|
|
ARM and Canonical to bring full Ubuntu desktop experience to low-poer, ARM technology
|
|
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)
