JNOISEMETER(1)						      General Commands Manual						    JNOISEMETER(1)

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 manufac-
       turers 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 Jaromir Mike <mira.mikes@seznam.cz> for the Debian project (but may be used by others).

								  August 2, 2010						    JNOISEMETER(1)