eye hard-coded to 4-speaker setup

[cwbishop]

Need to make this more flexible, but need to be on miller PC to test or Wu's PC to test.

In SIN_TestSetup, see line
opts.player.noise_mixer = fillPlaybackMixer(opts.player.playback.device, db2amp(-8 - db_correction_4speakers).*eye(4,4), 0); % Reduce noise output by -8 dB to create +8 dB SNR.

eye(4,4) is hard-coded to work with 4 data channels (from wav files) mapped to 4 output channels (sound card output channels).

Needs to be more flexible