Difference between revisions of "OpenTasks/Audio/Audio Output Selection"

OpenTasks/Audio/Audio Output Selection (view source)

Revision as of 14:47, 7 March 2011

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 This all results in a rather confusing way to select the output device -- for engine developers, for audio output developers, and for the user in the GUI. Clearly, this is a bad thing for all involved parties and hence the goal of this task is to fix this mess.
-Let's start by reviewing the current state of affairs. Engines specify during a startup a set of [http://doxygen.scummvm.org/dd/d2e/mididrv_8h.html#0b8636a2c41edcf47a84282ba7a00971 audio output flags]. These flags try to describe the output ''types'' supported by the engine resp. the current game. We then use some (rather simple and stupid) "detection" method based on these to determine the output ''device'' to use. Unfortunately, this detection does not include all the above mentioned output types, for example it does not allow to specify "Amiga", which recently turned out to be a major problem for the WIP SCI engine, since its Amiga games support both MIDI (in this case, MT-32 MIDI) and Amiga sound output. Now there is no (clean) way of checking whether the user wanted to use Amiga sound or MT-32 MIDI. This results in an unclean way of checking in the engine what exactly the selected output type is (this might be required to load special music data files for the different output types). To further complicate things, the current "AdLib" music device is actually an MIDI emulation device (useful on low end system which don't properly support MIDI output on their own), but on the other hand says it's an AdLib ''type'' ''device'', causing several engines which only use MIDI for audio output, and which don't contain any AdLib data, to still specify the MDT_ADLIB audio flag.
+Let's start by reviewing the current state of affairs. Engines specify during a startup a set of [http://doxygen.scummvm.org/dd/d2e/mididrv_8h.html#0b8636a2c41edcf47a84282ba7a00971 audio output flags]. These flags try to describe the output ''types'' supported by the engine resp. the current game. We then use some (rather simple and stupid) "detection" method based on these to determine the output ''device'' to use. Unfortunately, this detection does not include all the above mentioned output types, for example it does not allow to specify "Amiga", which recently turned out to be a major problem for the work-in-progress SCI engine, since its Amiga games support both MIDI (in this case, MT-32 MIDI) and Amiga sound output. Now there is no (clean) way of checking whether the user wanted to use Amiga sound or MT-32 MIDI. This results in an unclean way of checking in the engine what exactly the selected output type is (this might be required to load special music data files for the different output types). To further complicate things, the current "AdLib" music device is actually an MIDI emulation device (useful on low end system which don't properly support MIDI output on their own), but on the other hand says it's an AdLib ''type'' ''device'', causing several engines which only use MIDI for audio output, and which don't contain any AdLib data, to still specify the MDT_ADLIB audio flag.
 For the user, we currently always show all concrete output ''devices'' (except digital sound!) in the GUI, without distinguishing between output types and output drivers. We also always show everything, without considering what a given game actually supports. So currently the user can select "AdLib" sound for a game which does not contain any AdLib sound data. This is rather annoying.
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 To overcome these issues it would be best to change our audio output configuration API to use different layers of configuration:
-* The first layer would be the output ''type''. As described above, this could include General MIDI, MT-32 MIDI, AdLib (or generic OPL* output), (PC) Speaker (this might also include PCjr and CMS, to be determined later on), Amiga, FM-TOWNS, Digital Output etc. Note that this is no final list, rather making this list concrete is part of the task! For example, one might want to have a single MIDI output type, and introduce a notion of ''subtype'' to distinguish between GM, MT-32 and Roland MIDI.
+* The first layer would be the output ''type''. As described above, this could include General MIDI, MT-32 MIDI, AdLib (or generic OPL* output), (PC) Speaker (this might also include PCjr and CMS, to be determined later on), Amiga, FM-TOWNS, Digital Output etc. Note that this is no final list; making this list concrete is part of the task! For example, one might want to have a single MIDI output type, and introduce a notion of ''subtype'' to distinguish between GM, MT-32 and Roland MIDI.
 * The next layer would probably be a concrete output implementation, i.e. output drivers. Examples include Windows MIDI or ALSA for MIDI. Note that a single driver could potentially support multiple types: A user may usually use the Windows MIDI driver for GM output, via software synthesis, but may also own a real MT-32. This user may then wish to use the MT-32 via the Windows MIDI driver. Hence, depending on the output device resp. the user choice, the Windows MIDI driver would accept any MIDI variant. On the other hand, the MT-32 software emulator might only support MT-32 data.
 * One could also insert an extra conversion layer which tries to (optionally) translate between the various MIDI variants: So if a game requires MT-32 output but the user only has a GM device available, the MT-32 data could be translated on the fly (as good as possible) to GM before being sent to the actual output driver. Such conversions currently happen in engines. Sharing this code might be beneficial; on the other hand, it might turn out that each engine knows best how to perform this translation, in which case we should leave it up to the engines to perform such a conversion (or not). Once again, deciding this is part of the task.