Direct Digital Synthesis (DDS) is an electronic method for digitally creating arbitrary waveforms and frequencies from a single, fixed source frequency. Waveform quite literally means the shape and form of a signal, such as a wave moving across the surface of water, or the vibration of a plucked string. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ...

Overview

A basic DDS circuit consists of an electronic controller, a random-access memory, a frequency reference (usually a crystal oscillator), a counter and a digital-to-analogue converter (DAC). Two operating steps are required to make the device work: we shall call these programming and running. Different types of RAM. From top to bottom: DIP, SIPP, SIMM 30 pin, SIMM 72 pin, DIMM, RIMM RAM redirects here. ... A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. ... In general, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal. ... In electronics, a digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analogue signal (current, voltage or charges). ...

Programming

In the programming step, the electronic controller fills the memory with data. Each item of data is a binary word representing the amplitude of the signal at an instant of time. The array of data in the memory then forms a table of amplitudes, with time implied by the position in the table. If, for example, the first half of the table were filled with zeroes and the second half with values of 100%, then the data would represent a square wave. Any other wave shape can be created simply by altering the data. In computing, word is a term for the natural unit of data used by a particular computer design. ...

Running

In the running step, the counter (properly called the phase accumulator) is instructed to advance by a certain increment on each pulse from the frequency reference. The output of the phase accumulator (the phase) is used to select each item in the data table in turn. Finally, the DAC converts this sequence of data to an analogue waveform.

To generate a periodic waveform, the circuit is set up so that one pass through the table takes a time equal to the period of the waveform. For example, if the reference frequency is 1 MHz, and the table contains 1000 entries, then a complete pass through the table with a phase increment of 1 will take 1000 / 1 MHz = 1 ms, so the frequency of the output waveform will be 1/(1 ms) = 1 kHz. Wiktionary has related dictionary definitions, such as: period Wiktionary has related dictionary definitions, such as: periodic Period and periodic may refer to: Period (music) Period (rhetoric) Historical period Menstrual cycle, relating to the reproductive system Full stop, also known as a period, that marks the end of a sentence Science...

This system can generate a higher output frequency simply by increasing the phase increment so that the counter runs through the table more quickly. In the example above, the phase increment is equal to 1, so the next possible frequency is obtained by setting the increment to 2, resulting in a doubling of output frequency. To obtain a finer control of frequency than this, the standard phase increment can be set to, say, 10. This then allows slightly higher or lower output frequencies. For example, increasing the increment to 11 would increase the output frequency by 10%, and reducing it to 9 would decrease the output frequency by the same proportion. The more precision required over the frequency, the more bits are needed in the counter and DAC.

Implementation details

Practical implementations usually set the size of the lookup table to be a power of 2 and work with 32-bit phase accumulators and phase increments. Usually the upper 8 or 10 bits of the counter are used as lookup table index (lookup table size is 256 or 1024, respectively). The remaining lower bits can be used as a parameter or index to interpolate between the adjacent entries in the lookup table. Often linear interpolation suffices. The source frequency usually comes from a crystal of 1 MHz to 100 MHz. This article is about interpolation in mathematics. ...

The highest frequency that can be generated this way depends on the size of the lookup table and the frequency. In order to generate a reasonable representation of the waveform, at least a minimum number of samples must be taken from it. If the phase increment becomes too large, then the counter would step through the lookup table too fast and the result may be a severe distortion of the output signal.

Implementations exist in both software and hardware. Due to the realtime nature of DDS, software implementations are usually limited to audio frequencies.

Applications of DDS are: function generators, mixers, modulators, and sound synthesizers. It has been suggested that this article or section be merged with signal generator. ... For the musical use of modulation, see modulation (music). ... Wavetable synthesis is used in digital musical instruments (synthesizers) to produce natural tone-like sounds. ...

References

Papers

• Tierney, J., Rader, C.M., and Gold, B. "A Digital Frequency Synthesizer," IEEE Transactions on Audio and Electroacoustics AU-19:1, March 1971, 48-56

Books

• Goldberg, Bar-Giora, Digital Techniques in Frequency Synthesis, New York: McGraw-Hill, 1996. ISBN 0-07-024166-X

External links

• Direct and Indirect Frequency Synthesis in the 0.5–20GHz Frequency Range (Slides from a talk)
• Direct Digital Synthesis Primer (Viewgraphs)
• Tutorial on Digital Signal Synthesis (Essay)
• Thesis on high speed DDS (French)