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2x CX Cards (RF Capture) // H/W Left & Right audio on a secondary bracket for baseband capture.

Software Install / Buy Hardware Kits & Parts / Build your own Hardware

This doc is the official vhs-decode standardised workflow implementation of Rene Wolfs "cxadc-clock-generator-audio-adc" or just "clockgen mod" setup wiki which is flushed out with more theory.

This doc focuses on the basics of what goes whare and getting setup to make captures, and use them with the decode workflow.

To avoid any worries of permissions of configuration we want to set the system to use the same config on every re-boot, to do this we use the udev rules

File System --> etc --> udev --> rules.d

/etc/udev/rules/

Inside this folder using admin privileges made place a cxadc.rules file with your desired configuration.

This example config should be ample for most users current setups.

Clockgen Capture --> FM RF & Audio Files --> HiFi-Decode (if present) --> Auto Audio Align (Linear & or HiFi) --> vhs-decode

• Namazso's CXADC Web Server
• Current "local" capture server script

./local-capture.sh --add-date --video=0 --hifi=1 --compress-video --convert-linear --resample-hifi --compress-hifi VHS_PAL_SP_Tape_001

sudo apt install jq curl

sudo apt update && sudo apt install -y build-essential cmake libogg-dev

Install FLAC 1.5.0

wget https://github.com/xiph/flac/releases/download/1.5.0/flac-1.5.0.tar.xz
tar -xf flac-1.5.0.tar.xz
cd flac-1.5.0
mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Release
make -j$(nproc)
sudo make install
sudo ldconfig

This web server capture workflow can be done locally on a single computer.

However being a "server" also allows for captures to networked storage pools, but has more reliable buffering than the direct script for lower end systems.

./local-capture.sh --add-date --video=0 --hifi=1 --compress-video --compress-hifi --resample-hifi --convert-linear tape_capture_name

This will output a 4 file set ready for use with the decode workflow.

• filename-2025-11-21_12_09_49_00_00-headswitch.u8
• filename-2025-11-21_12_09_49_00_00-video.flac
• filename-2025-11-21_12_09_49_00_00-hifi.flac
• filename-2025-11-21_12_09_49_00_00-linear.flac

• Direct capture creates 2 40msps files
• Resampling uses SoX on the HiFi RF to make a 10msps 8-bit file

If your system is slow and or very low-end direct capture is the safest option, if you're noticing time differences between decoded HiFi and baseband captures of HiFi for example.

• Direct Capture Script (Without HiFi Down-sampling)
• Capture Script (With HiFi Down-sampling)
• Collect Info Script
• SoX Benchmark Script

Synchronised capture is all scripted into a single .sh file to run it simply download it and define the output path

Example:

./cxadc-vhs-sync-capture.sh /home/harry/Desktop

The Resulting Output:

RF Output is:

xxxx.xx.xx-xx.xx.xx-rf-audio-10msps.u8

xxxx.xx.xx-xx.xx.xx-rf-video-40msps.u8

Audio is 24-bit 3 channel 46875sps (46khz) file the mapping is:

Left/Right/Headswitch

You will want to map this to 2 separate audio tracks for muxing into a final video file.

The file set is automatically date stamped yyyy.hh.dd-hh.mm.ss

For the next step to be easy and fluid captures over 1 hour long, using the bash script method typically end up being split into multiple .wav files by the encoder for the baseband audio capture.

You can consolidate these together in an audio editor or DAW such as Ocen Audio, Audacity, Adobe Audition, iZotope RX with relative ease or you can use FFmpeg and or SoX.

SoX:

sox input-1.wav input-2.wav combined.wav

FFmpeg:

ffmpeg -i input1.wav -i input2.wav -i input3.wav -i input4.wav -filter_complex '[0:0][1:0][2:0][3:0] concat=n=4:v=0:a=1[out]' -map '[out]' output.wav

After decoding your Video RF and or HiFi FM audio you will want to sync your decoded audio data with your video file or Baseband (normally Linear audio) captures made by audio ADC's running off the same clock source like the Clockgen Mod or MISRC.

There is a few ways to do this, as it is only a simple process of reading the post decoded .JSON file for total duration and offsets from dropped fields/frames of information, but the standard workflow currently uses Rene Wolfs implementation for Windows, Linux and MacOS, however this may change to a more universal python replacement later as Mono is considered an extra dependency.

• Download The Latest Binary (Windows Native)

• Install Mono (For Linux / MacOS usage)

• Simple Usage Automation Script

With a standard configuration you just need to alter the following:

• -i baseband.wav

Input PCM or FLAC audio file.

• --json capture.tbc.json

Input TBC JSON from decoded video RF.

• baseband_aligned.flac

Output name at the end (also s16 will do 16-bit if s24 is replaced)

• --stream-sample-rate-hz

Input rate of your audio file in Hz i.e 46875 for clockgen baseband or 48000 for hifi-decode or 44100 for ld-decode.

• --rf-video-sample-rate-hz

Capture rate of your base file this normally will be 40000000 (40Msps) or 20000000 (20Msps) & 16000000 (16Msps) for down-sampled captures.

Ensure CXADC is installed and working with your 1-2 CX Cards.

cxvalues

If 2 sets of cards values show up, then both cards are good and ready to go for modification, likewise after modifying cards use this to check on every re-boot that both show up.

Install alsamixer

sudo apt install asmixer 

Run alsamixer with:

alsamixer

Sample rate modes are the following:

28.63msps, 20msps, 40msps, 50msps

To open the terminal control window use:

alsamixer 

Press F6 to change devices.

Navigate with ↑ & ↓ keys.

Hit Enter with CXADC+ADC+ClockGen selected.

Set both cards to 40MHz mode.

Press Space to enable audio control.

That is about it, you only play with these settings when doing manual overrides for stock settings.

Fix Capture Script Permissions

chmod +x cxadc-sync-capture.sh

Test Firmware / Clock Gen Firmware / Reset Firmware

1. Ensure Pi Pico is not plugged into the mainboard of the clockgen.

2. Equip a USB-C or Micro-B cable depending on your Pi Pico Type.

3. The Pi Pico displays as a USB storage device if working.

4. Simply drag and drop the test firmware onto the file system, and it will automatically flash

After a successful flashing, the Pi Pico will show up as a microphone 3ch 48khz device to any systems sound manager application.

As the clockgen mod is powered and driven by the Pi Pico, it requires both power and data from a Micro-B or USB-C cable (depending on Pico board used)

• Internal USB 2.0 to Micro-B or USB-C

Normally PC's and Servers have always a single or 2-3 internal USB 2.0 headers for basic devices.

As the audio ADC is unbalanced, and the clock gen board is sensitive to EMI due to high frequency clock signals being generated it's recommended to shield the setup.

You can print a PCI bracket, or you can drill out metal brackets.

Copper Tape + 3D Printed Case

Check USB Devices with

lsusb

Check PCI & PCie Devices with

lspci

Test

arecord -D hw:CARD=CXADCADCClockGe -c 3 -r 46875 -f S24_3LE --samples=1000 test.wav

Result

arecord: main:831: audio open error: Device or resource busy

Solution:

Restart, try again

• PC Wont boot

This can be 2 key issues, bad SMA cable, or bad jig.

The Jig boards for the CX Cards are very sensitive, if a board is damaged or component then the board will not work.

Solution:

Reflow parts, if fail use another board, or replace all SMD components on PCB Jig.

• There is a full debugging section here

Pre-made PCB boards off the Kofi Store.

Direct PCB Fab - Here's the Gerber's

I like PCBway but you can use JLCPCB or any domestic fab house or make them at home even have fun!

Is it worth using a PCB fab to populate parts?

No not really doing it yourself is very easy and the price of having 2 boards populated, you can buy the tools and learn a invaluable skill of basic 0805 SMD soldering which is the most common value or size of SMD parts used today, or if would like to support the work and save some time then you can order full kits pre-made.

Cables:

• 2x 50Ohm SMA to SMA 30-50cm Cable
• 1x 50Ohm SMA to BNC bulkhead 30-50cm.
• 2x BNC to BNC 50Ohm cables (Video RF + HiFi RF)
• 1x RCA or XLR to RCA cable (Linear or HiFi Ref)

Tools:

It's worth reading the Hardware Installation Guide as this fully covers what tools are recommended to build boards and do RF Tappping!

• 3rd Arm Clip Holder or Vice Clamp
• Soldering Iron / Bevel Type Tip
• 63/37 0.8mm - Mechanic SX-862 or better (i.g Siliverline or Kestral 60/40)
• Side Cutters
• 99.9% IPA
• Desoldering Gun or ENGINEER SS-02 Solder Sucker
• Desoldering braid

Parts:

• C - Capacitor
• D - Diode
• R - Resistor

Part Sizes

• SMA's are edge mount style connectors.
• SMD SOD-123 - Diodes
• 0805 SMD - Resistor & Capacitor

The top PCIe card is card 0 and bottom is card 1 for video and hifi respectively shortest distance to CPU will always define card 0, with the modded bracket providing secure bulkheads for standard RCA cables and a BNC to SMA bulkhead for head-switching & sync.

Front & Back

L 40mm X W 40mm X H 11mm heatsinks are readily available.

AliExpress Black / Silver

Amazon UK Silver / Black

• J30: SMA Edge Mount
• C1: 1.5 nF (6V or greater, ceramic X5R or X7R)
• C2: 10 nF (6V or greater, ceramic X5R or X7R)
• R1: 1K Ohm (5% tolerance or better)
• R2: 1K Ohm (5% tolerance or better)
• R3: 10K Ohm (5% tolerance or better)
• 2mm thick 25mm wide double-sided foam tape.
• 26AGW wire in 4 colours Link

To solder wires to C85 add some flux and add extra solder to the pads before hand and ensure the wires are tinned but long enough to pinch and pull into the side of the capacitor, doing top/bottom separately as not to make a cold joint or knock the capacitor off.

Cut any exsess wire off with a pair of flush or flat cutters, but do not press them into the PCB as you can scratch it and damage traces.

Gen 2 PCB using SMA for Headswitch/Sync input and the 2 clock output pads for CX Card 1 & 2, compatible with the original 3D printed case.

There is 3 off shelf boards used for this:

AliExpress is most likely the cheapest place to source these but are very available on Amazon and other vendors globally.

• Raspberry Pi Pico as the interface and control board via USB (Clockgen PCB is based around the 1st party green PCB but you can use Micro-USB or USB-C port versions)

• Adafruit Si5351A for generating the shared clock source with 5 potential inputs.

• PCM1802 PCB ADC for L/R input baseband Linear or HiFi audio capture.

Parts:

• SMA Edge Mount or Vertical (On Headswitch/Ground Pads)
• C1: 10 µF
• R1: 1K Ohm
• D1: 3.3V zener diode
• D2: Generic switching diode

Sockets:

Header Socket Pin Size: 2.54mm

• U1: 2x 20-pin Header Socket (Pi Pico)
• U2: 1x 9-pin Header Socket (PCM1802)
• U3: 1x 7-pin Header Socket (Si5351A)

Board Parts:

• 1x 1m XLR Audio Cable (Left/Right & Ground via shielding)
• 1x Pair of RCA or TRS audio connectors.
• 1x Small Zip-Tie or Cable-Tie (to secure audio line)

Interface:

• USB 2.0 Internal to Type-A or to Micro-B (or USB-C) cable

Secondary:

• BNC Bulkhead to SMA
• Copper Tape
• Electrical Tape
• Heatshrink (optional)
• Metal PCI Bracket & Step Drill Bits (or 3D printer and filament)

To check what board you have, check for continuity between the following points (as pictured):

• 3.3v and Mode0 Bridge + Pad
• 5v and Mode0 Bridge + Pad
• Right Leg of the voltage regulator and 3.3v

If you've got 3.3v (or 5v) to the pads on the back, and the right leg of the voltage regulator goes to the 3.3v pin, then you've got a fully good PCM1802! If you've got no connection between 3.3v (or 5v) to the pads on the back and the right leg of the voltage regulator goes to the 3.3v pin - you've got Board Type 1. If you've got 5v (or 3.3v) to the pads on the back, and the right leg of the voltage regulator does NOT go to the 3.3v pin - then you've got Board Type 2.

For all board types, you'll need to bridge the MODE0 Pads as well as the MODE1 pads as well as solder on the 8pin 2.54mm header.

This variant of the PCM1802 Board requires a bodge wire bypass mod as the 3.3v line needs to be connected to the pads on the back.

Setup (as pictured)

• MODE0 with 3.3v Bridge Mod
• MODE1 bridged pads

This board is missing a trace from the 3.3v regulator to the 3.3v pin. You will need to run a wire to bridge the two together from the bottom right pin on the regulator to the 3.3v pin as pictured.

• Tin & strip back XLR cable end
• Ensure cable is past the configuration pad's as not to bridge anything.
• Secure XLR cable with cable tie on the same hole used for 3.3v boge wire.

• Insulate the signal lines with tape or ideally a few mm thick of silicone.

• Wrap entire back end of connectors with metal foil tape to shield it to ground.

RCA Left/Right Audio & Headswitch Input BNC connected to the Clockgen Mainboard.

1. Turn off system
2. Find two available PCIe slots with 1x gen 2 or better bandwith.
3. Find one USB 2.0 motherboard header use a adapter cable to Connect your clockgen mainboard
4. Connect your cards to the slots and connect your SMA clock lines
5. Turn the power on and follow the software guide portion of this page for configuration.

There is also a spin-off of the clockgen mod just using wires and a Pi Pico on its own, but only the mainboard based version is the official "Clockgen Mod" workflow that is standardised for production.

• Rene Wolfs original GitLab

• Namazso's simplified version

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