make-json

#!/usr/bin/python3
# Given a .tbc file using PAL 4xfSC subcarrier-locked sampling, generate a
# .tbc.json that describes it.
#
# Usage: make-json input.tbc [output.tbc.json]

import json
import optparse
import os
import sys

# Parse command-line options
parser = optparse.OptionParser(usage="usage: %prog [options] TBC-FILE [JSON-FILE]")
options, args = parser.parse_args(sys.argv[1:])

# Subcarrier frequency in Hz
fSC = 4433618.75

# Working out activeVideoStart/End:
#
# The active area should be 922 samples wide, for 4:3 aspect ratio against 575
# vertical lines.
#
# The first sample in the .tbc is the left edge (not the middle) of the field 1
# line 1 sync pulse -- sample 955 going by EBU Tech 3280's numbering -- so 0H
# occurs 2.5 samples into the .tbc. The "digital active line" therefore starts
# at sample 1135-955 = 180 and ends at 180+948 = 1128.
#
# Drop (948-922)/2 = 13 samples on either side, and the active region on line 1
# is 193 to 1115.

# Parameters we're trying to generate (based upon ld-decode output)
videoParameters = {
    "sampleRate": fSC * 4,
    "isSubcarrierLocked": True,
    "isSourcePal": True,
    "fsc": fSC,

    "black16bIre": 16384.0,
    "white16bIre": 54016.0,

    "fieldWidth": 1135,
    "fieldHeight": 313,
    "colourBurstStart": 104.0,
    "colourBurstEnd": 144.0,
    "activeVideoStart": 193.0,
    "activeVideoEnd": 1115.0,

    "numberOfSequentialFields": 0, # We'll fill this in later
    }

# Parse positional args
if len(args) == 1:
    input_tbc = args[0]
    output_json = input_tbc + ".json"
elif len(args) == 2:
    input_tbc = args[0]
    output_json = args[1]
else:
    parser.error("no input/output filenames specified")

# Measure the length of the video
input_len = os.stat(input_tbc).st_size
field_size = 2 * videoParameters["fieldHeight"] * videoParameters["fieldWidth"]
numFields = input_len // field_size
if (input_len % field_size) != 0:
    print("TBC length", input_len, "is not a multiple of field size",
          field_size, file=sys.stderr)
    sys.exit(1)

# ld-decode computes medianBurstIRE as sqrt(2) * RMS(samples in colourburst in
# IRE form), i.e. half the P-P amplitude of the colourburst. This is used as
# part of the chroma gain calculation in ld-chroma-decoder to compensate for HF
# rolloff.
medianBurstIRE = (3.0 / 7.0) * 100.0 / 2

# Generate JSON info for fields
fields = []
for i in range(numFields):
    field = {
        # ld-chroma-decoder cares about these values:
        "isFirstField": (i % 2) == 0,
        "seqNo": i + 1,
        "medianBurstIRE": medianBurstIRE,

        # And doesn't care about these, so provide dummy values:
        "syncConf": 100,
        "diskLoc": i + 1,
        "decodeFaults": 0,
        "vitsMetrics": {"wSNR": 42.0, "bPSNR": 42.0},
        "vbi": {"vbiData": [42, 42, 42]},
        "audioSamples": 42,
        }
    fields.append(field)

# Write the JSON
videoParameters["numberOfSequentialFields"] = numFields
with open(output_json, "w") as f:
    json.dump({
        "videoParameters": videoParameters,
        "fields": fields,
        }, f, indent=2)