D5 Headphone Output; Trigger Inputs; Analog-Digital Conversion; Test Procedures - Alesis D4 Service Manual

4.21 D5 Headphone Output

Headphone outputs are taken from the main out lines after the volume potentiometer. These
signal are passed through a gain stage (approximately X10) consisting of U5 and it's associated
circuitry. The signal is R.F. filtered {C42, C43} before passing on to the output jack. Note that supply
lines to U5 are taken from the raw +/-12V. This is to provide the maximum possible amount of
headroom to this output. A lower output impedance than the D4 provides for a little extra current when
using 8Ω "consumer" headphones.

4.30 Trigger Inputs

The trigger input circuitry is much simpler than it might first appear. Note that due to the fact that
the trigger inputs are the same, only "typical" designations appear on the D4 schematic. A chart of
actual part designations appears below the trigger input circuit on the schematic. The input from JT1 is
noise filtered by CT0 and CT1. The two sections of an LM339 act as a differential buffer amplifier,
providing isolation for the input signal. CT2 provides further filtering and smoothing. The signal is then
sent to the analog switches U22 {U11} (trigger ins 1-6) and U23 {U13} (trigger ins 7-12) where it is
multiplexed with the other trigger inputs before analog to digital conversion. Trigger input selection is
handled by the 8031 via memory mapped I/O (see section 3.12) latch U10 {U20} (TRG A0-TRG A4).
The D5s trigger input circuitry is virtually identical to that of the D4. Note that on the D5 schematics all
triggers are shown separately.

4.30 Analog-Digital Conversion

Successive approximation is an empirical approach to the process of analog to digital
conversion. The idea is to divide the process into short, manageable sections. Each significant binary
weight (starting with the Most Significant Bit) is taken in turn, thus requiring only 8 comparisons to
achieve a final value. U2 {U18} is used to buffer the 8031 output ports. These signal are summed
through the binary weighted resistor network consisting of R1-R5 and the resistor SIP R7 {R104-R114,
R137}. This signal is compared to the filtered and shaped trigger input signal. The result of this
comparison is sent back to the 8031. Once the 8031 has obtained a value for all 8 bits, it stores the
final result in memory for further processing, and continues with the next trigger input. Note that when
the 8031 examines the states of the trigger inputs, it doesn't use a specific amplitude to trigger on,
rather it looks for a large change in amplitude over a short period of time. This helps to prevent false
triggering from the other drums in the kit.

5.00 Test Procedures

The D4 contains a number of built in "hidden" routines to assist with troubleshooting and repair
of the D4. These routines include:
Self Test-Initiates internal diagnostics routines. (Power up while holding the "MIDI" and
"DRUMSET" buttons simultaneously. Use the "MIDI" button to advance to the next test.)
Reinitialization-Clears memory and resets software parameters to initial values. (Power up
while holding "OUTPUT" and "VOICE" simultaneously.)
Check Version-Shows current software version. (Select "DRUM SET" Mode. Press
"CURSOR RIGHT" and "CURSOR LEFT" simultaneously.)
Alesis D4/D5 Drum Modules Service Manual 7 02/19/03