|
DSP drives Velodyne subwoofers
TI 'C2000 powers new generation
|
|
By
Henry
Davis
|
|
Audio DesignLine
(10/17/2005 5:24 AM EDT)
|
|
You’ve probably heard subwoofers driving down the road. A loud thump, thump, thump is one sure sign of an in-car subwoofer. But there’s many more reasons to use a subwoofer in any audio system than just blasting folks into next week.
Most audio systems include woofers. Often, the woofer has a small cone that handles both low and midrange frequencies. But, woofers generally can’t deliver full-range, high definition bass. A quality subwoofer can reproduce low frequency sounds with full impact and clarity. Like many other products, there is a difference in sound quality between subwoofers. Good low-frequency extension with enough output for the room combined with very good definition (low distortion), can make or break an audio system. Choosing a subwoofer is critical to creating a truly enjoyable audio experience, whether it’s music or full-on home theater movie sound.
What Makes A Subwoofer Work
Traditional subwoofers are similar to full-frequency range speakers in basic design. The fundamental difference is size and purpose. A subwoofer is designed to reproduce the lowest frequencies present in any music or movie. Most subwoofers have a dedicated amplifier to permit room-filling sound without drawing on the main amplifier, and that’s a necessity when the subwoofer puts out over 1000 watts.
A well-engineered subwoofer must match a series of electrical and mechanical systems to achieve the desired performance:
the bass speaker (the subwoofer driver)
the amplifier
the cabinet, and
an electronic crossover network
The ultimate goal is to produce bass that is not only heard but felt as well. Feeling is especially important because it is a critical factor in invoking an emotional reaction to the audio material.
Velodyne has taken subwoofer design into a whole new realm with Digital Drive subwoofers. Where many subwoofers suffer from 20-30% distortion, the Digital Drive subwoofers reproduce low frequencies with less than 0.5% distortion. Automatic room equalization has long been believed to be the “final frontier” in perfecting audio reproduction in the home environment. Digital Drive provides room equalization with no extra equipment by integrating the process into the subwoofer. By manipulating the audio spectrum digitally, there is virtually no limit to the way the signal can be customized for use in the listener’s environment with near perfect results.
Figure 1. The TMS320C2000 forms the backbone of the Velodyne subwoofer design. Equalization and feedback control are simultaneoulsy calculated by the DSP at up to 16k samples per second. The result is very low distortion.
To understand the design of the subwoofers, it’s important to understand the room equalization process. Every room, and how speakers and subwoofers are placed in it, introduces its own set of peaks and valleys in the frequency response. The primary culprit is audio signals reflected off walls and ceilings creating standing waves. Absorption of certain frequencies by furniture, carpets, drapes, and the like adds to the complexity of the problem. Unfortunately no two rooms are the same, making “canned” equalization solutions impossible. Both speaker placement and listening position can have a dramatic impact on how the system sounds. This is especially true in the bass audio spectrum, since most energy is dedicated to reproducing frequencies under 200 Hz. Traditionally room equalization requires some specialized equipment such as a spectrum analyzer. However, the cost of a spectrum analyzer is out of reach for most consumers. Alternatively, an SPL meter, combined with a series of test tones and charting the results is another approach. This method is manual, tedious, error-prone and time consuming. While some higher-end subwoofers include parametric equalizers, there is still the challenge to determine the room response, how to compensate for it, and to be able to experiment with different factors to get acceptable room response.
The Velodyne approach to room equalization strives to simplify the process so that consumers can perform the process themselves without relying on professionals. For professionals, the approach speeds the installation process. Digital Drive technology measures the subwoofer’s frequency response in any room using a specially calibrated microphone, digitally corrected, and stored in the unit’s memory to assure near ideal performance in the consumer’s listening area without any additional equipment. This is accomplished with an on-screen graph of the system’s actual frequency response and feedback displayed on the TV screen as the user makes digital adjustments to subwoofer equalization, phase, and crossovers. The consumer or custom installer can also modify additional over-laid curves to obtain optimum performance for various source materials such as action/adventure movies, rock, pop, jazz, or classical music. Custom settings are also available to match the consumer’s taste.
With this technology:
Crossovers, filters, phase, and contours are all controlled digitally through the use of a Digital Signal Processor.
Energy Recovery amplifiers used are digital switching class D, delivering high power in a compact space with little heat generation.
High Gain Servo technology is all-digital, including a digital accelerometer that delivers a digital representation of cone movement that is checked up to 15,800 times per second compared to the previous correction rate of 3,500 times per second.
Parametric Equalizers (EQs)
Unlike other subwoofers that contain either one or possibly three parametric EQs, Digital Drive
supplies eight EQs. Each EQ is fully adjustable for Frequency (F), Level (+ or – dB), and Q.
“Q” in this case means quality of the filter – in other words, how narrow a frequency range the
filter applies to. The higher the Q, the more narrow a frequency range is filtered.
Low Pass Crossover - Low pass crossover passes only the low frequencies of the incoming signal to subwoofer. It is adjustable between 20 and 199 Hz and can have a slope of 6, 12, 18, 24, 30, 36, 42, or 48 dB/Octave.
Subsonic Filter - Subsonic filtering was once called a rumble filter, but still performs the same function limiting the lowest frequencies that the subwoofer plays. It is adjustable between 20 and 199 Hz and can have a slope of 6, 12, 18, 24, 30, 36, 42, or 48 dB/Octave.
Phase - Phase of the subwoofer is adjustable in 15 degree increments between 0 and 180 degrees.
Polarity - Reverses the phase 180 degrees and is set to either + or -.
Listening Mode Presets - For each preset, there is a “contour” (extra EQ) with a Frequency, Level and Q setting. This lets the subwoofer react differently for different types of movies and musical source content. Additionally, each preset provides full customization of all the other settings such as volume, slope, crossovers, and EQs. There are factory default contours for Action-Adventure, Movies, Rock-Pop, and Jazz-Classical. Preset 5 is for user customization, and Preset 6 is set as a defeat of all equalization to demonstrate the effects of Digital Drive room equalization.
Volume - While there is an overall system volume, each preset can also have a volume differential applied automatically when selected.
Theater/Music Adjustment - There has been a long-standing debate over whether a subwoofer is designed to be “musical”(i.e. it is tight and fast for music), or “theatrical” (i.e. it impressively reproduces explosions and other low frequency information found in movies). The theater/music setting lets the user choose the kind of subwoofer – theatrical or musical. The effect of the setting is to control the amount of loop gain in the servo system, thus controlling the amount of distortion in the reproduced audio signal.
Night Mode - This limits the subwoofer’s output.
RS-232 Input - Digital Drive Subwoofers can accept RS-232 input.
Daisy Chaining Subwoofers Together - More than one subwoofer can be “daisy-chained” together. The EQ settings for the first subwoofer in the chain automatically are passed down to the other subs in the chain. This means the room must be adjusted only once for as many subwoofers as are desired in the chain.
Programming Updates - The RS-232 in port is also used to provide programming updates to Digital Drive units. This is useful to resolve technical problems or provide new features to the Digital Drive Subwoofer. A Windows application drives the update via a serial port of an IBM PC compatible computer.
Inadvertent Settings Modification Protection - After 15 minutes of inactivity or if the EXIT key is pressed on the remote, the subwoofer reverts to a locked out state to prevent inadvertent changing of subwoofer settings.
Connecting and Using Digital Drive
Digital Drive subwoofers accept RCA level (LFE or full spectrum), speaker level, and balanced
input. To employ the Digital Drive feature and optimize the subwoofer(s) for the listening
environment, the microphone is placed at the user’s preferred listening position. The onscreen display is activated by tuning the user’s home theater to the channel where the video signal is coming in from the subwoofer. The main setup screen displays the frequency response curve and equalizer settings. Entering this screen activates a test sweep signal that sweeps out tones from 15 – 200 Hz in lockstep with the update on the frequency response graph. The test tones are generated only while on this screen. By pressing NEXT on the frequency response graph screen, a settings screen is displayed that allows for adjustment of crossovers, subsonic filter, phase, polarity, volume, theater/music, auto-on options and night mode maximum volume.
Room Equalization
Room equalization can be a tricky thing. In general, the goal is to get the signal response curve +/- 3 dB across the spectrum, but this may not always be possible. There are several tools
available to equalize the subwoofer to the room: subwoofer placement; crossover settings; phase
settings; and the eight bands of EQ provided. To find the optimum equalization, users first start by experimenting with different subwoofer placement areas in the room if possible. There will be dramatic differences in response just by moving the subwoofer a few inches. This will be especially true if the subwoofer is near a wall or corners due to increased reflection of sound waves. The subwoofer is easier to calibrate if there are peaks but no significant valleys in the response curve. Peaks are generally much easier to compensate for than valleys, and there may be subwoofer and/or listening positions where non-correctable valleys are seen due to standing wave cancellation. One feature of the Digital Drive system is that if multiple room placement options are not available, the absolute best response can be realized from whatever position is available for the woofer. Once the best subwoofer position has been found, the EQs can then be used to further flatten the response. There are eight EQs, and not all need to be set for every environment. Again, a flat frequency response line is not required, but one that shows a reasonable curve with no more than +/- 3dB is optimal. Users should be aware that there may be peaks (generally above 80 Hz or so) that are little affected by adjusting a Digital Drive EQ. Because satellite speaker response overlaps with the subwoofer, the satellites are most likely the culprit and consequently no amount of subwoofer adjustment may be enough to compensate. In this case, it might be best to move the satellites if possible, move the subwoofer again, or use any equalization that might be included with the user’s home theater or music setup.
Technical Background
Velodyne’s current generation of Digital Drive is based on the Texas Instruments TMS320LF2407 Digital Signal Processing (DSP) chip. Analog-to-Digital converters are used to convert the audio signal to a perfect representation digitally within quantization error, the signal is filtered and manipulated based on customer specifications and room requirements, and the signal is then reconverted back to an analog signal for amplification. The ‘LF2407 includes a 10-bit A/D converter, but Velodyne uses an outboard 20-bit converter for higher signal resolution. The TMS320LF2407 is a fixed-point DSP, and provides 30 MIPS. An extra 32K memory chip is also used. An integrated Serial Communications Interface (SCI) is on the DSP and provide asynchronous communication between the subwoofer and a PC. The DSP also includes a JTAG compliant interface. Notably, there is adequate processing power left after the signal processing tasks to control an NTSC display directly using the on-chip interface.
The accelerometer used in Velodyne’s High Gain Servo system has also been designed to include
an A/D chip on the accelerometer hybrid. This means that a digital signal is generated from the
accelerometer itself, thus reducing noise in the servo-loop system. The accelerometer bit stream
is checked by the software approximately 15,800 times per second (at maximum music setting for
theater/music) and corrections applied accordingly, further reducing distortion.
The complete loudspeaker system signal path comprises a traditional closed loop feedback control system. An input terminal receives the input representing the sound signal to be reproduced. This input is applied to a summing point. The summing point signal feeds to a frequency-compensation network. The output signal of this network then drives a power amplifier and subsequently the loudspeaker. The loudspeaker coil has an attached accelerometer which produces a digital output which is then fed back to the summing point where it is summed with the input drive voltage. The net result is a system in which the speaker cone velocity is directly measured and feedback into the control loop, reducing the distortion.
Figure 2. Closed loop control of the speaker is conceptually simple, but proprietary techniques add important functionality.
Low-frequency oscillation can occasionally develop in subwoofer designs. For example, such an oscillation can occur near the one Hz unity-gain crossover frequency. To avoid this low frequency instability problem, Velodyne employs a sealed accelerometer. This prevents any interaction between the air-pressure variations inside the speaker enclosure (such as due to cone movement) and the feedback signal produced by the accelerometer.
High frequency instability can also develop in a loudspeaker system. The interactions of acoustic waves in the speaker system are extremely complex, and depend upon a number of factors which are difficult to control in manufacture. It appears that high-frequency instability arises from the presence of standing waves corresponding to one or more of the loudspeaker system's principal resonant modes - Mode I and Mode II waves.
These waves are axially symmetric and normally "theta-independent" - independent of radial angle about the center of the cone. They propagate outwardly along the cone radially. A Mode I wave has one-half wavelength spanning the distance from the coil to the surround. A Mode II wave has a wavelength extending from the coil to the dust cap. Mode II waves are reflected back to the coil at the point of attachment between the dust cap and the cone.
Mode I waves resonate at approximately 400 Hz in a 15 inch speaker. Mode II waves resonate at about 800 Hz when the dust cap is about 5 inches away from the coil. Such waves cannot simply be eliminated electronically. Instead, the system must be "detuned" by introducing "theta-dependence" into the speaker cone construction, and/or by eliminating sudden discontinuities in the mechanical impedance of the cone, such as those occurring at the joint between the dust cap and the cone.
The basis for the Velodyne Digital Drive subwoofer technology was first delivered over twenty years ago and has undergone continuous improvement. Today analog circuitry has been replaced by high performance digital techniques employing TI DSPs. The result is an easier to manufacture and test product. Consumers benefit from new sound system optimization techniques that are easy to use.
|
|
|
|
CAREER CENTER
|
Ready to take that job and shove it?
|
|
SPONSOR
|
|
|
|
RECENT JOB POSTINGS
|
|
|
For more great jobs, career related news, features and services, please visit EETimes' Career Center. |
|
