Improve and update .hea example

spec/header-files.md

@@ -21,29 +21,52 @@ 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. |
+| 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:**
+### Examples
+
+**Basic header (uniform sampling frequency):**
+record 100 from the [MIT-BIH database](https://www.physionet.org/content/mitdb)
+```text
+100 2 360 650000
+```
+
+- `100`: Record name (must match the filename prefix).
+- `2`: Number of signals in the record.
+- `360`: Sampling frequency in Hz.
+- `650000`: Number of samples for each signal.
+
+**Multi-frequency header (unique sampling rates per channel):**
+
 ```text
-100 2 360 650000 12:00:00 01/01/2000
+12345 3 62.5 625 12:00:00 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`: Number of samples. For multi-frequency records this is the total frames for each signal.
+- `12:00:00`: Base time.
+- `30/01/1989`: Base date.
+
 ---
 
 ## Signal Specification Lines
@@ -67,12 +90,43 @@ Each signal has its own line immediately following the record line (for single-s
 | Block size (optional) | Number of samples per block (for formats supporting block I/O). |
 | Description (optional) | Free-text description of the signal (e.g., lead name `ECG Lead II`). |
 
-**Example:**
+### Examples
+
+**Basic header (uniform sampling frequency):**
+record 100 from the [MIT-BIH database](https://www.physionet.org/content/mitdb)
 ```text
-100.dat 212 200 11 1024 995 0 MLII
-100.dat 212 200 11 1024 995 0 V5
+100.dat 212 200 11 1024 995 -22131 0 MLII
+100.dat 212 200 11 1024 1011 20052 0 V5
 ```
 
+- `100.dat`: File name of the signal file.
+- `212`: Format samples are stored in (12-bit two's complement).
+- `200`: ADC gain (i.e. number of digital values per physical unit). 
+- `11`: ADC resolution (bits).
+- `1024`: ADC zero value.
+- `995`, `1011`: Initial value.
+- `-22131`, `20052`: Checksum (sum of all signal samples modulo 2^16).
+- `0`: Block size.
+- `MLII`, `V5`: Description (e.g., signal lead names).
+
+**Multi-frequency header (unique sampling rates per channel):**
+```text
+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`: File name of the signal file.
+- `16`: Format samples are stored in (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`: Description (e.g., signal lead names).
+
 ---
 
 ## Comments and Info Strings
@@ -87,4 +141,5 @@ Each signal has its own line immediately following the record line (for single-s
 
 - A header file may describe signals stored in multiple files or multiple signals in a single file.
 - Fields like sampling frequency, counter frequency, and base time/date improve time-aligned analysis but are optional.
+- Storage format options and details can be found at: [https://physionet.org/physiotools/wag/signal-5.htm](https://physionet.org/physiotools/wag/signal-5.htm)
 - Multi-segment records use a slightly different structure (described separately).