A digital crossover for an active-speaker-system:
With LADSPA plugins applied to virtual sinks you can have a configurable active setup. You can separate the sound for each of the speaker and apply equalisation and delay too.
Summary
- This is a rather static setup so you do have to reload the virtual sinks if you change the LADSPA-filter parameters.
- These are IIR filters, not FIR.
- I would use a passive crossover (bipolar capacitors) for the tweeter so when PA would shut down for some reason and your tweeter got the full audio spectrum, it would not get fried.
- I have an AMDX2 3800+ CPU and this setup is quite stable (since PA 0.9.16 CPU load has been about 2%). A kernel with high resolution timers (1000Hz) and realtime scheduling mode of PA is needed to keep the sinks in sync (under high system load) though.
Software
- I installed LADSPA plugins from Steve Harris (http://plugin.org.uk/) and CMT (http://www.ladspa.org/cmt/).
- I tried two soundcards, a multichannel onboard soundchip (Realtek RC888) and an X-Fi Extreme Music. You need a channel for each speaker-driver so for four-way stereo speakers you need a 7.1 card.
- The current pulse plugin for Mplayer and SDL seem broken, but the ESD-types are good alternatives. The Xine backend for Phonon in KDE4 doesn't support PA well either.
1. default.pa for a Realtek integrated audio with one surround device in ALSA:
I left out automatic hardware detection and loaded a custom four-channel alsa-module instead (card number 0 and its device surround40 - if you have two cards and 7.1 it may look like device=surround71:1 instead). The aux channels let PA override whatever the card "naturally" would want to do with them. Then I create LADPSA sinks for the crossover. Note that I have a mono setup so I use extra remap-sinks too (You have to set the combined sink to about 73% volume so the mono remapping doesn't generate clipping. Also descriptions for the sinks are changed because they get quite long by default.
### LADSPA crossover
load-module module-alsa-sink sink_name=front_stereo sink_properties=device.description=front_stereo device=surround40:0 channels=4 channel_map=front-left,front-right,aux0,aux1
load-module module-remap-sink sink_name=sub sink_properties=device.description=sub master=front_stereo channels=1 master_channel_map=front-left channel_map=mono
load-module module-remap-sink sink_name=low master=front_stereo sink_properties=device.description=low channels=1 master_channel_map=front-right channel_map=mono
load-module module-remap-sink sink_name=mid sink_properties=device.description=mid master=front_stereo channels=1 master_channel_map=aux0 channel_map=mono
load-module module-remap-sink sink_name=high sink_properties=device.description=high master=front_stereo channels=1 master_channel_map=aux1 channel_map=mono
load-module module-ladspa-sink sink_name=subHp sink_properties=device.description=subHp master=sub plugin=highpass_iir_1890 label=highpass_iir control=26,4
load-module module-ladspa-sink sink_name=subLp sink_properties=device.description=subLp master=subHp plugin=lowpass_iir_1891 label=lowpass_iir control=40,4
load-module module-ladspa-sink sink_name=lowHp sink_properties=device.description=lowHp master=low plugin=highpass_iir_1890 label=highpass_iir control=60,4
load-module module-ladspa-sink sink_name=lowLp sink_properties=device.description=lowLp master=lowHp plugin=lowpass_iir_1891 label=lowpass_iir control=290,2
load-module module-ladspa-sink sink_name=lowEq sink_properties=device.description=lowEq master=lowLp plugin=single_para_1203 label=singlePara control=-8,300,1.9
load-module module-ladspa-sink sink_name=midDelay sink_properties=device.description=midDelay master=mid plugin=cmt label=delay_0.01s control=0.0001,0.00006
load-module module-ladspa-sink sink_name=midHp sink_properties=device.description=midHp master=midDelay plugin=highpass_iir_1890 label=highpass_iir control=370,2
load-module module-ladspa-sink sink_name=midLp sink_properties=device.description=midLp master=midHp plugin=lowpass_iir_1891 label=lowpass_iir control=2000,4
load-module module-ladspa-sink sink_name=highDelay sink_properties=device.description=highDelay master=high plugin=cmt label=delay_0.01s control=0.0005,0.00015
load-module module-ladspa-sink sink_name=highHp sink_properties=device.description=highHp master=highDelay plugin=highpass_iir_1890 label=highpass_iir control=2100,4
load-module module-combine sink_name=jacks sink_properties=device.description=jacks slaves=subLp,lowEq,midLp,highHp
set-default-sink jacks
2. default.pa for an X-Fi audio card with multiple stereo devices ALSA (one for each output jack):
Here I left automatic hardware detection enabled (I use hw:0,0 for standard headphone output). Additionally loaded custom modules for hw:0,1 and hw:0,2. As above I create LADPSA sinks for the filters and combine them in the end.
### LADSPA crossover
load-module module-alsa-sink sink_name=rearJack sink_properties=device.description=rearJack device=hw:0,1
load-module module-alsa-sink sink_name=centreLfeJack sink_properties=device.description=centreLfeJack device=hw:0,2 channel_map=aux0,aux1
load-module module-remap-sink sink_name=sub sink_properties=device.description=sub master=rearJack channels=1 master_channel_map=front-left channel_map=mono
load-module module-remap-sink sink_name=low master=rearJack sink_properties=device.description=low channels=1 master_channel_map=front-right channel_map=mono
load-module module-remap-sink sink_name=mid sink_properties=device.description=mid master=centreLfeJack channels=1 master_channel_map=aux0 channel_map=mono
load-module module-remap-sink sink_name=high sink_properties=device.description=high master=centreLfeJack channels=1 master_channel_map=aux1 channel_map=mono
load-module module-ladspa-sink sink_name=subHp sink_properties=device.description=subHp master=sub plugin=highpass_iir_1890 label=highpass_iir control=26,4
load-module module-ladspa-sink sink_name=subLp sink_properties=device.description=subLp master=subHp plugin=lowpass_iir_1891 label=lowpass_iir control=40,4
load-module module-ladspa-sink sink_name=lowHp sink_properties=device.description=lowHp master=low plugin=highpass_iir_1890 label=highpass_iir control=60,4
load-module module-ladspa-sink sink_name=lowLp sink_properties=device.description=lowLp master=lowHp plugin=lowpass_iir_1891 label=lowpass_iir control=290,2
load-module module-ladspa-sink sink_name=lowEq sink_properties=device.description=lowEq master=lowLp plugin=single_para_1203 label=singlePara control=-8,300,1.9
load-module module-ladspa-sink sink_name=midDelay sink_properties=device.description=midDelay master=mid plugin=cmt label=delay_0.01s control=0.0001,0.00006
load-module module-ladspa-sink sink_name=midHp sink_properties=device.description=midHp master=midDelay plugin=highpass_iir_1890 label=highpass_iir control=370,2
load-module module-ladspa-sink sink_name=midLp sink_properties=device.description=midLp master=midHp plugin=lowpass_iir_1891 label=lowpass_iir control=2000,4
load-module module-ladspa-sink sink_name=highDelay sink_properties=device.description=highDelay master=high plugin=cmt label=delay_0.01s control=0.0005,0.00015
load-module module-ladspa-sink sink_name=highHp sink_properties=device.description=highHp master=highDelay plugin=highpass_iir_1890 label=highpass_iir control=2100,4
load-module module-combine sink_name=jacks sink_properties=device.description=jacks slaves=subLp,lowEq,midLp,highHp
set-default-sink jacks
daemon.conf:
I change the default resampler in ~/.pulse/daemon.conf as well. You may need to omit this if your CPU is too slow. My X-Fi sound-card wants 48KHz (I hear clicks on initialization when using 44.1kHz), so I put in a line for that too.
resample-method = src-sinc-best-quality
default-sample-rate = 48000
Enjoy! :)