🔬 Lab 4: Direct Digital Synthesis#

📌 Objectives#

  • The goal of this lab is to generate an audio waveform of a desired frequency with a high degree of accuracy.

📜 Synopsis#

You are to use Direct Digital Synthesis to reproduce your audio waveform. You are given a template which implements the Interpolation Block Diagram from the class lecture. Your task will be to create the lookup table, implement the equations for interpolation, and choose an appropriate phase increment to create the desired frequency.

💻 Procedure#

Requirements#

While you have the flexibility to design the waveform generator as you see fit, your system must meet the following requirements:

  • Use an update rate of 48kHz

  • Your function generator should be able to create a maximum frequency of 12000 Hz.

  • Your function generator should be able to create a minimum frequency between a 0.5 Hz and 1.0 Hz (this is also the minimum change in frequency).

  • The provided code uses a phase increment of Q8.8 to meet the above requirements.

  • When your phase increment X = 1.0, the output frequency should be 200 Hz +/- 20 Hz.

  • Produce a frequency which is equal to your birth month and birth day concatenated together. For example, if your birthday is July 6th, you should produce a frequency of 0706 = 706Hz. Note that your final frequency may be slightly off from this desired frequency. This is ok as long as your phase increment calculation is correct.

Hardware#

Provided Code: Lab04_cadet_code.zip

The provided VHDL code will allow you to reproduce 5 different frequencies, selectable by button press, via the audio codec’s line out interface. You can plug a speaker or headset into the line out jack and then use a spectrum analyzer to view the frequency of the output.

The code template breaks the design into the following components: Control Unit and Data Path.

Within the Data Path are:

  • ResultReg for storing the final result

  • LUT to hold the BRAM and return either the base or next value

  • Amplify to multiply the output for amplification

  • Interpolate to interpolate between the base and next value by the offset amount

  • IndexOffsetReg to hold the index.offset value and increment it by the phase increment

Each of the components above has its own test bench. Note that the overall Lab4_tb is not intended to be used at this time. For the other components, feel free to use the test bench to assist you in completing the lab.

TODO items#

To complete the lab you will need to make the following changes.

  • Complete the interpolate component by adding equations for delta x offset and base + delta x offset.

  • Create a LUT based on the requirements above.

  • Choose a phase increment to produce a frequency which is your birth month and day concatenated together. This site can be useful for converting to fixed point binary format.

🚚 Deliverables#

  • To be submitted via gradescope.

  • Submit a picture from a spectrum analyzer showing that you can produce the birth month/day frequency.

  • Answer the follow up questions.

Spectrum Analyzer#

You can use the spectrum analyzer here.