Recently I purchased a pair of Audiovector S-Sub Signature sub-woofers to supply my Audiovector S3 Super speakers with a firmer and fatter bass, which they certainly delivered. As with any setup the addition of two sub-woofers required a lot of tinkering with the setup to provide the best sound – but I was constantly faced with bass-boost problems at specific frequencies as my room was a bit too lively for the new setup.

At the hearth of my Hi-Fi setup sits a SqueezeBox Receiver which is feeding a Cambridge Audio DacMagic, but sadly neither the Receiver nor the backing SqueezeBox Server software has any default equalization features. So as any hacker would do, I put Google on the task but all I ended up with were solutions that didn’t work anymore (with the newer software).

But after many hours of tinkering, I finally have a way to combine SqueezeBox Server with the BruteFIR software to give a full parametric 128 band equalizer. Seeing how much effort it took on my part to get thus far, I thought I would share my findings with anyone interested.

Prerequisite

This hack only works if your SqueezeBox Server software is installed on a Linux based machine, as it requires the BruteFIR software and access to an underlying bash shell. This is necessary to pipe audio from the Server software, into a decoder, through BruteFIR, into an encoder and back into the Server software, but more to come later on the details.

For the interested parties, BruteFIR is a (near) real-time FIR filtering software with added logic modules for equalization and such. FIR filters is short for “finite impulse response filter” (Wikipedia). The reason for using a FIR filter I suspect despite it being considerably harder to compute than a IIR filter, is that they require no feedback and thus calculation errors will not pile up (remembering that a computer has limited calculation precession on real numbers).

So if you haven’t got BruteFIR installed yet, go fetch and install it now. You need to have the ‘brutefir’ binary accessible in your path. To check if you have this, try running:

$ which brutefir
/usr/bin/brutefir

If you have BruteFIR installed and in path, you will get a path back otherwise the empty string will be printed.

Writing the BruteFIR filter

The next step is to write the filter script for BruteFIR which will apply the equalization needed. I have placed all my configurations in the path “/etc/squeezeboxserver/brutefir/filters/”, my examples assume this path exists and that you place your script there as well. It also assumes the path “/etc/squeezeboxserver/brutefir/settings/” exists, so that it can place the convolver configuration in this directory. So change as needed if that is not the case. This is my BruteFIR equalization filter script (saved in “/etc/squeezeboxserver/filters/bass_eq.cfg”)

### DEFAULT GENERAL SETTINGS ###
float_bits: 64;             # internal floating point precision
sampling_rate: 44100;       # sampling rate in Hz of audio interfaces
filter_length: 1024,64;     # length of filters
#filter_length: 65536;
overflow_warnings: true;    # echo warnings to stderr if overflow occurs
show_progress: false;        # echo filtering progress to stderr
max_dither_table_size: 0;   # maximum size in bytes of precalculated dither
allow_poll_mode: false;     # allow use of input poll mode
modules_path: ".";          # extra path where to find BruteFIR modules
monitor_rate: false;        # monitor sample rate
powersave: true;           # pause filtering when input is zero
lock_memory: false;          # try to lock memory if realtime prio is set
convolver_config: "/etc/squeezeboxserver/brutefir/settings/wisdom";
# location of convolver config file
 
### EQUALIZER SETTINGS ###
logic: "eq" {
        {
        coeff: "equalizer";
        bands: 35, 38, 41, 44, 47, 50, 54, 56;
        magnitude: 35/0.0, 38/0.0, 41/-4.0, 44/-6.0, 47/0.0, 50/0.0, 54/-8.0, 56/0.0;
        phase: 35/0, 38/0, 41/0, 44/0, 47/0, 50/0, 54/0, 56/0;
        };
};
 
coeff "equalizer" {
      filename: "dirac pulse";
      shared_mem: true;
      blocks: 4;
};
 
### INPUT DEFAULTS ###
input "l_in","r_in" {
      device: "file" {path: "/dev/stdin";}; # module and parameters to get audio
      sample: "S16_LE"; # sample format
      channels: 2/0,1; # number of open channels / which to use
      delay: 0,0; # delay in samples for each channel
      maxdelay: -1; # max delay for variable delays
      mute: false,false; # mute active on startup for each channel
};
 
## OUTPUT DEFAULTS ##
output "l_out","r_out" {
       device: "file" {path: "/dev/stdout";}; # module and parameters to put audio
       sample: "S24_BE"; # sample format
       channels: 2/0,1; # number of open channels / which to use
       delay: 0,0; # delay in samples for each channel
       maxdelay: -1; # max delay for variable delays
       mute: false,false; # mute active on startup for each channel
       dither: true; # apply dither
};
 
### EQUALIZATION FILTER ###
# Add equalizer
filter "eq_l" {
       from_inputs: "l_in";
       to_outputs: "l_out";
       process: -1; # process index to run in (-1 means auto)
       coeff: "equalizer";
       delay: 0; # predelay, in blocks
       crossfade: false; # crossfade when coefficient is changed
};
 
filter "eq_r" {
       from_inputs: "r_in";
       to_outputs: "r_out";
       process: -1; # process index to run in (-1 means auto)
       coeff: "equalizer";
       delay: 0; # predelay, in blocks
       crossfade: false; # crossfade when coefficient is changed
};

The relevant lines are bands, magnitude and phase which control the EQ logic module of BruteFIR we use for the equalization. The line:

bands: 35, 38, 41, 44, 47, 50, 54, 56;

Specifies a comma seperated list of frequencies in Hz that you want the EQ to operate on. The line:

magnitude: 35/0.0, 38/0.0, 41/-4.0, 44/-6.0, 47/0.0, 50/0.0, 54/-8.0, 56/0.0;

Specifies the magnitude in dB of each band and lastly in line:

phase: 35/0, 38/0, 41/0, 44/0, 47/0, 50/0, 54/0, 56/0;

Specifies the phase of each band in degrees (0 – 180).

The maximum number of bands you can define is 128 and you don’t need to have even spacing between them as I have in my example (3Hz intervals). Thus it gives you a full parametric 128 band equalizer, pretty neat if you ask me.

Noteworthy is also that the above filter specifies a 16-bit Left Endian (sample: “S16_LE”) input format and 24-bit Big Endian (sample: “S24_BE”) output format with dithering enabled (this effectively upscales the audio to 24-bit using the dithering technique). To ensure the best quality upscaling and filtering in general it’s also using double precision floating point arithmetic (float_bits: 64), don’t confuse this with the CPU Architecture as double precision floats doesn’t require a 64-bit CPU!

Routing audio from SqueezeBox Server through BruteFIR

To our luck the developers of the SqueezeBox Server software had enough foresight to predict the need for custom real-time conversions of the audio streams, which is why they gave us “custom-convert.conf”. This file basically allows you to specify arbitrary shell commands as audio decoders where a range of variables are available for usage dependent on the track currently playing. We will use this to route the audio through BruteFIR. The file normally doesn’t exist by default, so you need to create the file “/etc/squeezeboxserver/custom-convert.conf” with the following content (assumes you have placed your BruteFIR filter script in the same location as me):

flc flc * *
        [flac] --decode --stdout --silent --force-raw-format --endian=little --sign=signed -- $FILE$ 2> /etc/squeezeboxserver/brutefir/decode.log | /usr/bin/brutefir -nodefault /etc/squeezeboxserver/brutefir/filters/bass_eq.cfg 2> /etc/squeezeboxserver/brutefir/brutefir.log | [flac] --stdout --silent -0 --force-raw-format  --endian=big --sign=signed --channels=2 --bps=24 --sample-rate=44100 - 2> /etc/squeezeboxserver/brutefir/encode.log
 
mp3 flc * *
        [lame] --mp3input --decode -t --silent $FILE$ - - 2> /etc/squeezeboxserver/brutefir/decode.log | /usr/bin/brutefir -nodefault /etc/squeezeboxserver/brutefir/filters/bass_eq.cfg 2> /etc/squeezeboxserver/brutefir/brutefir.log | [flac] --stdout --silent -0 --force-raw-format  --endian=big --sign=signed --channels=2 --bps=24 --sample-rate=44100 - 2> /etc/squeezeboxserver/brutefir/encode.log

This configuration tells SqueezeBox Server that we have supplied it with to extra decoders, one for flac->flac decoding and one for mp3->flac. You might be wondering how flac->flac can be either a decoder or a converter, but it is. SqueezeBox Server will figure out how to playback the outputted format of our decoders/convertes.

You may have noticed that the decoder ensures the input format to BruteFIR is little endian and the encoding of the output from BruteFIR is told to expect 24-bit Big Endian format which matches the filter configuration.

The next step is activating our custom decoders, but first you need to restart the SqueezeBox Server. This is often accomplished  by issuing:

$ /etc/init.d/squeezeboxserver restart

Enabling the “Equalizer” in SqueezeBox Server

To do this you need to navigate to the Server Settings web interface of your server and then navigate to Advanced -> File Types. You ought to see a screen similar to the picture to the right of this.

Here you need to select “flac/flac” for the File format “flac” under Stream format “flac” and set all other stream formats under “flac” to disabled. Likewise for File format “mp3″ under Stream format “flac” you need to select “lame/flac” and set all others to disabled.

This is to ensure that our BruteFIR “decoder” is always used when playing these two formats (I haven’t got any other formats in my collection, so you might need to find solutions for other formats if you use alac, aif or any other). The name “flac/flac” and “lame/flac” comes from the fact that our decoder setups defines flac -> flac and mp3 (lame) -> flac conversions.

You are finished

If everything has gone well, you should now be able to play a track via any SqueezeBox device and have your BruteFIR filter equalization applied to it. You can verify this by checking issuing:

$ top -U squeezeboxserver

Which displays all processes run by the user squeezeboxserver (default user for said software), and you should see one or more brutefir and flac (or lame) processes running while playing. You might experience a slight delay prior to each playback (I experience 25-100 ms, so it’s rather short), this is the time it takes to pipe through BruteFIR and everything. I don’t notice it that much, only when playing albums like Pink Floyd Animals, will I notice the short delay in-between tracks.

Problem solving

If though you are experiencing any trouble, you may need to tinker a bit more with the setup. Prior to getting to this setup I had an experience where I would get sound, but it would be speedup and contained a rather large amount of white noise and distortion. This was prior to setting the output format of BruteFIR to Big Endian, but after doing so my problem was fixed. Many of the websites I have scanned through while tinkering myself had the outset set to  ”S24_LE” (or “S16_LE” if they didn’t have any upscaling with dithering) meaning Little Endian output. If you have problems with my setup you may have luck in trying to switch to Little Endian output of BruteFIR, just remember to also  change the custom-convert.conf file (specifically the flac encoding command which the output of BruteFIR is piped into, my configuration specifies “–endian=big” which you will need to change to “–endian=little”). Phew that was surely quite a lot more text than I originally anticipated. But I hope someone can find good use for it and if you are having trouble getting it to work, don’t hesitate to throw a comment here on the blog.

Digital Room Correction or DRC

Well I think I need to mention, that what I have actually done is to take a piece of software meant for something a bit different and made it fit my purposes. BruteFIR is primarily used for applying Digital Room Correction to your playback, which means a correction filter based on a sweep-tone recording of your Hi-Fi equipment resulting in a near-linear performance in that specific setup.

So if by chance you get your hands on the required equipment to do such a recording of your setup, you can also add this to your BruteFIR filter for DRC and equalization. The recommendations I have found is to apply the equalization prior to the DRC filtering, as you might otherwise get problems with cutting.

Personally I haven’t been fortunate enough yet to get my hands on a professional studio quality recording mike and recording equipment to get a sweep tone recording of my setup, but mayhap some day I will get that opportunity.