Amplification
Hearing aid amplification​
The baselines for both tasks use a common amplification process. This compensates for frequency-dependent hearing loss our listeners have using a set of dynamic range compressors. The same amplification is also applied to the reference used in the objective metric HAAQI.
The amplification is a simulation of hearing-aid non-linear amplification. It is applied separately to the left and right signals. Each extract is split into octave-spaced bands with centre frequencies of 0.25-8 kHz using cross-over filters. Frequency-dependent gains are applied to the filtered bands. Gains are based on an A-weighted 65 dB presentation level using [an implementation of] the CAMFIT nonlinear prescription [1]. Compression ratios were determined by the difference in prescribed outputs at 55 and 85 dB input (cf. IEC 60118, 2022) with an upper ratio limit of 6:1 imposed to avoid extreme values. Compression attack and release times (11-14 and 80-100 ms, respectively, across centre frequencies) were based on average values measured using IEC 60118 procedures in hearing aid devices [2]. The filtered, amplified and compressed stimuli were then summed.
The amplification in the baselines is part of the enhancement block, so you are free to replace it with a different approach. This will decrease HAAQI scores, however, because the reference signal for HAAQI uses this default amplification. A better amplification stage might score higher in listening tests, however. Note, we are allowing teams to submit a system optimised for listening tests and another optimised for objective evaluation.
Metadata​
For each listener (e.g. L001) the compressor parameter json gives the compression ratios (cr) and gains (gain) for each frequency range and left and right ears. Other parameters in the compressor are fixed. see Listener Metadata for details.
Levels and clipping​
The original mixes from the scene generator are set to a level of -40 LUFS. This is the level designed to work well with our non-linear amplification. Consequently, when inputting signals to the compressor part of the music enhancer make sure they're roughly at this level. If you use very different levels you will find the HAAQI score will be much lower.
It is also good practice to monitor whether clipping is happening due to the amplification. A small amount of clipping might be acceptable to allow sufficient amplification for those with hearing loss, too much and there will be audible distortion.
Note, if you are making a causal system, be careful your normalisation process does not use information from the future i.e. you can't calculate the rms for the whole extract of music because that involves samples in the future.
References​
[1] Moore, B.C.J., Glasberg, B.R. and Stone, M.A., 1999. Use of a loudness model for hearing aid fitting: III. A general method for deriving initial fittings for hearing aids with multi-channel compression. British Journal of Audiology, 33(4), pp.241-258.
[2] Whitmer, W.M. and Akeroyd, M.A., 2011. Level discrimination of speech sounds by hearing-impaired individuals with and without hearing amplification. Ear and hearing, 32(3), pp.391-398.