Yamaha S90 ES Owner's Manual page 115

Single timbre tone generator (Voice/Performance mode) and multi-timbral tone
generator (Multi mode)
The internal tone generator block operates in two different ways (single timbre and multi-timbral) depending on the selected
mode. The difference between the two is whether they can handle multiple MIDI channels at the same time or not.
Single Timbre Tone Generator (Voice/Performance mode)
A "single timbre" tone generator receives data over a single MIDI channel and plays a single instrument Part.
This is the operating status of the internal tone generator in the Voice and Performance modes.
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To set the MIDI receive channel for single timbre operation (Voice and Performance modes), use the following operation in the Utility mode.
[UTILITY] [F5] MIDI [SF1] CH
Multi-timbral Tone Generator (Multi mode)
A "multi-timbral" tone generator simultaneously receives data over multiple MIDI channels and plays multiple instrument
Parts. This allows playback of multichannel MIDI song data—such as on a MIDI sequencer or computer—with each internal
Part being assigned to and played by a different track or channel. This is the operating status of the internal tone generator
in the Multi mode.
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To set the MIDI receive channel for multi-timbre operation (Multi mode), use the following operation in the Multi mode.
[MULTI/SEQ PLAY] (select Multi mode)
Maximum Polyphony
Maximum polyphony refers to the highest number of notes that can be sounded simultaneously from the internal tone generator of the
instrument. The maximum polyphony of this synthesizer is 128. When the internal tone generator block receives a number of notes
exceeding the maximum polyphony, previously played notes are cut off. Keep in mind this may be especially noticeable with Voices
that have a long decay or sustain.
Furthermore, the maximum polyphony applies to the number of Voice Elements used, not the number of Voices. When the Normal
Voices that include up to four Elements are used, the maximum number of simultaneous notes may be less than 128.
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When a Plug-in Board is installed, playing Plug-in Board Voices does not affect the maximum polyphony of the S90 ES. For details on maximum
polyphony for the Plug-in Boards, refer to the owner's manual of the particular board.
BasicRcvCh
[EDIT] Part selection [F1] VOICE
Internal Structure (System Overview)
[SF2] MODE ReceiveCh
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Owner's Manual