Audio Precision Technotes & WhitepapersHOME/DOWNLOADS/TECHNOTES
TechNotes/AppNotes
Technote 104: Introduction to the Six Basic Audio Measurements
Technote 104 discusses methods of making a set of basic measurements using an Audio Precision 2700 Series or ATS-2 audio analyzer.These are Level Frequency Response THD+N Phase Crosstalk Signal-to-Noise Ratio
Technote 103: Dolby Digital DVD Certification Tests v1.4 (View previous versions)
This Technote describes the use of a group of over 30 AP Basic macros that make Dolby Digital DVD testing quick and easy, using an Audio Precision analyzer system and input switcher. Both real-time Dolby Digital encoding (with Audio Precision's OPT-2711 Dolby Digital option) and DVD-Playback methods are supported. The macros and measurement results were designed to correspond to the organization of Dolby's DVD test results documents to help you gather most of the technical information you'll need for Dolby Digital DVD certification.
Technote 102: Using Audio Precision High Speed Tester (HST) v2.01
The AP High Speed Tester (HST) application was originally developed to test playback only devices on the production line. The objective was a fast, accurate and easy-to-operate test station with a limited graphical user interface that just indicated the test results and had the ability to save results to a log file.
In addition testing play-back devices, HST 2.0 can use the instrument's generator to drive the input of the device under test. Both input and output can be set to digital or analog, and limits, user prompts and sample rate can be defined easily via a new setup utility.
This flexibility allows HST 2.0 to test almost any type of audio device - amplifiers, DACs, ADCs, signal processors, MP3 / DVD players etc. - quickly and easily.
Technote 101: LabVIEW ActiveX Automation for Audio Precision Instruments
TECHNOTE 101 describes the basic principles and techniques needed to operate Audio Precision instruments using the instrument's Audio Precision control software as an Active X automation server within the LabVIEW environment. LabVIEW is a graphical programming development environment from National Instruments Corporation (NI). Engineers, scientist and technicians create virtual instruments (called VI in LabVIEW) to control and automate a wide variety of test and measurement instruments and to gather and manipulate the resultant test data. Although it is possible for LabVIEW to directly control GPIB versions of Audio Precision instruments, GPIB control of Audio Precision systems is not the subject of this TECHNOTE. Instead, TECHNOTE 101 deals with the capability of LabVIEW to control the standard APIB versions of Audio Precision PC-controlled instruments via the instrument control software, using Microsoft ActiveX automation.
Technote 29: Computing Group Delay from Phase Data Using APWIN vN/A
Group delay is a property of a device or a system: a plot of the change in phase of the response as a function of frequency; it is the negative derivative (slope) of the phase-vs-frequency characteristic of a device. Constant group delay across the frequency band means that all portions of a wideband signal arrive simultaneously. A pure time delay, equal at all frequencies, gives a level straight-line plot of phase versus frequency. In an audio component, this plot may vary with frequency, and the component is said to produce group delay distortion. Group delay is of interest to audio engineers, particularly in the design and test of low-pass filters used in digital audio and in loudspeaker design areas. For instance, an anti-aliasing filter will typically have a phase response curve which slopes sharply down at high frequencies. This means that the high-frequency components will be delayed longer in their passage through the filter, resulting in a loss of precision in musical transients and a more diffuse stereo image. It is possible to correct the group delay distortion of such filters by using an all-pass network, but this is seldom done in practice.
Technote 27: Polarity Test Procedure vN/A
This utility determines the signal polarity between the Analog Generator Outputs and the Analog Analyzer Inputs. A common error in manufacturing of loudspeaker systems is to connect voice coils with reversed polarity. Even when all drivers in a multi-way system are phased correctly with respect to one another, it is possible to have the interior wiring to the external connection terminals of the cabinet reversed. An individual driver reversed will cause a dip in frequency response near the crossover frequency to the adjacent driver, since the two speakers are then producing acoustical output of nearly identical amplitudes but out of phase. An entire system wired out-of-phase would presumably be undetectable in a monaural application, but unacceptable in stereo systems. With minor adjustments, this procedure can be run as a stand-alone test, or it can be incorporated into other test setups. The procedure can test a single path, or it can test stereo channels. Equipment required is DSP or Dual-Domain version of System One, System Two, or System Two Cascade, and a microphone if you are testing acoustic paths such as loudspeakers.
Technote 28: Measurement Microphone dBspl Calibration
This TechNote describes how to set up and use the dBr unit as dBspl. Automated AP Basic macros (procedures) are available to facilitate the calibration. These macros apply to the instrument analog analyzers. Additionally, two computational methods are described to help you understand the macros, and to help you calibrate a measurement microphone without the macros.
Technote 10: SWR-122 Series Switchers APIB Interface Description
System One SWR series switchers are normally controlled via the APIB interface and S1.exe software. This Technote describes the APIB interface used on the switchers and explains how to control them from a generalized parallel port. A sample BASIC program is included to program the switchers from a PC parallel printer port. This Technote assumes programming and hardware knowledge.
Technote 15: GPIB Communication from AP Basic
This Technote describes how to setup communication between AP Basic and GPIB instrumentation (using the National Instruments GPIB Interface.)
Technote 17: Computing THD Measurements from FFT Data Using THD.exe
Conventional analog total harmonic distortion tests are made with a notch filter that removes the fundamental and makes a root sum square summation of all of the harmonics. FFT techniques, on the other hand, provide the absolute amplitude of each individual harmonic. This program will extract the necessary harmonic data and the fundamental amplitude, compute the rss summation and express the result as a conventional THD ratiometric value.
Technote 20: Return Loss Measurements
This Technote gives an overview of the concept of return loss in telephony. Return loss is an expression of the impedance mismatch in a transmission circuit which can give rise to reflection of signal energy. Sample hybrid circuits and their use in return loss measurements using Audio Precision instruments are described.
AP Applied: Dolby and dts Confidence Testing v1
Every device that bears a Dolby or dts logo is required to go through a compliance test process to insure that it meets the respective technical requirements. This AP Applied describes compliance testing using the APx585 with HDMI, utilizing features such as self-generated encoded test signals (closed loop testing), report generation, automation, and more. Testing examples are shown for an HD receiver and a disc player.
AP Applied: APx Automation with LabVIEW v1
Describes using LabVIEW with APx in a lab or automated production environment. Examples using the APx LabVIEW virtual instrument driver are shown.
AP Applied: Serial Digital Measurements with APx v1
Serial digital signal analysis is essential in research and development for evaluating audio circuit designs at the chip and at the board level. This AP Applied reviews serial testing procedures, and describes the abilities of the DSIO for APx, as well as the PSIA-2722 for the 2700 Series.
AP Applied: Digital Protocol Analysis v1
Explains the various ways to view, measure, and manipulate digital metadata, primarily over the HDMI interface.
Technote 105: FM Radio Receiver Testing with APx Audio Analyzers v0
In this technote, we discuss how to test the analog sound quality of FM radio receivers using an Audio Precision APx500 Series audio analyzer. We also discuss how to test the Radio Data System (RDS), which allows text, such as song title and artist, to be transmitted digitally as part of the analog radio signal.
AP Applied: Acoustic Response v0
A look at some of the important features needed in an acoustic audio analyzer, the problems you may encounter in non-ideal testing environment, the needs of good production line testing, and the range of measurements and results that should be produced.
AP Applied: Ultra-High Bandwidth Analysis v0
A look at the traditional ways of examining out of band noise signals, as well as the advantages of using an ultra-high bandwidth audio analyzer with full 24 bit FFT resolution from DC to beyond 1 MHz.
Technote 106: Measuring PSRR (Power Supply Rejection Ratio) v0
In this technote we describe and compare two different methods to measure PSRR. Then we give instructions for using the APx PSRR Measurement Utility, which simplifies the calculations and graphing on APx analyzers.
This technote is included in the APx PSRR Measurement Utility download.
Legacy Material
Technote 21: Obtaining Frequency Response Measurements using an Audio Program as the test signal vN/A
The Audio Precision System Two's dual channel FFT signal analysis provides an alternate method of obtaining frequency response measurements. With power spectrum averaging of the FFT, the test signal can be regular audio program material. This ability to use music as the test signal can be useful in situations where conventional test signals would be annoying to listeners, such as in broadcasting or live performances. The only requirement for the source program is that it have sufficient energy, over time, at all frequencies in the bandwidth of interest. In other words a rock band or a symphonic orchestra would be a better choice than a single folk singer with a guitar.
Technote 19: Create an HTML-Based Help System for APWin Basic Procedures vN/A
You can easily create an HTML-based help system to guide production technicians through APWIN Basic procedures. HTML, or HyperText Markup Language, is the language used by web browsers to format and display web documents. HTML-based help system files can be viewed by anyone with Internet or Intranet access, or they can be local files on a single computer. They can be updated at one location and can grow as your procedures grow. You can save files as HTML from word processors such as Microsoft Word 97, or you can use web development tools such as Dreamweaver or Front Page to create more sophisticated web-based help documents. It is possible to design robust interactive web-based help systems, including such features as displaying animated demonstrations, capturing and distributing information, using a database, and providing web-based training with audio, video, animations, etc.
Appnote 1: CD & DVD Player Tests using System One or System Two vN/A
This expansion and revision of the original Audio Precision Compact Disc Player Testing Application Note (AN-1) includes many new test techniques as well as refinements and improvements on the original tests. Test and procedure files supplied with this application note provide new convenience and ease-of-use for testing CD players using these instruments. Files created for both System One and System Two using APWIN are available. Also included are specialized and difficult tests such as distortion at low amplitudes, decoder linearity measured to below the Least Significant Bit, quantization noise, dynamic range, SMPTE and twin-tone (CCIF) intermodulation distortion and Wow and Flutter as an impulse noise test.
Technote 1: Generating Impedance vs. Frequency Plots with System One
Describes how to generate Impedance versus Frequency swept data using System One. This is of particular interest to loudspeaker designers where the impedance response can provide very useful information on the performance of a speaker. Includes a BASIC program that does the calculations.
Technote 1a: Generating Impedance vs. Frequency Plots with APWIN
This Technote is a rewrite of Technote #1 to support System One and System Two using Windows APWIN software. An additional method using an external amplifier and current sense resistor is provided.
Technote 2: Generating Group Delay vs. Frequency Plots
Plots of group delay versus frequency are useful in filter design and loudspeaker design applications. This Technote includes a BASIC program that will use a phase versus frequency plot as generated by System One to produce a group delay plot. The BASIC program can be run in a DOS shell within S1.exe.
Technote 3: Remote Site Data Retrieval of Test Results Using Modems
Some measurement situations place the measurement equipment at some distance from the operator. Many broadcast applications require that System One be located at a distant, possibly unattended site. This Technote describes how to set up a modem communication link between the distant measurement PC and a local operator PC to enable manual or automatic transfer of measured data. The set up for two commercial communication programs, PC Tools Commute and Norton pcAnywhere, are described.
Technote 4: Computing Equalization Curves Using EQCURVE.EXE
Tests of recorded or transmission media often require an equalized generator signal. Modifying the response of a swept generator to reciprocate the pre-emphasis or de-emphasis characteristics of the device under test will prevent overload and yield a flat response at the output. This Technote includes a BASIC program that will compute a data file based on user supplied time constants. The data file can then be used as an S1.exe EQ curve.
Technote 5: Usage Instructions for COMBINEC.EXE
System One produces data files that are two or three columns wide with as many rows as measurement steps. Usually the first column is the source data and columns two and three are measured data. There are some applications where it is desirable to use some data from one file with data from another file to produce a third file. This Technote includes a C program that allows columnar data to be exchanged, moved or combined to produce a new data file.
Technote 6: Computing Statistical Data From Nested Sweeps Using NSWEEP.exe
When several plots of a parameter are acquired, it is often useful to reduce the large volume of data to statistical summaries such as maximum, minimum, average or delta. This Technote includes a program that will operate on an S1.exe nested sweep file to produce the statistical data mentioned above. The program can be run in an S1.exe DOS shell.
Technote 7: Programmers' Reference to S1.exe internal *.tst file format
S1.exe *.tst files save instrument setups and test data in a binary format for space and speed efficiency. It is possible to examine such a file (or *.lim, *.eq, *.swp and *.ovl files) to extract this data. This Technote explains the internal structure of these files. It assumes intermediate programmers knowledge and is intended as a reference source, not a step-by-step explanation.
Technote 8: Programmers' Reference to S1.exe DSP Waveform File Format
System One DSP programs allow a SAVE WAVEFORM operation to store the time record data to disk. This Technote describes the internal structure of the *.wav files so produced. This would allow a knowledgeable programmer to extract data from this file for external post processing. It assumes intermediate programming expertise.
Technote 9: Restructuring Multitrack Data Using MTRACKFR.exe
Frequency response measurements on a Multitrack tape recorder are best made by successively measuring each track at the first frequency, then step to the second frequency and measure all tracks, and so on until all frequencies are measured. This produces a nested sweep file that plots amplitude versus track rather that amplitude versus frequency. This Technote contains a program that will restructure such data into the preferred format
Technote 11: Remote Test Data Retrieval: Send a FAX
This Technote describes how to set up the PC that controls System One to be able to automatically send a fax to a designated fax machine with the results, in both tabular and graphical form, of the measured data. The measuring PC can be at a remote unattended location as the fax capability can be configured to operate completely automatically on time, event of data out-of-limits conditions. The description shows how to integrate commercial fax modem cards and software.
Technote 12: Creating Arbitrary Waveforms Using Time Record data
System One DSP can produce signals based on a stored waveform loaded into its memory. A file with a .wav extension is downloaded to the DSP memory in System One to create the output. A utility called MAKEWAVE.exe is supplied With System One DSP that will convert an ASCII file that contains a time record into the .WAV format for use with the DSP generator. This Technote describes the format of the .waa ASCII files and how to create arbitrary waveforms. It assumes some programming knowledge.
Technote 13: PC Sound Card Testing Using DSP-RIFF.exe Waveform Conversion Utility
The multimedia expansion in the PC industry has created the need to test the audio performance of sound cards. This is a classic dual-domain application requiring testing of the D-to-A and A-to-D sections. This utility will translate either direction between the Microsoft RIFF .WAV file format and the Audio Precision DSP.WAV file format. This allows a System One test signal created by MAKEWAVE to be played back though the D-to-A, and a captured PC .WAV file to be analyzed on System One.
Technote 14: Testing Reduced Bit-Rate Codecs Using the System One DSP Program CODEC.dsp
Reduced bit rate codecs cannot be properly tested using conventional sine wave test signals. Using multitone test signals and FFT analysis, the codec can be stressed to properly characterize how it will perform with program material. CODEC.dsp uses multitone testing techniques and internal automatic masking curve generation to facilitate objective and repeatable evaluation of codecs.
Technote 18: Multitone Waveform Creation Utility for Use with FASTTEST v1
This Technote describes the Makewave utility, an AP Basic procedure that simplifies the process of creating multitone waveform files to use with the analog and digital generators in System One and System Two instruments. The download includes the utility.
Technote 30: .WAV file Import
This APWIN utility allows you to load any .wav files compatible with the Microsoft RIFF file format into APWIN and analyze them in the frequency or time domain.
Whitepapers
Measuring Switch-mode Power Amplifiers vN/A
Switch-mode audio power amplifiers are becoming increasingly popular due to their smaller size, lower weight, and improved efficiency. Their advantages are obvious in low power battery operated personal audio players and laptop computers. However they are also progressively displacing more traditional linear designs in mainstream applications such as home entertainment systems, automotive sound systems, and professional installations where high quality audio is important. Measuring the performance of switch-mode amplifiers presents some new and unique challenges. They inherently generate ultra-sonic artifacts and spurious signals with slew rates that can provoke non-linear behavior within the input stages of high quality audio test and measurement equipment. Absolutely worthless and inaccurate results can result unless effective measures are taken to prevent this non-linear behavior. Written by Bruce Hofer.
Limitations in making audio bandwidth measurements in the presence of out of band noise vN/A
Audio analyzers are optimized to make high quality measurements up through their maximum specified bandwidths. The presence of significant energy above these bandwidths can have a profound effect on the audio measurements and possibly induce errors. This white paper provides valuable information on avoiding the possibility of errors when testing digital to analog converters and other audio devices that contain significant energy above the audio band. Written by Bruce Hofer
Testing Challenges in Personal Computer Audio Devices vN/A
The Personal Computer audio environment has evolved over the years to become a significant entity within the field of acquisition and rendering of audio information. The personal computer is a highly sophisticated interactive environment that is much more complex than a conventional dedicated home audio device, leading to new problem areas. These include, but are not limited to, stochastic interrupts, network accesses, disc I/O and disparate hardware qualities. While the environment of a highly matrixed multi-tasking concurrent operating system offers many opportunities to overcome quality issues, the PC, due to the media-rich tools and feature sets, is becoming the entertainment capture and rendering device of choice for future generations. Presented at the 114th Convention, 2003 March 22–25 Amsterdam, The Netherlands. Wayne Jones, Michael Wolfe, Theodore C. Tanner Jr., Daniel Dinu
Measuring Digital Audio vN/A
Transmitting and storing audio signals in the digital domain is well-established in the broadcast industry. Analog audio has given way to the AES3 and Sony/Philips Digital Interface Format (SPDIF). AES3 data streams are also embedded in SDI television signals. Handling audio in the digital do- main offers many advantages over analog methods. An analog signal incurs progressive degradation as it passes through a chain of circuits. Converting the analog signal into digital and converting back to analog as late in the chain as possible overcomes this degradation.
Fundamentals of Modern Audio Measurement vN/A
Characterizing professional and consumer audio equipment requires techniques which often differ from those used to characterize other types of equipment. Sometimes this is due to the higher performance requirements. Other times it is due to the peculiarities of the audio industry. Other fields deal with some of the same measurements as those in audio. From level and THD to jitter and noise modulation, no other field has the breadth of requirements found in high performance audio.
Measuring Distortion in Switching Amplifiers v0
Measuring Distortion in Switching Amplifiers by Bruce Hofer
Fast Audio-Band Measurement Using Log-Swept Chirp Signals v0
Fast Audio-Band Measurement Using Log-Swept Chirp Signals by Dr. Tom Kite, AP VP of Engineering
