Liquidsoap svn : Stream contents

Stream contents

In liquidsoap, a stream may contain any number of audio, video and MIDI channels. As part of the type checking of your script, liquidsoap checks that you make a consistent use of stream contents, and also guesses what kind of stream your script is intended to work on. As with other inferred parameters, you do not necessarily need to read about stream contents typing if you're still learning the ropes of liquidsoap, but you might eventually need to know a little about it.

The content of a stream is described by the audio, video and MIDI arities. An arity might be fixed or variable. Fixed arities are usual natural numbers, described a number of channels that does change over time. For example, the stream type (2,0,0) describes streams that always have 2 audio channels and no channel of another type. Variable arities describes numbers of channels that vary over time. For example, the stream type (*,0,0) describes a stream which contains only audio, but whose number of channels might change at anytime – think of playing files, some of which being stereo, some mono, and some videos without any audio content. The stream type (*+1,0,0) also describes a variable number of audio channels, but with the guarantee that there will always be at least one.

In liquidsoap script language, there are three sorts of objects that rely on stream types: sources, requests and encoding formats. A source produces a stream, and it is important what kind of stream it produces when composing it with other sources. A request is an abstract notion of file, often meant to be decoded, and it is useful to know into what kind of stream it is meant to be decoded. Finally, a format describes how a stream should be encoded (e.g., before output in a file or via icecast), and the stream content is also useful here for the format to make sense.

In this page, we explain how liquidsoap uses stream types to guess and check what you're doing.

Global parameters

You might have noticed that our description of stream contents is missing some information, such as sample rate, video size, etc. Indeed, that information is not part of the stream types, which is local to each source/request/format, but global in liquidsoap. You can change it using the frame.audio/video.* settings, shown here with their default values:

set("frame.audio.samplerate",44100)
set("frame.video.width",320)
set("frame.video.height",240)
set("frame.video.samplerate",25)

Checking stream contents

Checking the consistency of use of stream contents is done as part of type checking. There is not so much to say here, except that you have to read type errors. We present a few examples.

For example, if you try to send an ALSA input to a SDL input using output.sdl(input.alsa()), you'll get the following:

At line 1, char 22-23:
  this value has type
    source(audio=?A+1,video=0,midi=0)
    where ?A is a fixed arity type
  but it should be a subtype of
    source(audio=0,video=1,midi=0)

It means that a source with exactly onevideo channel was expected by the SDL output, but the ALSA output can only offer sources producing audio. By the way, ?A+1 where ?A is fixed means that the ALSA input will accept to produce any number of channels, fixed once for all: it will attempt to initialize the soundcard with that number of channels and report a runtime error if that fails.

Conversions

The above example did not make much sense, but in some cases you'll get a type error on seemingly meaningful code, and you'll wonder how to fix it. Often, it suffices to perform a few explicit conversions.

Consider another example involving the SDL output, where we also try to use AO to output the audio content of a video: output.ao(output.sdl(single("file.ogv"))). This won't work, because the SDL output expects a pure video stream, but AO wants some audio. The solution is to split the stream in two, dropping the irrelevant content:

s = single("file.ogv")
output.sdl(drop_audio(s))
output.ao(drop_video(s))

Currently, the video dropping is useless because AO tolerates (and ignores) non-audio channels.

If you want to support both mono and stereo (and more) files within the same playlist, you'll need your playlist or single instance to have type source(*+1,0,0). But this content type won't be supported by most operators, which require fixed arities. What you need to do is use audio_to_stereo which will normalize your variable arity audio into a fixed stereo audio.

The last conversion is muxing. It is useful to add audio/video channels to a pure video/audio stream. See mux_video, mux_audio and mux_midi.

Type annotations

You now have all the tools to write a correct script. But you might still be surprised by what stream content liquidsoap guesses you want to use. This is very important, because even if liquidsoap finds a type for which it accepts to run, it might not run as you intend: a different type might mean a different behavior (not the intended number of audio channels, no video, etc).

Before reading on how liquidsoap performs this inference, you can already work your way to the intended type by using type annotations.

For example, with output.alsa(input.alsa()), you'll see that liquidsoap decides that stereo audio should be used, and consequently the ALSA I/O will be initialized with two channels. If you want to use a different number of channels, for example mono, you can explicitly specify it using:

output.alsa((input.alsa():source(1,0,0)))

Guessing stream contents

When all other methods fail, you might need to understand a little more how liquidsoap guesses what stream contents should be used for each source.

First, liquidsoap guesses as much as possible (without making unnecessary assumption) from what's been given in the script. Usually, the outputs pretty much determine what sources should contain. A critical ingredient here is often the encoding format. For example, in

output.icecast(%vorbis,mount="some.ogg",s)

%vorbis has type format(2,0,0), hence s should have type source(2,0,0). This works in more complex examples, when the types are guessed successively for several intermediate operators.

After this first phase, it is possible that some contents are still undetermined. For example in output.alsa(input.alsa()), any number of audio channels could work, and nothing helps us determine what is intended. At this point, the default numbers of channels are used. They are given by the setting frame.audio/video/midi.channels (whose defaults are respectively 2, 0 and 0). In our example, stereo audio would be chosen.