pre uc480 integration
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Thomas Ales [M S E]
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# SRAS Scan Binary Format — Version 4
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Each `.sras` file contains **one complete scan**: all GR rotation angles and all
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Y rows. Files are named `{prefix}.sras`.
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---
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## File Layout
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```
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[Global Header — 43 bytes]
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[Angle Table — n_angles × 4 bytes (float32 per angle)]
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[Row Table — n_rows × 4 bytes (float32 per row)]
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[Preamble Blocks — n_channels × (uint16 length + UTF-8 WFMOutpre string)]
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[Background Block — uint32 n_bg_samples + n_bg_samples × int8 bytes]
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[Waveform Data — n_angles × n_rows × n_channels × n_frames × samples_per_frame × bps bytes]
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```
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All multi-byte integers and floats use **big-endian** byte order
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(`>` in Python's `struct` module).
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---
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## Global Header (42 bytes)
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| Offset | Size | Type | Field | Description |
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|--------|------|-----------|--------------------|--------------------------------------------------|
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| 0 | 4 | `4s` | `magic` | Always `SRAS` (0x53 0x52 0x41 0x53) |
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| 4 | 1 | `uint8` | `version` | Format version — `4` |
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| 5 | 2 | `uint16` | `n_angles` | Number of GR rotation angles |
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| 7 | 2 | `uint16` | `n_rows` | Number of Y rows per angle |
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| 9 | 4 | `float32` | `x_start_mm` | X scan start position in mm |
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| 13 | 4 | `float32` | `x_delta_mm` | X scan width in mm |
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| 17 | 4 | `float32` | `velocity_mm_s` | Stage scan velocity in mm/s |
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| 21 | 4 | `float32` | `laser_freq_hz` | Laser repetition rate in Hz |
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| 25 | 4 | `uint32` | `n_frames` | A-scans per row (= FastFrame count per channel) |
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| 29 | 4 | `uint32` | `samples_per_frame`| Time samples per waveform |
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| 33 | 8 | `float64` | `sample_rate_hz` | Oscilloscope sample rate in Hz (e.g. 6.25e9) |
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| 41 | 1 | `uint8` | `bytes_per_sample` | Bytes per ADC sample: `1` = int8, `2` = int16 |
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| 42 | 1 | `uint8` | `n_channels` | Number of channels recorded (currently `3`) |
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**Total header size:** 43 bytes — verified:
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`struct.calcsize(">4sBHHffffIIdBB") == 43`.
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---
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## Angle Table
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Immediately after the header: **n_angles** big-endian float32 values, one per
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GR angle (degrees, 0–180).
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```
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angle[0], angle[1], …, angle[n_angles - 1]
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```
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---
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## Row Table
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Immediately after the angle table: **n_rows** big-endian float32 values, one
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per Y row (mm).
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```
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y_mm[0], y_mm[1], …, y_mm[n_rows - 1]
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```
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---
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## Preamble Blocks
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Immediately after the row table: **n_channels** length-prefixed UTF-8 strings,
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one per channel in `SCAN_CHANNELS` order (CH1, CH3, CH4). Each block is:
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```
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uint16 length — byte length of the following UTF-8 string
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bytes preamble — WFMOutpre response string from the oscilloscope
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```
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The preamble captures per-channel scaling constants (YMULT, YOFF, YZERO) needed
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to convert raw ADC values to volts.
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---
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## Background Block
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Immediately after the preamble blocks: a single CH1 waveform captured with the
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**Helios (generation) laser enabled** and the **Genesis (detection) laser
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disabled**. This provides a noise/background reference for subtraction during
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post-processing.
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```
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uint32 n_bg_samples — number of samples in the background waveform
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int8[] bg_data — raw ADC samples (same encoding as waveform data)
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```
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`n_bg_samples` equals `samples_per_frame` under normal acquisition settings.
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---
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## Waveform Data
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Immediately after the background block. Data is stored in **angle-major, row-minor**
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order. Within each row, channels are interleaved in ascending channel-index
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order, with each channel's FastFrame data written in frame order.
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```
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for angle in 0 … n_angles-1:
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for row in 0 … n_rows-1:
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for channel in [CH1, CH3, CH4]: # 3 channels, fixed order
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for frame in 0 … n_frames-1:
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samples[0 … samples_per_frame-1] # bps bytes each
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```
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Each sample is a raw signed ADC value. With `bytes_per_sample = 1` this is
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**int8** (−128 … +127). With `bytes_per_sample = 2` this is **big-endian
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int16**.
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Total data size:
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```
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n_angles × n_rows × 3 × n_frames × samples_per_frame × bytes_per_sample
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```
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> **Incomplete files:** If a scan is aborted the file is closed immediately and
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> the data block will be shorter than the expected size. Readers should check
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> `file_size >= header + angle_table + row_table + data` before reshaping.
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---
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## Spatial Mapping
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The *k*-th waveform (frame) in a row corresponds to the *k*-th laser pulse that
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hit the sample. The physical X position of that pulse is:
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```
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x_k = x_start_mm + k * (velocity_mm_s / laser_freq_hz)
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```
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---
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## Python Read Example
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```python
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import struct, numpy as np
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from pathlib import Path
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HDR_FMT = ">4sBHHffffIIdBB"
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HDR_SIZE = struct.calcsize(HDR_FMT) # 43 bytes
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def read_sras(path):
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with open(path, "rb") as f:
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hdr = struct.unpack(HDR_FMT, f.read(HDR_SIZE))
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magic, ver, n_angles, n_rows, xs, xd, vel, freq, nf, spf, sr, bps, n_ch = hdr
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assert magic == b"SRAS" and ver == 4, "Not a v4 SRAS file"
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angles = np.frombuffer(f.read(n_angles * 4), dtype=">f4")
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y_positions = np.frombuffer(f.read(n_rows * 4), dtype=">f4")
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# Preamble blocks (one per channel)
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preambles = []
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for _ in range(n_ch):
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(plen,) = struct.unpack(">H", f.read(2))
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preambles.append(f.read(plen).decode("utf-8"))
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# Background waveform block (v4+)
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(n_bg,) = struct.unpack(">I", f.read(4))
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background = np.frombuffer(f.read(n_bg), dtype=np.int8)
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dtype = np.int8 if bps == 1 else ">i2"
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data = np.frombuffer(f.read(), dtype=dtype).reshape(
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n_angles, n_rows, n_ch, nf, spf
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)
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return {
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"angles_deg": angles,
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"y_positions_mm": y_positions,
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"x_start_mm": xs,
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"x_delta_mm": xd,
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"velocity_mm_s": vel,
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"laser_freq_hz": freq,
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"sample_rate_hz": sr,
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"n_channels": n_ch, # 3: CH1, CH3, CH4 (see Acquisition Settings)
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"preambles": preambles, # WFMOutpre strings, same order as n_channels
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"background": background,# shape: (n_bg_samples,) — CH1 noise reference
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# shape: (n_angles, n_rows, n_channels, n_frames, samples_per_frame)
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"data": data,
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}
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```
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---
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## Acquisition Settings (fixed by sc3_aui_app.py)
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| Parameter | Value |
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|----------------------|------------------------------|
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| Oscilloscope trigger | CH2, rising edge, 1.24 V |
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| Trigger offset | 0 % (trigger at left edge) |
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| Sample rate | 6.25 GS/s (160 ps/sample) |
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| Channels recorded | CH1, CH3, CH4 |
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| Stage X velocity | 100 mm/s |
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| Stage X acceleration | 1500 mm/s² |
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| Stage X trigger out | Logic-high at max velocity |
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| Acquisition mode | FastFrame, Normal trigger |
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---
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## Version History
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| Version | Change |
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|---------|--------|
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| 1 | One file per row; header included `angle_idx`, `row_idx`, `angle_deg`, `y_mm`. |
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| 2 | One file per scan; global header with `n_angles`/`n_rows`; separate angle and row tables; three channels (CH1, CH3, CH4) per row. |
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| 3 | Added preamble blocks (WFMOutpre strings) after the row table, one length-prefixed UTF-8 block per channel. |
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| 4 | Added background waveform block (CH1, Helios ON / Genesis OFF) after the preamble blocks; stored as `uint32` sample count followed by raw `int8` ADC bytes. |
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