Mask Roms; Analog Signal Paths; D4 Audio Output; D4 Headphone Output - Alesis D4 Service Manual

Two bits are required for each sample read (6 Mask ROMs, 0-5). This, plus the fact that there
are sixteen voices available means that 32 bits total are required for each "sample output cycle".
Latches U14-U17 provide the storage area needed to hold all 32 "extra" address bits (which are
provided by the software via memory mapped I/O [see section 3.12]). Binary counters U19-U20 are
used to synchronize the decoding process. U13 is used to decode which latch the current voice's bit
pair will come from, while U18 is used to mask the bit pair itself from the other outputs of the current
latch. U30 does the actual decoding of the bit pair, providing the single Chip Enable needed to access
the desired sample. Note that 1/2 of U43 is used as a one shot multivibrator to extend the time that the
CLear line of the binary counters (U19-U20) is held low slightly longer than SNH0 is low. This is to
ensure that the 8031 has time to communicate necessary parameters to the ASIC before the decoder
starts it's processing.
Since the D5 uses much larger Mask ROMs, this type of decoding scheme is not necessary. The
D56IFT ASIC contains all the extra hardware needed to handle extra address lines not dealt with in the
DM3AG ASIC.

3.30 Mask ROMS

The Mask ROMs contain the all of the sample information. The 18 bit address buss allows for 2
megabytes per Mask ROM. Since Mask ROMs rarely fail, most problems in this area will consist of
opens or shorts along the data and address busses. Mask Roms in the D5 have a 20 bit address buss
(thus only 2 are needed).

4.00 Analog Signal Paths

4.10 D4 Audio Output

Output from the DAC is routed to the four individual output (Main and Aux, Left and Right) by an
analog switch (U39). Output selection is controlled directly by the DM3AG ASIC. Each output section
consists of an output sample capacitor (C96-C99), and a buffer/anti aliasing filter amplifier (U42,
associated resistors and capacitors).

4.11 D4 Headphone Output

The left and right headphone outputs are a sum of the main and aux outputs sent though a pair
of X10 inverting amplifiers. The output impedance is fixed by the sum of the op-amps output
impedance, and R141/R142 in parallel with C34/C35, at roughly 150Ω. This makes the headphone outs
more compatible with 600Ω pro headphones than the 8Ω consumer variety.

4.20 D5 Audio Output

The outputs of the '4319 DACs are buffered and low pass filtered by U2 (and surrounding
resistors and capacitors) before being routed to the outputs via A.C. coupling capacitors {C11, C3, C35,
C31}. Note that only the main outs run through the volume potentiometer just before being sent to the
outputs. Output impedances are fixed at roughly 300Ω. C1, C2, C29, and C30 provide R.F. filtering at
the outputs.
Alesis D4/D5 Drum Modules Service Manual 6 02/19/03