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DIY Digital Crossovers. Neil Davis July 18, 2009. Approaches. Analog Works well, but hard to achieve flexibility Stopped doing this ~2004 DSP separates DCX-2496 (or equivalent) + amps Difficult to integrate with loudspeaker measurement tools Big, expensive & not DIY

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Diy digital crossovers l.jpg
DIY Digital Crossovers

Neil Davis

July 18, 2009


Approaches l.jpg
Approaches

  • Analog

    • Works well, but hard to achieve flexibility

    • Stopped doing this ~2004

  • DSP separates

    • DCX-2496 (or equivalent) + amps

    • Difficult to integrate with loudspeaker measurement tools

    • Big, expensive & not DIY

  • Integrated: amps with embedded DSP

    • Uses amps developed for home theater systems (multi-channel) or for LCD TV’s

    • Can be designed as a plate amps for making active speakers

    • DIY: lots of flexibility and more fun


Focus area integrated dsp l.jpg
Focus Area: Integrated DSP

  • Audiodevelopers product range:


Amps with dsp example l.jpg
Amps with DSP--Example

Lots of Class D amps have digital biquads!



What s a biquad l.jpg
What’s a Biquad?

  • …a second-order recursive linear filter, containing two poles and two zeros:

Implementation:


Ok so what s a biquad l.jpg
OK, So What’s a Biquad?

  • One biquad can be configured as:

    • 2-pole low-pass filter (or one pole)

    • 2-pole high-pass filter

    • 2-pole shelving filter, low or high

    • Peaking filter for EQ—variable F, Q and Gain

    • Notch filter

    • Bandpass filter

    • All-pass filter

    • Phase inverter


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Cascading Biquads

  • Cascading filters allows making higher order filters or crossovers

  • It takes 8 biquads to make a 4-pole 3-way L-R crossover:

HFP

Q=.7

F=3000

HFP

Q=.7

F=3000

LFP

Q=.7

F=3000

LFP

Q=.7

F=3000

HFP

Q=.7

F=300

HFP

Q=.7

F=300

LFP

Q=.7

F=300

LFP

Q=.7

F=300


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Unusual value

  • Apogee DDX-8001: $5

    • 8 processing channels , with drivers for class D output stages (30-100W ea)

    • Each channel has 10 biquads

    • Commonly used in many HT amps

    • Similar chip: TI TAS5518, (Panasonic SA-XR series)

  • TAS3103: $8

    • 3 processing channels, each with 12 high-resolution biquads, plus digital delay

  • TAS5706: $6

    • 2.1 amplifier, 20W/channel; 7 biquads/channel + 2 high-res biquads for sub channel


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How Do You Use These Things?

  • Need to calculate the biquad coefficients (b0, b1, etc)

    • Requires math--trig and complex variables

  • Need to program the biquads

    • Most are controlled by I2C bus, so need a microprocessor

  • Need to model the response to build loudspeakers

    • Combine driver response with filter transfer functions to view system response

  • Manage on-chip resources: volume, channel mapping, etc


Software hw overview l.jpg
Software/HW Overview

  • Unavoidable feature: microprocessor to control the amplifier

  • Microprocessor stores amp data, so PC only needed for designing the speaker

PC

Amplifier

Load Driver Data

Calculate Coefficients

Audio Interface (Analog, USB, SPDIF)

USB

Micro-processor

Amps

Crossover Designer



Early version 2005 or so l.jpg

Connectors

Modules

Early version—2005 or so

SPKR

Sub Out

LM3886

Analog In

L

TAS3004

R

Analog LPF & HPF (Sub)

SPKR

LM3886

±12V

EEPROM

PC Intfc

Low Voltage Power Supply

Power Supply

I2C

3.3V

Power

Budget Computer Plate Amp Version 1


Bcpa 2 l.jpg

Connectors

Modules

BCPA-2

  • Digital amps run cooler and use smaller transformers

    • Easier to install into a speaker cabinet, and lower cost

    • Small: can still use small low-cost prototyping PCB’s

    • Frustrating: both TAS3004 and AD1994 are now obsolete (*sigh*)

SPKR

Analog In

TAS3004

Tweeter

AD1994

Mid or Woofer

SPKR

I2C

EEPROM

Microcontroller MC9S08 (QG8)

18V Power Supply

Low Voltage

USB Intfc

I2C (out)

USB

Power

Budget Computer Plate Amp Version 2


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Connectors

Connectors

Modules

Modules

STA328 (DDXi2161)

  • “Analog-free”: all-digital signal path

  • 2-channel for active speakers; USB version for computer speakers

SPKR

SPKR

Filter

SPDIF In

DDXi2161

Filter

SPDIF

USB In

DDXi2161

SPKR

Filter

PCM2707

SPKR

Filter

Microcontroller MC9S08 (QG8)

Filter

Sub

Low Voltage

Microcontroller MC9S08 (QG8)

Low Voltage

USB Intfc

I2C (out)

USB

USB Intfc

Power

I2C (out)

USB

Power

SPDIF 2-Channel

USB 2.1 (40/15/15W)


Multichannel amps ddx8001 l.jpg
Multichannel Amps (DDX8001)

  • Working on a 4+2 board—should be done soon

    • 50/50/15/15/15/15 with DSP: good for 3-way’s

    • Has digital delay (each channel) for making steerable line arrays

  • Picture shows a different version (eval board + control board)

    • Works in stand-alone mode or with PC-based software



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Two Versions, maybe 3

  • Biquad Crossover Designer (BCD)

    • Smaller amps: 2-channel or 2.1

  • BCD Multichannel

    • 3-ways to 5-way systems with flexible channel routing


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Driver Response charts

Biquad charts

System Response

SystemAmp() = Sys_amp_Data

SystemPhase() = Sys_phase_Data

Plotdata2() = SumTarget

Plotdata() = Biquad_Data

Driver_summed, System_Response

If biquad windows not opened

Volume Updates

System_complex()

Update_response

Response File

Filter_calc, Filter_prod

If biquad windows opened

Systemresponse()

Biquadresponse()

Resample

Offset and Phase

Driver_with_offset()

Biquad Updates

Driver_complex()

Driver_data()

Tweeter

Load_drivers

Mid

Graphs

FRD_data()

Inputs

Processing

Woofer

Bold indicates Complex


Bcd 2 2 1 amps l.jpg
BCD (2-2.1 Amps)

  • Now in 3rd major revision, (…but none complete)

    • Designed to support multiple amp chips

      • Code for TAS3004 and STA328—others are “placeholders”

    • “Hardwired” for a maximum of 3 channels

    • Current version uses XML structure for saving state


Driver input l.jpg
Driver Input

  • Reads FRD or CLIO text files for driver response

  • Allows adjustment of acoustic center offset

  • Still working on Min phase algorithm…

  • System Response graph updated when biquads are programmed


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Biquads (one per channel)

  • Easy to tweak response and hear effect in real time

    • Data gets calculated and sent to the amp for any change

    • Has equations for many “textbook” filter types


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Other Features

  • Includes screens to control registers in the chips

  • Allows saving settings in EEPROM

    • Controls what the amp loads at power-on

    • Allows rapidly switching loads without re-calculating


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BCD Multichannel

  • Uses logical channels rather than physical channels



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Prototypes (currently free)

  • Send email via PE Techtalk for ExpressPCB files

    • Be ready for the “SMD challenge” (actually fairly easy with decent soldering tools)

    • Need to program microprocessor—I will do that for cost of postage

  • Download latest posted SW version:

    • http://www.audiodevelopers.com/Software/BCD.zip

    • http://www.audiodevelopers.com/Software/BCD-Multichannel.zip

    • Need Microsoft .NET framework version 3.5, SP1

    • Need to download the mschart controls:

      • http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=130f7986-bf49-4fe5-9ca8-910ae6ea442c

    • Need FTDI D2XX drivers:

      • http://www.ftdichip.com/FTDrivers.htm



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Rushed into Service…

  • Uses Spherex Xbox360 amp, not fully modded

    • Still need to isolate digital audio lines…CPU on the board is partially in control

    • Amp is 6-channel DDX-8001 circuit, but is configured “funny”: bridged sub & single-ended surrounds

  • Software to control the amp isn’t complete

    • “Hardwired” (assembly code) as 5-way with 4-pole LR crossovers at 100, 500, 3000 and 7000Hz

    • All drivers arbitrarily set at same volume level

    • No programmable delay control yet (circuitry done)

    • No omni/dipole switching control yet (relays inside)

    • Probably 2-3 weeks away from being done


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Unusual Features

  • “Ring arrays” with no comb filtering due to crossover frequencies

  • Super-tweeter uses 8cm headphone elements

    • Also designed to accommodate a circular Heil

  • Approx 600 Lumens of light output

    • Total of 28 1W LED’s in each cabinet, but still needs proper resistors for higher output

    • 3-channel color organ feature not enabled yet (but it works…)

  • Fully programmable channel mapping, crossover frequencies, delays, slopes and volume levels

    • Could be an all-day event in itself


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Proposed Follow-Up

  • “Biquad class” at our cabin in Western MD

    • Plenty of PC’s to design speakers (at least 4)

    • I can bring lots of speakers and amps to play with, or bring your own speakers

  • MarthaFest

    • Compare topologies (2-way to 5-way)

    • Compare crossover frequencies and slopes

    • Compare omni to dipole

    • Listen to effect of time delay compensation

    • Drink beer, experiment, drink beer, etc