Fullscreen HTML5Med Res (??MB)HTML4

Review by BOB FLYNN Audio Precision System One Analyser In 1985, a new company, Audio Precision, Inc, intro­duced an automated test set for the measurement of audio equipment. Ten years later, the system has been considerably upgraded & is still regarded as the world’s best. Here are our impressions of the system after several months’ use. Since the earliest days of audio, engineers have needed to know the performance of the equipment they have designed. As time has passed, the range of tests has become more extensive and detailed. For example, the basic parameters to describe the performance of a power amplifier may be as follows: Gain, Fre­quency Response, Power Output, Signal-to86  Silicon Chip Noise ratio, Total Harmonic Distortion and Intermodulation Distortion. Parameters such as gain, bandwidth and power output are relatively easy measurements to make, requiring no more than a sinewave oscillator with a variable output and flat across the audio band, an RMS-reading voltmeter (with a bandwidth greater than that of the device being measured), a dummy load and an oscilloscope so you can spot the onset of distortion (ie, clip­ping). Signal-to-Noise ratio requires the above gear and an AC millivoltmeter, although for really quiet equipment, you need a millivoltmeter which will read down to microvolts. For example, a noise measurement of -115dB below 20V equates to a reading of only 35µV. For these tests, you also need bandwidth limiting and weighting filters (eg, for A-weighted tests). For harmonic and intermodulation distortion, the test equipment list grows longer. Originally, for harmonic distortion tests, you needed an audio generator to provide the test signal and a harmonic analyser. Setting the generator’s frequency was never a bother but trying to match the notch filter in the ana­lyser to that frequency was another story. In our lab, we have an old Radford “Distortion Measuring Set” and this is quite a beast to drive. As well as a large handspan dial labelled “Rejection Frequency Tuning” and a “Rejec­tion Frequency Range” switch, it has two knobs marked “Coarse Tuning”, two marked “Medium Tuning” and two more marked “Fine Tuning”; seven tweaks to drive you up the wall! Besides the inevitable drift in the generator frequency, you were also fight­ing the analyser’s own drift. Finding a true null with this device was almost a miracle. Later test sets combined the audio generator in the same case as the distortion analyser. The tuning of the notch filter was then coupled to the setting of the test signal frequency. Servo circuitry kept the notch filter in track with the genera­tor, to relieve the operator of the fiddly task of nulling. Further circuitry automatically adjusted the voltmeter so that the analyser input level was automatically set at 100%. So it could be said that these instruments were partly automated. This was a big improvement over previous instruments but making a “THD versus frequency” test, for example, still meant the operator setting each plot frequency on the generator, wait­ing till the analyser settled, then plotting each reading on graph paper. Producing a series of such plots under different operating conditions could take many hours or even days. Automated testing All this changed in 1985, when Audio Precision produced their System One audio test set which was controlled by an IBM PC. The test set reviewed here is the System One, Model SYS-22. The SYS-22 is a stereo analog audio test set comprising a 2-channel generator and 2-channel analyser. Available options for the System One include weighting filters, tone burst generation, intermodulation distortion measurement, wow and flutter measure­ment, and a digital signal processor (DSP). External options include the DXC-127 DC (a DC voltage source, a DC voltage and resistance meter and a digital input-output device) and SWR-122 switcher modules (these Fig.1: this is the first screen used to set up the System One. The lefthand panel is for the Generator while the middle panel sets up the Analyser. Fig.2: the second control screen for the System One. This screen sets up all the settling parameters. allow multi-point and multi-channel test­ing of equipment). Our unit has the options for inter­ modulation distortion, tone burst tests and the A-weighting filter. Perhaps the most important other option is DSP (Digital Signal Processor) and this enables analysis of digital audio equipment such as CD players, Mini-Disc and DAT recorders and also has Fast Fourier Transform (FFT) analysis to allow full audio spectrum analysis. System One is large, heavy and initially at least, quite inscrutable. It has no switches or knobs, no dials or displays or, in fact, anything that the user can directly control. It is total­ly controlled via an interface card which is installed in an accompanying IBM PC computer. The System One measures 438mm wide, 133mm high and 432mm deep and it weighs about 15kg. The upper left half of the front panel accommodates the generator output sockets, comprising two three-pin male XLR sockets and six banana jacks. Below these sockets are three BNC connectors, for a sync output, a trigger input and a monitor output (eg, oscilloscope). The righthand side of the panel is devoted to the analyser inputs, with two three-pin female XLR sockets, five banana sockets and a BNC connector. Below these are five BNC sockets arranged in a two groups. Three of these are monitor outputs, while the other two are for external filters. Outputs of the generator may be balanced or unbalanced, floating or June 1995  87 Fig.3: this control screen is set up to measure the power bandwidth of an amplifier at 1% rated harmonic distortion. Fig.4: a self-test of the System One showing the overall frequen­cy flatness, using a measurement bandwidth of 500kHz. The specification is 20Hz to 20kHz ±0.05dB. grounded. In floating mode, source impedance can be 50Ω, 150Ω or 600Ω. In unbalanced mode, it can be 25Ω or 600Ω. The analyser inputs are balanced and the impedance can be set at 150Ω, 600Ω or 100kΩ. The low impedance terminations are automati­ cally removed if the input exceeds 30V. Along with the test set and previously mentioned interface card, the System One comes with a set of program discs and a comprehensive user’s manual. System One may be 88  Silicon Chip run on any IBM or IBM compatible machine through to 80486 machines. The computer needs at least 640Kb of memory, DOS 2.2 or later versions and a Her­cules, CGA, EGA or VGA monitor. We installed the software and the interface card in a 386SX computer and experienced no problems. Connection between the computer and the System One is made via a supplied cable fitted with 25-pin D connectors. The software runs under DOS (ie, non-Windows) and as sup­plied with our version, the program contains about 80 test files and 13 procedure files. Test files are compiled to carry out a specific test; eg, THD+N versus Frequency, THD+N versus Amplitude, Crosstalk etc. The user can generate custom test files or make modifications to the supplied tests. A procedure is a file that will execute a series of tests and sub-procedures in a fixed order. Procedures are particularly useful for production tests and quality control. Once a procedure has been established by the production manager, non-technical staff can run tests on a pro­duct and every time the procedure is run the test parameters remain the same. With everything ready to go we were keen to put the system through its paces. A good place to start is SYS­22CK.PRO, a proce­dure file with eight tests to check key parameters of the system. After this procedure is finished, a report of the response of the instrument to each test is shown. If any of the parameters is outside the specifications, then the instrument is in need of recalibration or servicing. The command S1 starts the program and the Audio Precision logo appears with a command line below it. This command line shows a menu of fourteen single word commands; eg, Run, Panel, Load, Save etc. One letter commands are enough to produce action and entering (L)OAD, brings up the next command line with a further 10 commands such as Test, Limit, Procedure, etc. Entering (P)RO­CE­DURE displays a list of the Procedure files. Picking SYS22CK loads that procedure, then entering (R)UN followed by (P)ROCEDURE starts the series of eight tests. The result of each test is displayed on the monitor in either graphical or tabular form. The total time to run the eight tests and the tolerance report was less than 90 seconds. As you would expect, the instrument comfortably exceeded the speci­fications for all tests. Sample tests The following is not intended to be a blow by blow descrip­tion of how to use System One but rather to give a rough idea of what needs to be done to set up a test. If a test is loaded and (P)ANEL is entered then the screen shown in Fig.1 appears. The left hand panel titled GENERATOR shows the generator setup with the key functions such as WAVEFORM in the leftmost column. If the highlighting bar is moved, either by the keyboard cursor keys or the mouse, to cover a word to the right of any of the key func­tions, then the command line at the bottom of the screen shows the options available for that field. For example, if NORMAL, to the right of WAVEFORM, is highlighted then the COMMAND options available are NORMAL, BURST, TRIG, GATE. Highlighting the figure next to FREQUENCY allows you to key in the generator frequency, or by pressing the + key, increasing the frequency by the factor shown next to FREQSTEP if “*” is selected to the right of FREQSTEP. Similarly, highlighting the figure to the right of AMPLITUDE allows you to key in some other signal voltage or by pressing the plus key, increasing the ampli­tude by the voltage shown next to AMPSTEP if + is selected to the right of AMPSTEP. Highlighting the field to the right of OUTPUT gives you the option of selecting OFF, (channel) A, B, A&B, A&-B. In other words, you are setting up the generator with the key­board instead of switches and potentiometers. The centre screen panel is titled ANALYSER and here again by highlighting the words to the right of the key functions, the analyser can be set up. Immediately to the right of MEASURE is a field allowing you to choose channel A or channel B. Further to the right again is a field giving measurement options: AMPLITUDE, BAND­PASS, BANDREJECT, THD+N , SMPTE , CCIF , DIM W+F, 2-CHAN and CROSSTALK. Similarly, to the right of the function READING are the options %, dB, PPM, X/Y and OFF. READING is just what it says; the parameter being measured. LEVEL can be set to V, dBm, dBu, dBv, dBr, W, OFF. This is the signal level into the analyser before any filtering or tailoring. To the right of BANDWIDTH the first field allows you to choose the low frequency cutoff and the next field the high frequency cutoff. The righthand panel titled SWEEP (F9) DEFINITIONS allows the user to set up the tests with sweeps of frequency or ampli­tude measurements versus time. To the right of DATA-1 near the top of the panel can be selected the parameters to be plotted, ANLR (analyser), GEN (generator), DCX (not The Audio Precision test set can be fitted with a large range of options, including DSP analysis for equipment such as DAT & MiniDisc recorders. Fig.5: a self-test of the System One showing the residual harmo­nic distortion & noise between 5kHz and 100kHz, with a measure­ment bandwidth of 500kHz. fitted) or DSP (not fitted). Further to the right can be selected RDNG, LEVEL, FREQ, PHASE, NONE. DATA-2 can be changed to SOURCE-2, HOR-AXIZ, or STEREO. SOURCE-2 allows two types of parameters to be swept in the one test. Hor-axis permits two measured values to be plotted against one another. Stereo generates consecutive sweeps, the first through one chan- nel after which the generator output and analyser inputs are switched by System One and the same sweep is performed on the second channel. SOURCE-1 is the swept independent variable and can be set to GEN, ANLR, SW1, DCX, DSP, EXTERN. With GEN selected, for example, the next field to the right gives the following options: FREQ, AMPL, TB-ON, TB-INT, TB-LVL or NONE. Frequency allows you to June 1995  89 adequate and ENABLE (the top line) to SWEEP. When the power output versus frequency sweep is now made the generator amplitude will be varied to maintain the measured distortion level at 1%. Printouts Fig.6: a self-test of the System One showing the residual harmo­nic distortion and noise of both channels between 20Hz and 20kHz. Note that below 5kHz, the distortion is less than .0005%! set the sweep START and STOP to the range required. Amplitude allows you to set the START and STOP amplitudes of the generator and the output will be swept with a fixed frequency signal but varying in amplitude. NONE gives a single point measurement with tabular display. More options Pressing “page down” displays a second screen, as shown in Fig.2. When you operate a manual test set, if you alter the generator frequency and the analyser is an automatic frequency tracking instrument, you have to wait for the instrument reading to “settle” (ie, to stabilise) before you take the reading. With System One, SWEEP SETTLING allows the selection of parameters that effect the settling as it performs a sweep. If, while a sweep is being run, the trace stops at some point in its travel, then a “T” will appear at the bottom of the graph before the sweep continues. The T indicates a “time out” meaning that, at that point in the trace, the required settling parameter was not achieved. This can be due to noise in the analyser signal. Some adjustment of the parameters in the SWEEP SETTLING panel will be required. Pressing “page down” a second time displays a third screen, entitled REGULATION, as shown in Fig.3. This allows testing of a device while varying either the test signal amplitude or frequen­cy, while making a sweep. For example, say you need to measure the power bandwidth of an amplifier at a distortion level of 1% across the audio band. To achieve this, set “REGULATE ANLR RDNG TO” 1% and “BY VARYING GEN AMPL” to the HI BOUND and LO BOUND levels you think are Key Specifications Total System THD+N ���������� <.0015% from 20Hz to 20kHz, with 80kHz measurement bandwidth; <.001% from 20Hz to 20kHz, with 22kHz measurement bandwidth Total System Flatness �������� ±0.05dB, 20Hz to 20kHz Total System IMD ��������������� <.0018% SMPTE; <.002% DIM; & <.0005% CCIF Analyser Residual Noise ���� <1.5µV (-114dBu) with 22kHz measurement bandwidth. 90  Silicon Chip Having made a test and deciding that the displayed graph is the one to keep, what methods are available to keep a record? If you have a dot matrix or HP LaserJet printer connected to your computer, then a screen dump can be made by typing a <*> (aster­isk). However, the printout will only be as good as your moni­tor’s resolution. If though, you start the program with <s1/g> and then after running your test you press the escape key, the command line appears at the bottom of the screen. Typing (S)AVE brings up another menu. Pressing (G)RAPH­ ICS then allows you to save the graph as a .GDL (Graphics Display List) file. There are also two utilities in the program: Post and Plot. Post allows you to convert the .GDL file to a Postscript or EPS file and Plot will let you convert it to an HPGL plotter file. Well, having had the use of System One for some months now, what are my impressions? At first, while very pleased with the instrument’s performance, I could not help feeling that I had been removed a couple of steps from the testing procedure. Maybe it was the fact that I was now setting up the generator and the analyser on a keyboard, with no more twiddling of switches and pots and no more waiting for the instruments to settle. I cannot say I missed plotting the results on graph paper with a pencil though. This feeling of being remote from the testing soon passed. The more you use System One, the more things you find it can do. Now I would hate to have to return to the old manual way of doing the job. This is truly automated audio testing. The only thing that the operator needs to be aware of is interference from the monitor’s radiated timebase. While the System One is very well shielded, there is a need to take care to keep it out of the device being tested. The System One and other distortion test sets by Audio Precision, Inc are distributed in Australia by I.R.T. Electronics Pty Ltd, 26 Hotham Parade, Artarmon, NSW 2064. Phone SC (02) 439 3744.