TECHNICALLY SPEAKING... By Steve Somers, Vice President of Engineering Digital Signal Processing Reshaping Your Perceptual Environment One Digital Sample at a Time t was the 60’s when I fondly recall watching the sci-fi TV show The Outer Limits. It competed for Iattention with Rod Serling’s The Twilight Zone. In TTZ, strange things just happened to people. TOL set the stage with a “control voice” stating in part: "There is nothing wrong with your television set. Do not attempt to adjust the picture. We are “controlling transmission. If we wish to make it louder, we will bring up the volume. If we wish to make it softer, we will tune it to a whisper. We will control the horizontal. We will control the vertical… You are about to experience the awe and mystery which reaches from the inner mind to... The Outer Limits.” Both series broadcast in black and white, What is DSP, Really? of a time-varying, real world analog which in retrospect really magnified their Underneath it all, DSP is all about creating event. Given sufficient discrete samples, warp from reality. If TOL had been in focus. Focusing on something is akin those samples may be ingested by a color, perhaps the control voice could to honoring its message or its intrinsic processor, manipulated mathematically, have corrected the color and tint for me beauty. For example, DSP can remove and reconstructed as a new version of too. I’m certain that all of you survivors unwanted noise so we can focus on the that analog signal with such accuracy so of NTSC’s early days can relate. As the unique quality of a sound. DSP can filter as to be indistinguishable, or enhanced, control voice for this article, I hope to out a range of frequencies, much like from the original. DSP unintentionally maintain the awe and dispel some of the the adaptiveness of the human ear, to directs new focus on how users interface mystery about digital signal processing. allow focus on a specific message. DSP with and use products more than on its can remove the mundane and the clutter technical functionality. The typical DSP Controlling, filtering, re-blending, much like a photographer crops an image product feature list is often complicated enhancing, compressing, and replaying to focus attention on one visual concept. with so many choices that the user the everyday analog events of our lives interface must not only be graphical, but is the new reality that we lump into the We’ve been filtering signals for years in it must guide the user’s setup choices phrase “digital signal processing”, or DSP the analog domain. Why is digital signal and decision-making method to a greater in daily tech-speak. DSP is implemented filtering so important? The difference is degree. in various forms and means different analogous to the accuracy of cutting things to different people. While the most something with an axe compared to DSP State of the Art common inference ties DSP to audio cutting with a focused laser beam. Analog One question that invariably comes up processing, any manipulation of digital bits filtering is coarser with commensurate is: “So, what is the state of the art in representing picture or sound information difficulties controlling the filter response. DSP?” It’s a valid question, but tends is a form of digital signal processing. DSP Digital signal processing manipulates to focus on the hardware and not on is now the default acronym associated numbers that lie within specific ranges what the hardware is doing. DSP is with manipulation of audio signals; an which support very precisely designed about processing numbers rapidly attribution tied to the reality that digitally digital filters having stable, predictable to perform calculations required by processing audio information occurred response. DSP can “surgically remove algorithms designed to address a specific earlier than video because the audio or modify” information with relative ease problem. Just as in traditional computer frequency range is much lower than the compared to analog methods. programming, the DSP engineer writes basic video frequency range. Therefore, an algorithm, or process, for solving a audio signals initially required less digital The strength and the foundation of DSP is problem. The problem solution is usually processing, which could be implemented the manipulation of the numbers, or digital modeled into one or more equations with lower cost processors. data, which represent discrete samples that can be systematically solved by continued on page 26 ­www.extron.com­ 25 TECHNICALLY SPEAKING... TECHNICALLY SPEAKING... Digital Signal Processing — continued a microcomputer through iteration. result is the prime process in a DSP. A multiple MAC units, local instruction cache The faster that the microcomputer can DSP can typically perform one MAC memory, data address generation units, perform the required calculations, the in one clock cycle while a standard and circular addressing. closer the solution approaches what we microcomputer may require four or more consider to be real time. For audio signals, clock cycles. Processing Power… Approaching ‘The it must be fast enough that we do not Outer Limits’ perceive the result as abnormal. Figure 1 shows the basic architectural One of the most common design difference between a DSP and a considerations with a DSP is the data How does a digital signal processor microprocessor. The shared bus in the format used for calculations… that compare to a regular microcomputer? typical microprocessor necessitates is, fixed-point versus floating-point Are they really the same thing? DSPs incremental fetching of program steps, processing. A fixed-point processor has are specialized, highly streamlined then data. Further, the data must be its binary point, which corresponds to implementations of microcomputers with moved into registers and the appropriate the decimal point in our base 10 number unique features. All microcomputers can operations executed. The result is system, fixed at one position within be programmed to perform digital signal obtained at the cost of several steps and the data word. Floating-point allows processing functions in the general sense. considerable additional time. In the DSP movement of the binary point by utilizing The microcomputer limitations become architecture, program and data are usually a mantissa and an exponent; whereby evident with the type of algorithm typically separate with dedicated bus pathways. the binary point can “float” based on the needed for the DSP solution, such as Both program steps and data can be exponent value. with audio filtering. The difference boils fetched and moved simultaneously, thus down simply to efficiency and speed effecting much faster processing. Fixed-point format limits the processor’s of execution. The internal architecture number range for a given data word. The of a regular microcomputer, while very There are many facets that contribute to fixed point is maintained by using a single capable at performing looping calculations the fast architecture of a DSP. Among scaling factor. For example, the value of and multiplications characteristic of them are: high memory bandwidth, π, 3.14, can be represented in fixed- filter algorithms, does not make efficient use of its memory or management of program steps for DSP-type calculations. Microprocessor Architecture A microcomputer would have to run at several times the speed of a typical DSP Program General for even the simplest filtering operations. and Data Bus Purpose When addressing multiple algorithms, Memory Microprocessor the microcomputer solution becomes too slow or non-functional very quickly. Microcomputers are optimized for control applications where sequential timing of events is necessary without significant DSP Architecture mathematical operations. DSP Algorithms Dictate DSP Engine Program Bus Design Memory So, the design of a DSP is dictated by DSP the types of algorithms, or calculation Core sequences, it most often executes. DSP architecture is optimized to handle Data multiple calculation sequences in parallel Memory Bus while minimizing clock cycles. At the heart of a DSP is one or more MACs, or “multiply and accumulate” units. Multiplication and accumulation of the Figure 1: Basic architectural difference between basic microprocessor and a DSP ­26­­­­­­­­­­­­ExtroNews­21.2­­­|­­­Summer­2010 TECHNICALLY SPEAKING... point notation by the integer value 314 Bit Depth and Latency incurred while performing operations. divided by the scale factor 100. Numbers The topic always comes up, so let’s talk For one-way audio processing and larger than one data word require about it. Of course, under the hood, it’s broadcasting, latency may not be concatenation of an additional data important that a DSP system uses lots of important. In telephony, latency is very word, or words, for the range required. bits per word. More bits is better, right? important because it affects the quality Concatenation requires additional No system designer is going to look of the conversation between two people. calculation time, which affects processing ‘cool’ if he designs an 8-bit DSP solution. In conversation, the listener issues speed accordingly. This situation is one Moreover, are there any out there? In a unconscious utterances subtly notifying factor relating directly to processing fixed-point DSP, if there are more bits the talker that both are engaged in the latency in DSP. The primary concern with needed to maintain numeric fidelity, and conversation. Video conferencing and fixed-point designs is the maintenance of I love that term, we can concatenate live broadcasting likely demand the most “numeric fidelity”, or managing number more data words to make up any level control over latency since action and overflow in the processor. Fixed-point of processing precision needed. So discussion must remain in sync to realize DSPs typically garner lower cost what’s to give up? Speed, of course. a ‘normal’ experience. Live audio systems than their floating-point counterparts.