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Creating speaker crossovers with dsptoolkit

While you can always use SigmaStudio to design your individual DSP program for HiFiBerry DSP boards, for simple use cases, this might be overkill.

Therefore, we provide a simple way to create your own n-way crossovers using a configuration file

What you need

DSP toolkit

First you need to have the DSP toolkit in at least version 0.10 installed. Get it at https://github.com/hifiberry/hifiberry-dsp

A DSP profile that supports a mixing/crossover matrix

A mixing/crossover matrix allows you to mix each input channel onto each output channel and also apply filtering. In SigmaStudio it looks like this:

crossover matrix

We provide an example profile here: https://raw.githubusercontent.com/hifiberry/hifiberry-dsp/master/sample_files/xml/4way-iir.xml

Installing the DSP profile

First, you need to install the profile onto the DSP:

dsptoolkit install-profile 4way-iir.xml

You don't even have to download the profile by yourself as dsptoolkit directly accepts URLs:

dsptoolkit install-profile https://raw.githubusercontent.com/hifiberry/hifiberry-dsp/master/sample_files/xml/4way-iir.xml

Create a crossover settings file

The settings files defines the filters used on each channel of the mixing/crossover matrix

A very simply example looks like this:

IIR_L1: pass
IIR_L2: mute
IIR_L3: mute
IIR_L4: mute
IIR_R1: mute
IIR_R2: pass
IIR_R3: mute
IIR_R4: mute

This settings will pass the left channel to output 1 of the DSP board and the right channel to output 2 without any processing. This is basically a normal stereo full range configuration.

For a crossover, you need to use low-pass and high-pass filters. Let's do a simple 2 way setup with a crossover at 2kHz. The connection to the DSP board is as follows:

  • woofer left: channel 1
  • woofer right: channel 2
  • tweeter left: channel 3
  • tweeter right: channel 4

On the Beocreate 4 channel amplifier it is recommended to connect woofers to channels 1 and 2 as these can provide twice the power of channels 3 and 4

IIR_L1: lp:2000Hz
IIR_L2: mute
IIR_L3: hp:2000Hz
IIR_L4: mute
IIR_R1: mute
IIR_R2: lp: 2000Hz
IIR_R3: mute
IIR_R4: hp: 2000Hz

With this settings configuration,

  • output 1 receives a signal from the left channel filtered with a 2kHz low-pass filter
  • output 2 receives a signal from the right channel filtered with a 2kHz low-pass filter
  • output 3 receives a signal from the left channel filtered with a 2kHz high-pass filter
  • output 4 receives a signal from the right channel filtered with a 2kHz high-pass filter

Applying the crossover

Save you crossover file as a file, e.g. crossover.txt. Then use dsptoolkit to apply the settings on the DSP.

dsptoolkit apply-settings crossover.txt

This will apply the settings just in the DSP RAM, not store them permanently. This is recommended especially when you develop crossovers as it is the fastest way to apply settings and fix potential problems. Sometimes things can go wrong, e.g. if you have mixed up channels. In this case, just edit the file and apply the settings again.

In rare cases it can happen that something is completely wrong and you fear that your speakers might be damaged (e.g. if you have accidentally applied a huge volume increase). In these cases, the fastest way to return to the default settings is to reset the DSP:

dsptoolkit reset

This will load the standard settings from the DSP's EEPROM.

Optimizing the crossover

In most cases a crossover like this won't work perfectly as this would only be correct if woofers and tweeters have exactly the same efficiency. Usually the tweeters will be some decibels too loud. You can correct this with an additional "vol" filter:

IIR_L1: lp:2000Hz
IIR_L2: mute
IIR_L3: hp:2000Hz,vol:-3dB
IIR_L4: mute
IIR_R1: mute
IIR_R2: lp: 2000Hz
IIR_R3: mute
IIR_R4: hp: 2000Hz, vol:-3dB

This would reduce the volume of the tweeters by 3dB.

Further optimizations

There are a lot more optimisations you can do here:

Adding a subsonic filter

Filtering low frequencies that the speaker can't really handle is often a good idea:

IIR_L1: lp:2000Hz,hp:30Hz,hp:30Hz

This adds a 4th order high-pass filter to the woofer channel that removes very low frequencies. Depending on your speaker a value between 20 and 100Hz might be used

Filtering specific frequencies

The frequency response of almost any loudspeakers isn't really flat. So you might add some filters to increase or decrease the volume at a specific frequency:

IIR_R4: hp: 2000Hz, vol:-3dB, eq:5000Hz:2:+2dB

Note that you will need measurement equipment to correctly design these filters.

Delays for specific channels

It might be necessary to delay a channel a bit (often the tweeter). If the profile supports delays, you can configure them also with settings:

delay1: 100

Note that the delay is defined in samples. At 48kHz the lengths of a sample is roughly 0.02ms. So a delay 100 means approximately 2ms delay. If your profile is using a different sample rate, you need to do the calculations according to your sample rate (which is the internal DSP sample rate, NOT the sample rate of music you want to play back).

Write the settings to the EEPROM

If your crossover is working as expected, you should save it to the EEPROM of the DSP board. This ensures that the crossover is still correctly configured after a reboot.

dsptoolkit store-settings crossover.txt

Check if is stored correctly by resetting the DSP:

dsptoolkit reset

Merge the settings into an existing DSP profile

If you want to share your settings or apply the profile including these settings on another system, it can help to merge the settings into an existing XML DSP profile. This can also be done with dsptoolkit by just adding the file name of the XML profile file:

dsptoolkit store-settings crossover.txt profile.xml

Note that this will NOT apply the settings directly to the DSP, but only to the XML profile file.