How to Write (And Read) Audio Specifications an Audio Precision White Paper by David Mathew Ap.Com
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How to write (and read) audio specifications an Audio Precision White Paper by David Mathew ap.com Introduction About this document At Audio Precision, the analyzers we make are This document discusses how to write audio recognized around the world as the standard in audio specifications, and how to read them. test and measurement. Specifications, then, are very • First, a look at our recommendations for writing important to us. Engineers use our equipment to specifications. compose their own specifications for the audio devices What should good specifications look like? they design. They use our equipment to verify that their • manufacturing processes are correct, and that their • What are typical values? products meet the goals they have set. They use our • Appendix: audio standards. equipment to characterize their products’ competitive • Appendix: THD and THD+N. advantages, and they publish the specifications they • Glossary: explanations of some of the derive as advertising bullet points. So we are very careful measurements, standards and units found in in how we spec our own analyzers, and we have audio specifications. developed some strong opinions about how speccing ought to be done. Specifications are necessary for every manufacturer, for Recommendations for writing every product. Internal engineering specs define the specifications product and provide benchmarks for manufacturing We consider these guidelines as we write our own quality assurance. Published specifications, usually less specifications, and think about them when we read complete and rigorous than the engineering specs, specifications from other manufacturers. Each of these inform the user of the device’s capabilities and recommendations is discussed in more detail following. limitations with the goal of safe and proper operation. 1. Decide which characteristics are important. Of course, a published spec is also a powerful sales tool 2. Use the “best practice” method of for an audio device. What we read in an advertisement measurement. If a standard applies, use the or even a data sheet might be, well, shaded a bit, in an standard’s recommendation. attempt to place a device in an attractive position 3. Spec conservatively. without actually lying. This tension between truth and 4. State ranges and conditions; annotate when marketing is especially evident in audio products more detail is required. targeted toward home entertainment use, as ordinary 5. Express in standard units. Display secondary consumers are typically less informed than audio units if applicable. engineers, and less adept at interpreting arcane lists of 6. Express graphically (x-y, polar, waterfall, etc.) numbers. when appropriate. In the professional and commercial audio fields, the 7. Use uniform units, formatting, and numerical tendency to use dubious power output ratings is expressions throughout the product line. certainly much lower than in the consumer goods sector; but a cursory sampling of pro audio, industrial audio and communications audio product specifications reveals incompleteness, inconsistency and just plain sloppiness, both in which specs are chosen and how they are expressed. 1 Decide which characteristics are devices you’ll also see separation, crosstalk, and important. interchannel phase. Balanced devices may spec common mode rejection, and devices with mechanical drives will Many comments can be made about any audio device. show “wow and flutter” figures. But which characteristics are really telling in describing a device’s key features, its performance, its compliance Please refer to the glossary for explanations of with industry standards or safety regulations? measurements, standards and units of measurement. Key features: interesting, but not performance Measure according to standard or best benchmarks The number of inputs and outputs, their nominal levels practice. If a standard or regulation exists for a measurement, use and connector types, the signal processing and codecs its recommendation and indicate which standard you available, the controls and displays are often included in used. If not, most measurements have a “best practice” a list of specifications. So are application-specific details method. For unusual testing methods or characteristics like constant voltage outputs or IR remote compatibility, new to the industry, consult the current literature and market-specific features such as the type of mains clearly state your method, testing conditions and so on. connector and mains voltage range. Physical characteristics such as size and weight, environmental The role of standards limits and safety regulation compliance often make the Some measurements are complicated and difficult to list as well. These are important in choosing a feature set, perform correctly. Other measurement techniques, or but they don’t indicate actual performance levels of the their reporting, may be disputed in the industry or can be device. manipulated in the service of marketing. Cleary, standardization of measurement methods and reporting Performance specifications benefit both the manufacturer and the customer by The details of an audio device’s tested performance are provide a standard reference. the heart of a set of specifications. These are the numbers that are weighed for bragging rights, for market Standards are revised and even withdrawn and replaced position, and ultimately, for a sale. from time to time. Spec using the latest standard that is appropriate. In general, the results of four categories of measurements form the core of audio performance Refer to the Audio Standards appendix for more specifications for any audio device: information. 1. Level, as volts (V), watts (W), or sound pressure level (SPL). Spec conservatively. Establish that the measurement used for a spec is not a 2. Frequency Response, as a statement or a graph; or at least, Frequency Range. “best result” of a “golden unit” but is typical of the device, achievable by any properly adjusted unit. 3. Distortion, as total harmonic distortion (THD),total harmonic distortion plus noise It’s always possible to tweak a device (by carefully (THD+N), and intermodulation distortion (IMD) selecting components, for example) so that it will surpass of various flavors. the typical performance of the devices. Do not use such 4. Noise, as an absolute voltage, as signal-to-noise results for specifications. ratio (SNR), or as dynamic range. It is also possible to find a “sweet spot” in a These will be expressed differently for different classes of characteristic, and report that as a performance device or applications, but they should always be in the specification. That is misleading, even if the specification specifications in one form or another. For multichannel specifically mentions the sweet spot. 2 For example, the graph above shows the residual THD+N External conditions and device configuration conditions, versus amplitude for one of our analyzer systems. For too, will affect device performance. What is the mains audio analyzers, this is a key specification, one we would voltage? The ambient temperature? Is the device volume like to tout. control set to low, medium or high? Are all channels being driven, or only the channel being measured? We claim ≤ –112 dB residual THD+N in a 22 kHz bandwidth. You can see that for most amplitudes above If there isn’t room for the conditions in the brochure, 400 mV the THD+N is well below this. If you are working there certainly is in the data sheet. Annotate at a 2 V amplitude, it is about –116 dB. We find, at least specifications when more detail is required. Use in our end of the business, that it is important not to footnotes or endnotes. mislead our customers. Here is an egregious but not uncommon example: An (The sharp inflections in the trace are artifacts of expensive audiophile amplifier states its THD+N this way: switching between ranging circuits; the rise at the left is “THD+Noise <1%” the inevitable approach of the noise floor.) What does that mean? There is no level or bandpass State ranges and conditions. filtering mentioned. It’s like saying “I’m taller.” THAN Stating ranges and conditions is absolutely essential. WHAT? Many specifications mean nothing if these parameters The table below shows the THD+N results for one are not provided. Yet, most manufactures do not include channel of a home theater receiver, made under varying this information in their specifications, or what they do conditions. Rated power (100 %) is 94 W into 4 Ω. include is incomplete. Are they saving ink? Note: the lower power unfiltered readings are high Every level measurement is made at some frequency or primarily due to the hum and noise component range of frequencies. Every frequency measurement is (N). made at some level or range of levels. Be very clear as to the level and frequency of the stimulus signal, and to the It’s clear that THD+N stated with no conditions is level range, bandwidth and filtering in the analyzer. meaningless. THD+N Bandwidth A-weighting Power Stimulus Filter Frequency 0.65 % no no 100 % 1 kHz 1.85 % no no 10 % 1 kHz 1.92 % no no 10 % 10 kHz 1.06 % no no 10 % 100 Hz 5.66 % no no 1 % 1 kHz 0.25 % yes no 100 % 1 kHz 0.01 % yes no 10 % 1 kHz 0.02 % yes no 1 % 1 kHz 0.26 % no yes 100 % 1 kHz 0.01 % no yes 10 % 1 kHz 0.01 % no yes 1 % 1 kHz 3 Express results using standard units. What should good specifications Do not mix units. look like? Here is one of the biggest problems in interpreting specifications: simply decoding the units chosen for the The characteristics that are deemed important differ specification, and comparing them to a similar across classes of devices. Power is important for specification stated in different units. One unit claims amplifiers; low noise is essential for microphone THD+N at –81 dB. Another touts a mere 0.0089 % preamplifiers, and intelligibility is key for THD+N.