Alesis QS6, Руководство по обслуживанию

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Alesis QS Series Keyboards Service Manual V1.00 11/19/06
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For further information about MIDI, consult MIDI SPEC available from the International
MIDI Association.
2.31E PC Serial Connector
While technically a part of standard H8 I/O, Computer Serial I/O is complex enough to
warrant it’s own section (See Section 2.50).
2.32 The SG ASIC
In order for the unit to play a sound sample, the H8
µ
P (See Section 2.31B) must
receive a command via the keyboard (via the Keyscan ASIC) or through MIDI. The
Microprocessor then tells the SG ASIC three things:
1. The note value (e.g., C3, D#4, A6, ...).
2. The velocity of the note value.
3. The type of sound to be played (dependent on PROGRAM, MIX, MIDI Channel, etc.).
The SG ASIC then retrieves the correct 16-bit sound information from the SOUND ROM
according to the instructions sent by the H8. In addition, the SG ASIC performs the duties
necessary to create the envelope of the voice (e.g., Attack, Decay, Sustain and Release) as
well as other audio functions such as changing filters.
SG ASIC instructions are passed to it by the H8
µ
P through memory mapped I/O (See
Section 2.31B).
There is one clock signal for the SG ASIC. It is received at pin 19. The clock is
generated from crystal M2 via U18A. The clock signal is also frequency halved , and output via
pin 12. This 12.288MHz clock is used in the QS6 by the 74HC390 (U17) {to derive other
system clock signals (1MHz, 614.4MHz)}. System signals in the QS7, QS8 and QSR are
derived slightly differently providing for better software control (See Section 2.54). Note also
that the QS7, QS8 and QSR all utilize a digital VCO (U11 U7) to provide extremely stable
clocks to the sound generation system.
The SG ASIC can send sound information to the FX ASIC by two possible ways. The
first is the 8 bit data buss. The second is via optical information from EOPTOUT (pin 3), to the
FX ASIC. Currently, the optical buss is used for all communication.
2.32A Sound Generation
Sound is generated with the use of Sample ROM playback. The playback is
accomplished by the SG ASIC while the sounds themselves are contained either in the MASK
ROMs (2 Megabytes each) or in a PCMCIA CARD attached to the system. As far as sound
generation is concerned, the only distinguishable difference between the two is their relative
location in the SG ASIC’s memory map, and as such will be referred to simply as Sound ROM.
Besides actual sonic data, Sound ROM also contains data about the samples Start, End, and
Loop points, start and end volumes, sample rate, and data smoothing coefficients.
When the H8 receives a command to play a sample (keyboard, MIDI, or PC Serial) it
notifies the SG ASIC what note value and velocity have been requested. The SG ASIC then
retrieves data for 2 samples. The first sample is the requested sample. The second is the
sample at the next higher address. This is necessary because current technology does not
allow for a sample to be taken at possible note value (the amount of necessary memory jumps
prohibitively when this is attempted). Instead, a number of “key” samples are provided and the
SG ASIC’s DSP is called upon to interpolate for the correct note pitch value. The notes that
each individual sample is responsible for is called a “Keygroup”. Note that the pitch
interpolation also takes into account any variations in pitch that may occur due to modulation
(i.e. pitch wheel or LFO Modulation).