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\\ oO // Did you know that... by mike/zeroteam / \__/
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\/ \__/You must have noticed that sampled music (not only) in the MQM 5 demo has the option of playback on a D/A. But do you know what it is?
In the former Czechoslovakia was popular parallel interface based on a chip i8255. This is also due to the fact that no other chip was available. The 8255 has three gates A, B, C and each can be set as input or output. In addition, gate C can be divided into 4-bit halves and each half can be set to input or output separately.
The first interface that appeared was UR-4 (univerzalni rozhrani = universal interface) from Tesla Kolin. The interface contained a reset button, kempston compatible joystick with atari pinout and FRB connector with 8255 I/O gateways. The addressing of the interface was chosen as: gateway A on port 31, B on port 63, C on port 95 and CWR at 127. This configuration has become the de facto standard for connecting peripherals.
The UR-4, however, dates from the pre-Didaktik era and was intended mainly for the original ZX Spectrum computer. Didaktik computers became the most common ZX Spectrum-compatible computers in Czechoslovakia. Let's take a look at the individual models and their D/A connection options.
Didaktik Gama:
Gama is actually ZXS Issue 3 with added 8255 interface. Since the socialist economy couldn't produce defective memory chips (cheap joke, I know), Gamas were fitted with pieces with both halves of the memory working. They took advantage of this in Didaktik and they connected the memory bank selection to bit 0 of the C port. This increased the size of the available memory to 80kB, but also made it impossible to use the three-channel D/A. But there is a solution, which we will describe later.
Didaktik M:
Basically, Didaktik M has only a simple interface for the Kempston joystick. Because of this Didaktik produced an "Interface M/P" that included an 8255 chip and ROM with drivers for Printers.
Didaktik Kompakt:
The Kompakt is the combination of the Didaktik M and the D80 floppy disk drive in a compact body. 8255 controller is present.
Didaktik 40/80:
The D40/D80 floppy disk drives also included a parallel interface. Apparently because of the Gama, it was possible to block the built-in 8255 circuitry from interfering with the 8255 in the computer. However, the circuit could be unblocked by software and thus also on the Didaktik Gama was able to use the D/A (with a bit of black magic, first unblock 8255 in the D40, switch the ports to output, lock the D40, switch the internal 8255 in the Gama to the input ports and thus stop responding to the transmitted data, and again unblock D40).
As you can see, owning a computer with a parallel interface was not a problem. Interestingly, all the wiring was pin-nearly compatible. Didactics added two inventors on the unconnected UR-4 pins, and Didaktik Gama added one more on the PC0 and +5V pins.
But why own a DAC? The reason is the Sample Tracker from CBM.
Although the most common use of the tracker is with AY chip, the program also supports output to Beep and 4- and 8-bit D/As. Because of the linear volume progression, the 4-bit D/A sounds better than the AY chip. That wouldn't be the main reason for building the converter. It is until you use the 8-bit samples, the quality of the sound is really somewhere else. The D/A also includes a switch between 4bit and 8bit modes. This is because of the loudness output when 4bit samples are OUTed only on the lower 4 bits. The schematic can be found in the manual at e.g. spectrumcomputing or WOS (look for Sample Tracker 3T). And one last piece of information, the stereo is of course wired as ACB :-)
If you would like to listen to music in D/A quality and you don't want to dig Didaktik or UR-4, I'm planning to make a zone of D/A music on the next Forever party.
mike/zeroteam
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