RF Compression & Decompression Guide

FLAC Compression & De-Compression Guide

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What is RF Compression & Down-Sampling?

RF Captures of tape formats are normally oversampled, meaning in the case of formats like VHS there signal range tops out at 8mhz or 16msps so when using CX Cards or DomesDayDuplicators at rates of 28-40msps we are technically oversampling, its not that this is a bad thing as capturing more then the minimum allows for some redundancy from drops during capture to bit-flips in active storage devices but for archives going to solid mediums like optical discs, ware currently space is at a premium down sampling allows us to cut RF captures down on a per format basis to the minimum needed.

The benefits of this also include, better SNR as there is less noise outside of the signal range, faster decoding times and smaller final files.

Basics of RF and RF Compression

FM RF captures commonly called samples or sampled data etc is just like audio data just electrical signal values digitised into binary bits of information, but this is brightness and colour information in a different waveform pattern than normal sound waves, as such we can use lossless audio codecs like FLAC to compress captured FM RF data down A LOT or lossy ones to just break the files in fun ways too!

In terms of software support 8-bit and 16-bit singed/unsigned are what's common and universally used by most tools, including the decode suit.

But lets say our ADC captures 10-bit / 14-bit etc then normally you get scaled 16-bit data, then you have the DdD which uses the 10-bit packed format (which ld-lds-reader enables reading of in real-time to the decoders).

FLAC compression essentially drops those extra 6 values to 0 in the case of 16-bit files alongside its standard compression methods.

Now in the real world 8-bit is more then enough for tape formats, its the msps or million samples per second rate that actually matters.

How does it work?

RF Data at 40msps in 10-bit packed --> ld-lds-converter --> 16-bit singed unpacked data --> Down Sampling via SoX or GNURadio with lowpass filtering --> 16msps 8-bit unsinged FLAC output

Down-Sampling is a method of cutting file size down with controlled loss of data more accurately removing background noise or dead space for example a 10-bit ADC only has 10-bits of information but can only be saved in a 16-bit format FLAC will remove those 6 extra bits of essentially 0 value information, alongside filtering cut off removing anything else recorded outside of the real-world usable signal range for said particular audio or video format, simmer to how a XLR microphone will have a cut off for 75hz to remove buzzing sounds from lights or power transformers.

This process can be done for low bandwidth formats like HiFi FM signals without much care as only 5msps or less is needed but for video formats its more a stricter practice and the bandwidth also changes between PAL/NTSC systems on a per format basis.

Compression Rates

40msps 10-bit Packed (DdD) (16-bit) (2.81GB/min) VHS PAL SP

Runtime	File Size	Storage Medium Note
45min	126.45GB	128GB Sony Quad Layer BDXL Max
60min	168.6GB
75min	210.75GB
90min	252.9GB
120min	337.2GB
150min	421.5GB
3-hour	505.8GB
240min	674.4GB
5-hour	843GB
6-hour	1011.6GB

40msps 10-bit FLAC compressed (DdD) (16-bit) (1.48GB/min) VHS PAL SP

Runtime	File Size	Storage Medium Note
45min	66.6GB
60min	88.8GB
75min	111GB	128GB Sony Quad Layer BDXL Max
90min	133.2GB
120min	177.60GB
150min	222GB
3-hour	266.4GB
240min	355.2GB
5-hour	444GB
6-hour	532.8GB

*Rates based off 60sec live capture.

*Rates are relative not absolute

Imgsli Slider Comparison

16msps 8-bit with FLAC compression for VHS NTSC

Runtime	File Size	Storage Medium Note
45min	15 GB
60min	19.44GB
75min	24.30GB	25GB M-Disk/GlassMasterDisc
90min	29.16GB	S-VHS/VHS-C tape max
120min	38.88GB
150min	48.6GB	50GB M-Disk / 50GB GlassMasterDisc Max
3-hour	58.32GB	VHS SP Max
240min	77.76GB
5-hour	97.2GB	100GB M-Disk Max
6-hour	116.64GB	128GB Sony Quad Layer BDXL Max / VHS LP Max

*16msps does work on PAL samples but can have issues with some edge cases, i.e damaged tapes or dropped sample captures, 18msps is considered the safe mimium, and 20msps is considerd the hard safe mimium for all VHS flavors, 24msps for SVHS and Video8/

*Rates are relative not absolute

DomesDay Duplicator (DdD)

ld-compress DdD-capture.lds

Will just compress your capture to a lossless 40msps 16-bit FLAC file in .ldf this is the standard Linux command.

The DdD samples data at 40msps 10-bit

You have 2 working capture modes

16-bit Scaled Signed (uncompressed)

10-bit packed (compressed)

10-bit packed is the better option due to space savings before FLAC compression to 16-bit however the data format is not flexible as software expects standard 16-bit/8-bit formatted samples.

Linux Scripts

Copy the scripts below into a text document, save and then add .sh extension to the end of the file, you can use these in any directory witch the script file is put inside of, you can name the scripts however you like but the below example is clear enough.

./40msps-DdD-16msps-8bit.sh DdD-capture.lds

Will make DdD-capture_NTSC_16msps_8-bit.flac

The scripts below automatically append the sample rate/bit-depth/TV System accordingly.

.lds 10-bit Packed to FLAC

There is two drag and drop ld-compress scripts ld-compress.bat & ld-compress-nvidia-gpu.

Simply drag and drop your RF capture on the script and it will automatically run the compression commands on your file.

s16 16-bit Signed to FLAC

Since this is uncompressed data it is easy to handle, the captures save to .raw but should be renamed to .s16.

ffmpeg INPUT.s16 -i -f s16le -ar 40k -ac 1 -acodec flac -compression_level 11 -f ogg OUTPUT.flac

Drag and Drop Script 16-bit-singed-to-flac.bat

@echo off
title Compressing : %~n1
ffmpeg -i "%~1" -f s16le -ar 40k -ac 1 -acodec flac -compression_level 11 -f ogg "%~dp1%~n1.ldf"

Linux Down-Sampling Scripts

20msps 8-bit

#!/bin/bash

echo "Conversion of 10-bit 40msps .lds to 8-bit 16msps PAL .flac has 
started"

$1 sox -r 40000 -b 8 -c 1 -e signed -t raw - -b 8 -r 16000 -c 1 -t flac $1_20msps_8-bit.flac sinc -n 2500 0-9650

NTSC - VHS/Betamax (16msps 8-bit)

#!/bin/bash

echo "Conversion of 10-bit 40msps .lds to 8-bit 16msps PAL .flac has 
started"

ld-lds-converter -i $1 | sox -r 40000 -b 16 -c 1 -e signed -t raw - -b 8 -r 16000 -c 1 -t flac $1_16msps_8-bit.flac sinc -n 2500 0-7650

PAL - VHS/BetaMax (18msps 8-bit)

#!/bin/bash

echo "Conversion of 10-bit 40msps .lds to 8-bit 18msps PAL .flac has 
started"

ld-lds-converter -i $1 | sox -r 40000 -b 16 -c 1 -e signed -t raw - -b 8 -r 18000 -c 1 -t flac $1_18msps_8-bit.flac sinc -n 2500 0-8670

NTSC/PAL - Umatic/SVHS/SuperBeta/ED-Beta (24msps 8-bit)

#!/bin/bash

echo "Conversion of 10-bit 40msps .lds to 8-bit 18msps PAL .flac has 
started"

ld-lds-converter -i $1 | sox -r 40000 -b 16 -c 1 -e signed -t raw - -b 8 -r 24000 -c 1 -t flac $1_24msps_8-bit.flac sinc -n 2500 0-9400

VHS HiFi FM Audio - (S)VHS-HiFi 40msps to 10msps 8-bit

#!/bin/bash

echo "Conversion of 10-bit 40msps .lds to 8-bit 18msps PAL .flac has 
started"

ld-lds-converter -i $1 | sox -r 40000 -b 16 -c 1 -e signed -t raw - -b 8 -r 24000 -c 1 -t flac $1_VHS_HiFi_5msps_8-bit.flac sinc -n 2500 0-3050

SoX Commands

Inital applicaiton call command

Linux: sox

Windows "C:\Program Files (x86)\sox-14-4-2\sox.exe"

Export Spectrogram

sox Input.flac -n spectrogram

SoX can export a handy spectrogram chart of RF & Audio waveform data.

28msps 8-bit to 18msps 8-bit FLAC

sox -r 286363 -b 8 -c 1 -e unsigned -t raw infile.u8 -b 8 -r 18000 -c 1 -t flac outfile_18msps_8-bit.flac sinc -n 2500 0-8670

40msps 8-bit to 18msps 8-bit FLAC

sox -r 40000 -b 8 -c 1 -e unsigned -t raw infile.u8 -b 8 -r 18000 -c 1 -t flac outfile_18msps_8-bit.flac sinc -n 2500 0-8670

40msps 16bit to 10msps 8-bit FLAC

sox -r 40000 -b 8 -c 1 -e signed -t raw infile.s16 -b 8 -r 10000 -c 1 -t flac outfile_10msps_8-bit.flac sinc -n 2500 0-8670

Note

Video8/Hi8 hifi is in the same modulated signal as video just on different carriers so you use 1 file for both video/hifi decoding.

RF Compression & Resampling Commands - Windows Users

Download ld-tools for windows rename the folder to ld-tools-suite-windows and place it in the C:/ directory or your boot drives main directory for scripts to work.

Download Windows scripts for CXADC Captures can be found here!

Manual Dwnloads SoX / FLAC / FFmpeg

You will need tools installed as PATH which is easy to do with chocolatey.

PowerShell as an administrator

Install Choco (chocolatey package manager)

Set-ExecutionPolicy Bypass -Scope Process -Force; [System.Net.ServicePointManager]::SecurityProtocol = [System.Net.ServicePointManager]::SecurityProtocol -bor 3072; iex ((New-Object System.Net.WebClient).DownloadString('https://community.chocolatey.org/install.ps1'))

Then you can install these tools system-wide (PATH) without any hassle.

Install FFmpeg

choco install ffmpeg

Install FLAC

choco install flac

Install SoX

choco install sox.portable

Windows Resampling Scripts

WIP - Currently there is issues with sox/flac scripting for self contained use.

Naming & Formatting Your Files

You can rename the .flac to your media format name using standard designators & standard naming guide.

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