C way and use void* but there are
> advantages to the type safety
> I guess a proposal for the callback could be:
> result on_audio_cb(float* data_out, size_t data_out_size, const
> audiohw::format& audio_format);
Not exactly, this depends on the format, and the channel number.
> How do you plan to handle synchronous input/output audio?
> What I mean by this is that the consumption and production of audio is
> synchronized so when you get 20msec of audio from the input you must also
> output 20msec.
we have to see that but i think this is 2 separate callback, one
sending the datas, the other pulling, and i think both may have
different buffer sizes

> This has benefits to various audio algorithms, and can be achieved across
> different devices with special clock monitoring and resampling techniques.
> It is generally the case for input/output on the same physical audio
> hardware that they are synchronized, but is not across different hardware
> as often the audio card clocks can be un-synchronized.
> This decision can affect how to structure the open() call. For example, you
> may open both input/output at the same time if you want to support this.
This is a good question in the first steps no
> There are a few options the first is my preferred though is more difficult
> to use:
> * You can define for synchronous handling, that the record/playback/monitor
> callbacks will always be called in order
> * You could do what portaudio does and define the callback to have (but
> that doesn't work well with the previous design)
> result on_audio_cb(float* data_out, const float* data_in, size_t data_size,
> const audiohw::format& audio_out_format, const audiohw::format&
> audio_in_format);
> How do you plan to handle asynchronous errors in the audio device?
> Maybe an error code passed to the on_audio_cb() or a seperate callback that
> can be registered for errors?
probably separate
> On Wednesday, 24 April 2013, adrien courdavault wrote:
>> Hello.
>> I make this new thread to be clearer.
>> There is currently no way to manage audio endpoints cconnection easily.
>> It looks like some people might find this usefull (as I do), and I've
>> been suggested on the boost dev list to try to detail this, as an
>> extension to Boost.ASIO.
>> For this reason I create this thread.
>> I'm trying to make a very basic first draft of the concepts and see if
>> this may be a good idea.
>> I attached here the first things I've written. This is very short and
>> general.
>> I would like to know:
>> * do you think I'm going in the right direction by seing this as
>> Boost.ASIO extension.
>> * do you have suggestions
>> * would someone like to participate ?
>> Thank you
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