Standard Tape Flux vs Frequency
If you need to do a calibration, but the speed and/or equalization of the machine that you have does not match the MRL calibration tape that you have, you can use our Magnetic Tape Equalization Calculator to find the corresponding levels for your machine.
The calculation was developed using IEC Standard 60094-1, 1981, "Magnetic Tape Sound Recording and Reproducing Systems, Part 1, General Conditions and Requirements", which gives the definition and formula for calculating the standard flux levels vs frequency in Clause 15 on page 17. It gives a table of the standard "Time Constants" for each standard speed and use in Clause 15.1 on page 19, and the actual standard amplitude/frequency response of the recorded short-circuit flux level in decibels in Table 1, on page 27 of IEC 60094-1. We have reproduced that table at iec94-flux-v-frequency-table.pdf.
The MRL publication "Choosing and Using MRL Calibration Tapes for Audio Tape Recorder Standardization" also gives the formula for calculating the flux level vs frequency on page 2 (Sec 1.2.3), and the standard transition frequencies for use in that formula in Table 1 "Standard Equalizations Per IEC, NAB, and AES Standards", on page 3
"Choosing and Using...", in Sec. 2.3.3 (page 7 and 8) also gives eight conversion tables for different speeds and equalizations, in case you need to calibrate a system with one speed and equalization standard, but you only have a Calibration Tape for another speed or equalization standard.
Command Line version
MagTapeEq
The original command line version of the Magnetic Tape Equalization Calculator was called MagTapEq. It is no longer actively supported because it was developed using the Forth programming language and would only run on DOS (and maybe Windows?). We recommend using the web version if at all possible, but provide the original version for historical interest, and to document the conversion algorithm.
The "MagTapEq" program can be used to calculate a table of standard recorded flux vs frequency by using "serial" 0 as the "tape you wish you had" -- for instance, enter 11 0 to calculate the NAB flux vs frequency at 15 in/s. To calculate the standard reproducing equalization (assuming that you have an "ideal" reproducing head), enter 0 10 to calculate for an IEC1 reproducer at 15 in/s.
Instructions:
- Download two files: "magtapeq.seq" (the Forth source code), and "magtapeq.exe" (the executable code). You can get both wrapped in a single zip file using the download button at the bottom of the page.
- Open "magtapeq.seq" using a text editor and read the instructions therein. It gives general instructions for using the executable file, including a table of serial numbers for each speed and standard equalization for open-reel systems.
- Run "magtapeq.exe" from the command line, and pass it the "serial" number of the tape you have, and the "serial" number of the tape you wish you had (both found in magtape.seq). For example: magtapeq 11 0
- The output will display on the screen, or write to a file or a printer, the tables (like in Table 5 on p 8 of "Choosing and Using...") giving the frequencies and necessary levels.
- Type "bye" to exit the program.
Note:
"magtapeq.exe" is the complete F-PC forth, with software floating point and some other functions compiled on top, with "magtapeq.seq" compiled on top of that. So you have the complete "works" in "magtapeq.exe" , and the 3/4 baked instructions in the .seq file. Running magtapeq.exe doesn't take any programming knowledge, but it is a command line program, which can be very frustrating if you end up needing to talk to it, but not knowing the words to say.