5.4.10 Replay Equalisation

5.4.10.1  The signal representation in most analogue audio formats is deliberately not linear in terms of frequency response. Correct replay, therefore, requires appropriate equalisation of the frequency response.

5.4.10.2  The most common of the equalisation standards for audio replay of analogue tape are as set out below (Table 1 Section 5.4). It should be noted that equalisations have developed over time. The current standards are given in bold type, together with their date of introduction. Earlier recordings must be replayed by applying the respective historical standards and simple additional circuits may be utilised. The overlapping of old and new standards should be taken into account when decisions are to be made for tapes recorded in times of transition. Prior to that there were a number of manufacturers’ standards.

Table 1 Section 5.4 Common Equalisation Standards for Audio Replay of Analogue Tape⁴

5.4.10.3  At 15 ips and 7.5 ips there is a choice in replay equalisation for reel tapes even for tapes which were recently recorded according to the current standards. However, these are the two most common recording speeds, and care must be taken when choosing a replay equalisation to ensure that it corresponds with the record equalisation. Apart from the standards mentioned in table 1 section 5.4 there are a small number of more current standards intended to achieve better performance but which are different from the commonly accepted standards. At 15 ips Nagra tape recorders have the option to use a special equalisation called NagraMaster. The US version of NagraMaster had time constants ∞ and 13.5 µs, the European version of NagraMaster had time constants 8 and 13µs. Ampex used “Ampex Master Equalization” (AME), also at 15 ips but officially only on particular 1/2 inch mastering recorders introduced in 1958 and sold for several years following (MRL 2001). Logging machines and some popular semi-professional portable equipment were able to record at the very slow speed of ¹⁵⁄₁₆ ips (2.38 cm/s). However, it appears that there is no agreed exchange standard for these tapes and any equalisation would have adhered to proprietary conventions.

5.4.10.4  Sometimes any lack of documentation may require the operator to make replay equalisation decisions aurally. Cassette replay equalisation corresponds to the tape type, and care must be taken to ensure that the correct replay equalisation is used. Many tape recordings, specifically private recordings and those of cultural or research institutions that lacked technical support, have been made on un-aligned tape recorders. Unless there is objective evidence that would allow alternate settings, with regard to equalisation, tapes must be treated as properly aligned.

4. Note, IEC refers to IEC Pub 60094-1 4th edition, 1981, NAB to the NAB reel to reel standard 1965 (IEC2), or cassette standard 1973, DIN refers to DIN 45 513-3 or 45 513-4 and AES to AES-1971, and BS to the British Standard BS 1568). Thanks to Friedrich Engel, Richard L. Hess and Jay McKnight for generously supplying information on tape equalisation.