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| author | Peter Meerwald <pmeerw@cosy.sbg.ac.at> |
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| date | Fri, 25 Jun 2010 16:00:21 +0200 |
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Open Source Line Echo Canceller (OSLEC) ======================================= [[introduction]] Introduction ------------ Oslec is an open source high performance line echo canceller. When used with Asterisk it works well on lines where the built-in Zaptel echo canceller fails. No tweaks like rxgain/txgain or fxotrain are required. Oslec is supplied as GPL licensed C source code and is free as in speech. Oslec partially complies with the G168 standard and runs in real time on x86 and Blackfin platforms. Patches exist to allow any Zaptel compatible hardware to use Oslec. It has been successfully tested on hardware ranging from single port X100P FXO cards to dual E1 systems. Hardware tested includes TDM400, X100P, Sangoma A104, Rhino E1 etc. It also works well on the link:ip04.html[IP04 embedded Asterisk platform]. There is also a project underway to use http://peter.schlaile.de/mISDN/[OSLEC with mISDN]. Oslec is included in many distributions, including Debian, Gentoo, Trixbox, Elastix, and Callweaver. [[testimonials]] Testimonials ------------ So how good is Oslec? + The http://www.trixbox[Trixbox] community have been testing Oslec, here is a http://www.trixbox.org/forums/trixbox-forums/open-discussion/need-people-echo-problems[Trixbox forum thread] that has feedback from many people. Start about half way down the page, or search on the string "night and day" :-) + The http://callweaver.org/wiki/Zaptel[CallWeaver] project use and recommend Oslec. + Here is some feedback from Oslec users: [Gordon Henderson, posted in the Asterisk-user mailing list July 21 2008] ________________________________________________________________________ I switched to OSLEC after testing HPEC on TDM400 boards, and found that it worked much better and wasn't limited to the restricted mechanism Digium uses for licensing (unlikely as it sounds, I have some clients who do not have a connection to the public Internet, and never will for their phone system) It also passes the wife test which HPEC didn't. It's also free (OS as in Open Source), which HPEC isn't, although that wasn't my primary reason for using it - ease of use and "workability" was. _____________________________________________________________________ [Michael Gernoth, Author of the Zaptel MG2 echo canceller] ________________________________________________________________________ Thanks for OSLEC, it's so much better than my hacked KB1 which is called MG2 :-) _____________________________________________________________________ [David Gottschalk] ________________________________________________________________________ Once I managed to get OSLEC installed (it was a big job for me, because I was missing many of the dependancies and knowledge), it immediately fixed the echo problems I had been unable to get rid of to date despite lots and lots of tweeks, tests and fiddling. Even more suprisingly, it appears to converge almost immediately from my initial test. This code is the best since thing since, well, Asterisk !!! _____________________________________________________________________ [Nic Bellamy, Head Of Engineering; Vadacom Ltd] ________________________________________________________________________ Short version: KB1 < MG2 < OSLEC < HPEC In the open source line echo canceller category, OSLEC is so far the best I've seen. It converges reasonably fast, and thanks to it's dual-path approach, will converge, albeit a bit slowly, on nasty echo paths where cancellers like KB1 and MG2 will fail entirely. The nonlinear processor and comfort noise generator provide for a far more pleasant listening experience than KB1/MG2. It's not quite up to the grade of ADT's G.168 canceller used in Digiums HPEC product in terms of convergence speed or tail coverage. _____________________________________________________________________ [Pawel Pastuszak, Astfin.org] ________________________________________________________________________ Since I installed the first version of OSLEC I was hooked.... It's been several months now since the alpha version was released; after using OSLEC Mark and I agreed that we had to have OSLEC as the main echo cancellation in the Astfin distribution. ________________________________________________________________________ [Pavel Selivano, Parabel Ltd] ________________________________________________________________________ I've been working with Oslec since March of 2007. Oslec has had some problems, but, thanks to David, they are solved. Now, I can say Oslec is time-tested. The best, I can note - it is still under development (with feed-back). Thank you David for your good deed. BTW: comparison to Intel IPP's echo canceler have shown, Oslec is very good :-) ________________________________________________________________________ [[news]] Oslec News ---------- + Saturday, June 7, 2008: We have worked out how to use <<rhino,Oslec with Rhino>> PRI cards. Thanks Juan Manuel Coronado! + Friday, May 16, 2008: Oslec Road Map. I have kicked off a discussion of the http://sourceforge.net/mailarchive/forum.php?thread_name=1210895745.7127.60.camel%40localhost&forum_name=freetel-oslec[development roadmap] for Oslec. Is hardware echo cancellation headed for extinction? + Friday, May 16, 2008: Oslec chosen as the **default** http://sourceforge.net/mailarchive/forum.php?thread_name=20080515131239.GJ27523%40xorcom.com&forum_name=freetel-oslec[echo canceler for Debian]. Thanks Tzafrir for your work on packaging Oslec for Debian! + Tuesday, May 13, 2008: link:/blog/?p=64[Oslec versus the SPA 3000] ATA. A blog post comparing the SPA 3000 echo cancellation to Oslec on a problem FXO line. + Wednesday, February 11, 2008: Oslec mailing list created. Due to popular demand (well, Tzafrir mainly!) there is now an https://lists.sourceforge.net/lists/listinfo/freetel-oslec[Oslec mailing list]. + Sunday, February 10, 2008: Zaptel 1.4.8 patch checked into SVN, thanks Tzafrir. [[install]] Installing OSLEC with Asterisk/Zaptel -------------------------------------- Notes: + This process also installs a system for sampling echo signals that is helpful for developing oslec. + I assume Asterisk is already installed and tested. + I assume you are running Linux 2.6 and Zaptel 1.2.13 or a later 1.2/1.4 version of Zaptel. + I assume you are using a Digium TDM400 line interface card. Change the "insmod wctdm.ko" line below to match your line interface hardware (e.g. wcfxo for a X100P). + For Zaptel 1.4.x replace the 1.2.13 text below 1.4.x (tar ball and patch file). + For Linux 2.4 replace the "insmod moduleXYZ.ko" lines below with "insmod moduleXYZ.o". 1/ Download, build and install oslec: $ cd ~ $ wget http://www.rowetel.com/ucasterisk/downloads/oslec-0.1.tar.gz $ tar xvzf oslec-0.1.tar.gz $ cd oslec-0.1 $ make $ insmod kernel/oslec.ko Optional: If you want the latest and greatest, replace the "wget" step with: $ svn co http://svn.astfin.org/software/oslec/trunk/ oslec NOTE: There are patches for Zaptel 1.2.13, 1.2.18, 1.2.24, 1.4.1, 1.4.3, 1.4.4, 1.4.7.1, 1.4.8, 1.4.9.2 and 1.4.11. It's quite easy to port to other versions, please feel free to send me a new patch should you get Oslec working with another Zaptel version. If you can't see a patch for your Zaptel version try http://svn.astfin.org/software/oslec/trunk/kernel[Oslec SVN] for the latest patches. 2/ Build, patch and install Zaptel. First obtain zaptel-1.2.13.tar.gz and: $ tar xvzf zaptel-1.2.13.tar.gz $ cd zaptel-1.2.13 $ ./configure $ patch < ../oslec-0.1/kernel/zaptel-1.2.13.patch (see note 1 below) $ cp ../oslec-0.1/kernel/dir/Module.symvers . (see note 2 below) $ make $ insmod zaptel.ko $ insmod wctdm.ko $ ./ztcfg $ asterisk NOTE: 1: Use -p1 option for zaptel-1.4.9.2 and above NOTE: 2: The "cp Module.symvers" step above is optional on many systems; it stops warnings like WARNING: "oslec_echo_can_create" [zaptel.ko] undefined!". However on some Linux machines zaptel will not compile without this step. 3/ These options in zapata.conf are important: --------------------------------------------------------------------- echocancel=yes echocancelwhenbridged=no ;echotraining=400 --------------------------------------------------------------------- The *echocancelwhenbridged=no* allows faxes to pass from FXS to FXO ports without interference from the echo canceller. This option is important for fax signals. Make sure *echotraining* is disabled when using Oslec - this is not supported and if enabled will cause the channel to be silent (i.e. no audio will pass through). 4/ The settings above have been shown to reliably cancel echo in 95% of cases. If you can still hear echo you may have one of the rare cases where your echo is longer than 16ms. To configure Oslec with a 32ms tail: --------------------------------------------------------------------- echocancel=256 echocancelwhenbridged=no ;echotraining=400 --------------------------------------------------------------------- Then restart Asterisk. [[pbxinaflash]] PBX in a Flash Install ---------------------- Here are the http://pbxinaflash.com/forum/showthread.php?t=100&page=3[Oslec install procedure] for PBX in a Flash. Check the 7-24-08 post in this thread by Alex728 for Zaptel-1.4.11 instructions. Thanks JD Austin and Alex. Matt Keys has suggested that the PBX in a Flash install procedure also worked well for Ubuntu server. [[rhino]] Rhino PRI cards --------------- The rhino-2.2.6 driver for Rhino PRI cards has software echo cancellation disabled by default. This means there is no echo cancellation unless you opt for the Rhino hardware echo cancellation module. However the driver is easy to hack to enable software echo cancellation, and hence Oslec. In the rhino-2.2.6 driver, file r1t1_base.c, r1t1_receiveprep function, around line 800: --------------------------------------------------------------------- static void r1t1_receiveprep(struct r1t1 *rh, int nextbuf) { /* int x; for (x=0;x<rh->span.channels;x++) { zt_ec_chunk(&rh->chans[x], rh->chans[x].readchunk, rh->ec_chunk2[x]); memcpy(rh->ec_chunk2[x],rh->ec_chunk1[x],ZT_CHUNKSIZE); memcpy(rh->ec_chunk1[x],rh->chans[x].writechunk,ZT_CHUNKSIZE); } */ zt_receive(&rh->span); } --------------------------------------------------------------------- **Solution 1:** Simply un-comment the code and rebuild the driver: --------------------------------------------------------------------- static void r1t1_receiveprep(struct r1t1 *rh, int nextbuf) { int x; for (x=0;x<rh->span.channels;x++) { zt_ec_chunk(&rh->chans[x], rh->chans[x].readchunk, rh->ec_chunk2[x]); memcpy(rh->ec_chunk2[x],rh->ec_chunk1[x],ZT_CHUNKSIZE); memcpy(rh->ec_chunk1[x],rh->chans[x].writechunk,ZT_CHUNKSIZE); } zt_receive(&rh->span); } --------------------------------------------------------------------- **Solution 2:** Bob Conklin from Rhino has suggested this fix that they have in SVN trunk: --------------------------------------------------------------------- static void r1t1_receiveprep(struct r1t1 *rh, int nextbuf) { if (!rh->dsp_up) zt_ec_span(&rh->span); zt_receive(&rh->span); } --------------------------------------------------------------------- This version automatically falls back to software echo cancellation if the DSP-based hardware echo cancellation is not present. Thanks Bob! [[status]] Status ------ Current status is considered *Stable*. As of 3 January 2008: + Thousands of people (estimated) using Oslec on x86 platforms now. Oslec is stable and works well. Oslec has been included in many distributions, including Debian, Gentoo, Trixbox, Elastix, and Callweaver. + Oslec works well on many different types of hardware, from single port $10 X100P cards to dual E1s. + Oslec has been partially optimised for the embedded Blackfin DSP/RISC CPU and now runs in real time on the link:ip04.html[IP04 Open Hardware IP-PBX]. Running on several hundred IP04s now. + Real time runs well on FXO/FXS lines where the other Zaptel echo cancellers (including the latest MG2) struggle. No tweaks like levels, fxotrain, or even opermode required so far. Hardware used for tests was a TDM400 and X100P card. + The simulation code passes many but not all of the G168 tests. Some problems with high level signals near 0dBm0 and some of the G168 echo models. Passes maybe 90% of the tests attempted so far, however not all of the tests and/or range of test conditions have been attempted yet. The fails are close calls (like a few dB off), not complete breakdowns of the algorithm. + Oslec has been specifically developed to work with low cost line interface hardware like the X100P. The X100P has problems with 60Hz hum and DC offset in the rx signal, which interfere with the echo canceller algorithm. However with the addition of some high pass filters good results with X100P cards have been obtained using Oslec. Line interfaces based on chips such as those from Silicon Labs (e.g. the TDM400) have no DC offset or hum which is one reason they tend to perform better. + Oslec has been optimised to deal with specific problems encountered with soft phone clients. Due to the high quality microphones and sound blaster hardware used on PCs, more low frequency energy is present compared to normal telephone signals. When this low frequency energy is sent to a hybrid it can force the hybrid into a non linear mode. Echo cancellers assume a linear hybrid so any non-linearities make the echo canceller fall over. The solution is to high pass filter energy sent to the hybrid to ensure excessive low frequency energy is removed. This means inserting a HP filter in the tx path. [[directories]] Directories ----------- The http://svn.astfin.org/software/oslec/trunk/[Oslec SVN repository] combines several projects (mainly spandsp) used to develop, test, and run oslec. spandsp : A subset of spandsp to support testing and development of Oslec. The echo canceller and G168 test suite source code live in here. user....: User mode apps, e.g. sampling of echo signals and a unit test for measuring the execution speed of Oslec. kernel..: Builds Oslec into a kernel module. Also contains patches for Zaptel to allow the use of Oslec. [[diagnostics]] Run Time Information -------------------- Set up a call that uses an analog port, then check out /proc/oslec for real-time stats: [root@homework kernel]# cat /proc/oslec/info channels....: 1 mode........: [13] |ADAPTION|NLP|CNG| Ltx.........: 0 Lrx.........: 211 Lclean......: 211 Lclean_bg...: 211 shift.......: 0 Double Talk.: 1 MIPs (last)....: 1 MIPs (worst)...: 7 MIPs (avergage): 1 You can turn the various mode switches off and on, for example: echo 9 > /proc/oslec/mode turns off comfort noise, but keeps ADAPT and NLP on. The mode switches are listed in the http://svn.astfin.org/software/oslec/trunk/spandsp-0.0.3/src/spandsp/echo.h[echo.h] header file (#define ECHO_CAN_USE_*). The /proc interface only monitors the first Zaptel call you bring up, see http://svn.astfin.org/software/oslec/trunk/kernel/oslec_wrap.c[oslec_wrap.c] for more information. There is a GUI for run-time control of Oslec, called the Oslec Control Panel. For example you can Enable and Disable the echo canceller in real time, as you are speaking. If you Reset the echo canceller you can hear how long it takes to converge again. NOTE: The command line tool *dialog* must be installed on your system to use the Oslec Control Panel. $ cd kernel $ ./oslec-ctrl-panel.sh image::/images/echo/oslec_control_panel.png[Oslec Control Panel] [[speed]] Execution Speed and Optimisation -------------------------------- In the user directory there is a http://svn.astfin.org/software/oslec/trunk/user/speedtest.c[speedtest.c] program that measures how many MIPs the echo canceller consumes and estimates how many cancellers can run in real time. The current x86 code is vanilla C and could be greatly improved with MMX or SSE optimisations, some of which are already coded in spandsp. There is also much that can be done to improve execution speed on the Blackfin. Steve's spandsp code includes some support for MMX and SSE2 optimisation. Define USE_MMX or USE_SSE2 in the Makefile to use this option. At this stage only the fir.h filter code is optimised, oslec would run much faster if lms_adapt_bg() in echo.c was also ported to MMX/SSE2. With USE_MMX defined, the speedtest.c program dropped from 33 MIPs to 20 MIPs per channel for 256 taps/32ms. If using MMX in the kernel (e.g. with zaptel) make sure you compile zaptel with CONFIG_ZAPTEL_MMX to ensure the FPU state is saved in the right places. To enable MMX/SSE modify this line of oslec/kernel/Makefile: ---------------------------------------------- all:: $(MAKE) -C $(KDIR) EXTRA_CFLAGS='$(KINCLUDE) \ -DUSE_MMX -DUSE_SSE2 -DEXPORT_SYMTAB -O6' \ SUBDIRS=$(PWD) modules ---------------------------------------------- Thanks Nic Bellamy for help in testing MMX and Ming-Ching Tiew for providing the correct command line for compiling with MMX/SSE. Some notes on further optimisation: 1/ How do lots of MIPs in ISR affect total system performance? For example if we are using 25% of MIP in ISR does * still run OK? Find the CPU load knee different for user versus kernel mode cycles consumption. 2/ Estimated Oslec MIPs are 5(N)(Fs), N is the filter size (number of taps), Fs=8000 is the sampling rate. Factor of 5 is comprised of 1 for each FIR filter (forgeround and background), 2 for LMS, 1 for overhead. This suggests that when optimised around 10 MIPs for a 32ms tail. 3/ There are some more notes on optimisation for the Blackfin in http://svn.astfin.org/software/oslec/trunk/spandsp-0.0.3/src/echo.c[echo.c]. [[credits]] Background and Credits ---------------------- Oslec started life as a prototype echo canceller and G168 test framework from Steve Underwood's http://www.soft-switch.org/[Spandsp] library. Steve wrote much of the DSP code used in Asterisk, and the Zaptel echo cancellation code is heavily based on his work. Using the Spandsp G168 test framework, a high performance echo canceller has been developed and carefully tested. Working together with alpha testers the performance was brought to a beta state in June 2005. <<links,More>>. Since then many thousands of people have installed Oslec. Thanks to Steve Underwood, Jean-Marc Valin, and Ramakrishnan Muthukrishnan for their suggestions and email discussions. Thanks also to those people who collected echo samples for me such as Mark, Pawel, and Pavel. Thanks Nic Bellamy for help with testing, explanation of long path issues, and MMX support. Thanks Tzafrir for testing and patches/enhancements and Dmitry for help with multithreaded and locking issues. Thanks Vieri for finding a circuit with a tail > 32ms, and Patrick for submitting a Zaptel 1.2.18 patch. Thanks Dave Fullerton and Carlton O'Riley for Zaptel 1.4.3/1.4.4 patches. Thanks Bill Salibrici for finding a memory leak. Thanks Michael E. Kromer for submitting a Zaptel 1.4.7.1 patch, and Tzafrir for the Zaptel 1.4.8 patch. Thanks Russ Price for the Zaptel 1.2.24 patch. Thanks Chris Notley for the 1.4.11 patch. Thanks to Peter Schlaile for porting Oslec to mISDN. Thanks also to Kristijan Vrban for sending me some ISDN hardware so I can help with the ISDN/Oslec testing! Thanks to Tzafrir and Faidon for helping debug the muted audio problems on 64 bit systems. Thanks Rudolf E. Steiner for testing Oslec on SMP and multiple E1 systems. [[sample]] Sampling Zaptel Echo -------------------- Introduction ~~~~~~~~~~~~ I have developed a system for sampling echo on running Zaptel/Asterisk systems. If you do experience any echo with Oslec, please use this system to sample the echo, then email the sample files to me. See the http://www.rowetel.com/blog/?p=18[Part 1 blog post] for more information on sampling echo. The nice thing about is that it doesn't interfere with your running Asterisk system. If you hear echo at any time you can fire up a console and run "sample" to capture real-time data from the Zaptel port. There are other ways to capture audio from a running Asterisk system, for example using the built in Asterisk play and record applications. However this system is designed to capture *exactly* the signals being fed to & from the echo canceller - preserving the exact timing of the signals and with no intermediate buffering. This is very important for echo cancellation work - the algorithms depend on an exact timing relationship between the transmit and receive signals. The system simultaneously captures the receive signal both before *and* after the echo canceller - something that is difficult to do with the built in Asterisk functions due to the location of the echo canceller deep in the Zaptel driver. If you would like to help further develop Oslec, please mailto:david_at_rowetel_dot_com[send me] your echo samples! I would welcome any samples of your echo signals, for example where the echo canceller isn't working well, and also cases where it does work well. By comparing the two cases we can learn a lot about the strengths and weaknesses of the algorithm. Installing ~~~~~~~~~~ 1/ Install Oslec ("HowTo - Run OSLEC with Asterisk/Zaptel" section above). 2/ If you would like to use zaptap without oslec change the selected echo canceller in zconfig.c and rebuild/install zaptel. 3/ Compile sample.c: # cd user # make 4/ Create a device node. I used major number 33 as it was free on my PC: # mknod -m 666 /dev/sample c 33 0 If 33 is not free choose a free major number and change the #define SAMPLE_MAJOR in zaptel.c and recompile. Note it is important to "insmod zaptel.o" from inside the zaptel directory and don't forget the ".o" (.ko for Linux 2.6). Otherwise insmod will use the previously installed version of zaptel and "sample" won't work. 5/ Make the call to the Zaptel port you wish to sample. Run sample while talking: # ./sample test 1 5 this will create test_tx.raw, test_rx.raw & test_ec.raw. There will be a few messages on the console as the driver does it's thing, you can check these with dmesg. 6/ You can check your samples by playing them back through your sound card, for example: # play -f s -r 8000 -s w test_ec.raw 7/ To convert the raw files to wave files (this is more convenient for playing and processing with the Oslec simulation): # sox -t raw -r 8000 -s -w -c 1 test_ec.raw test_ec.wav # play test_ec.wav 8/ There is an Octave script pl.m to help plot the samples: # cp /your/test/samples ~/oslec/user # cd ~/oslec/user # octave octave:1> pl("test") Configuration ~~~~~~~~~~~~~ + Zaptel/Asterisk 1.2.13, 1.4.0 or 1.4.1. However other versions should work OK. + Linux 2.4 or 2.6 Kernel + Digium TDM400 hardware + My zapata.conf for the FXO port under test is something like: --------------------------------------------------------------------- signalling=fxs_ks echocancel=yes ;echocancelwhenbridged=yes ;echotraining=400 group=2; conext=incoming channel =>4 --------------------------------------------------------------------- Note echo training is switched off, although this didn't make much difference to the FXO Port when using the built-in Zaptel echo cancellers. (helped the the FXS port though). Oslec does not support echo training. [[support]] Support ------- https://lists.sourceforge.net/lists/listinfo/freetel-oslec[Oslec mailing list]. [[further]] Further Work ------------ Here are some ideas for further work. 1/ Make info screen on control panel update each second automatically. 2/ When e/c is reset during a call it converges faster than at start of calls. This suggests e/c is wandering off into dumb states before the call is connected. Perhaps disabling adaption until the call is connected would help with faster convergence 3/ Preserving the state of the e/c between calls is also a very good idea, current zaptel design destroys the e/c at the end of a call. 4/ Switch on and test SSE and MMX filter code for x86. Code LMS update for x86 in SSE or MMX. 5/ Add a feature to /proc/oslec to extract the current estimated impulse response. Use gnuplot or Octave to plot it in real time. 6/ Attempt to speed convergence. 7/ Set up test scripts for greater coverage of G168 tests. 8/ If necessary develop a sparse approach to handle 128ms tails. [[Simulation]] HowTo - OSLEC G168 Simulation ----------------------------- The simulation form of Oslec is useful for Oslec development. It is much easier to develop using a non-real time, user mode program than saying 'Hello,...1,...2,....3" down a telephone line. The simulation dumps internal states while it is running, which can be plotted and analysed using Octave. 1. Read spandsp-0.0.3/README and make sure you have installed the dependencies like libtiff-devel, libaudiofile-devel, fftw-devel. 2. Make, build, run a test: $ cd spandsp-0.0.3; $ ./configure --enable-tests $ make $ cd tests $ ./echo_tests 2a 3. You can change the echo path model with -m [1..7] (default 1) 4. You can change the ERL with -erl [0..whatever] (default 10.0) 5. You can plot the internal states using Octave: $ cd spandsp-0.0.3/echo_tests $ octave octave:1> echo_dump The st= and en= statements at the top of echo_dump.m control which part of the waveform is plotted. 6. There are some sequences of tests set up in the script files: $ ./g168_quick.sh (useful for a quick sanity test) $ ./g168_tests.sh (more comprehensive set of tests) 7. For more options: $ ./echo_tests -h 8. Counting passes (say if u want to see how many tests pass after making a change): $ ./g168_tests.sh > lms16bit.txt $ cat lms16bit.txt | grep PASS | wc -l [[links]] Further Reading --------------- + http://svn.astfin.org/software/oslec/trunk/spandsp-0.0.3/src/echo.c[echo.c] is the heart of the Oslec echo canceller. There are several blog posts documenting the development of Oslec: + http://www.rowetel.com/blog/?p=18[Part 1 - Introduction] discusses the myth of hardware echo cancellation and the concept of an echo canceller developed using echo samples collected by a community. + http://www.rowetel.com/blog/?p=21[Part 2 - How Echo Cancellers Work] is an easy to read introduction to echo cancellation for C programmers. + http://www.rowetel.com/blog/?p=22[Part 3 - Two Prototypes] discusses two algorithms that were developed as candidates for Oslec. This is a fairly "hard core" DSP post - some familiarity with echo cancellers is assumed. + http://www.rowetel.com/blog/?p=23[Part 4 - First Phone calls] talks about the very first real-world phone calls made using Oslec. + http://www.rowetel.com/blog/?p=33[Part 5 - Ready for Beta Testing]. Walks through some of the alpha testing bugs and how they were fixed, discusses open development methods and the need for 128ms tails. Some useful links: + Ochiai, Areseki, and Ogihara, "Echo Canceller with Two Echo Path Models", IEEE Transactions on communications, COM-25, No. 6, June 1977. http://www.rowetel.com/images/echo/dual_path_paper.pdf[download]. + The classic, very useful paper that tells you how to actually build a real world echo canceller: Messerschmitt, Hedberg, Cole, Haoui, Winship, "Digital Voice Echo Canceller with a TMS320020. http://www.rowetel.com/images/echo/spra129.pdf[download]. + A nice http://en.wikipedia.org/wiki/Least_mean_squares_filter[introduction to LMS filters]. + Good introduction to http://www.cisco.com/univercd/cc/td/doc/cisintwk/intsolns/voipsol/ea_isd.htm[echo on VOIP calls] from Cisco.