NAME

       jnoisemeter - Jnoisemeter is a small app which measure audio test signals.

SYNOPSIS

       jnoisemeter [options]

DESCRIPTION

       This manual page documents briefly the jnoisemeter

       This manual page was written for the Debian distribution because the original program does
       not have a manual page.

       jnoisemeter is a small app designed to measure audio test signals and in particular  noise
       signals through Jack Audio Connection Kit.

       The  simplest use is to measure the S/N ratio of your sound card. If you can calibrate the
       input levels of your soundcard it can also  be  used  (with  some  external  hardware)  to
       measure noise levels of any type of audio equipment, including preamps and microphones.

       jnoisemeter has the following filters:

        FLAT
         No filtering at all, the signal is passed directly
         to the detector.

        20KHZ
         This is 4th order Chebyshev lowpass filter having
         a noise bandwidth of exactly 20 kHz. This means
         that if the input signal is white noise, the RMS
         output level is the same as for a 'perfect' 20 kHz
         lowpass filter. This is the normal filter to use for
         'unweighted' measurements, as any noise measurement
         should always use a well-defined bandwidth.
         Future version may use a higher order filter.

        IEC A and C
         The well-known standard noise weighting filters used
         to obtain dB(A) and dB(C) measurements.

        ITU-R468
         This is a filter optimised for measuring low-level
         background noise. It rises 6 dB/oct at low frequencies,
         has a peak of around +12 dB at 6.3 kHz, and falls off
         radiply after that. It should be used together with the
         ITU-R468 detector described below.

        ITU-R468 (Dolby variant)
         This is the same filter as the previous one with around
         5.6 dB less gain. See below for why this exists.

       jnoisemeter  also  provides a DC blocking filter (first order highpass, 5 Hz). This may be
       necessary when using the FLAT and 20KHZ filters, the others are DC-blocking anyway.

       jnoisemeter has three detectors:

        RMS
         Root-mean-square (i.e. 'power') meter. The time constant
         is 125 ms as per IEC standard, or 1 second in slow mode.

        AVERAGE
         This measures the average of the absolute value. The one
         used in jnoisemeter is actually a VU meter. A 10 times
         slower version is also provided.

        ITU-R468
         This is a 'pseudo-peak' detector designed specifically
         to measure noise and S/N ratios. For a peak meter it is
         quite slow, as it should be for noise measurements, but
         at the same time it is much more sensitive to short
         impulsive noise than its speed would suggest.
         The original rationale for this was the presence of
         impulsive noise (generated by the electromechanical
         telephone exchanges of those days) on long analog audio
         lines. Today long distance audio lines are all digital,
         but a detector such as this one is also ideal to reveal
         the typical short noise bursts and 'crackle' originating
         in computers and other digital equipment.

       All three detectors will show 0.0 dB for a 'digital full scale' sine  wave  (i.e.  peaking
       +/- 1.0).

       The ITU-R468 standard

       This uses both the ITU filter and detector, and is probably the 'best' standardized way to
       measure noise. It is used by e.g. the manufacturers of quality microphones, in  particular
       the  European  ones. It produces a result that is on average about 10 dB higher than an A-
       weighted RMS measurement.

       There is a 'variation' of this standard called 'ITU-ARM'. This was devised by  Dolby  Inc.
       at  the time they were selling noise reduction technology for magnetic tape recorders. The
       traditional A-weighted measurements would show very  little  S/N  ratio  improvement  when
       using  Dolby-B. For this reason Dolby wanted to adopt the ITU-R468 method (which shows the
       difference quite clearly) but without the apparent 10 dB loss in S/N  ratio  as  this  was
       deemed  bad  for  marketing. The solution adopted was to lower the gain of the filter, and
       use an average detector instead of the pseudo-peak one. Despite the 'ITU-ARM' name this is
       not an official standard, and not approved by the ITU.

OPTIONS

         -h
              Display short info

         -n <name>
              Name to use as jack client

       SEE ALSO
              jackd(1).

AUTHOR

       jnoisemeter was written by Fons Adriaensen <fons@kokkinizita.net>.

       This  manual  page  was  written  by  Jaromír  Mikeš <mira.mikes@seznam.cz> for the Debian
       project (but may be used by others).

                                          August 2, 2010                           JNOISEMETER(1)