`channel`¶

Project-wide impairments of the pressing: properties of the disc and the channel it is read through, rather than of one capture. Optional — omit the block for an ideal channel.

channel:
  timing_offset_samples: -0.3
  edge_smear:
    fraction: 0.12
    tau_samples: 3.0

These two apply to any standard and any capture chain. The impairments that come from LaserVision itself — the FM-beat herringbone, the IEC 60856 group-delay pre-distortion, the replay response and the field-2 misregistration — are in the LaserVision channel, laserdisc.channel.

Every project that models one pressing must use the same channel: block. The capture noise layer, the chroma error and the dropouts describe one capture and can differ between projects; the channel cannot.

Keys¶

The block must set at least one of timing_offset_samples and edge_smear. An empty channel: block, or any other key, is a parse error; fm_beat here is rejected with an error pointing to laserdisc.channel.fm_beat.

Key	Type	Required	Range	Description
`timing_offset_samples`	float	No	−8.0 to 8.0	Sub-sample timing offset in 4fsc samples. Positive values make the signal arrive later
`edge_smear`	map	No	—	Readout edge smear; see `edge_smear`

`edge_smear` keys¶

Key	Type	Required	Range	Description
`fraction`	float	Yes	0.0 to 0.5	Fraction of the signal that is smeared
`tau_samples`	float	Yes	above 0, up to 100	Time constant of the smear, in 4fsc samples

Behaviour¶

Configuration	Result
Block absent	No channel stage; output is unchanged
`timing_offset_samples: 0.0` only	No processing; output is bit-identical to the block being absent
Integer offset	An exact shift of the frame by that many samples
Fractional offset	A band-limited delay of the frame
`edge_smear.fraction: 0.0`	No smear; output is bit-identical to leaving `edge_smear` out

`timing_offset_samples`¶

The offset moves the whole sampled signal in time relative to the sampling grid — sync, VBI, VITS and picture alike — as two players would when their capture clocks lock with a slightly different phase. A stacking tool has to align captures with different offsets before it can average them.

The integer part is an exact index shift.
The fractional part is a 32-tap Kaiser-windowed-sinc fractional-delay filter (β = 8) normalised to unit DC gain, so flat levels pass unchanged. Up to 6 MHz its response is within 0.001 dB and 0.0002 sample of an ideal delay on every standard; content closer to Nyquist is progressively attenuated.
Each frame is shifted circularly within itself. The frame boundary lies in vertical blanking, so the samples that wrap are sync and equalising pulses only.

`edge_smear`¶

A fraction of the signal passes through a one-pole low-pass, as a player's readout trails each sharp transition:

H(z) = (1 − c) + c · (1 − a) / (1 − a z⁻¹), with c = fraction and a = exp(−1 / tau_samples)

After a step, the output reaches 1 − c·aⁿ⁺¹ at sample n. Flat levels pass unchanged (the DC gain is 1), and the filter never overshoots.
It applies to Y and C over the whole frame, VITS included, so a stacking tool can measure the smear from a VITS bar edge.
Each frame is treated as circular: the filter state is primed over the frame's own last ⌈20·tau_samples⌉ samples, so the result depends on that frame alone and is the same whatever order frames are rendered in.

Common rules¶

The smear and the delay are applied in that order, after the LaserVision channel: the FM beat and the replay filter belong to the disc and the player's replay path, so the readout smear and the sampling offset apply to them.
Each step rounds to the internal fixed-point step. Y is clamped to the same legal range as noise injection; C is centred on zero and is left for the output stage to quantise, so saturated chroma is not clipped.
Both run on the Y and C signals after generation and before noise, so both noise layers are added afterwards and keep the configured spectrum. The shared (pressing) layer is then delayed by timing_offset_samples, as the picture was, so it stays on the picture's samples; the band-limited delay leaves its spectrum unchanged to within 0.001 dB up to 6 MHz. The capture layer is not delayed.

Stage order¶

The channel stage runs once per frame, between generation and noise:

Generation
Channel: the LaserVision channel (FM beat, then group delay and replay response), then edge smear, then timing offset
Chroma error
Line timing
Noise (capture layer, then shared layer)
Dropouts
Output

Rules¶

timing_offset_samples outside −8.0 to 8.0 is a validation error.
edge_smear.fraction outside 0.0 to 0.5, or edge_smear.tau_samples not above 0 or above 100, is a validation error.
edge_smear without both of its keys is a parse error.
A non-numeric value is a parse error.

channel¶