mel_hertz.3

speech signal process tools

.\" Copyright (c) 1998 Entropic, Inc. All rights reserved.
.\" @(#)mel_hertz.3	1.1 9/4/98 ESI/ERL/EG
.ds ]W (c) 1998 Entropic Research Laboratory, Inc.
.TH MEL_HERTZ 3\-ESPSsp 9/4/98
.SH NAME
mel_to_hz \- Convert a mel-scale value to the equivalent frequency in Hertz.
.br
hz_to_mel \- Convert a frequency value in Hertz to the equivalent mel-scale value.
.SH SYNOPSIS
.ft B
double 
.br
mel_to_hz (melValue)
.br
double	melValue;
.sp
.br
.ft B
double 
.br
hz_to_mel (freqValue)
.br
double	freqValue;
.SH DESCRIPTION
.PP
These functions are reciprocal functions that warp a linear-scaled frequency
value in Hertz to the equivalent mel-scale value and back.
.PP
The mel-scale value
.I m
corresponding to a frequency value in Hertz 
.I f
is given by:
.IP
.if n \{\
m = 1127.01 log (1 + f/700)
\}
.if t \{\
.IR m " = 1127.01 log (1 + " f "/700)"
\}
.LP
where the constant shown as 1127.01 is actually 1000/log(1700/700),
a value chosen so that 1000 mel corresponds to 1000 Hz.
(There is no single universally accepted definition of the mel scale.
The one used in
here
is consistent with the one used in HTK [1].)
The conversion from a frequency value in Hertz 
to a mel scale value is done in the
obvious manner by
using the exponential function.
.SH EXAMPLE
.nf
double bigNumber = MAXFLOAT, hertz, mel;

/* mel_to_hz can exceed MAXDOUBLE */
errno = 0;
hertz = mel_to_hz(bigNumber);
if (errno)
  perror("mel_to_hz error");

/* reasonable frequency values are fine */
hertz = 5000.0;
mel = hz_to_mel(hertz);
printf("mel equivalent of 5000 Hz is %f", mel);

/* This makes the variable hertz = 5000.0 */
hertz = mel_to_hz(mel);
printf("mel value converted back to hertz is %f", hertz);
.fi
.SH WARNINGS
.PP
A negative input value to either of these functions is invalid. Both
functions print an error message and return a negative value for a negative
input whose magnitude is greater than 1. 
.SH BUGS
None known.
.SH SEE ALSO
.nf
\fIbark_to_hz\fP(3-ESPS), \fIhz_to_bark\fP(3-ESPS), \fImelspec\fP(1\-ESPS)
.fi
.SH REFERENCES
.PP
[1] Steve Young, Julian Odell, Dave Ollason, Valtcho Valtchev,
and Phil Woodland,
.I "The HTK Book,"
Entropic, 1997.
.SH AUTHOR
Code extracted from \fImelspec\fP(1\-ESPS) by David Burton; manual page by David Burton.