Jnoisemeter is a small app designed to measure audio
test signals and in particular noise signals.

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.


There are various 'standard' ways to measure noise
(and S/N ratio). All amount to some combination of a
particular weighting filter and a particular detector.


FILTERS
-------
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.

All filters except the last one have 0 dB gain at 1 kHz.
The Dolby version of the ITU filter has 0 dB gain at 2 kHz.

Apart from these filters 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.


DETECTORS
---------

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.


Enjoy !

-- 
FA
