[linux-dvb] Re: Linux DVB API V4 / Audio API

[Johannes Stezenbach <js@convergence.de>]

Hi,

I've comitted a new audio.h for the V4 API. So far it
seems that not many people care (PC hardware doesn't need
any of the new features). Anyway, feedback is welcome.

Johannes


(deliberate full quote:)
On Thu, Oct 23, 2003 at 07:16:31PM +0200, I wrote:
> Hi all,
> 
> I've collected some requirements for the V4 audio DVB API.
> In V3 we currently have:
> 
>     demux ---> decoder ---> output/mixer
>     
>     ^^^^^^^^^^^^^^^^^^      ^^^^^^^^^^^^
>      covered by DVB API     some other API (e.g OSS)
> 
> The V3 audio decoder API also had some ad hoc extensions to support
> DVD playback for the EM8300 and Margi drivers.
> 
> 
> In V4 we want better support for audio post-processing, mixing
> and output control. Since I think that existing Linux audio APIs
> like OSS and ALSA don't fit well for embedded DVB applications,
> I propose to include the following in the Linux DVB API:
> 
> 
>   demux ---> decoder --> post-proc -> |-------|
>          |           |                |       |
> 	   |           |--------------> |       |  |-> TV out
> 	   |                            |       |  |
> 	   |-> decoder --> post-proc -> | mixer |--|-> VCR out
>                                       |       |  |
>   ext -----> decoder --> post-proc -> |       |  |-> head phones
>                                       |       |
>   pcm -----------------> post-proc -> |-------|
> 
> 
>         ^............^.............^...........^   S/P-DIF out
> 
> 
> We have the following devices:
> 
> o audio decoder
>   - decodes audio PES or ES in various formats
>     (MPEG-1, MPEG-2, mp3, AC3, ...)
>   - performs synchronization to the STC (lip-sync)
>   - input can be from demux, from external input (I2S or SPDIF)
>     or memory
> o audio post processor
>   - performs 5.1 to stereo downmix
>   - performs 2ch to 5.1 Dolby Prologic decoding
>   - stereo to 3D stereo (SRS, virtual Dolby)
>   - dynamic compression
>   - ...
> o mixer and output control
>   - possible input signals are all decoder outputs, all
>     post-proc outputs and external PCM inputs
>   - generates different mixes for each output, including
>     volume and balance control
>   - may include bass/treble control, equalizer or similar
>   - may include a test tone generator ("beep")
> o S/P-DIF output
>   - this is a separate device so it can be connected at
>     various points in the signal path
>   - S/P-DIF output may be synched to the STC (lip-sync)
> 
> 
> A possible simplification which would not sacrifice functionality
> for current hardware would be to integrate decoder and post-processor
> into one device. That means it won't be possible to have more than one
> post-processor per decoder.
> 
> A post-proc can support a number of different modes, and in each
> mode there will be a distinct set of parameters. If a decoder
> does not use a post-proc the mode will simply be set to "none".
> Similarly, to feed PCM to a post-processor (or directly to
> the mixer) one would set the decoder to "pass though". This can
> also be used for PCM playback from memory.
> 
> If the hardware has more decoders than post-processors they are
> assigned dynamically, and the attempt to enable a post-processor
> for a decoder may return EBUSY.
> 
> Output from the decoder may be mono, stereo oder 5.1ch, and
> one can only select a matching post-processor mode.
> 
> The outputs from the decoders and post-processors are fed
> into the mixer (so the mixer can e.g. output 5.1 on main
> and a stereo downmix on headphones).
> 
> The S/P-DIF output device can be connected at various points
> in the signal path to output an undecoded stream or a PCM audio
> stream. If the output signal is PES from the demux it
> can be synced to the STC.
> 
> Sample rate conversion is handled implicitely by the driver.
> The API only allows to specify input sample rates and output
> sample rates. If a rate conversion along the signal path is
> not supported by the hardware, an error code will be returned.
> 
> The ioctls for the decoder + post-prcessor, mixer and spdif
> output device are defined in audio.h.
> 
> 
> I'm currently not sure how to proceed. The basic device
> structure might work for a variety of DVB audio systems,
> but the differences between audio HW are so large that
> it is difficult to have a generic API to manage all possible
> paramters.
> Also, the mixer could get awfully complicated because
> actual HW has many restrictions on internal signal routing.
> 
> o we could try to define paramter sets for common functionality
>   (e.g. 5.1 to stereo downmix or Dolby Prologic are common
>   post-processors and would have common paramter sets), and leave
>   everything else for private extensions;
>   I'm inclined to go that way.
> o or we try to have a generic inferface with lots of capabilities etc.
>   (sounds nice, but how?)
> 
> 
> Please comment.


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