The description in the official documentation:

Audio emphasis filter creates or restores material directly taken from LPs or emphased CDs with different filter curves. E.g. to store music on vinyl the signal has to be altered by a filter first to even out the disadvantages of this recording medium. Once the material is played back the inverse filter has to be applied to restore the distortion of the frequency response.

The filter accepts the following options:

  • level_in

    Set input gain.

  • level_out

    Set output gain.

  • mode

    Set filter mode. For restoring material use reproduction mode, otherwise use production mode. Default is reproduction mode.

  • type

    Set filter type. Selects medium. Can be one of the following:

    • col

      select Columbia.

    • emi

      select EMI.

    • bsi

      select BSI (78RPM).

    • riaa

      select RIAA.

    • cd

      select Compact Disc (CD).

    • 50fm

      select 50µs (FM).

    • 75fm

      select 75µs (FM).

    • 50kf

      select 50µs (FM-KF).

    • 75kf

      select 75µs (FM-KF).

[me@host: ~]$ # for restoring material:
[me@host: ~]$ ffmpeg -y -i input -filter_complex "aemphasis=mode=reproduction:type=cd" output
[me@host: ~]$ # otherwise:
[me@host: ~]$ ffmpeg -y -i input -filter_complex "aemphasis=mode=production:type=cd" output