```
# This code is based on the Matlab implementations of PLP and Rasta
# feature calculations by Daniel P. W. Ellis of Columbia University /
# International Computer Science Institute. For more details, see:
# http://www.ee.columbia.edu/~dpwe/resources/matlab/rastamat/
mel2hz <- function(z, htk=FALSE){
if(!is.numeric(z) || z < 0)
stop("frequencies have to be non-negative")
if(htk){
f <- 700 * (10^(z/2595) - 1)
} else {
# Mel calculation like in Slaney's Auditory Toolbox
f_0 <- 0
f_sp <- 200/3
brkfrq <- 1000
# Starting Mel value for log region
brkpt <- (brkfrq - f_0)/f_sp
# The magic 1.0711703 which is the ratio needed to get from 1000 Hz to
# 6400 Hz in 27 steps.
logstep <- exp( log(6.4)/27 )
linpts <- (z < brkpt)
f <- 0 * z
# Calculate f separately for linear- and log-spaced frequencies
f[linpts] = f_0 + f_sp * z[linpts]
f[!linpts] <- brkfrq * exp( log(logstep) * (z[!linpts] - brkpt) )
}
return(f)
}
```

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