AVR Dragon provides on-chip debugging feature through debugWire interface.  The debugWire utilizes the ISP six-pin header to control the target device as shown in the following link:

http://www.atmel.no/webdoc/avrdragon/avrdragon.section.zrr_osd_lc.html

However, the target application does not work correctly with SPI feature during debug run.

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Typical analog noise generators make signal by amplifying AC component coming out from zener current of transistors reversely biased between emitter and base (here’s an example circuit).

Any bipolar transistor would work as such a noise source, but noise quality in listening is different from part numbers.  2SC828A is well known as good noise source, but it’s been obsolete for long.  So for Analog2.0, I have been recommending 2SC3311 instead.  But this part becomes obsolete as well.  Now I have to find another one.

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As a part of my current experiment, I’m running following piece of code:

   if (digitalRead(pinDeviceReadReady) == HIGH) {
    Serial.println("DATA READY");
    spiSend(0x20); // command "read request"
    readBuffer.deviceId = spiReceive();
    Serial.println(readBuffer.deviceId);
    readBuffer.wireId = spiReceive();
    Serial.println(readBuffer.wireId);
    readBuffer.length = spiReceive();
    Serial.println(readBuffer.length);
    for (int i = 0; i <= readBuffer.length; ++i) {
       readBuffer.data[i] = spiReceive();
       Serial.println(readBuffer.data[i]);
    }
  }

where

inline void spiSend(uint8_t data)
{
  // wait for device ready to write
  while (digitalRead(pinDeviceWriteReady) == LOW);
  SPI.transfer(data);
}

inline uint8_t spiReceive()
{
  while (digitalRead(pinDeviceReadReady) == LOW);
  uint8_t data = SPI.transfer(0);
  return data;
}

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I’m going to try making a digital communication bus for synthesizer modules.

Analog synth has a very simple control language which is voltage.  Any message is translated to voltage that can be read by any modules that accept voltage input. So for example, VCO output is basically audio output but also can be used as control voltage of some other modules, such as VCO cross modulation.

This simple data exchange methodology makes analog synthesizer very versatile and flexible.  However, as a drawback, patch wiring would become too complicated as you make complex module network.

One solution for making the wiring simple is to use a single common data bass where all modules are connected, and exchange data selectively using some software.  Apparently, making such a bass for analog signals is impossible or extremely difficult. So I’m going to try making it using a digital bass.

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I did basic study of voltage controlled envelope generator based on analog circuit approach.  However, I found the implementation quite complex, although its quality was good.  So, I next tried another approach that has simpler circuit.  This version calculates the EG curve using micro processor, and makes analog output using PWM with minimum external filtering circuit.

This version is inspired by Tom Wiltshire‘s Voltage Controlled ADSR Envelope Generator.

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I’ve started studying voltage controlled envelope generator designs, since I have several use cases of it.

An envelope generator usually is implemented by an RC charging circuit with potentiometers as resistors.  However, such design does not capable of quick parameters change.  So voltage control (or digital control) functionality is necessary for better articulation.  Also, non-potentiometer control is crucial to polyphonic voices.

There are several approaches to design voltage controlled envelope generators.  This article describes about an analog approach which I tried first.

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You need to use well matching transistor pairs to make a good quality transistor ladder VCF.

Above is the classic methodology to find matching transistor pairs.  I tried this procedure with 2SC1815 and 2N3904 I have.  Next version of Analog2.0 boards will support the both types of transistors, so I wanted to know how different they are in VCF perspective.

Here is the result:

The result shows that 2N3904 has less variability.

This comparison might not be fair since my 2SC1815 transistors are very cheap ones purchased from Akizuki-Denshi, so these might have different quality control in manufacturing than usual retail transistors.  But at least I learned following:

• 2N3904 is good enough to be used for VCF (Naturally.  Actually, 2N3904 is more common than 2SC1815 to be used for VCF).
• It’s always better to check matching of transistors when you make a VCF.