Following on from Experimenting with a programmable RF Modulator (making a controller for it and testing harmonics), I have continued my work on this topic with these goals in mind:
- Make a neater VBIT-Pi assembly by integrating the modulator, VBIT-Pi hat and Raspberry Pi into a single unit
- Control a Sky IO Link
Raspberry Pi control
The Raspberry Pi already has I2C support, and it is fairly straightforward to use it from the command line and in a Python script. In my application, the Raspberry Pi is already being used with a VBIT-Pi hat, an open-source hardware teletext generator which adds teletext data to an external video signal, which has two ICs on it connected to the I2C bus. Fortunately, the modulator IC's I2C address is different to the two ICs, so it's simply a case of wiring up the I2C pins to the same pins used on the VBIT-Pi.
The i2cdetect command can be used to test that all the I2C devices are connected:
pi@raspberrypi:~ $ i2cdetect 1
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n]
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- 25 -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- 44 -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- 65 -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
The modulator can then be tested from the command line with a command like this: i2cset -y 1 0x65 0x80 0x10 0x25 0xF4 i (this will set the output to C21)
I modified the VBIT-Pi to remove the composite video output, since that was now wired into the modulator, and add an audio input for the modulator instead. I also added a micro USB port (wired to a GPIO pin via a resistive divider) with the intention of connecting that to the USB port on the set top box that will supply the video signal, so that the modulator is only active when the set top box is on.
I wrote the script to control it in Python. To keep things simple, I only implemented the maths for UHF channels, even though the modulator supports VHF channels.
When pressed into service, the Pi can be set up to run the script at startup. During testing, it can be started from the command line with python3 ./rf-mod.py and terminated with Ctrl+C. If you have no need for the external enable, then all the code concerning the GPIO interrupt can be removed.
# RF modulator example code
# This example controls an MC44BS373CA RF modulator
# from a Raspberry Pi. The GPIO set as an input
# can be used to enable and disable the RF modulator
# on demand, and if connected to the USB port on a
# set top box, will automatically turn the modulator
# on when the set top box is on.
import time
import smbus
# Set I2C bus number (may vary according to the Pi used)
bus = smbus.SMBus(0)
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
# GPIO 23 set up as input.
GPIO.setup(23, GPIO.IN)
device_address = 0x65
# Set desired channel here (UHF only)
desired_channel = 38
# Set to 1 to enable test pattern or 0 for normal behaviour
test_screen_enable = 0
desired_frequency = (desired_channel - 21) * 4 * 8 + 1885
desired_n = (desired_frequency << 2) & 0x3ffc
desired_n_h = (desired_n >> 8) | (test_screen_enable << 6)
desired_n_l = desired_n & 0xff
config_values = [0x80, 0x10, desired_n_h, desired_n_l]
def configureModulator(gpioChannel):
#gpioChannel only needed for setting up event handler; it doesn't do anything
try:
time.sleep(1)
bus.write_i2c_block_data(device_address, config_values[0], config_values[1:4])
print("RF Modulator configured: C", desired_channel, " System I")
except:
print("I2C Bus Error (Modulator probably not powered)")
#Configure now just in case modulator is already powered
configureModulator(0)
try:
GPIO.add_event_detect(23, GPIO.RISING, callback=configureModulator, bouncetime=2000)
except KeyboardInterrupt:
GPIO.cleanup() # clean up GPIO on CTRL+C exit
#Loop forever - should not be resource intensive
while True:
time.sleep(1000)
GPIO.cleanup() # clean up GPIO on normal exit
This works very well and reliably. The output of the modulator is fed into the house AV distribution system, making it available at every TV point in the house. In retrospect, it may be more useful to use the set top box signal to control a multiplexer which switches the video input between the set top box and Raspberry Pi's composite video out, instead of turning the modulator off. The Pi could then be used to display an "in vision" service or other video content when the set top box is turned off.
Sky IO Link
The Sky IO Link is an RF modulator in a small box which was introduced alongside later Sky+HD boxes where the built-in modulator was removed to reduce costs. This modulator is still made today (2022) by various manufacturers and sold at a low price (~£10), which makes it a great choice for any project requiring a modulator.
The modulator is similar to the ones that were previously built into the Sky boxes, but with an Abilis modulator IC rather than the Freescale one used previously, but the IC claims to be a drop-in replacement, so it should work with the code I've already developed.
The short cable is terminated with a 10-way mini DIN connector, and probing around suggests the following pinout: (wire colours on my unit shown)
The VBIT-Pi's open-source PCB layout could be modified to include the mating connector, which would allow neat and tidy integration with the Pi.
For testing, I wired the connector up to the Pi's GPIO header and connected the video input to the Pi's video output:
- 4 (Power supply) - 2 (5V)
- 5 (GND) - 9 (GND)
- 6 (SCL) - 5 (GPIO 1 / SCL 0)
- 7 (SDA) - 3 (GPIO 0 / SDA 0)
The SCL and SDA pins require pull-up resistors, and should be pulled up to 3.3V (not 5V as shown in my photo, but I got away with that bodge).
I ran my Python script from earlier (changing the SMBUS port to 0 for the original Pi in use here) and it worked perfectly.
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