[Speex-dev] Speex echo canceller on TI C55 DSP

[Jean-Marc Valin]

> I built and ran the same test on the TI C64 simulator, and the echo was 
> canceled nicely (about 10:1 reduction in the peak amplitude during the 
> second of two brief speech bursts).  So, my problem must again be related to 
> the 16-bit processing on the C5X DSPs.

Good. At least we've narrowed it down a bit.

> Also, the line where it is hanging is:
>          st->power_1[i] = 
> FLOAT_SHL(FLOAT_DIV32_FLOAT(MULT16_32_Q15(M_1,r),FLOAT_MUL32U(e,st->power[i]+10)),WEIGHT_SHIFT+16);

Actually, I just found a 16-bit bug in FLOAT_DIV32_FLOAT. Could you
update svn and let me know if it works?

> and it is e that is in the denominator, not r (sorry for the confusion).  I 
> can now run the simulations side-by-side and look for differences.

Can e really go to 0? I don't see how.

	Jean-Marc

> - Jim
> 
> Le lundi 08 mai 2006 à 20:05 -0400, Jim Crichton a écrit :
> > > I've just been made aware of these problems (look for the thread "speex
> > > echo cancellation limitations"). It's on my short-term TODO list.
> >
> > I saw the other thread, my problems happened in different (but similar)
> > routines.
> >
> > >> If fftwrap.c, I ifdefed out the spx_fft_float and spx_ifft_float
> > >> routines,
> > >> because there were not used and required smallft.c (which is not so 
> > >> small
> > >> at
> > >> all) to be added to the build.
> > >
> > > Right, need to cleanup that part...
> > >
> > >> With these changes, the link was successful, using testecho.c with some
> > >> modifications for the C55 environment.  The code and data memory
> > >> requirements were a lot more than I had hoped (>20kbytes of dynamic 
> > >> data
> > >> memory for block size=128, tail length = 1024), and I will probably not
> > >> be
> > >> able to fit it in the production build without some trimming.
> > >
> > > Yes, there may be a bit of memory reduction possible here. Of course,
> > > decreasing the tail length is also a rather easy way.
> > >
> > >> When I run the build, it goes into an infinite loop in FLOAT_DIV32 
> > >> (mdf.c
> > >> line 660), which occurs because adapt_rate is < 0, which happens when
> > >> FLOAT_EXTRACT16 gets the input {0x7ff0, 0xfffb}.  The rounding is 
> > >> causing
> > >> the result to go negative.  I worked around this by changing
> > >
> > > I think that was mentioned in the previous thread...
> > >
> > >>       return (a.m+(1<<(-a.e-1)))>>-a.e;
> > >> to
> > >>       return (((spx_uint16_t) a.m)+(1<<(-a.e-1)))>>-a.e;
> > >
> > > Is that sufficient to remove all the overflows at this place?
> >
> > The rounding takes the value to exactly 0x8000, and it is followed by a
> > right shift, so you just need to avoid the sign extension.
> >
> > >> I have not had time to trace this, but it looks like a similar problem,
> > >> where the result of MULT16_32_Q15(M_1,r) is negative, and
> > >> FLOAT_DIV32_FLOAT
> > >> bombs.  Maybe the best thing to do next is to instrument the routines 
> > >> in
> > >> pseudofloat.h which have loops, but I will not get to that for a day or
> > >> two.
> > >
> > > Yeah, r is never supposed to be negative and the float routines assume
> > > that.
> >
> > No, it was a divide by zero, as explained in my second post.  I will try a
> > build on the C6x DSP to see if this is a 16 vs. 32-bit problem.  I sent 
> > the
> > test files off-list.
> >
> > >> 1.  speex_echo_state_init takes about 20M instructions, which is a 
> > >> little
> > >> frightening,
> > >
> > > That's the fft initialization that calls a lot of float cos() functions.
> > > If you have a fixed version of cos() you can use it there, otherwise a
> > > fixed table (for a certain size) would work.
> > >
> > >> and the calls to speex_echo_cancel take about 630K instructions
> > >> for 128 samples.  Given your recent experience with the fixed point
> > >> canceller, does this sound rational?  The MIPs for the canceller are
> > >> similar
> > >> to the MIPs for the encoder running 8kbps, complexity 1.
> > >
> > > The order of magnitude seems right. It may be possible to reduce that a
> > > bit, though. If you have an optimized FFT, you could replace kiss_fft
> > > with it and get a big improvement right there.
> >
> > Yeah, but then I have to try to actually understand the algorithm.  I am 
> > not
> > sure that those brain cells are still alive.
> >
> > >> 2.  The testecho example uses a frame length and tail size that are
> > >> powers
> > >> of two (128, 1024).  Are there any implications to using sizes which 
> > >> are
> > >> not
> > >> powers of two?  It would be most convenient to use the encoder frame 
> > >> size
> > >> (160), and some multiple of that for the tail size.  How does the frame
> > >> size
> > >> affect performance (I understand that the tail length determines what
> > >> echo
> > >> signals are cancelable)?
> > >
> > > Non powers of two will be a bit slower because of the FFT, but that's
> > > all. I made sure the echo canceller works with 160, precisely because
> > > it's the frame size used by Speex. Note that I don't recommend using
> > > frames more than 20 ms long (at any sampling rate).
> > >
> > >> 3.  Do you have any suggestions for code/data memory reduction for the
> > >> canceller, other than to make the tail length no longer than necessary
> > >> (this
> > >> is a line echo canceller for a local phone, so I should be able to keep
> > >> it
> > >> to 40ms).  I was surprised by the size of the FFT code, but I guess 
> > >> that
> > >> it
> > >> is doing much more than the radix2 version in the TI library.
> > >
> > > The FFT code has more than just the radix two, so you can save there. It
> > > wasn't meant to be an optimized FFT, so if TI supplies you with one,
> > > it's probably a good idea to use it (that's what fft_wrap is for). Also,
> > > given that the memory use is almost directly proportional to the tail
> > > length, reducing that one to 40 ms will make a huge difference.
> >
> > Thanks for the advice.
> >
> > - Jim
> >
> >
> >
> 
> 
>