Some notes about analog multichannel playback ============================================= Analog multichannel playback was implemented by Eduard Hasenleithner <eduard@hasenleithner.at> Created: 2001-07-05 If you have any questions regarding this topic, don't hesitate to mail me :) Usage ===== Multichannel playback is now possible with the emu10k1 driver. An audio application requests the number of channels with the SNDCTL_DSP_CHANNELS ioctl, and the new code is activated. The voices start with fx8, and increment for each additional voice. For example, five channel output uses fx8 .. fx12 as dsp input. Since no routes are present for this input, nothing can be heard! In order to route the channels to the speakers following commands must be issued: emu-dspmgr -a"fx8:Front L" emu-dspmgr -a"fx9:Front R" emu-dspmgr -a"fx10:Rear L" emu-dspmgr -a"fx11:Rear R" emu-dspmgr -a"fx12:Analog Center" emu-dspmgr -a"fx13:Analog LFE" The reason for not using the voices from fx0 on is just convenience. One must not delete all routes before setting up the new routes for multichannel output. It is possible to use fx0 as base. To set a different fx number as base invoke emu-config -m 0 for setting fx0 as base. Every other fx can also be set. Attention: fx15 is unusable. So for 6ch output, this parameter should be no higher than 9! Using fx15 would need extra code in the driver which is not yet included. To level the output of ch5 and ch6 (SbLive5.1, Center and LFE) a new OSS mixer control has been included. The control name is SOUND_MIXER_IMIX and has the name "Mix" in aumix. Although this is a stereo control it is intended for setting the Center and LFE individually. How does it work? ================= The emu10k1 chip has the possibility to play 64 voices. This is more than sufficient to replay 5.1 surround. But the chip can not replay this the same way as stereo. For stereo streams, two voices point to the same input buffer. A flag in one of the emu10k1 register signals, that the voice should only read every second sample. Using an offset of one sample, two voices can be combined to replay stereo. But this method does not work for replaying more than two channels. Doing this, required some kind of software emulation. What I did was to allocate the needed number of mono voices along with separate voice buffers. In the write function, the input stream gets split up into the individual voice buffers. Are there problems? =================== Although this works very well it has several drawbacks. These drawbacks do not interfere with the usual DVD playback, but should be considered when doing something else. So, remember following facts: *) fragment_size The fragment size ioctl does not set the fragment size of the user-space buffer, but the fragment size of the individual voice buffers. Therefore the fragment size reported to the user program is num_channels * requested_fragment_size. *) memory mapped access memory mapped access is simply not possible. As this access needs support from hardware, there is no way to do it. Aside from that, all other things should work. But it is possible that I did not modify all IOCTLs to care for the multichannel extension I made. If you need one IOCTL, and it don't works, email me. Some internals ============== This section is not intended for end users, but only for some person, who wants to modify the code. The former structure of the source code had almost no preparations for implementing multiple channels. Therefore, following two structures had to be modified: *) woinst woinst is now responsible for more than one voice. The voice member is now an array. All bytes values in this structure refer to a single voice. To report them to user space they must be multipled by the number of voices. The number of _separately_ allocated voices is contained in num_voices. So, for stereo, num_voices is still 1! *) waveout_buffer waveout_buffer contains all values with the metrics for the first voice. All other voices are dependend on this first voice and are assumed to have the same values. Instead of the three variables emupageindex, addr and dma_handle, a new structure, waveout_mem is present in waveout_buffer. This new structure is inserted as an array of waveout_mem. With this modification, it is possible to allocate more than one buffer. So, every function which relys on the values of waveout_buffer must take the number of voices into account.
