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diff --git a/.moc/moc/README_equalizer b/.moc/moc/README_equalizer new file mode 100644 index 0000000..3c78cc8 --- /dev/null +++ b/.moc/moc/README_equalizer @@ -0,0 +1,184 @@ +Preamble +--- +This document is meant to give you an overview on the idea of having a +parametric equalizer for sound enhancement and how you can create your +own presets. Also the interaction with the equalizer in MOC is described. + +I would like to improve this document to make it more usable; so if you +have any comments and/or ideas feel free to contact me. + +- Hendrik Iben (hiben<at>tzi(dot)de) + + +Content +--- +0. Document History +1. Motivation +2. Usage +3. Preset Format +4. Creating Presets +5. TODO +6. References + + +0. Document History +--- +07.09.2008 - Initial version +15.03.2011 - Reformatted + + +1. Nuts and Bolts / Motivation for Implementing the Equalizer +--- +The equalizer is an implementation of a biquadratic peaking equalizer +filter looked up from the Audio EQ Cookbook[1]. + +It happens to be a parametric equalizer and this means that, different +from other equalizer implementations, the number of bands* is not fixed. +When I started the idea of implementing the equalizer I looked around +in the source of other audio playback software and found that a lot of +them are recycling the code used by the famous XMMS[2] audio player. +I also would have liked to recycle the code but I decided against it +for two reasons: + +The first reason is that there is almost no documentation on the algorithm +used. Maybe the signal processing folks have fun finding out what makes +this thing work but I was totally lost. So I decided that I wanted to +*know* what I am doing if I do it. + +As for the second reason, the code used by XMMS is totally optimized for +integer arithmetic. There is no problem with this in general but I had +the goal of implementing something that was as accurate as I could and +I wanted to use floating point arithmetic. + +So I am no signals processing guy, but I have -- I think -- a solid +understanding of the matter. I sat down and started to read about +equalizing, audio processing and signal theory in general. After some +time I found a mathematical description and a C implementation of +biquadratic filters in the Audio Cookbook. I made an implementation of +the XMMS equalizer and the biquadratic filter using Octave[3] to compare +the outcome of both filters. I was a bit surprised how different filters +can be but in the end succeeded (?) in finding a quite good biquadratic +filter set that would produce results not unlike the XMMS equalizer. + +Although I did not use the XMMS-code I think that people will be more +happy to accept this equalizer if they can use their presets with it. +There is some conversion needed, but it's a straightforward process. +I converted all presets provided by XMMS into presets for this mixer. +They should be available at [4]. + +* A band is a chosen center frequency where a filter has most impact. + If you look at WinAmp / XMMS / Beep Media Player you will find that + they settled on a common set of 10 bands. + + +2. Using the Equalizer +--- +The default keys for the equalizer are: + +'e' - Refresh equalizer +'E' - Toggle equalizer (on/off) +'k' - Select next preset +'K' - Select previous preset + +Each of these actions results in a message displayed in the message area. +This message will be overridden by the next action. + + +3. Preset Format +--- +Presets for the equalizer are to be placed in a directory called 'eqsets' +in MOC's home directory (e.g., $HOME/.moc/eqsets). There is no convention +for the filename, but it will serve as the name in the selection process. + +File format in pseudo EBNF: + + EQSET + ((<CF> <BW> <AMP>)|(0 <PREAMP>))* + + CF: Center frequency (sane values are from ~20 to ~20000). + BW: Bandwith in Octaves. This defines how fast the bands + influence vanishes over the frequencies. + AMP: Amplification factor (in dB) to apply to the band. + PREAMP: Specifies an amplification factor applied before equalizing. + +So a valid equalizer set would be: + + # this is a comment + EQSET + # amplify audio by 1.4dB + 0 1.4 + # damp frequencies at 100Hz by -4dB, filter bandwidth 1.5 octaves + 100 1.5 -4 + # amplify frequencies at 4000Hz by 2dB, filter bandwidth 1.5 octaves + 4000 1.5 2 + +There is no order to stick to when specifying frequencies. + + +4. Creating Your Own Presets +--- +For a start you should have a look at the converted presets[4]. The +bandwidths used in the conversion have been extracted by taking a look +at the filters signal response (implementation and analysis in Octave). +I tried to do this as accurately as possible but I don't know if I made +a mistake. They sound correct though... :-) + +You might note that there is never a positive amplification factor in +the presets although there are in the original preset. The reason for +this is that I used the maximum amplification in the preset as zero +amplification and adjusted the other values accordingly. + +In general, when creating a preset get used to the following idea: Do not +amplify the frequencies you want but damp those that are of no interest. +This has the same effect but avoids clipping and this equalizer type seems +to be very prone to clipping. Also be very careful with pre-amplifying +the audio for the same reason. + +With that said, the next confusing thing is the bandwidth definition. +Every band needs a defined bandwidth in octaves where the bandwidth +defines where the filter's effect has been reduced by 3dB*. This means +that if you define a band at 1000Hz with a bandwidth of 1.5 octaves and +an amplification of -10dB, at 353.6Hz** and at 2828.4Hz the amplification +will be reduced to -7dB. + +If unsure, stay in between 1.0 and 2.0. Just keep in mind that if two +bands overlap you might get an undesired amplification. + +When designing presets, just save the preset and select it in MOC. After +each change press the refresh key (default 'e'). This will re-create the +equalizer reflecting your changes. + +If your preset is not found, have a look at the output of MOC's server +thread. Parsing errors are emitted there. + +* 3dB is commonly used for bandwidth. -3dB equals about 70.7% of + original amplification. +** 353.6 =~ 1000*(2^-1.5), 2828.4 =~ 1000*(2^1.5) + + +5. TODO +--- +- The equalizer is currently not optimized in any way. + +- It converts all sound data into floating point values to perform the + equalization and converts them back afterwards. A better approach + would be either to provide integer algorithms for equalization or to + leave the audio data in floating point format. + +- There is no sorting for the presets; their order is defined by reading + the directory content. + +- Maybe it would be nice to add a name to the preset different from the + filename. + + +6. References +--- +[1] Cookbook formulae for audio EQ biquad filter coefficients + http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt +[2] X Multimedia System + http://www.xmms.org/ +[3] GNU Octave + http://www.gnu.org/software/octave/ +[4] Converted WinAmp / XMMS Equalizer sets + http://www.informatik.uni-bremen.de/~hiben/moc/eqsets.tar.gz |