by Henning S. Madsen [Ortofon Manufacturing A/S, Valby, Denmark ]
KEEPING TRACK of the test records available in the world is almost a full time job, particularly if one wants to know what is cut into the records and what this can tell you when testing a particular phonograph cartridge. Ortofon has quite a library but fortunately only a few of them are used frequently. The important measurements are: frequency response and crosstalk, output, square wave response, distortion, tracking angle, and tracking ability.
These measurements can be conducted on various test records, but how does one choose from the big selection? Ortofon has been cutting quite a few test records themselves, and therefore it is natural to standardize on some of our own, but on top of these, quite a few other records are frequently used. In reviews and technical articles test results are reported, and it is necessary to be able to repeat these measurements. Since Ortofon cartridges are sold in most countries of the world, we end up using German DIN records as well as American and Japanese test records.
After a very brief description of how a magnetic cartridge works, our test record collection is listed. Finally the instrumentation which is necessary for carrying out various measurements is discussed together with typical results.
The conclusion deals with the difficulties encountered in making measurements involving records. A standard measuring method applying some kind of a piezoelectric shaker is suggested.
Two earlier articles about test records can be recommended (ref. 1 and 2). They give many fundamentals.
Magnetic Phono Cartridge Principle
Fig. 1--Cantilever assembly in magnetic circuit.
A magnetic phonograph cartridge is a transducing element capable of changing the mechanical movement to which the stylus is subjected in the record groove into electrical signals. The most common method is via a tiny rod magnet mounted at the end of the cantilever opposite the stylus in a magnetic airgap. Figure 1 illustrates how the armature is pivotly mounted at its center of gravity. Variations in the airgap cause a varying magnetic flux through the pole pins, which in turn induces an electrical voltage across the two surrounding coils connected in series.
The guiding mathematical formula for such a magnetic transducer is given below (ref. 3):
e = Tv
where v is the velocity; T is the electro-mechanical coupling factor which only depends on the magnetic circuit, and e is the voltage. This tells us that the magnetic pickup is a velocity transducer and not an amplitude transducer as the crystal pickup.
This is important to know when talking about frequency response measurement. A flat frequency response measurement requires a test record with a constant velocity sweep from 20 Hz to 20 kHz or more. We will return to this subject later. One special magnetic cartridge is the moving-coil cartridge. It works by moving a few turns of a coil in a homogenous magnetic field. A step-up transformer or pre-preamplifier is normally required. Also this kind of transducer is of the velocity type.
Test Record Library
Table 1 lists the records presently in Ortofon's library. A description of what type of measurements can be performed with the records is also given, though a detailed specification is not included. Some of the test records have been the subject matter for technical papers. In these cases the reference literature is given.
Most Frequently Used Records
The output level at 1 kHz of both channels at a standard velocity of 3.54 cm/s rms is a rather important figure. The laboratory standard record is the CBS STR 100. It is a very simple measurement requiring only a record player and an electronic voltmeter. In between can be connected a left/right switching box with the standard electrical load of 47-Kohms built in. In this way also the separation or crosstalk can be measured.
Why is this measurement important? It is because it correlates with many other important figures like frequency response and tracking ability. If the pivot point is not correctly mounted, the moving mass is increased and a drop at 20 kHz results but at the same time the output voltage is either increased, that is when the pivot point is moved closer to the stylus allowing greater movement of the armature in the airgap, or decreased when the pivot is moved away from the stylus. The first situation also reduces the tracking ability at low frequencies because the compliance is reduced. One can see that a high output normally characterizes a cartridge with a low performance. On the other hand, there is a minimum output which can be tolerated in order to keep the signal-to-noise ratio high.
That the difference between the channels is not too great is measured at the same time. All these measurements are included in the performance test in the quality control at Ortofon. The OR 1013 is presently the test record which every single cartridge must play before leaving the factory. The relative level at 7.5 kHz, 15 kHz and 20 kHz is also tested.
The tracking ability at 300 Hz is also tested with this record. The laboratory standard is the German HiFi record No. 2 which has increasing levels from 20 um peak to 100 ram peak at 300 Hz. A combination of watching the signal on a dual-beam oscilloscope and listening with headphones is used. Figure 2 shows the set-up.
Fig. 2--Set-up for measuring tracking ability.
Fig. 3--Set-up for measuring frequency response and crosstalk.
Fig. 4--Typical frequency response and crosstalk without low-end compensation.
Fig. 5--Bruel & Kajaer measuring set-up with filter.
Frequency response is a very common measurement in the laboratory. Usually the OR 2015 test record is used. It has a very fast sweep. The measuring set-up is shown in Fig. 3 and a typical result is shown in Fig. 4. The curve is not flat in the low end since it is not possible to cut a constant velocity of 3.54 cm/s down to 20 Hz as the cutterhead is not able to cut such large amplitudes. The RIAA characteristic is adopted on this record in the low end. Another test record, the CBS STR-100, has a constant amplitude up till 500 Hz and then a constant velocity from 500 Hz-20 kHz.
A more complicated measuring set-up is required if one wants the response to be flat down to 20 Hz. It involves a filter, and the crosstalk can only be measured if it is measured with a spectrometer. Brüel & Kjaer has a special arrangement for this measurement (Fig. 5 and ref. 4). With the advent of the CD-4 four channel system, new test records with frequency sweep up to 50 kHz were required. Ortofon has, in cooperation with the two other Danish companies, Brüel & Kjaer and Bang & Olufsen, cut the new test record QR 2010 which has a very fast sweep like the OR 2015, Measuring CD-4 cartridges can be done on the same measuring setup if the switching box has a built-in alternative electrical load of 100 K-ohms as required for CD-4.
The JVC record TRS 1005 is commonly used in the laboratory for measuring CD-4 cartridges. It has a sweep from 1 kHz to 50 kHz. Distortion measurements are perhaps the most powerful measurements to establish the different cartridges' ability to reproduce sound from records. Distortion can be divided into harmonic distortion and inter modulation distortion. The latter of these is most frequently used and several test records are available.
At Ortofon the German DIN record 45542 is most frequently used. It has two different tones, 300 Hz and 3000 Hz. The low tone has a level of 12 dB higher than the high tone. If the low tone is filtered out, only the high tone should remain but some trace of the low tone will usually remain. See Fig. 6. These low-pitched variations can be detected with a wow-and flutter meter, as shown in Fig. 7. Figure 8 shows the result for two Ortofon cartridges.
Fig. 6--3 kHz with 300 Hz modulation.
Fig. 7--Set-up for measuring frequency intermodulation distortion (FIM)
Fig. 8--Non-linear distortion (FIM). 0 dB: v(300 Hz) = 8 cm/sec, v(3000 Hz) = 2 cm/sec.
Fig. 9--Set-up for measurements incorporating a wave analyzer.
Fig. 10--High-frequency trackability. Test record TTR 103.
Fig. 11--Intermodulation. Two-tone glide.
A recent test record from Shure, TTR 103, has tone bursts of 10.8 kHz with a repetition frequency of 270 Hz (ref. 5). This record will tell about tracking ability at higher frequencies because distortion is just a more technical term for tracking ability. The measuring setup is shown in Fig. 9 with typical results in Fig. 10.
RCA presented long ago their two glide-tone intermodulation test record (ref. 6). With the wave analyzer set at the difference tone, the distortion is measured up to 20 kHz at four different levels. Figure 11 shows typical results.
The harmonic distortion measurement has more recently become a very sophisticated test since new measuring equipment has made it possible to measure the second, third or any higher harmonic during a sweep. The measuring set-up involves a heterodyne analyzer and a tracking frequency multiplier, as shown in Fig. 12. Figure 13 shows typical results.
The tracking angle is the angle between the record surface and a line from stylus tip to the pivot point. For many years this angle has been 15° as a standard, but recently a change to 20° has been recommended because 15° was very difficult to meet. It is very important that the angle with which the record is cut coincides with the tracking angle during replay, since the distortion increases tremendously if this is not the case. The way to measure the tracking angle is by means of distortion.
Again, the German DIN record 45542 is adopted. Two different tones are cut into the record simultaneously with different angles from 6 to 30 degrees.
When measuring with a wave analyzer adjusted to the sum frequency, one gets a minimum output when there is a coincidence between the pick-up tracking angle and the angle with which the groove is cut. See Fig. 9.
Square-wave response. As when testing loudspeakers and amplifiers, a square-wave response will tell a lot about the performance of a cartridge. The CBS record STR Ill has 1 kHz square waves. Figure 14 shows the result as watched on an oscilloscope. The overshoot and ringing should be as little as possible as this is a measure of difference between actual sound and reproduced sound.
Among miscellaneous records used for testing, the Pacific Transducer Corp. 102M and 103L are very informative. They have 70 Hz-10 kHz and 70 Hz 15 kHz sweep respectively, repeated at high rate so that the whole response can be watched on an oscilloscope. Output level, frequency response, and tracking ability can be detected at a glance.
The absolute phase of a cartridge is not important as long as only stereo is involved, but the four-channel CD-4 cartridge must have a positive response when the stylus moves to the periphery of the record. JVC has come out with the test record TRS 1004 for testing the absolute phase. It has a square-wave cut with a shorter positive cycle than negative cycle. For correct phasing, the shorter wave must always go positive if the measuring equipment is not changing the phase.
Fig. 12--Set-up for harmonic distortion sweep.
Fig. 13--VMS-cartridge, 1P. Frequency response, 2nd and 3rd harmonic. Test record OR 2009.
Fig. 14-Typical square-wave response.
New Standard Equipment
The foregoing were the various measurements and tests frequently performed on phonograph cartridges using test records. One big problem with these tests is that records are never 100 per cent the same. They change with wear, temperature, pressing, and material. In order to get reproducible results, one has to go through elaborate testing with standard cartridges. It would be of great help if someone would develop some kind of shaker which could be internationally adopted as a measuring standard. At Ortofon we often use a cutterhead directly but this cutterhead and the cartridge are both magnetic devices and this causes big shielding problems. The ideal shaker should be a piezoelectric device.
Of course it will always be necessary also to use test records since the action between the stylus and the moving groove never could be completely duplicated with a shaker, but a reference instrument which was the same for all parties involved would certainly be helpful.
The ultimate test is the listening test. Ortofon is paying an increasing attention to listening. What selections are we using? Well, this would lead into a new subject which gives material enough for a new article.
References:
1. Arnold Schwartz, Test Records, dB, April, 1969.
2. C.G. McProud, How we test a phono cartridge, Audio, August, 1972.
3. F. V. Hunt: Electroacoustics, The Analysis of Transduction and its Historical Background Harvard U.P., Cambridge, Mass., 1954.
4. Brüel & Kjaer. Instruction Manual for level recorder type 2305.
5. C. Roger Anderson and Paul W. Jenrick: A Practical High Frequency Trackability Test for Phono Pickups, JAES, Vol. 20. No. 3.
6. J.G. Woodward and R. E. Werner: High-Frequency Intermodulation Testing of Stereo Phonograph Pickups, JAES, Vol. 15, No. 2.
(Audio magazine, Mar. 1974)
Also see:
Trackability--1973 (Aug. 1973)
The Phono Cartridge Electrical Output Network (March 1981)
Which Tracks Best--A pivoted or a radial Tonearm? (June 1982)