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I used to have a cassette tape/radio unit (boom box format) device.

I used to use this to record mix tapes from my dual turntable and mixer setup. The resulting tapes were - for me - ideal for playing in the car because

  1. The tape itself ran a tad slow which - when you played back on another deck speeded up the music by roughly the same amount as was used in the classic era of music radio in the USA - which I loved (yeah, I know philistine and all that).

  2. The AGC in the blaster input would almost infinitely expand sounds all the way down to rumble or surface noise if I let a record run out, but would also tame loud sounds nicely. In short it absoluteley murdered the dynamic range - but for car listening I found this was just excellent (again, for me, I know others will blench at this 8-) ).

So, to my question regarding SoX. I have found out that speed 1.02 gets me the speed up I am trying to reproduce. I also found a set of dynamics that reproduce the limit function of classic FM radio - but I also want to reproduce the infinite expander that I once had on the cassette deck.

I did manage to get close to it using Cool Edit 2000 compander, but that's long ago stopped working and the Audacity compander doesn't (IMO) do as well. I can't pretend to understand the way that Sox dynamics work, so I wondered if anyone could post be a command that might get me there? If it would help, I can post an example of a digitised mix tape from that era (file of about 80MB) on my site so you can hear the music junction dynamics I am trying to reproduce.

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This should be close to what you want:

The following example might be used to make a piece of music with both quiet and loud passages suitable for listening to in a noisy environment such as a moving vehicle:

sox asz.wav asz-car.wav compand 0.3,1 6:−100,−60,−20 −5 −90 0.2

The transfer function (‘6:−100,...’) says that very soft sounds (below −100dB) will remain unchanged, so you will still have "silence" between passages cranked right up. You can play with that -100 if needed.

From http://sox.sourceforge.net/sox.html

Sounds in the range −100dB to 0dB (maximum volume) will be boosted so that the 60dB dynamic range of the original music will be compressed 3-to-1 into a 20dB range, which is wide enough to enjoy the music but narrow enough to get around the road noise. The ‘6:’ selects 6dB soft-knee companding. The −5 (dB) output gain is needed to avoid clipping (the number is inexact, and was derived by experimentation). The −90 (dB) for the initial volume will work fine for a clip that starts with near silence, and the delay of 0.2 (seconds) has the effect of causing the compander to react a bit more quickly to sudden volume changes.

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