update hea example

Author: briangow

spec/header-files.md

@@ -21,28 +21,35 @@ A header file contains:
 
 Each line must be under 255 characters, and fields are separated by spaces or tabs (except where otherwise noted).
 
+Detailed documentation on header files can be found at: [https://physionet.org/physiotools/wag/header-5.htm](https://physionet.org/physiotools/wag/header-5.htm)
+
 ---
 
 ## Record Line
 
 The first non-comment line is the **record line**, which provides metadata about the overall record. It includes:
 
-| Field | Description |
-|:------|:------------|
-| Record name | Identifier for the record (letters, digits, underscores only). |
-| Number of segments (optional) | If present, appended as `/n`. Indicates a multi-segment record. |
-| Number of signals | Number of signals described in the header. |
-| Sampling frequency (optional) | Samples per second per signal. Defaults to 250 if omitted. |
-| Counter frequency (optional) | Secondary clock frequency, separated from sampling frequency by a `/`. |
-| Base counter value (optional) | Offset value for counter, enclosed in parentheses. |
-| Number of samples (optional) | Total samples per signal. |
-| Base time (optional) | Start time of the recording (`HH:MM:SS`). |
-| Base date (optional) | Start date (`DD/MM/YYYY`). |
+| Field | Description                                                                |
+|:------|:---------------------------------------------------------------------------|
+| Record name | Identifier for the record (letters, digits, underscores only).             |
+| Number of segments (optional) | If present, appended as `/n`. Indicates a multi-segment record.            |
+| Number of signals | Number of signals described in the header.                                 |
+| Sampling frequency (optional) | Samples per second per signal. Defaults to 250 if omitted.                 |
+| Counter frequency (optional) | Secondary clock frequency, separated from sampling frequency by a `/`.     |
+| Base counter value (optional) | Offset value for counter, enclosed in parentheses.                         |
+| Number of samples (optional) | Total samples per signal when `samps_per_frame`=1; total frames otherwise. |
+| Base time (optional) | Start time of the recording (`HH:MM:SS`).                                  |
+| Base date (optional) | Start date (`DD/MM/YYYY`).                                                 |
 
 **Example:**
 ```text
-100 2 360 650000 12:00:00 01/01/2000
+12345 3 62.5 625 30/01/1989
 ```
+- `12345`: Record name (must match the filename prefix).
+- `3`: Number of signals in the record.
+- `62.5`: Sampling frequency in Hz.
+- `625`: Total frames for each signal. When `samps_per_frame` =  1 (default), this will be the total samples.
+- `30/01/1989`: The date (`base_date`).
 
 ---
 
@@ -69,9 +76,20 @@ Each signal has its own line immediately following the record line (for single-s
 
 **Example:**
 ```text
-100.dat 212 200 11 1024 995 0 MLII
-100.dat 212 200 11 1024 995 0 V5
+12345.dat 16x4 200/μV 12 0 0 2178 0 ECG
+12345.dat 16x2 16/mmHg 12 0 0 3497 0 ICP
+12345.dat 16x1 2500/Ohm 12 0 0 1366 0 RESP
 ```
+- `12345.dat`: Filename of the signal file.
+- `16`: Storage format (16-bit integers).
+- `x4`,`x2`,`x1`:  4, 2, and 1 samples per frame, respectively. This indicates that the ECG signal has `157 * 4 = 628` samples while the ICP signal has 314 samples and the RESP signal has 157 samples. 
+- `200/μV`, `16/mmHg`, `2500/Ohm`: ADC gain (i.e., number of digital values per physical unit).
+- `12`: ADC resolution (bits).
+- `0`: ADC zero value.
+- `0`: Initial value.
+- `2178 `, `3497`, `1366`: Checksum (sum of all signal samples modulo 2^16).
+- `0`: Block size.
+- `ECG`, `ICP`, `RESP`: Signal labels (e.g., lead names).
 
 ---