Synthstrom Deluge: hold long-decay silent-sustain amplitude envelopes at the zone level

[douglas-carmichael]

Problem

Several source formats fake a slow, sustaining pad with a near-silent amplitude sustain plus a very long decay — e.g. a SoundFont 2 pad whose amplitude decays over tens of seconds — relying on that long decay to keep the note audible while the key is held.

The Deluge's decay/release rate table only reaches about 5.9 seconds, so when the converted envelope keeps the silent sustain but clamps the decay to what the Deluge can represent, the note collapses to near-silence within a couple of seconds. Converted pads then play far too quietly compared to the source.

Concretely: an E-mu Orbit patch such as JUNO PHASE or HIWEED (sustain ≈ -100 dB, decay ≈ 44 s) plays at full level in the original and in TX16Wx, but was almost inaudible after conversion to the Deluge.

Fix

In DelugeCreator, when the amplitude sustain is (near) silent and the decay is longer than the Deluge can represent, write the sustain as a hold at the level implied by the zone's own attenuation (10^(gain/20)) instead of the silent sustain. The note then sustains at the source's intended level rather than dropping out. Envelopes that already sustain at an audible level, or whose decay fits within the Deluge's range, are unchanged.

This is intentionally narrow — it only rewrites the sustain value for the silent-sustain + over-long-decay case; normal envelopes take the existing envelopeSustain path untouched.

Notes

• Scoped to the amplitude envelope only. The related filter-envelope carry-over is already handled by the filter cutoff key-tracking work merged upstream.
• Verified against the original patches and TX16Wx on hardware: the converted JUNO PHASE / HIWEED pads now play at the correct volume on the Deluge.
• CHANGELOG updated.