FM/AM Broadcast Processor and Audio Maximizer for Edge Devices, specifically for Raspberry Pi.

96% single core utilization with the provided settings on the Raspberry Pi Zero W

For use with the RPI DAC+ module https://www.raspberrypi.com/products/dac-plus/

Initial Setup

• Download: git clone https://github.com/radiopushka/VostokRadioLite
• Configuration cd VostokRadioLite and then edit the Makefile change $(PI3_FLAGS) to $(PI_FLAGS) if using the PI zero or Zero 2W.
• Compilation: simply run make
• Installation: cp ./vtkradio /usr/bin or where ever you want to put it.
• Configuration: the sample_cfg file has all the configuration options available, pass it to vtkradio: vtkradio ./sample_cfg and it will show you the active configuration.
• Setup: if your pilot is not on 19khz sharp, you might have to overclock/underclock, trim the DAC+ clock via alsamixer, so that the two oscillators are in sync.

Configuration file guide

• The configuration file format expects no spaces between the key, the equals sign, and the value.
• Please follow the file format as specified, do not file bug reports if you are not sure the format is exactly correct

Audio Section

[audio]
iface=default
oface=default
discard_samples=0

• (discard_samples) this lets you adjust the sample rate if the input rate is too fast, you increase this value. Does not actually drop samples but instead performs FIR resampling.
• This section specifies what alsa interface to use for the playback (oface) and for recording (iface) for the real time processing

MPX Section

[MPX]
pilot_amp=0.15
stereo_ratio=0.3
negmod=-0.02
c1_mpx=1
c2_mpx=1
lookahead=10
release=0.001
antialias=1
a0=0.1
a1=0.8
a2=0.1

• This section contains the percentage of the pilot tone with respect to the signal(pilot_amp), the percentage of the 38khz modulation with respect to the signal(stereo_ratio).
• It also contains (negmod) which provides negative amplitude modulation of the pilot tone if the sound card or anything outside modulates the pilot tone, causing annoying ultrasonic artifacts in the audio. Set it to a negative value for positive modulation. This is a problem with the DAC+, the PI DAC+ contains an onboard headphone amplifier which draws current from the local power rail. Hence, it affects the amplitude of all outputs and causes negative pilot tone modulation. This parameter is to compensate for it.
• The (c1_mpx) and (c2_mpx) fields specify whether to use the right or left channel or both for MPX instead of mono audio.
• The (antialias) flag can be set to 0 to disable the alias filter that is applied after the FFT limiter. It is highly recommended to have it on(set to one).
• The (a0,a1,a2) define the convolution window that anti-aliasing uses. For now only a 3 point window is available.
• The (release) defines the speed at which the final limiter disengages
• The (lookahead) defines how many samples ahead of time the limiter starts acting

Limiter Section

[limiter]
pre_amp=0.7
lookahead=3
post_amp=1
limit=3.5e+9
save_harmonics=0.8
bins=9
basscut=220

• (pre_amp) specifies the pre-amp constant of the incoming audio before the agc(the agc comes before the complex limiter)
• (lookahead) specifies how many samples to process for each FFT coefficient transition inside the limiter(basically, lookahead like a regular limiter)
• (post_amp) specifies the pre-amp after the limiter before the MPX section
• (limit) specifies the maximum of the sum of amplitudes of all the frequency bins in the limiter.
• (save_harmonics) say if 1khz is really loud and everything else is quiet, setting this to a low value will bring the quiet part up and limit 1khz, setting to a high value will do the opposite. Be careful setting this to a low value.
• (bins) the number of bins to split the frequency range of 1 - 16 khz in to. only values 5,9,15,30,45 are accepted. higher values will give less of a "sound through the tube" effect but will require more processing power.
• (basscut) cuts off bass before the limiter (for transmitters that are not able to properly transmit bass)

AGC section

[agc]
attack=0.06
release=0.004
target=6.2e+9
noise_th=4e+6

Bass Boost section

[bassboost]
boost=0.4

• This controls how much bass to add. Do not add too much bass if your transmitter has attenuated bass; this will make the other parts of the audio sound distorted by the bass.

EQ section

[eq]
0=0.005
1=0.1
2=0.7
3=0.9
4=1
5=1
6=1
7=1.1
8=1.11

• For each bin specified in the limiter, you can set its relative presence in the audio.

Release section

[release]
0=0.0000001
1=0.00001
2=0.001
3=0.01

• The limiter has a special attack method, but setting the release values to low values can help with frequencies that sound too clipped.

System Requirements

• Raspberry PI Zero W V1.1 Minimum, but Raspberry PI Zero 2 W recommended for the ability to use scp and file transfer
• Alpine Linux for the PI Zero W 1 since we will use nearly 100% of all resources
• The DAC mentioned in the title
• An FM transmitter that has a flat frequency response from 0Hz to 57khz (you can get away with poor bass response, but not recommended).

Miscellaneous

Video

https://youtu.be/JOmtsO-myVg?si=u7Xn-yfBd7oQSrS0