Neil Davis

1
DIY Digital Crossovers

• Neil Davis
• July 18, 2009

2
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 TVs
• Can be designed as a plate amps for making active
  speakers
• DIY lots of flexibility and more fun

3
Focus Area Integrated DSP

• Audiodevelopers product range

4
Amps with DSP--Example
Lots of Class D amps have digital biquads!
5
Some Amps with Biquads
Designator Plate amp? Biquads per chan Coeff size spdif Delay analog in biquads DAC power amps Watts per chan power supply channels sub out sub amp
BCPA-1 yes 7 24 none none TAS3004 TAS3004 TAS3004 LM3886 40 on-board 2 analog
BCPA-2 yes 7 24 none none TAS3004 TAS3004 TAS3004 AD1994 20 trans or wall-wart 2 no
BCPA-3 yes 7 28 CS42526 AD1953 CS42526 AD1953 AD1953 AD1994 20 trans or wall-wart 2.1 analog TDA8920
BA-4 maybe 5 24 optional TPA5050 CS4525 CS4525 CS4525 CS4525 20 wall-wart 2.1 PWM H-Bridge PS
BA-5 yes 7 26 CS42526 TPA5050 CS42526 TAS5706 --- TAS5706 20 trans or wall-wart 2.1 PWM H-Bridge PS
PPA-2 yes 4 24 CS42526 none CS42526 STA328 --- STA328 x 2 30 ? 4 no STA328
SA-1 no 7 26 CS42526 TPA5050 CS42526 TAS5518 --- TAS5518 50 switchers 8
SA-2 maybe 10 24 CS42526 TPA5050 CS42526 STA308 --- STA5XX 40 switchers 4 or 8
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Whats a Biquad?

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

Implementation
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OK, So Whats 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 EQvariable 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 F3000
HFP Q.7 F3000
LFP Q.7 F3000
LFP Q.7 F3000
HFP Q.7 F300
HFP Q.7 F300
LFP Q.7 F300
LFP Q.7 F300
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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

11
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
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Amps
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Early version2005 or so
SPKR
LM3886
Sub Out
Analog In
L
TAS3004
R
Analog LPF HPF (Sub)
SPKR
LM3886
12V
EEPROM
PC Intfc
Power Supply
Low Voltage Power Supply
I2C
3.3V
Power
Budget Computer Plate Amp Version 1
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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
  PCBs
• Frustrating both TAS3004 and AD1994 are now
  obsolete (sigh)

SPKR
Analog In
Tweeter
TAS3004
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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STA328 (DDXi2161)

• Analog-free all-digital signal path
• 2-channel for active speakers USB version for
  computer speakers

SPKR
Filter
SPDIF In
DDXi2161
SPDIF
SPKR
Filter
Microcontroller MC9S08 (QG8)
Low Voltage
USB Intfc
I2C (out)
USB
Power
SPDIF 2-Channel
USB 2.1 (40/15/15W)
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Multichannel Amps (DDX8001)

• Working on a 42 boardshould be done soon
• 50/50/15/15/15/15 with DSP good for 3-ways
• 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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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

AudioDevelopers Product 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 BCD
BCPA-3 7 yes AD1953 AD1994 20 2.1 BCD
BA-4 5 yes CS4525 CS4525 20 2.1 BCD
BA-5 7 yes TAS5706 TAS5706 20 2.1 BCD
PPA-2 4 no STA328 STA328 x 2 30 4 BCD
SA-1 7 yes TAS5518 TAS5518 50 8 BCD-multichannel
SA-2 10 yes STA308 STA5XX 40 4 to 8 BCD-multichannel
EQ-1 12 yes TAS3013 --- --- 3 BCD-EQ
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Biquad charts
Driver Response charts
System Response
Plotdata2() SumTarget
SystemAmp() Sys_amp_Data SystemPhase()
Sys_phase_Data
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
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BCD (2-2.1 Amps)

• Now in 3rd major revision, (but none complete)
• Designed to support multiple amp chips
• Code for TAS3004 and STA328others are
  placeholders
• Hardwired for a maximum of 3 channels
• Current version uses XML structure for saving
  state

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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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DDX-8001 Example
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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 microprocessorI 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-Multic
  hannel.zip
• Need Microsoft .NET framework version 3.5, SP1
• Need to download the mschart controls
• http//www.microsoft.com/downloads/details.aspx?di
  splaylangenFamilyID130f7986-bf49-4fe5-9ca8-910a
  e6ea442c
• Need FTDI D2XX drivers
• http//www.ftdichip.com/FTDrivers.htm

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Crippled Marthas
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Rushed into Service

• Uses Spherex Xbox360 amp, not fully modded
• Still need to isolate digital audio linesCPU 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 isnt 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 LEDs 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 PCs 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