VHS FM RF / Baseband Composite PAL / 4fsc YUV Decoded Picture
FM RF Archival is a simple basic preservation scheme to interface, capture and preserve analogue media formats on celluloid (magnetic tape) or optical (pitted) substrate recording mediums
Source Media Reading --> FM Waveform/Signals on test points --> PCM Sampling (computer) --> FLAC Lossless compression (40-70%)
Software Decoding --> Baseband Composite (combined) or S-Video (Y/C separated) on file --> Chroma-Decoding (comb-filtering) --> YUV Sampling (digital video files).
Digital sampling is a simple concept, your taking wave of voltage information and giving it a high/low value so a 1 or 0 to map the signal into a waveform, higher the sampling more accurate the wave is to a point of 1:1 digital replica.
Detail level per each simple is defined by bit depth so 8-bit means 256 values of information per each sample thinkable in terms of steps of detail and 10-bit means 1024 values.
Bandwidth or sampling speed the amount of information an ADC can accept and convert to useful information is defined by sample rate in SPS or MSPS normally samples per second and million samples per second respectively.
Below is the Munday Demo Tape Google Drive / Internet Archive loaded into an audio DAW OcentAudio.
Highlighted is a "frame" of signal information made by 2 separate fields:
Highlighted is a "line" of signal information:
With cutting it down to just 2 fields, inside the DAW you can decode a single frame of information from the exported FLAC file:
1mhz = 2msps of minimum sampling so a 2:1 ratio
Less is worse more is better.
The best layman's example of this is HiFi 20hz to 20khz (Witch is around the average human hearing range) CD audio is digital 44.1khz just over 2:1 sampling, so 48khz covers the entire range of human speech, ware as 192khz would cover a large array of multiple sound information points like a orchestra.
Oversampling is just the practice of capturing far more information than the potential of the source information, this just wastes space in simple terms and is always the preference to undersamping ware signal information is lost.
Four times the frequency of SC (sub-carrier).
The digital sampling rate of a composite video signal with respect to the sub-carrier frequency of an NTSC or PAL analogue video signal.
The 4fsc frequency sample rate is defined based off full spec composite
FPS: 29.97
14.31818182 MHz (4x 3.579545455 Mhz)
Full Signal Frame: 910x525
Active Picture Area: 720x486 & 720x480
FPS: 25
17.734475 MHz (4x 4.43361875 Mhz)
Full Signal Frame: 1135x625
Active Picture Area: 720x576
A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information.
Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.
In simple terms, let's say you have a 5mhz signal, inside this you have audio at 1.2mhz for left and 1.8mhz for right 2.2mhz has a timecode signal and 4.5mhz has the video signal all these signals are modulated
Some good real world examples are the HiFi carrier positions on common videotape formats.
Left 1.3Mhz / Right 1.7mhz
Left 1.5Mhz / Right 1.7Mhz
Left is 1.38Mhz A head & 1.53Mhz B head
Right is 1.68Mhz A head & 1.83Mhz B head
Exurbs From Digital Video and HD Algorithms and Interfaces 2nd Edition (By Charles Poynton 2012-02-07)
Pages 162 to 180
The following pages give a clear explanation of what 4fsc, S-Video, and Chroma Sampling are, witch are the core surface concepts to understand the processing chain of software tape decoding and how analogue is presented in the digital domain.
Four times the frequency of subcarrier, this is normally based off the composite signal standard for PAL/NTSC.
The 4fsc frequency sample rate is typically:
14.3 MHz (28.6 MSPS) in NTSC.
17.7 MHz (35.4 MSPS) in PAL.
In simple terms the same system used for D2/D3 tape.
Page 171
YUV 4:2:2 10-bit - The Industry StandardPage 172
A question asked a lot in the digital user meets analog world is why 4:4:4 10-bit is not used for analog video sampling, this section aims to clear this up in practical and conceptual terms.
Nyquist is Nyquist - pixel peeping on a PC will always be in the RGB domain pretty much - so you are always doing a 4:2:2/4:4:4 to RGB conversion to pixel peep on a PC.
You will see a difference in many cases between 4:2:2 and 4:4:4 in this domain (because you can have chroma differences at pixel not pixel-pair resolution)
but those differences don't mean there is source information there. It is often just more noise. However much, intuitively, it sounds that sampling a 300kHz bandwidth (VHS Chroma) signal 20 times more than you need to rather than 10 times more than you need to will improve things - it won't.
Once you're past Nyquist for your signal, you're capturing it all.
As long as you stay within the same sampling structure after this - you don't get loss from conversion. That's why the broadcast and post industry is standardised on 4:2:2 to avoid conversions between 4:2:0, 4:1:1, 4:4:4 etc.
@Marshalleq
I get what you're saying but I don't think that's how it works.
Chroma subsampling is not pre-defined holes that always know where to find themselves in subsequent conversions.
I did some testing and could clearly see loss over generations. It' s particularly bad in low resolution as there's less detail to begin with.
Stephen_Neal — 09/05/2025 04:46 AM
Nyquist is Nyquist though - with 4:2:2 sampling of VHS chroma you are capturing a 0.3MHz bandwidth signal in a channel designed for 2.75MHz bandwidth signal. That's already massive overkill of the order of 10x more than you need. You are oversampling to a ridiculous degree with 4:2:2 capture of VHS. With 4:4:4 it's just pointless. You're just more accurately capturing noise.
You can fully capture a 2MHz luminance, 0.3MHz chrominance bandwidth VHS signal with 1.4:0.22:0.22 sampling... 2:0.5:0.5 (i.e. 2.875MHz luminance, 687.5kHz chroma bandwidth) would be overkill and accurately capture the source.
VHS for example can be stored in 320x576 25i 4:1:1 YUV with FFV1 at under 20mbps lossless in terms of luma/chroma, but if this file was to be used in editing it would run into the conversion problems
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