RF Capture Decoding Guide

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Welcome to the 21st, its the future.

Software decoding simply is just doing what was done on dedicated components, then on dedicated IC's and finally on FPGAs and all in one IC's, its not magic and its not black box hardware either its entirely software emulating the ability's of hardware with an order of magnitude more flexibility.

Do you need to know about programming?

No.

Do you need to know how to use basic systems on a computer.

Yes.

Do you need to have a basic understanding of a VCR does and what a TBC does?

Yes.

It makes life easier and decoding a thoughtless copy-paste clean, new tape, clean, hit enter and decode workflow after hardware hook-up.

What does decoding do?

For FM Video this creates a 4fsc sampled Composite .tbc or S-Video .tbc & _chroma.tbc pair ready for inspection inside ld-analyse during and after decoding, then its chroma-decoded to standard video files in the Next Page TBC-Video-Export Guide.

ld-decode produces only a Composite .tbc and .pcm for stereo/mono analog audio, .efm for digital data, and digital audio data is decoded to .dts or .ac3 with ld-efm-decoder.

For FM Audio (HiFi-Decode & RTLSDR)

HiFi-Decode Produces a 192khz PCM sampled audio stream 2:1 compressed with FLAC so a .flac audio file ready for playback or muxing into your final video files.

RTLSDR-Decode is a realtime GNURadio based decoder/encode script that creates a 8msps .u8 RF file, a 48khz .flac audio file and decodes the signal live with a 1-3sec delay.

Running the decoders

There are 4 decoders are bundled inside vhs-decode repository and windows binary version.

The prefix to run these decoders are as such:

Linux/MacOS

vhs-decode / ld-decode / cvbs-decode / hifi-decode

cd vhs-decode - Enter the directory

Call the decoder with vhs-decode or ./vhs-decode to run locally for example.

Example:

./vhs-decode --debug -p -t 8 --tf vhs --cxadc --recheck_phase my-tape.u8 my-tape-decoded

Windows

decode.exe vhs / decode.exe ld / decode.exe cvbs / decode.exe hifi

Followed by universal arguments you can read the Full Command List for there use and effects.

Example:

decode.exe vhs --system pal --threads 8 -Turkish_Video_AG5620_2023-12-06_02-28-09.lds Turkish_Video_AG5620_2023-12-06_02-28-09

VHS is the assumed format by default, and 40msps the DdD rate is assumed by default, but these defaults should never be taken for granted as its bad habit when dealing with outher formats, but are helpful for short hand quick use.

Mandatory Settings

• TV System i.g --system pal, --system ntsc, --system palm, --system ntscj (PAL-M - Brazil / NTSC-J - Japan)

• Tape format i.g --tape_format VHS or SVHS, UMATIC, UMATIC_HI, Hi8, Video8, BETAMAX, BETAMAX_HIFI, EIAJ, TYPEC, VCR.

• Input Sample Rate -f or --frequency this is the msps value of your RF capture file. (40msps is the assumed/internal rate)

• Input file & Out file Name.

Basic Commandline Control:

Ctrl+C Will kill the current process, use this to stop the decode manually.

Press Ctrl+C to copy then Ctrl+P to past the command use <+> to move edit position on the command line to edit the name or command and Enter to run the command.

Basic Decoding Command Examples

There is 2 ways to write commands, long hand and short hand such as --tape_format & --tf or -f & --frequency each version is documented together in the commandlist master doc.

--debug for a slight speed hit gives more logging and better error reporting.

VHS-Decode:

Example Long: ./vhs-decode --debug --tape_format vhs --system pal --cxadc --threads 8 --recheck_phase my-tape.u8 my-tape-decoded

Example Short: decode.exe vhs --debug -p -t 8 --tf vhs --cxadc --recheck_phase my-tape.u8 my-tape-decoded

LD-Decode:

Note: NTSC AC3 Audio discs need --AC3 enabled to decode that data to a .efm.

ld-process-efm also needs a dedicated flag but for DTS audio, AC3 is automatic.

Example Long: ./ld-decode --system pal --threads 8 DdD_8.5_Gain.ldf First_Decoded_LD

Example Short: decode.exe ld -p -t 8 DdD_8.5_Gain.ldf First_Decoded_LD

CVBS-Decode:

CVBS-Decode, decodes raw RF captured composite signals, you capture the normal composite video signal out of a device, in raw RF values then save it to file.

-A - Automatic is mandatory for decoding ot work currently.

Example Long: ./cvbs-decode --debug --threads 8 --system pal -A --cxadc CX_Blue_Level_0db_6db_off_CXADC.u8 CX_Blue_Level_0db_6db_off_CXADC

Example Short: decode.exe cvbs --debug -t 8 -p -A --cxadc CX_Blue_Level_0db_6db_off_CXADC.u8 CX_Blue_Level_0db_6db_off_CXADC

You have full control over the baseband processing, fine control over how colour is processed with virtual vector scope and scan-line ossiliscope with the chroma-decoder in ld-analyse, you have a full 625/525 lines of image information to work with.

HiFi-Decode:

HiFi-Decode provides the FM audio decoding for VHS, SVHS, BetaMax, Video8, Hi8

A single command decoder for HiFi RF to make playable FLAC files

Example Long: ./HiFi-decode --system pal -t 8 HiFi_Capture_40msps.s16 Tape-name_HiFi_Decode_192khz.flac

Example Short: decode.exe hifi -p -t 8 HiFi_Capture_40msps.s16 Tape-name_HiFi_Decode_192khz.flac

RTL-SDR Decode:

Primiative version of HiFi-Decode using a GNURadio script, using plug and play USB SDR units, such as the RTLSDR.

Ideal for testing HiFi tap points or tracking/physical calibration, and capture for DdD users.

Entirely GUI based and self contained read its doc here RTLSDR.

Audio Alinement

This can be dome manaully to a refracne capture in a editing application or autmatically.

Linear.wav / tbc-media.json / hifi_decode_192khz.wav

Notes

The following commands are your key commands for speading things up with vhs-decode.

--no_resample for example can speed up tape decodes drastically for non 40msps captures i.e 20msps as it skips internal re-sampling to 40msps.

--level_detect_divisor Currently, 1-6 value, 1 every sample, 2 every other sample and so on, Changes vsync serration code only use every nth sample when determining sync/blank levels higher value lower accuracy. - set to 6 for speed.

--use_saved_levels Experimental Skips doing most of the level detect code on each frame and only does it at the start or if there are issues detecting syncs. Provides a speedup in decoding and seems to work okay on captures that only have one single recording of media.

VBI Data Decoding

ld-process-vbi input.tbc / ld-prcoess-vits input.tbc Will decode most Vertical Blanking Interval Data.

This data is decoded and stored in the updated .json file.

This covers Closed Captions, VITC Timecode & VITS Test Signals currently.

TBC-Video-Export automatically runs this and embeds the .json into mkv files for safety.

Viewing TBC Files in ld-analyse

Full Wiki Page For ld-analyse

Your avalible frames will update every few 100, as the data is written to the JSON file.

You can open it as a gui application or call it with ld-analyse in the terminal.

Determining Capture Quality

There is 2 ways to assess the quality of a decode.

Black Signal To Noise Ratio

(Open window & screenshot it currently)

Visually you can tell how noisy the signal is by how black the VBI space is if its a clear grey colour this means a high SNR.

LaserDiscs will normally have a curve to the graph ware as tape will mostly be level per each recorded segment.

Sudden dips in the graph are ware there are dropouts, interference or weak signal.

60~70 dB - Live Composite

40~50 dB - Great Signal

30~40 dB - Good Signal

20~30 dB - Weak Signal

10~20 dB - Poor Signal

Export Dropout Analysis

(Open window & screenshot it currently)

Visually Checking

Visual Video Errors Doc / VBI Data Identificaiton Doc

Very Bad Signal Example

Normal Good Signal Example

*note this example contains a dropout.

Clean Macrovison Example

Adjusting your TBC in ld-analyse

As ld-analyse reads the .tbc & _chroma.tbc files and combines them together in real-time with the ld-chroma-decoder this allows you to play with different chroma decoders and filtering settings on a basic level.

With ld-analyse can adjust JSON metadata alongside reading it, you can ignore this step however it is useful to learn to make better finer adjusted exports.

PAL Transform 2D for VHS & 3D for CVBS sources for example.

This is to the same effect as SoX combining commands on some media consider it good enough for checking but make a short video export with python3 tbc-video-export.py --length 300 input-tbc-name script to see the real media result.

• Black Level Adjust

• White Level Adjust

You can adjust these sliders and save an updated .json with these values that the tbc-video-export.py script will read.

You can manually set the 16:9 Flag for widescreen content (iswidescreen inside the .json)

Vector Scope & Chroma Decoder

NOTE: Final output will be determend by fully running the chroma-decoder so run a 10sec test export.

For media with test bar signals you can select 75% or 100% bars you can adjust your image with the chroma-decoder.

EBU 75% Colour Bars	EBU 100% Colour Bars	SMPTE 75% Color Bars	SMPTE 100% HDTV Color Bars

• Chroma Gain (Intensity)

• Chroma Phase (Hue Control)

• Luma Noise Reduction (PAL/NTSC)

• Chroma Noise Reduction (NTSC Only)

Resource Usage

Chroma-Decoder can use around 20GB~33GB of ram with FFmpeg it will work with less.

decoders however are cpu bound and will use normally less then 500MB of ram, and up to 6 threads.

HiFi-Decode Notes

There is the full hifi-decode and the universal RTLSDR HiFi-Decode which is a GNU radio script.

FM Audio decoding has been normal for decades if you ever have used a car you know it has a radio and it picks up and decodes FM audio signals from local stations.

HiFi decode allows you to have full control over the de-modulation and noise filtering process this can remove and improve on sound quality drastically in some cases and or save you money if you don't have the best amplifiers or a decent audio interface capture solution, if you have good hearing you will notice the difference quickly with even 15USD Behringer ''studio'' headphones.

You can use an cheep RTLSDR or the DdD & CX Cards for HiFi capture, it does not require very high bandwidth.

Linear Audio will have to be captured via standard ADC solutions, such as line in or ADC board with the external clock mod for CX Cards.

As decent quality conventional capture workflow hardware is mandatory on the linear audio front, this can range from an GV2-USB to an Blackmagic Analog to SDI setup or Zoom F3 field recorders for example, but modern 24-bit ADCs in basic recorders are very affordable and higher SNR then linear tape audio in most cases.

Format Notes

Tape decoding can be broken into 3 types.

Composite Modualted - Is always 1 TBC

Colour-Under - Is 2 TBC files or a Y + C S-Video Pair

VHS, SVHS, Betamax, Umatic, Umatic SP, EIAJ, Video8, Hi8, Philips VCR, Video2000 - Colour Under

SMPTE-A, SMPTE-B, SMPTE-C, 2" Quaduplex, LaserDisc - Composite Modualted

Betacam, Betacam SP, W-VHS - Duel Channel Component (Support not yet implimented)

W-VHS / HDVS, Hi-Vision are all under the MUSE system that we need more samples of.

Page End

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