Clockgen Mod
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This doc is a 1 page version of Rene Wolfs "cxadc-clock-generator-audio-adc" clockgen mod setup wiki which is flushed out with more theory, this doc focuses on the basics of what goes ware and how to actually make captures, and also features the sane use of the SMA connector, there is also a spin off of this just using wires and a Pi Pico on its own.
Important
This Guide Assumes you have CXADC Installed and have tested your cards before starting modification of them and know the basics of capture with a CX Card, and have at least overview info about its mods.
The clockgen + ADC mod setup typically assumes external amplifiers are employed, allowing for optimal signal to noise ratio of the raw signal's intended to be captured without need for the CX Cards gain levels to ever be modifyed.
Important
C31 Mod & BNC Mod is normally recommended and assumed deployed with this mod.
To avoid any worrys of permissions of configration we want to set the system to use the same config on every re-boot, to do this we use the udev rules.
/etc/udev/rules/
Inside this folder using admin privliges made place a cxadc.rules file with your desired configuration.
This example config should be ample for most users current setups.
- 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 most safe option if your noticing 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
Synchronized 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
This web server capture workflow can be done locally on a single computer. This also allows for captures to networked storage pools, but has more reliable buffering then the direct script for lower end systems.
./local-capture.sh --add-date --video=0 --hifi=1 --convert-linear Your-Tape-Name
Important
Ensure both the script & the capture server itself has read and write permissions for the video group.
Caution
Only stop/exit with pressing q if you do CTRL + C as you would for a normal script, it will break USB audio device access and will require restarting.
Important
Read this segment if you are confused by multiple standard audio files from long duration captures!
For the next step to be easy and fluid captures over 1 hour long typically end up being split into multible .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
Auto Audio Align
After decoding your Video RF and or HiFi FM audio you will want to sync your PCM audio data with your video file.
With a standard configuration you just need to alter the following:
-
-i baseband.wave- Input PCM Audio File -
--json capture.tbc.json- Input TBC JSON from decoding -
baseband_aligned.flac- Output name at the end (also s16 will do 16-bit if s24 is replaced)
Tip
Below is premade commands for usage:
"ffmpeg" -i baseband.wav -filter 'channelmap=map=FL-FL|FR-FR' -f s24le -ac 2 - | mono VhsDecodeAutoAudioAlign.exe stream-align --sample-size-bytes 6 --stream-sample-rate-hz 46875 --json capture.tbc.json --rf-video-sample-rate-hz 40000000 | ffmpeg -f s24le -ar 46875 -ac 2 -i - -af aresample=48000 -sample_fmt s32 baseband_aligned.flac
"ffmpeg" -i baseband.wav -filter_complex "[0:a]channelsplit=channel_layout=2.1:channels=FL[FL]" -map "[FL]" -f s24le -ac 1 - | mono VhsDecodeAutoAudioAlign.exe stream-align --sample-size-bytes 3 --stream-sample-rate-hz 46875 --json capture.tbc.json --rf-video-sample-rate-hz 40000000 | ffmpeg -f s24le -ar 46875 -ac 1 -i - -af aresample=48000 -sample_fmt s32 baseband_aligned.flac
ffmpeg -i baseband.wav -filter 'channelmap=map=FL-FL|FR-FR' -f s24le -ac 2 - | VhsDecodeAutoAudioAlign.exe stream-align --sample-size-bytes 6 --stream-sample-rate-hz 46875 --json capture.tbc.json --rf-video-sample-rate-hz 40000000 | ffmpeg -f s24le -ar 46875 -ac 2 -i - -af aresample=48000 -sample_fmt s32 baseband_aligned.flac
ffmpeg -i baseband.wav -filter_complex "[0:a]channelsplit=channel_layout=2.1:channels=FL[FL]" -map "[FL]" -f s24le -ac 1 - | VhsDecodeAutoAudioAlign.exe stream-align --sample-size-bytes 3 --stream-sample-rate-hz 46875 --json capture.tbc.json --rf-video-sample-rate-hz 40000000 | ffmpeg -f s24le -ar 46875 -ac 1 -i - -af aresample=48000 -sample_fmt s32 baseband_aligned.flac
Sample rate modes are the following:
28.63msps, 20msps, 40msps, 50msps
To open the terminal control window use:
alsamixer
Press F6
Navigate with ↑ & ↓ keys.
Hit Enter with CXADC+ADC+ClockGen selected.
Press Space to enable audio control.
That is about it, you only play with these settings when doing manual overrides for stock settings.
Install asmixer
sudo apt install asmixer
Run alsamixer with:
alsamixer
Fix Capture Script Permissions
chmod +x cxadc-sync-capture.sh
Test Firmware / Clock Gen Firmware / Reset Firmware
-
Ensure Pi Pico is not plugged into the mainboard of the clockgen.
-
Equip a USB-C or Micro-B cable depending on your Pi Pico Type.
-
The Pi Pico displays as a USB storage device if working.
-
Simply drag and drop the test firmware onto the filesystem and it will automatically flash
Note
If the unit blinks then its all good to reset and proceed.
Note
To reset an Pi Pico Hold the button on it while unplugged then plug into usb, and use the flash_nuke.uf2 file to reset it to clean.
After a successful flashing, the Pi Pico will show up as a microphone 3ch 48khz device to any systems sound manager application.
Internal USB headers can be used for the data connection to the Pi Pico
Normally PC's and Servers have internal USB 2.0 header's 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 its 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.
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, or if would like to support the work and save some time then you can order the pre-made boards off Harry's Kofi-Store.
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 recommneded to build boards and do RF Tappping!
- 3rd Arm Clip Holder or Vice Clamp
- Soldering Iron / Bevel Type Tip
- 60/40 Leaded Solder (i.g Silverline or equivalent)
- 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
(With 3D printed bracket & metal drilled bracket)
Important
Ensure that the 3 parts C22, R12, C19 below the crystal package are removed.
Front & Back
Important
Ensure orientation of opening fins are suited for the airflow of your case/fan setup.
L 40mm X W 40mm X H 11mm heatsinks are readily available.
Tip
You can order pre-assmbled jigs or bare PCBs here!
Warning
Ensure all parts are populated in the correct orientation as shown in the image below.
- 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
Caution
This PCB when connected to a CX Card will be under tension, ensure strong adhesive tape is used or use glue/epoxy and ensure both surfaces are cleaned with 99.9% IPA and dry before adhering.
Tip
You can order pre-assmbled mainboards or bare PCBs here!
Warning
Always check via line indicator on SMD ports the polarity of diodes are in-line with indicator on PCB
Gen 2 PCB using SMA for Headswitch input and 2 clock lines, compatible with the orignal 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 venders 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 (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 Audio Cable (A 1m XLR cable is ideal as its shielded)
- 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
- Duct-Tape
- Metal PCI Bracket & Step Drill Bits (or 3D printer and filament)
Note
The Si5351A kit headers are not recommended for socketed connectors, its recommended to solder headers then the board to said 2.54mm headers directly to ensure a secure connection.
Caution
Ensure all parts are populated on the PCB as shown in the top-down photo below if parts are missing you will have issues!
Note
The PCM1802 Board requires a bodge wire bypass mod in most cases if 3.3v lines are not connected, its always good to test with a multi-meter if possible.
Parts
- PCM1802 Board
- 8Pin 2.54mm Headder
MODE0 with 3.3v Bridge Mod
Tip
When soldering the ground the the RCA lugs you can have it slightly shorter to sang proof the cable so no tugging or tension is on the signal wires.
Previous Page CX Cards
Sub-Page RF Capture Guide
Next Page Hardware Installation Guide
- FAQ - Frequently Asked Questions
- Diagram Breakdowns
- Visual-Comparisons
- VCR Reports / RF Tap Examples
- Download & Contribute Data
- Speed Testing
- Visual VBI Data Guide
- Closed Captioning
- Teletext
- WSS Wide - Screen Signalling
- VITC Timecode
- VITS Signals
- XDS Data (PBS)
- Video ID IEC 61880
- Vapoursynth TBC Median Stacking Guide
- Ruxpin-Decode & TV Teddy Tapes
- Tony's GNU Radio For Dummies Guide
- Tony's GNU Radio Scripts
- DomesDay Duplicator Utilities
- ld-decode Utilities