Creating a radio effect using a convolution filter

Creating the effect

Creating the convolution impulse response

UH720SX-NB

UH720SX-NB

The first step in creating the effect is to reverse engineer a 2-way radio signal path. The radios used were a pair of small handheld FM CB radios by Uniden. Sequoia contains an “impulse response extractor” tool. In order to use this tool, the first step is to generate the “reference sweep” signal. Once generated, the reference signal needs to be passed through the signal path and then the resulting signal recorded.

Impulse Response Extractor tool

Impulse Response Extractor tool

This is the the impulse response extractor tool where we generate the “reference signal” or in Sequoia terminology, the “Chirp”. The chirp is then laid into a DAW track for playback.

Seqouia Impulse Response Reference Signal ("Chirp")

Seqouia Impulse Response Reference Signal (“Chirp”)

Reference signal: [audio:http://104.198.4.5/wp-content/uploads/2013/01/Reference1.mp3|titles=Reference Signal]

Once we have the chirp playing back at a suitable level, we need to play this signal through our target signal path and record the result. To achieve this, we’ll play-back the chirp signal through a speaker and transmit the acoustic audio through one of the pair of handheld walkie-talkies to the other situated in a quiet room. A Schoeps MK41 and a Sound Devices 552 has been set up in the other room to record the signal and relay an AES/EBU feed back to the DAW where it can be recorded in a separate track alongside the reference signal.

Processed Chirp signal

Processed Chirp signal

Processed signal: [audio:http://104.198.4.5/wp-content/uploads/2013/01/Processed1.mp3|titles=Processed Signal]

Closer analysis of the resulting signal file identifies some artifacts that will need to be added to the signal manually, for instance there is a significant and audible noise floor to be considered and additionally, there is a “cutoff-noise” aspect to the signal which occurs when the transmission stops and the receiver “Squelch” takes a small period of time to cut in. As long as both of these elements can be captured, they can be added to the final sound for additional authenticity.

In order to be able to process the impulse response, the recorded signal file must be trimmed to the same size as the original reference file.

As Sequoia is a fairly full-featured DAW, it can also be used to extract some more useful information from this signal. For instance, using the “Spectral Cleaning” tool and some carefully positioned markers, it is possible to determine that the target signal chain is only able to usefully transmit frequencies between approximately 368Hz and 4.2kHz

Inspecting the processed signal, we see that we have a signal envelope that only encompasses part of the sweep signal. By placing markers at the start and end of this envelope, we are able to inspect the frequency components of the sweep signal and determine what frequencies were being transmitted when the receiver began to respond. This information might be useful if we want to band-limit the input signal for any reason.

Trimmed Signal

Trimmed Signal with Frequency Analysis markers

Frequency Range Analysis

Frequency Range Analysis

Now that we have our original “chirp” and our “processed chirp” we can go back to the impulse response extractor tool and let it work its magic…

There are a couple of parameters that we need to configure before generating the impulse response. Firstly, we need to set a reasonable decay cutoff threshold. Sequoia allows a maximum threshold of -50dBFS.

IRE

 

We set up the “original chirp” object name and the “Reverbed chirp” object name, set our Decay cutoff threshold and check the “Normalise Impulse Response” and “Adapt Length to Power of 2” checkboxes and then push the “Generate” button, creating a New Wave Project.

The resulting Impulse Response looks as follows:

Impulse Response

Impulse Response

Resulting Impulse Response: [audio: http://104.198.4.5/wp-content/uploads/2012/12/IR_unidenFM01.mp3|titles=Impulse Response]

Spread the word. Share this post!