Advanced Parameter Adjustment

This document contains advanced modify-able options inside of ld-decode, vhs-decode, hifi-decode.

These are called "advanced parameters" but are more accurately, just tinkering with core components of the decoders.

Note

Tweaks are on a per recording and per VCR basis and results can vary drastically from different recordings & formats.

VHS-Decode (Tape Decoding)

--params_file <filename> Changes the decoder settings to use an external .json file with the correct formatting

Example:

Create a new text file, use this template and then save it as a custom-params.json file.

Note these values vary by tape format & TV System.

{
   "sys_params":{
      "burst_abs_ref":5000
   },
   "rf_params":{
      "video_lpf_freq":3000000
   }
}

VHS Tune Tool

vhstune.py is a filter adjustment tool used with tbc files that have been raw exported.

Use --export_raw_tbc when decoding export a raw TBC without deemphasis applied for filter tuning.

This can be used to derive a custom-params set of values to load into vhs-decode for a full decode.

User Methods

Current State as of 12.10.2022 (DD.MM.YYYY)

PAL

Itewreed#2179's adjustments for Teletext with VHS-Teletext

Hardware Used for reference:

CX Card is a WinTV-Nova S, a 33µF electrolytic cap is between tap and the rg174 wire. From there it goes to the amp that originally is for digital CCTV with a bw of 8MHz and then into the CX card.

rfparams["deemph_mid"] = 210000
RFParams_PAL_VHS["video_bpf_low"] = 1400000
RFParams_PAL_VHS["video_hpf_extra"] = 1120000
RFParams_PAL_VHS["video_lpf_freq"] = 4500000

"It might be, that this is a good setting for my VCR I use for RF capture. So the settings could be different for other VCRs hardware and could give different results."

Discord Note

if self.options.double_lpf: # Double up the lpf to possibly closer emulate # lpf in vcr. May add to other formats too later or # make more configurable. self.Filters["FVideo"] *= filter_video_lpf

Smooths out high-frequency noise in the chroma signal.

Discord Note

vhsdecode/addons/chromaAFC.py

stock luma filter settings, chromaafc.py settings: (self.fsc_mhz - .44) / self.out_frequency_half, (self.fsc_mhz + .34) / self.out_frequency_half,

and --nld

Changes chroma filtering

Discord Note

Modificaiton to addons/chromaAFC.py

Top is:

(self.fsc_mhz - .84) / self.out_frequency_half, (self.fsc_mhz + .74) / self.out_frequency_half,

Middle is (stock values):

(self.fsc_mhz - .64) / self.out_frequency_half, (self.fsc_mhz + .54) / self.out_frequency_half,

Bottom is:

(self.fsc_mhz - .44) / self.out_frequency_half, (self.fsc_mhz + .34) / self.out_frequency_half,

Deprecated Adjustments

Discord Note

NTSC

Titan91#1526's adjustments for NTSC video.

1. Set RFParams_NTSC_VHS["video_bpf_low"] = 1500000 in vhs.py
2. Set rfparams["deemph_gain"] = 12 in vhs.py
3. Set rfparams["deemph_mid"] = 280000 in vhs.py (I have not confirmed this)
4. Disable double band pass filtering to improve sharpness slightly by commenting out lines 1554 and 1558 of process.py

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