I keep being interested in allpass filters. An allpass changes a signal’s phase only, leaving the magnitude spectrum untouched. This article is about a 2nd-order allpass design I’ve been working on for phase alignment of crossovers in active loudspeakers.
Author Archives: Richard Taylor
Dither and ecasound
Ecasound is an incredibly versatile audio processing tool but it doesn’t have dither. I wish it did.
I just posted version 0.0.4 of my LADSPA plugins for doing crossover/equalization of active loudspeakers. Just a couple of changes to note:
- First-order low- and high-pass filters are now available.
- The build system has been moved to cmake; hopefully this makes it easier to build the plugins on other platforms. Also some code cleanup and restructuring. (All thanks to Florian Franzmann.)
There’s no need to upgrade if these changes don’t affect you.
At some point the LADSPA API will become deprecated and my plugins will have to be migrated to LV2. Any volunteers?
Lateral Reflections in Rectangular Rooms (Code)
Several people have asked for the code that I used in a previous post to calculate and plot first-reflection delay times as a function of listener- and loudspeaker-placement. So here it is.
Running the script below will produce pdf or postscript graphics output that looks like this:
An R Script for ABX Testing
I couldn’t find a computer program I liked for doing ABX tests and so ended up writing my own. The full program listing (in the R language) is given below in case someone finds it useful. Continue reading
Digital Allpass Filters for Crossover Phase Equalization
When two loudspeaker drivers are crossed over we typically want their on-axis acoustic outputs to sum in-phase. For example the 4th-order Linkwitz-Riley (LR4) crossover is designed for this. In practice, however, things rarely work out so well because there are potentially many sources of (often nonlinear) phase distortion, e.g. equalization filters, physical offset of the drivers, band-pass characteristic of the drivers, and acoustical interaction with the baffle.
Crossover phase alignment is often corrected by delaying the signal to one of the drivers. This typically aligns the drivers’ acoustic phases only up to zeroth-order, which is a bit rough. It’s possible to do much better: one can design an allpass phase-equalization filter that gives accurate alignment throughout the crossover region. My aim here is to briefly demonstrate the technique. Continue reading
Optimal Toe-In Angle for a Dipole Loudspeaker
I’ve been doing some work on determining the best toe-in angle for a dipole loudspeaker. The mathematical details are in this draft manuscript, but here is a brief summary.
Attenuating Lateral Room Reflections
By toeing-in a dipole one can selectively attenuate various room reflections by placing them at or near a dipole null (i.e. 90° off-axis). Shown here are two scenarios for the first lateral reflection paths in a room, with the polar response of an ideal dipole superimposed. Only the dipole orientation changes, the reflection paths are the same in both cases:
The first case is typical; the side-wall reflection is severely attenuated (about 30dB) since it is radiated near the dipole null. The front reflection is helped a bit, but it’s only down 3dB (plus about 4dB due to path length). In practice a mirror placed at the side wall reflection point will show an image of the loudspeaker on-edge. Continue reading
Pluto 2.1 with Software DSP
About a year ago I implemented a software-DSP version of the Linkwitz Labs Pluto 2.1. My aim was to replace the original analog signal processor (ASP) with a small, standalone PC that can double as a media server: like a Squeezebox that also performs xover/eq duties.
Pluto is an outstanding loudspeaker, and really deserves to be built/heard by more people. Part of my motivation is to provide a lower-cost, easier-to-build alternative to the ASP.
I own the original ASP version of Pluto 2.1 as well: in several hours of level-matched A/B listening tests I haven’t been able to hear any difference — except that my DSP version currently has a less powerful amplifier on the woofers, which limits the peak output somewhat.
If you want to go the software-DSP route yourself, follow the 2-way crossover instructions in my DSP how-to article. I’ll post the proprietary Pluto-specific configuration files in the Owners’ area of the Orion/Pluto Forum. Continue reading
Digital Crossover/EQ with Open-Source Software: HOWTO
The following setup can be used to implement a 2- or 3-way digital crossover (with equalization) for active loudspeakers. It runs on a linux PC and uses only free, open-source software. My design goal was a small, self-contained unit that can live on a shelf beside an amplifier: it runs without need of a display, keyboard, or mouse, and it looks like this:
The PC is the small box on the left. It’s silent, always on, and requires no user interaction.
One could use this system to implement the xover/eq for the Linkwitz Labs Pluto 2.1 or LX521. For those interested, I’ll post specific configuration files in the Owners’ area of the Orion/Pluto/LX521 Forum: together with this how-to, those files can be used to build a DSP version of the Analog Signal Processor (ASP) for those loudspeakers. I’ve written a long article detailing my development of the LX521 implementation; my Pluto implementation is similar. Continue reading
Using MusicIP with mpd
Lately I’m using mpd instead of a squeezebox as my network music player of choice. The only thing I miss about squeezebox is its integration with MusicIP. MusicIP is a great piece of abandon-ware. It analyzes your digital music collection to discover acoustic “similarity” between tracks. Given a seed song (or songs), it creates a playlist of “similar” songs — sort of like Pandora, but operating on your own music collection. (The notion of “similar” here is based on some undisclosed metric; MusicIP is most regrettably closed-source.) I’ve used MusicIP for years now and I’m still impressed by how well it works. It’s an integral part of how I navigate my music collection.
I wrote a simple script to act as a middle man between MusicIP and mpd. It gets the current song from mpd, asks MusicIP to generate a playlist with this song as the seed, and passes the result back to mpd. You can run it from any machine on your network. Linking this script to a desktop icon gives a nice way to generate a MusicIP mix with one click. Continue reading