Neil DavisJuly 18, 2009

DIY Digital Crossovers

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

Audiodevelopers product range:

Amps with DSP--Example

Lots of Class D amps have digital biquads!

Some Amps with Biquads

Designator

Plate amp?

# Biquads per chan

Coeff size spdif Delay

analog in biquads DAC

power amps

Watts per

chanpower supply

channels

sub out

sub amp

BCPA-1 yes 7 24 none none TAS3004 TAS3004TAS300

4LM3886 40 on-board 2

analog

BCPA-2 yes 7 24 none none TAS3004 TAS3004TAS300

4AD1994 20

trans or wall-wart

2 no

BCPA-3 yes 7 28CS4252

6AD1953 CS42526 AD1953 AD1953 AD1994 20

trans or wall-wart

2.1analo

gTDA892

0

BA-4 maybe 5 24 optionalTPA505

0CS4525 CS4525 CS4525 CS4525 20 wall-wart 2.1 PWM

H-Bridge + PS

BA-5 yes 7 26CS4252

6TPA505

0CS42526 TAS5706 --- TAS5706 20

trans or wall-wart

2.1 PWMH-

Bridge + PS

PPA-2 yes 4 24CS4252

6none CS42526 STA328 ---

STA328 x 2

30 ? 4 no STA328

SA-1 no 7 26CS4252

6TPA505

0CS42526 TAS5518 --- TAS5518 50 switchers 8

SA-2 maybe 10 24CS4252

6TPA505

0CS42526 STA308 --- STA5XX 40 switchers 4 or 8

What’s a Biquad?

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

Implementation:

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

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:

HFPQ=.7

F=3000

HFPQ=.7

F=3000

LFPQ=.7

F=3000

LFPQ=.7

F=3000

HFPQ=.7

F=300

HFPQ=.7

F=300

LFPQ=.7

F=300

LFPQ=.7

F=300

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

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

Unavoidable feature: microprocessor to control the amplifier

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

PC

Load Driver Data

USB

Crossover Designer

Calculate Coefficients

Amplifier

Micro-processor Amps

Audio Interface (Analog, USB,

SPDIF)

Amps

Early version—2005 or so

LM3886

LM3886

Power Supply

SP

KR

PowerConnectorsModules

TAS3004

EEPROM

Analog In

Low Voltage Power Supply

Sub Out

PC

Intfc

I2C

L

R

3.3V

±12V

SP

KR

Analog LPF & HPF (Sub)

Budget Computer Plate Amp Version 1

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*)

18V Power Supply

SP

KR

Power

ConnectorsModules

Low Voltage

Analog In S

PK

R

AD1994

Tweeter

Mid or Woofer

EEPROM

Microcontroller MC9S08 (QG8)

USB Intfc

USB I2C (out)

TAS3004

I2C

Budget Computer Plate Amp Version 2

STA328 (DDXi2161)

“Analog-free”: all-digital signal path 2-channel for active speakers; USB version for computer

speakers

SPDIF 2-Channel

USB 2.1 (40/15/15W)

SPKR

Power

ConnectorsModules

Low Voltage

SPDIF In SPK

R

DDXi2161

Microcontroller MC9S08 (QG8)

USB Intfc

USB I2C (out)

SPDIF

Filter

Filter

SPKR

Power

ConnectorsModules

Low Voltage

USB

In

SPKR

DDXi2161

Microcontroller MC9S08 (QG8)

USB Intfc

USB I2C (out)

PCM2707

Filter

Filter

Filter Sub

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

Software

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

AudioDevelopersProduct

# Biquads per channel

Digital Delay Audio Processor Power amps

Watts per channel Channels

Design Software

BCPA-1 7 no TAS3004 LM3886 40 2

BCD

BCPA-2 7 no TAS3004 AD1994 20 2

BCPA-3 7 yes AD1953 AD1994 20 2.1

BA-4 5 yes CS4525 CS4525 20 2.1

BA-5 7 yes TAS5706 TAS5706 20 2.1

PPA-2 4 no STA328 STA328 x 2 30 4

SA-1 7 yes TAS5518 TAS5518 50 8 BCD-multichannelSA-2 10 yes STA308 STA5XX 40 4 to 8

EQ-1 12 yes TAS3013 --- --- 3 BCD-EQ

Tweeter

Mid

Woofer

System Response

Response File

Offset and Phase Biquad

Updates

Biquad charts

Driver Response

charts

Plotdata() = Biquad_Data

Plotdata2() = SumTarget

Biquadresponse()

Systemresponse()

SystemAmp() = Sys_amp_DataSystemPhase() = Sys_phase_Data

Driver_complex()

Driver_with_offset()

Bold indicates Complex

System_complex()

If biquad windows not opened

FRD_data()

Driver_data()

If biquad windows opened

Volume Updates

Graphs

Inputs

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

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

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

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

BCD Multichannel

Uses logical channels rather than physical channels

DDX-8001 Example

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

Crippled Marthas

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

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

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