CAR FAC - Cascade of Asymmetric Resonators with Fast Acting Compression

The code below implements a model of the cochlea developed by Dick Lyon.

It is described in detail in his book. Page numbers in the code comments refer to the pages of this book.

Following on from the previous notebook on the CAR model, we now incorporate the Outer Hair Cell model in the Basilar membrane too. To do so, we update the BM resontator to look like this:


As before $X$ is the input of the section, coming from the sound source for the first section in the cascade, and from the output $Y$ of the previous section for each of the following sections. $a_0$ and $c_0$ are are the coefficients of the filter, controlling the resonant frequency of each section. The resonant frequency will be highest for the first section and drop exponentially along the cochlea. $h$ controls the distance between the poles and the zeros in the frequency domain. $g$ adjusts the dc gain.

What is different is that now $r$, which controls the radius of the poles and zeros and thus the damping, is now modified online by the OHC model. First, the temporal derivative of the BM displacement is taken to calculate the BM velocity $v$ and then passed through a nonlinear function. The output of this function is multiplied by $d_{rz}(1-b)$ where $b$ is a static parameter between 0 and 1 for the moment. This signal then modifies $r$ from its minimum value $r_1$. The nonlinear function is such that $r$ increases as $v$ decreases.

In [1]:
%matplotlib notebook
from pylab import *
from scipy import signal

First let's set the stimulation parameters:

In [2]:
fs = 32000.0                  # sample frequency
dur = 2                       # simulation duration
npoints = int(fs*dur)         # stimulus length

Next, we create a log-sine-sweep using the chirp function from the scipi.signal library:

In [3]:
# create a log-sine-sweep
f0 = 20                       # sweep start frequency
f1 = 20000                    # sweep end frequency
t1 = arange(npoints)/fs       # sample times
gain = 1.0                    # gain of input signal
stimulus = gain*signal.chirp(t1, f0, t1[-1], f1, method='logarithmic', phi=-90)

Now we define the number of cochlear filters and the range of Basilar Membrane positions to spread these filters over with equal spacing.

In [4]:
nsec = 100                    # number of sections in the cochlea between
xlow = 0.1                    # lowest frequency position along the cochlea and
xhigh = 0.9                   # highest frequency position along the cochlea

Once all these parameters are defined, we can calculate the parameters for the CAR model. The frequencies of the resonators are spaced according to the Greenwood formula, while the damping controls the poles. The choice of $h = c_0$ sets the zeros for each section about half an octave above the location of the poles. $g$ is calculated to provide 0dB gain at DC.

In [5]:
x = linspace(xhigh,xlow,nsec) # position along the cochlea 1 = base, 0 = apex
f = 165.4*(10**(2.1*x)-1)     # Greenwood for humans
a0 = cos(2*pi*f/fs)           # a0 and c0 control the poles and zeros
c0 = sin(2*pi*f/fs)

damping = 0.2                 # damping factor
r = 1 - damping*2*pi*f/fs     # pole & zero radius (actual)
r1 = 1 - damping*2*pi*f/fs    # pole & zero radius minimum (set point)
h = c0                        # p302 h=c0 puts the zeros 1/2 octave above poles
g = (1-2*a0*r+r*r)/(1-(2*a0-h*c0)*r+r*r)  
                              # p303 this gives 0dB DC gain for BM

Now we also need parameters for the OHC part:

In [6]:
scale = 0.1                   # p313 NLF parameter
offset = 0.04                 # p313 NLF parameter
b = 1.0                       # automatic gain loop feedback (1=no undamping).
d_rz = 0.7*(1 - r1)           # p310 relative undamping

From the block diagram at the top, we can see that the feedback signal is multiplied by $(1 - b)$ so that when $b = 1$, the feedback loop is effectively turned off. We'll start with this to verify that the code runs as in the previous notebook.

We also implement the Inner Hair Cell (IHC) model shown here:


First, the output of the BM filters is high-pass filtered at about 20Hz before being passed through a nonlinearity. A feedback loop provides gain adaptation that causes the IHC to respond more strongly to the onset of a stimulus than the sustained part of that stimulus. Finally, the output of the IHC is low pass filtered to reduce the IHC's response to frequencies above 8kHz.

In [7]:
f_hpf = 20                    # p328 20Hz corner for the BM HPF
q = 1/(1+(2*pi*f_hpf/fs))     # corresponding IIR coefficient 

tau_in = 10e-3                # p329 transmitter creation time constant
c_in = 1/(fs*tau_in)          # p329 corresponding IIR coefficient    
tau_out = 0.5e-3              # p329 transmitter depletion time constant
c_out = 1/(fs*tau_out)        # p329 corresponding IIR coefficient 
tau_IHC = 80e-6               # p329 ~8kHz LPF for IHC output
c_IHC = 1/(fs*tau_IHC)        # corresponding IIR coefficient 

Before we run the simulation, we initialise the variables as needed.

In [8]:
W0 = zeros(nsec)              # BM filter internal state
W1 = zeros(nsec)              # BM filter internal state
W1old = zeros(nsec)           # BM filter internal state at t-1
BM = zeros((nsec,npoints))    # BM displacement
BM_hpf = zeros((nsec,npoints))# BM displacement high-pass filtered at 20Hz
trans = ones(nsec)            # transmitter available
IHC = zeros((nsec,npoints))   # IHC output
IHCa = zeros((nsec,npoints))  # IHC filter internal state
BM[-1] = stimulus             # put stimulus at BM[-1] to provide input to BM[0]
BM[-1,-1] = 0                 # hack to make BM_hpf[nsec-1,0] work

Finally we simulated the whole system for each sample time and for all cochlear sections.

In [9]:
for t in range(npoints):
    for s in range(nsec):
        W0new = BM[s-1,t] + r[s]*(a0[s]*W0[s] - c0[s]*W1[s])
        W1[s] = r[s]*(a0[s]*W1[s] + c0[s]*W0[s])
        W0[s] = W0new
        BM[s,t] = g[s]*(BM[s-1,t] + h[s]*W1[s])
    BM_hpf[:,t] = q*(BM_hpf[:,t-1] + BM[:,t] - BM[:,t-1])
    z = (BM_hpf[:,t]+0.175).clip(0)   # p328
    v_mem = z**3/(z**3+z**2+0.1)      # p328
    IHC_new = v_mem*trans    
    trans += c_in*(1-trans) - c_out*IHC_new   
    IHCa[:,t] = (1-c_IHC)*IHCa[:,t-1] + c_IHC*IHC_new
    IHC[:,t] = (1-c_IHC)*IHC[:,t-1] + c_IHC*IHCa[:,t]
    v_OHC = W1 - W1old
    W1old = W1.copy()
    NLF= 1/(1 + (scale*v_OHC + offset)**2)
    r = r1 + d_rz*(1-b)*NLF
    g = (1-2*a0*r+r*r)/(1-(2*a0-h*c0)*r+r*r)

After playing the sweep signal through the cochlea, i.e., using it as the stimulus and simulating all time steps, we can get the frequency response and phase response of each cochlear filter by calculating:

In [10]:
# play the stimulus through the system and measure the output
output = BM_hpf

# use the FFT of the stimulus and output directly to calculate the transfer function
myFFT = fft(zeros((nsec,npoints)))
for s in range(nsec):
    myFFT[s] = (fft(output[s])/fft(stimulus))
In [11]:
fig = figure(1, figsize=(12,4)) # Bode plot of BM displacement 
ax1 = subplot(1,2,1)
freq = linspace(0,fs/2,(npoints)/2)
semilogx(freq,20*log10(abs(myFFT.T[0:(npoints)/2,:])+1e-10))  # note, 1e-10 offset to avoid division by zero in log10
title('BM gain (in dB)')
xlabel('f (in Hz)')
ax2 = subplot(1,2,2,sharex=ax1)
title('BM phase (in rad)')
xlabel('f (in Hz)')

figure(2, figsize=(12,3)) # IHC output
xlabel('t (in ms)')
title('IHC response')