MRL TEST TAPES
With the exception of loudspeakers and microphones, few items in the world of professional audio cause such emotive discussion and confusion as ‘line up’ tapes. This is, to some extent, for historic reasons and to arguments about what constitutes a ‘reference’ in this context.
Although over the years the international bodies have at least agreed on standard tape widths and speeds, there are still two commonly used international standards for tape equalisation, one of which is commonly referred to by any one of three names! Having accepted that it is unlikely that different equalisation standards will disappear for the forseeable future, it seems hopeful that at least agreement is possible on one point - that the reference for the signal recorded on the tape should be a value for the magnetic flux on the medium. Sadly, however, the value of such a reference fluxivity has at least three common ‘standards’, and to make matters lead to virtual despair, modern research has shown that, due to equipment limitations, the original flux measurements made in Germany in the 1950’s were 10% in error. Confused? Well, MRL have followed the only peaceful path and made calibration tapes to suit virtually any requirement. The range is extensive. Please contact Technical Support if your requirement is for a type that is not listed. Note that MRL also make a range of polarity calibration tapes for determining absolute polarity in any system. An extremely helpful guide to choosing and using MRL test tapes is available free on request, stock code 02-209.
The following table shows a selection of the most popular types:
1 1/4″ tapes are full width recordings not fringing compensated. 1/2″, 1″, 2″ tapes are full width recordings, fringing compensated.
2 IEC1 is also commonly referred to as IEC, DIN, CCIR. NAB may also be referred to as IEC2. AES as per “proposed recommended practice” in J.AES volume 19, p68, 1971, January, is also IEC2.
3 Spot frequency tapes also include a section for setting gain and azimuth. Two speed short calibration tapes have 100Hz, 1kHz, 10kHz and 16kHz tones, each recorded at 15in/s (38cm/s) with NAB equalisation and 30in/s (76cm/s) with AES equalisation. They are available in 5.5 and 11 minute total duration versions. To special order these can also be supplied with IEC equalisation for the 15in/s (38cm/s) section.
4 Level of the frequency response section relative to the reference fluxivity section.
5 Reference fluxivity of G320 nWb/m is consistent with the German flux measurements made in the 1950’s that are used in the UK and Europe to this day. The actual fluxivity of these recordings as measured by DIN 45520 or AES7/ANSI S4.6 is 290nWb/m.
The original tape flux measurements were made in Germany in the late 1950’s, using a transfer-to-dc method standardized in German Standard DIN 45520. These measurements are the basis for the reference fluxivity of 320 nWb/m used on German calibration tapes made by BASF and Agfa (now a part of BASF). In the late 1960’s Ampex used the ANSI method to measure the German tapes, and found that the German reference fluxivity was not 320 nWb/m, but only 290 nWb/m, which is about 1 dB low. Recent new measurements at MRL have confirmed that flux measurement by the transfer-to-dc method used in Germany gives exactly the same results as the ANSI method. So MRL have concluded that the original (1950’s) German measurement was in error by 10%.
The MRL Calibration Tapes made to conform to the old German measurements were previously identified by MRL as “320” nWb/m; this has now been changed to G320 nWb/m, indicating 320 nWb/m according to the original German measurement. The basis for this change has now been published as “Tape Flux Measurement Revisited” in the AES Journal, Vol 46 Nr 10, pp845...858 (Oct 1998), with supporting papers from Otto Schmidbauer (translation of his 1957 paper) “Determining the Magnetization of Magnetic Tape”, pp 859...864, and the translation of the German Standard for flux measurement DIN 45520(1957), pp 865...867.
Many organisations refer to the ‘peak’ flux as their ‘standard’. This means that flux level which would produce peak level eg PPM 6 or +8dB when replayed on a correctly aligned machine. If you wish to conform to that standard you would then want to know where to align your reproducer for a given reference fluxivity. This is given by 20 log (Flux P/Flux R) where Flux R is the reference flux and Flux P is the quoted ‘peak’ flux. For example if the flux quoted were 510nWb/m and the reference in use were 320nWb/m then the reference section would be aligned to produce 20 log (320/510) –4dB relative to the PEAK LEVEL, (commonly PPM 5 or +4dB).
Important note: We regret that under no circumstances can test tapes be returned for credit if the packaging seal is broken.