Download required project files

Download the required project files contained in CID_Decoder_Project_Files.zip; unpack the files to the same folder in which you plan to build your LabVIEW subVIs and top-level application VIs.

The .zip archive contains the following files:

Study the Caller ID signal audio recordings

Open the CallerID-1.wav audio recording in Audacity or an equivalent sound editor to view and listen to the signal. Use the zoom features to study the fine detail of the FSK signal, especially at the transitions between frequencies.

Repeat for the other .wav audio files included in the download distribution.

Are you able to discern any obvious differences among the various audio recordings?

SubVI construction

Possible approaches to analyze the signal generated by the telephone central office include: (1) process each sample as it arrives and generate the decoded message "on the fly," or (2) record the entire signal and then make repeated passes over the recording as needed to extract the message. Real-time implementation requires the former approach, while the latter "off-line" approach is easier to implement as a sequence of subVI calls, and is therefore the method of choice for this project.

Build the subVIs listed below. You may already have some of these available from previous projects.

Demonstrate that each of these subVIs works properly before continuing to the next part. The order in which you build the subVIs does not matter, however, the order presented roughly corresponds to the general processing flow that begins with the audio recording and ends with a collection of bytes.

Isolate and demodulate the FSK signal

Build a top-level VI that isolates and demodulates the FSK portion of the complete Caller ID audio signal. util_GetAudio to load the .wav audio file, and then pass this signal to cid_Demodulator.vi. Connect front-panel controls to the four parameter inputs of cid_Demodulator.vi. Create a mixed-signal waveform chart to display the four signals associated with cid_Demodulator.vi, namely: FSK signal (the original audio signal applied to the demodulator input), baseband signal (the demodulated output), phase error magnitude, and PLL locked.

The Figure 2 screencast video describes how to set up a LabVIEW "Mixed Signal Graph" to plot the waveform data type and Boolean 1-D array on a common timescale, much like an oscilloscope display with multiple analog and digital signals.

Figure 2: [video] Set up a "Mixed Signal Graph" to display both "analog" and "digital" signals on a common timescale

Set the demodulator VCO carrier frequency to the average value of the mark and space frequencies of the FSK signal.

Experiment with the remaining demodulator parameters VCO gain (start with values in the range 0.05 to 0.20), phase error LPF cutoff frequency, and comparator threshold for PLL lock to satisfy the following goals:

Report your four demodulator parameter values and plot the mixed signal graph that demonstrates the ability of your system to identify the time region over which the FSK signal is active.

Sample the baseband signal

Use regen_Sampler.vi to extract the FSK signal region from the baseband signal produced by cid_Demodulator.vi. Note that regen_Sampler.vi is used like a "gating" circuit here: the PLL locked signal is active over the entire time that the FSK signal is detected, therefore regen_Sampler.vi picks out every value from the original audio recording over which PLL locked is active.

Add the bit sync system to your VI using a zero-crossing comparator, util_EdgeDetector.vi (two instances), regen_BitClock.vi set to 1,200 Hz, and another instance of regen_Sampler.vi. Use the zero-crossing comparator to convert the FSK signal to a Boolean 1-D array. This signal is not a bitstream yet, but rather serves to identify the beginning of bit intervals. Use one edge detector to produce indicator pulses for each zero crossing of the baseband signal (both rising edges and falling edges). These indicator pulses serve as the synchronization for the bit clock oscillator, which produces a square wave synchronized to the baseband signal. The square wave transitions low-to-high at the beginning of the symbol interval and transitions high-to-low at the midpoint of the signal interval. Use an edge detector to produce indicator pulses at the midpoint of the symbol interval which control when the sampler should take samples from the isolated FSK signal.

Create another mixed signal waveform graph to display the isolated FSK signal, the thresholded version of this signal, the indicator pulses that synchronize the bit clock, the bit clock output, and "actual sampling instants" produced by the sampler.

Study your results to ensure that the demodulated baseband signal is sampled properly.

The Figure 3 screencast video shows how you can conserve front-panel real estate by placing two waveform graphs inside a tabbed control.

Figure 3: [video] Conserve front-panel real estate with a tabbed control

Decode the message bitstream

Use a zero-crossing comparator (specifically of the "less than zero" type) to convert the sampled baseband signal to a bitstream. Process this bitstream with cid_DetectStartBit.vi to extract only the data block portion of the bitstream.

Use util_BitstreamToText.vi to convert the bitstream to a sequence of 8-bit values ("string" data type), and then use cid_ParseMessage to convert the text string into an information cluster to be displayed with the custom front-panel control CID Display.

Include front panel indicators for the framing error and text out outputs from util_BitstreamToText.vi. The string indicator can be easily switched from ASCII display to hexadecimal display, as needed: right-click on the front panel indicator and choose either "Normal Display" or "Hex Display."

The framing error indicator should be dark for the entire data block region; some "left over" bits are likely after the data block ends due to the delay until the FSK demodulator's PLL locked output returns to F, and the resulting framing errors may be safely ignored.

Evaluate the completed system

Choose one of the Caller ID signal recordings, and run your VI to decode the FSK signal. When all goes well, the CID Display indicator will show "OK" for both the parsing process and the checksum calculation.

Confirm that the data block values are correctly extracted by manually decoding the hex values in the text string front panel indicator. More specifically, copy the hex values to a piece of paper, and work through the interpretation of each byte. For example, the first byte of the data block is 0x80, which indicates the Caller ID message is of the MDMF (Multiple Data Message Format) type. The next byte is an 8-bit unsigned integer that indicates message length; convert the hexadecimal value to decimal and report this value. Continue in this way to demonstrate that you understand the significance of each byte in the data block.

Try your Caller ID decoder on other signal recordings. Report the CID Display indicator values for each audio signal recording.