Original article by A.Stroiczek and DMV Verlag (CPC International 8/9'91)
converted for PC and into english by Tim Riemann (email: 
TCS-Software@t-online.de; please mail me if there are to much mistakes!!!)

CPC-Digiblaster
===============
The sound of the printer port

If we're looking at the 16bit computers like Amiga, Atari ST, Mac and PC
(with a soundcard) the CPC users can be jealous. We can help with an easy
circuit plan.

The CPC only can do rectangle frequencies. Amiga and other computers are
working with digital instruments which they give out with internal D/A con-
verters. If you look exactly the CPC has the possibility to give out digital
instruments. For this you need the volume register. This register can handle
16 different volumes. So it may be possible to play samples with 4bit. But
the soundchip dissapoints again.
To play a linear volumeincrease the power at the sound port is logarithmical.
This means that you have to change the values so that they are linear. So
you only have 3bit. The Amiga has a advantage with his 8bit samples. If you
say: "Okay, half bits are enough" you are wrong. With 3 bits you can use 8
different values, with 8 bits 256. That means you cannot play low sounds.
Okay, if you don't look at the very difficult programming the CPC is not as
good as his colleagues. But the meagre printer port allows you to change that.
To play 8bit samples you need a 8bit interface and a circuit which changes the
values in a power value. The printer port has 8bit and a 5V power supply (Bit
7 = STROBE).

The printer port makes it possible

To build a D/A unit you normally need a special IC which costs about 15 to 20
DM and it needs an own power supply. The circuit here costs about 3 to 12 DM
and does not need an own power supply. But because of this the output power
is not exactly linear.
You can connect the output of the circuit directly to an amplifier.

All good things are three

Of course, the D/A unit needs a small demo program. Because sampled instruments
are very long I only have printed the file 'DATA1.BAS'. Start it and hear a
little sound. Strangly it sounds much better when the upper rom is selected.

The programming

The program writes directly to the hardware of the CPC. You can use the port
with the adress &EFxx. Because the CPC has only a 7bit printer port (we need
8bit for the Digiblaster) we use the STROBE signal, which is inverted by the
hardware, so you have to invert your sound value.
Now we want to output the value &C5:

LD A,&C5     ;A=&C5
LD B,&EF     ;load the port adress
XOR &80      ;invert the 7th bit
OUT (C),A    ;out to the printer port

The accu is the register A.
The parallel port has the following connects:

PIN 1    /STROBE
PIN 2    D0
PIN 3    D1
PIN 4    D2
PIN 5    D3
PIN 6    D4
PIN 7    D5
PIN 8    D6
PIN 9    GND
PIN 11   BUSY
PIN 14   GND
PIN 16   GND 
to PIN 28 GND
PIN 33   GND

The pins that are not named are not connected.

Easy changing

How often you print out the values it is called the sampling rate. This value
shall be three times higher than the highest frequency.
It is possible to play some instruments at the same time. You have to add the
values and divide them. I thing that 4 channel sound is possible with the CPC.

A.Stroiczek

-----------------------------------------------------------------------------
Converted by Tim Riemann
There are mistakes in my translation? Please mail me:
****TCS-Software@t-online.de****
