5.4.5 Replay equipment: Professional Cassette Machines Professional cassette replay machines are unavailable new. Also, the second hand market for professional cassette machines is not as strong as that for reel machines making it difficult to locate appropriate equipment. This represents a critical problem for sound archives, many of whose collections hold large numbers of recorded cassette tapes. Thus it should be a matter of priority for any collection with cassette tapes to seek out and acquire professional cassette replay machines. The characteristics that distinguish a professional machine from a domestic machine, apart from the replay specification, include solid mechanical construction, the ability to adjust replay characteristics and head azimuth, and the provision of balanced audio outputs. Many high quality audiophile machines provide some of the above characteristics. The characteristics of a suitable archival cassette replay machine include the following: Replay speeds 17/8 ips (4.76 cm/s) (note that speeds of 15/16 ips and 3 ? ips may also be required for replay of specially recorded cassettes). Variation from speed better than 0.3%.Wow and flutter weighted better than 0.1%. Replay frequency response of 30 Hz to 20 kHz +2, -3 dB. Ability to replay Type I, II, and IV cassettes (as required). Most cassette machines will automatically select the correct replay equalisation by reading the holes or notches on the top of the cassette housing or shell to determine the tape type. A few machines do not read the notches but have a switch that the operator uses to select the appropriate equalisation. Type III cassettes may be problematic as they are enclosed in shells identical to Type I cassettes, while requiring the same replay equalisation curve as Type II cassettes.Where no explicit option to replay Type III has been provided by the playback machine, it may be necessary to use a deck with adjustable equalisation or to rehouse the tape in a Type II shell (see Section Cassette Enclosures).